Title of Invention

MAGNET POWDER, A SINTERED MAGNET AND A MAGNETIC RECORDING MEDIUM

Abstract

Magnet powder comprises a primary phase of a hexagonal ferrite containing A, M and R, where A represents at least one element selected from the group consisting of Sr, Ba, Ca and Pb, M represents Co, or Co and Zn, and R represents at lest one element selected from the group consisting of rare earth elements comprising Y, and Bi, wherein said magnet powder has at least two different Curie temperatures, said two different Curie temperatures are present within a range of from 400 to 470°C, and an absolute value of a difference therebetween is 5°C or more.

Full Text

Field of the Invention The present invention relates to a magnet powder, a sintered magnet and a magnetic recording medium, and generally to a hexagonal ferrite suitably used as a permanent magnet material such as a motor for an automobile, and particularly relates to a magnet material containing a hexagonal magnetoplumbite ferrite, and a process for producing the same. The present application is divided out of Indian Patent Application No.1689/CAL/98 (hereinafter referred to as the parent application). BACKGROUND OF THE INVENTION As an oxide permanent magnet material, a strontium (Sr) ferrite and a barium (Ba) ferrite, which are of a magnetoplumbite (M type) hexagonal structure, are mainly used, and they are produced as a sintered magnet and a bonded magnet. What are important among characteristics of a magnet are a residual magnetic flux density (Br) and an intrinsic coercive force (HcJ). Br is determined by the density of the magnet, the degree of orientation of the magnet, and the saturation magnetization (4??s) determined by the crystal structure. Br is expressed by the following equation: Br = 4??s x (degree of orientation) x (density) The Sr ferrite and the Ba ferrite of M type has a 4??s value of about 4,65 kG. The density and the degree of orientation each is about 98% at most in the sintered magnet, which provides the highest values. Therefore, Br of these magnets is limited to about 4.46 kG at most, and it has been substantially impossible to provide a high Br value of 4.5 kG or more. The inventor of the invention have found that the addition of appropriate amounts of La and Zn in an M type ferrite raises its 4??s value by about 200 G at most, and a Br value of 4.5 kG or more can be obtained, as described in US Patent Application Serial No, 08/672,848. In this case, however, since the anisotropic magnetic field (H????which will be described later, is decreased, it is difficult to obtain a Br value of 4.5 kG or more and an HcJ of 3.5 kOe or more at the same time. HcJ is in proportion to the product (H? x fc) of the anisotxopic magnetic field (HA = 2K1/Is) and a single magnetic domain grain fraction (fc), in which K1 represents a crystal magnetic D constant, which is determined by the crystal structure as similar to Is, The M type Ba ferrite has K1 of 3.3 x 106 erg/cm3, and the M type Sr ferrite has K1 of 3.5 x 106 erg/cm3. It has been known that the M type Sr ferrite has the largest K1 value, but it has -3- been difficult to further raise the K1 value. On the other hand, In the case where ferrite particles are in a single magnetic domain condition, the maximum HcJ is expected because the magnetization must be rotated against the anisotropic magnetic field to reverse the magnetization. In order to make ferrite particles into single magnetic domain particles, the size of the ferrite particles must be smaller than the following critical diameter (dc) expressed by the following equation: wherein k represents the Boltzman constant, Tc represents a Curie temperature, and a represents a distance between iron ions. In the case of the M type Sr ferrite, since dc is about 1 ?m, it is necessary for producing a sintered magnet that the crystal grain size of the sintered magnet must be controlled to 1 ?m or less. While it has been difficult to realize such a fine crystal grain and the high density and the high degree of orientation to provide a high Br at the same time, the inventor has proposed a new production process to demonstrate that superior characteristics that cannot be found in the art are obtained, as described in US Patent Application Serial No. 08/072,967. In this process, however, the HcJ value becomes 4.0 kOe when the Br value is -4- 4.4 kG, and therefore it has been difficult to obtain a high HcJ of 4,5 kOe or more with maintaining a high Br of 4.4 kG or more at the same time. In order to control a crystal grain size of a sintered body to 1 ?m or less, it is necessary to make the powder size in the molding step 0.5 ?m or less with taking the growth of the grains in the sintering step into consideration. The use of such fine particles brings about a problem in that the productivity is generally deteriorated due to increase in molding time and increase in generation of cracks on molding. Thus, it has been very difficult to realize high characteristics and high productivity at the same time. It has been known that the addition of A12O3 and Cr2O3 is effective to obtain a high HcJ value. In this case, Al3+ and Cr3+ have effects of increasing HA and suppressing the grain growth by substituting for Fe3+ having an upward spin in the M type structure, so that a high HcJ value of 4.5 kOe or more is obtained. However, when the Is value is reduced, the Br value is considerably reduced since the sintered density is reduced. As a result, the composition exhibiting the maximum HcJ of 4.5 kOe can only provide a Br; value of 4.2 kG. A sintered magnet of the conventional anisotropic M type ferrite has a temperature dependency of HcJ of about +13 Oe/°C and a relatively high temperature coefficient of about from +0.3 to +0.5%/°C, which sometimes bring about great reduction in HcJ on the low temperature side and thus demagnetization. In order to prevent such demagnetization, the HcJ value at room temperature must be a large value of about 5 kOe, and therefore it is substantially impossible to obtain a high Br value at the same time. Powder of an isotropic M type ferrite has a temperature dependency of HcJ of at least about +8 Oe/°C, although it is superior to the anisotropic sintered magnet, and a temperature coefficient of +0.15%/°C, and thus it has been difficult to further improve the temperature characteristics. Because a ferrite magnet is excellent in environmental resistance and is not expensive, it is frequently used in a motor in various parts of an automobile. Because an automobile may be used under severe conditions including intense cold and heat, the motor is required to stably function under such severe conditions. However, the conventional ferrite magnet has a problem of considerable deterioration in coercive force under low temperature conditions, as described in the foregoing. Even though a ferrite magnet satisfies such characteristics, it has a problem in that those having a low squareness (Hk/HcJ) in the demagnetization curve have a low (BH)max and a deteriorated change with time. If a magnet having a high degree of orientation, which is obtained by the production process using an organic solvent system, can be obtained by a production process using an aqueous solvent, the production becomes easy to provide advantages in productivity, and moreover it does not lead environmental contamination to make possible to omit apparatuses for preventing contamination. SUMMARY OF THE INVENTION An object of the invention according to the parent application is to provide a ferrite magnet and process for producing the same, which has a high residual magnetic flux density and a high coercive force that cannot be attained by the conventional M type ferrite magnet, is excellent in temperature characteristics of coercive force, has excellent magnetic characteristics in that decrease in coercive force does not occur particularly in a low temperature region, and is excellent in squareness in the demagnetization curve. Another object of the invention according to the parent application is to provide a ferrite magnet and a process for producing the same, which can be exhibit superior characteristics even though the content of expensive Co is reduced. Further object of the invention according to the parent application is to provide a ferrite magnet and a process for producing the same, which exhibits a high degree of orientation that can be realized by the organic solvent system even though it is produced by a production process using an aqueous system. Still further object of the invention according to the parent application is to provide a motor and a magnetic recording medium having excellent characteristics. Accordingly, the invention according to the parent application provides a process for producing a hexagonal ferrite sintered magnet comprising a primary phase of a hexagonal ferrite containing A, Co, R and Fe, where A represents Sr, 8a or Ca, and R represents at; least one element selected from rare earth elements including Y, and Bi, said process comprising adding a part or whole of constituent elements to ferrite particles comprising a primary phase of a hexagonal ferrite containing Sr, Ba or Ca ; molding mixture and sintering the molded material. The objects of the invention can be attained by one of the constitutions (!) to (26) described below. (1) Magnet powder comprising a primary phase of a hexagonal forrite containing Sr or Ba, Co ana R, where R represents at least one element selected from the group consisting of rare earth elements including Y, and Bi, wherein the magnet powder has at least two different Curie temperatures, the two different Curie temperatures are present within a range of from 400 to 470°C, and an absolute value of a difference therebetween is 5°C or more. (2) Magnet powder as described in item (1), wherein R represents at least La. (3) Magnet powder as described in item (1) or (2), wherein the hexagonal ferrlte is a magnetoplumbite ferrite. (4) Magnet powder as described in any one of items (1) to (3), wherein the hexagonal ferrite comprises A, R, Fe, and M, wherein A represents at least one element selected from the group consisting of Sr, Sa, Ca and Pb, provided that Sr or Ba are essentially included in A, R represents at least one element selected from the Accordingly, the present invention provides a Magnet powder comprising a primary phase of a hexagonal ferrite containing A, M and R, where A represents at least one element selected from the group consisting of Sr, Ba, Ca and Pb, M represents Co, or Co and Zn, and R represents at lest one element selected from the group consisting of rare earth elements comprising Y, and Bi, wherein said magnet powder has at least two different Curie temperatures, said two different Curie temperatures are present within a range of from 400 to 470°C, and an absolute value of a difference therebetween is 5°C or more. The present invention also provides a sintered magnet comprising a primary phase of a hexagonal ferrite containing A, M and R, where A represents at least one element selected from the group consisting of Sr, Ba, Ca and Pb, M represents Co, or Co and Zn, and R represents at least one element selected from the group consisting of rare earth elements comprising Y, and Bi, wherein said sintered magnet has at least two different Curie temperatures, said two different Curie temperatures are present within a range of from 400 to 470°C, and an absolute value of a difference therebetween is 5°C or more. The present invention further provides a magnetic recording uedium comprising a thin film magnetic layer comprising a primary phase of a hexagonal ferrite containing A, M and R, where A represents at least one element selected from the group consisting of Sr, Ba, Ca and Pb, M represents Co, or Co and Zn, and R represents at least one element selected from the group consisting of rare earth elements comprising Y, and Bi, wherein said thin film magnetic layer has at least two different Curie temperatures, said two different Curie temperatures are present within a range of from 400 to 470°C, and an absolute value of a difference therebetween is 5°C or more. group consisting of rare earth elements including Y, and Bi, and M represents Co, or Co and Zn, and proportions of the elements with respect to the total amount of the metallic elements are from 1 to 13 atomic% for A, from 0.05 to 10 atomic% for R, from 80 to 95 atomic% for Fe, and from 0.1 to 5 atomic% for M. (5) Magnet powder as described in any one of items (1) to (4), wherein the proportion of Co in M is 10 atomic% or more. (6) Magnet powder as described in any one of items (1) to (5), wherein the magnetic powder has an absolute value of a temperature coefficient of a coercive force within a range of from -50 to 50°C of 0.1%/°C or less. (7) A bonded magnet comprising magnet powder as described in any one of items (1) to (6). (8) A motor comprising a bonded magnet as described in item (7). (9) A magnetic recording medium comprising magnet powder as described in any one of items (1) to (6). (10) A sintered magnet comprising a primary phase of a hexagonal ferrite containing Sr or Ba, Co and R, where R (5) represents at least one element selected from the group consisting of rare earth elements including Y, and Bi, wherein the sintered magnet has at least two different Curie temperatures, the two different Curie temperatures are present within a range of from 400 to 470° C, and an absolute value of a difference therebetween is 5° C or more. (11) A sintered magnet as described in item (10), wherein R represents at least La. (12) A sintered magnet as described in item (10) or (11), wherein the hexagonal ferrite is a magnetoplumbite ferrite. (13) A sintered magnet as described in any one of items (10) to (12), wherein the hexagonal ferrite comprises A, R, Fe, and M, wherein A represents at least one element selected from the group consisting of Sr, Ba, Ca and Pb, provided that Sr or Ba are essentially included in A, R represents at least one element selected from the group consisting of rare earth elements including Y, and Bi, and M represents Co, or Co and Zn, and proportions of the elements with respect to the total amount of the metallic elements are -12- from 1 to 13 atomic% for A, from 0.05 to 10 atomic% for R, from 80 to 95 atomic% for Fe, and from 0.1 to 5 atomlc% for M. (14) A sintered magnet as described in any one of items (10) to (13), wherein the proportion of Co in M is 10 atomic% or more. (15) A sintered magnet as described in any one of items (10) to (14), wherein the sintered magnet has a squareness Hk/HcJ of 90% or more. (16) A sintered magnet as described in any one of items (10) to (15), wherein the sintered magnet has a degree of orientation Ir/Is of 96% or more. (17) A sintered magnet as described in any one of items (10) to (15), wherein the sintered magnet a ratio of a total X-ray diffraction intensity from c plane (?I(OOL)) to a total X-ray diffraction intensity from all planes (2 I(hkL)) of 0.85 or more. (18) A sintered magnet as described in any one of items (10) to (17), wherein the sintered magnet has an absolute value of a temperature coefficient of a coercive force within a range of from -50 to 50°C of 0.25%/°C or less. (19) A motor comprising a sintered magnet as described in any one of items (10) to (18). (14) (20) A magnetic recording medium comprising a thin film magnetic layer comprising a primary phase of a hexagonal ferrite containing Sr or Ba, Co and R, where R represents at least one element selected from the group consisting of rare earth elements including Y, and Bi, wherein the thin film magnetic layer has at least two different Curie temperatures, the two different Curie temperatures are present within a range of from 400 to 470° C, and an absolute value of a difference therebetween is 5° C or more. (21) A process for producing a hexagonal ferrite sintered magnet comprising a primary phase of a hexagonal ferrite containing A, Co, R and Fe, where A represents Sr, Ba or Ca, and R represents at least one element selected from rare earth elements including Y, and Bi, said process comprising adding a part or whole of the constitutional elements to ferrite particles comprising a primary phase of a hexagonal ferrite containing Sr, Ba or Ca; molding the mixture; and sintering the molded material. (22) A process for producing a hexagonal ferrite sintered magnet as described in item (21), wherein said part of the constitutional elements is at least one element selected from Co and R. (23) A process for producing a hexagonal ferrite sintered magnet as described in item (21) or (22), wherein -14 - in adding a part or whole of the constitutional elements. Si and Ca are further added. (24) A process for producing a hexagonal ferrite sintered magnet as described in any one of items (21) to (23), wherein in adding a part or whole of the constitutional elements, a dispersant is further added. (25) A process for producing a hexagonal ferrite sintered magnet as described in any one of items (21) to (24), wherein said part or whole of the constitutional elements is added at pulverization. (26) A process for producing a hexagonal ferrite sintered magnet as described in item (24) or (25), wherein said dispersant is an organic compound having a hydroxyl group and a carboxyl group or a neutralized salt thereof or a lactone thereof, an organic compound having a hydroxy-methylcarbonyl group, or an organic compound having an enol-type hydroxyl group capable of being dissociated as an acid or a neutralized salt thereof; and said organic compound has from 3 to 20 carbon atoms, where a hydroxyl group is bonded to at least 50% of carbon atoms other than carbon atoms double-bonded to an oxygen atom. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is an SEM photograph of the structure of a plane of the sintered magnet Sample No. 1 of the invention. -15- Figure 2 is an SEM photograph of the structure of c plane of the sintered magnet Sample No. 1 of the invention. Figure 3 is an SEM photograph of the structure of a plane of the Comparative Sample No. 3. Figure 4 is an SEM photograph of the structure of c plane of the Comparative Sample No. 3. Figure 5 is a graph showing a ?-T curve of Sample No. 1 of the invention. Figure 6 is a graph showing a ?-T curve of Sample No. 2 of the invention. Figure 7 is a graph showing a ?-T curve of Comparative Sample No. 1. Figure 8 is a graph showing degrees of orientation depending on the substituted amount of La and Co of the samples of the invention. Figure 9 is a graph showing HcJ-Br characteristics of the samples of the invention. Figure 10 is a graph showing degrees of orientation of the samples of the invention. Figure 11 is a graph showing degrees of magnetic orientation of depending on the density at a calcination temperature of l,250°C. Figure 12 is a graph showing HcJ-Br and Hk/HcJ at a calcination temperature of l,250°C. Figure 13 is a graph showing squareness Hk/HcJ of the sintered body samples at l,220°C of the samples of the invention. Figure 14 is a graph showing degrees of magnetic orientation (Ir/Is) depending on the substituted amount of the samples of the invention. Figure 15 is a graph showing magnetic characteristics of the samples baked at l,200°C, l,220°C and l,240°C. Function and Effective As a result of investigations with respect to improvement in the magnetic characteristics made by the inventors, it has been found that superior characteristics is obtained by a magnetoplumbite ferrite, as described in Japanese Patent Application No. 9-56856. However, the ferrite material having this composition only provides a squareness of from 80 to 90% by the conventional production process, where the addition is conducted at the mixing of the raw materials. On the other hand, the inventors has proposed a production process, in which a high degree of orientation can be obtained by an aqueous process, as described in US Patent Application Serial No. 08/984,087. However, even when such a process is employed, it is not sufficient as compared with a degree of orientation Ir/Is of from 97 to 98% obtained by a process using an organic solvent system proposed in US Patent Application Serial No. 08/072,967. As a result of earnest investigations made by the inventor taking these factors into consideration, it has been found that a magnet having a high squareness can be obtained by a magnetoplumbite ferrite having a structure exhibiting two different Curie temperatures, as described in Japanese Patent Application No. 9-56856 described above. Furthermore, the employment of this structure can reduce the content of Co. It has been also found at the same time that as one process for producing the structure, it is suitable to add a compound containing at least one element selected from Sr or Ba, R (where R represents rare earth elements including Y), Co and Fe (i.e., oxides of these elements and compounds that are converted into the oxides) on the step of preparing a slurry for molding, preferably on the wet pulverizing step or the dry pulverizing step. It has been also found that by employing this production process, a high degree of orientation that is obtained by the organic solvent system can be obtained by using an aqueous dispersant system. A preferred composition of the M type Sr ferrite of the invention is a composition containing at least optimum amounts of La and Co. As a result, while Is is not lowered, rather Is and K1 are simultaneously increased to increase HA, and thus a high Br value and a high HcJ value are realized. Specifically, in the sintered magnet of the invention, satisfactory characteristics can be obtained when the coercive force HcJ (unit: kOe) and the residual magnetic flux density Br (unit: kG) satisfy the following conditions at an ordinary temperature of about 25°C: When HcJ ? 4 Br + 1/3HcJ ? 5.75 (I) When HcJ Br + 1/l0HcJ ? 4.82 (II) It has been reported that the conventional Sr ferrite sintered magnet exhibits Br of 4.4 kG and HcJ of 4.0 kOe, but none has been obtained that has HcJ of 4 kOe or more and satisfies the equation (I). In other words, if HcJ is increased, Br must be low. In the sintered magnet of the invention, although the combination addition of Co and Zn lowers the coercive force lower than the case of the single addition of Co, in some cases lower than 4 kOe, the residual magnetic flux density is considerably increased. At this time, the magnetic characteristics satisfying the equation (II) are obtained. There has been no conventional Sr ferrite sintered magnet having HcJ of less than 4 kOe that satisfies the equation (II). -19- Because the ferrite of the invention has an anisotropy constant K1 and an anisotropic magnetic field (HA) larger than the conventional ferrite, a larger HcJ can be obtained with the same grain size, and the grain size can be reduced with the same HcJ to be obtained. For example, an HcJ value of 4.5 kOe or more can be obtained with an average grain diameter of the sintered body of from 0.3 to 1 ??m, and even in the case of from 1 to 2 ??m, an HcJ value of 3.5 kOe or more can be obtained. Accordingly, the time for pulverization and molding can be reduced, and the improvement in yield of the product can be realized. While the invention exhibits a greater effect of enhancing the HcJ when applied to a sintered magnet, ferrite powder produced according to the invention can be mixed with a binder, such as plastics and rubber, to form a bonded magnet. Furthermore, a coating type magnetic recording medium can be obtained in such a manner that a coating composition is prepared by mixing and kneading the magnet powder with a binder, and the coating composition is coated on a substrate comprising a resin or the like, followed by hardening if necessary, to form a magnetic layer. The magnet material of the invention has a small temperature dependency of HcJ, and particularly the magnet powder of the invention has a considerably small -20- temperature dependency of HcJ. Specifically, the sintered magnet of the invention has an absolute value of a temperature coefficient of HcJ within a range of from -50 to 50°C of 0.25%/°C or less, which can be easily reduced to 0.20%/°C or less. The magnet powder of the invention has an absolute value of a temperature coefficient of HcJ within a range of from -50 to 50°C of 0.1%°C or less, which can be easily reduced to 0.05%/°C or less. Owing to such good temperature characteristics of HcJ, the excellent magnetic characteristics satisfying the equation (V) can be obtained. Such superior magnetic characteristics under the low temperature environment cannot be attained by the conventional Sr ferrite magnet. A Ba ferrite represented by the following formula: is disclosed in Bull. Acad. Sci. USSR. phys. Ser. (English Transl.), vol. 25 (1961), pp. 1405-1408 (hereinafter referred to as Reference 1). In this Ba ferrite, M3+ is La3+, Pr3+ or Bi3+, and M2+ is Co2+ or Ni2+. While it is not clear as to whether Ba ferrite of Reference 1 is powder or a sintered body, this is similar to the Sr ferrite of the invention in the point of inclusion of La and Co. Fig. 1 of Reference 1 shows the change of saturation magnetization -21- depending on the change of x for a Ba ferrite containing La and Co, but in Fig. 1, the saturation magnetization is reduced with the increase of x. Although Reference 1 discloses that the coercive force increases by a few times, there is not disclosure of specific values. In the invention, on the other hand, by employing the composition, to which the optimum amounts of La and Co are added, for the Sr ferrite magnet, the considerable increase of HcJ and the slight increase of Br are realized, and the considerable improvement in temperature dependency of HcJ is also realized. In the invention, by adding the optimum amounts of La and Co to the Sr ferrite magnet powder, the HcJ is greatly increased and its temperature dependency is considerably reduced. It is firstly found in the invention that the combination addition of La and Co to a Sr ferrite provides such effects. A Ba ferrite represented by the following formula: is disclosed in Indian Journal of Pure and Applied,Physics, vol. 8, July 1970, pp.412-415 (hereinafter referred to as Reference 2). This ferrite is similar to the magnet material of the invention in the point of inclusion of La and Co. However, in Reference 2, the saturation -22- magnetization as when M2+ is Co2* is such low values of 42 cgs unit at room temperature and 50 cgs unit at OK. While specific values are not disclosed, Reference 2 states that it cannot be a magnet material due to a low coercive force. It is considered this is because the composition of the ferrite of Reference 2 deviates the scope of the invention (the amounts of La and Co are larger than the invention). An isometric hexagonal ferrite pigment represented by the following formula: Mx(I)My(II)MZ(III)Fe12-(y+z)O19 is disclosed in Japanese Patent Application Kokai No. 62-100417 (hereinafter referred to as Reference 3). In the formula, M(I) is a combination of Sr, Ba, a rare earth metal, etc. with a monovalent cation; M(II) is Fe(II), Mn, Co, Ni, Cu, Zn, Cd or Mg; and M(III) is Ti, etc. The hexagonal ferrite pigment disclosed in Reference 3 is similar to the magnet material of the invention in the point that a rare earth metal and Co are simultaneously contained. However, Reference 3 does not disclose any example in that La and Co are simultaneously added, and there is no disclosure that the simultaneous addition of them improves the saturation magnetization and the coercive force. Furthermore, in the examples of Reference 3 where -23- Co is added, Ti is simultaneously added as the element of M(III). Because the element of M(III), particularly Ti, functions as an element lowering the saturation magnetization and the coercive force, it is clear that Reference 3 does not suggest the constitution and the effect of the invention. An optomagnetic recording medium comprising a magnetoplumbite barium ferrite characterized by substituting a part of Ba with La and a part of Fe with Co is disclosed in Japanese Patent Application Kokai No. 62-119760 (hereinafter referred to as Reference 4). This Ba ferrite is similar to the Sr ferrite of the invention in the point of inclusion of La and Co. However, the ferrite of Reference 4 is a material for "optomagnetic recording", in which information is written as a magnetic domain in a magnetic thin film by utilizing a heat effect of light, and the information is read out: by utilizing a optomagnetic effect, which is of a technical field different from the magnet material of the invention. Furthermore, in Reference 4, Ba. La and Co are essential in the compositional formula (I), and in the formulae (II) and (III), there is only disclosed that an unidentified tetra-valent metallic ion is added thereto. On the other hand, the ferrite of the invention is the Sr ferrite, in which Sr is essential, and the optimum amounts of La and Co are -24- added thereto, which is different from the composition of Reference 4. That is, as explained with respect to Reference 1, the Sr ferrite of the invention realizes the considerable increase of HcJ and the slight increase of Br, and also realizes the considerable improvement in temperature dependency of HcJ, by using the composition of the Sr ferrite magnet containing the optimum amounts of La and Co. The effects can be obtained when the combination addition of La and Co is applied to the Sr ferrite, and is realized in the composition of the invention, which is different from Reference 4, DETAILED DESCRIPTION OF THE INVENTION The magnet material of the invention comprising a primary phase of a hexagonal magnetoplumbite ferrite containing Sr or Ba, Co and R, where R represents at least one element selected from the group consisting of rare earth elements including Y, and Bi, wherein the magnet material has at least two different Curie temperatures Tcl and Tc2, the two different Curie temperatures are present within a range of from 400 to 470°C, and an absolute value of a difference therebetween is 5°C or more. Thus, the magnet material has two different Curie temperatures, whereby the squareness Hk/KcJ is markedly improved. The Curie temperature can be obtained from an inflection point of a magnetization a - temperature T curve -25- of a magnet. Specifically, it is obtained from the temperature of the point of intersection of the tangent line of the low temperature side curve at the inflection point of the ?-T curve and the temperature axis. The two different Curie temperatures Tc1 and Tc2 exhibit the absolute value of the difference therebetween of 5°C or more, preferably 10°C or more. The upper limit thereof is not particularly limited, but it is generally about 465°C. These Curie temperatures fall within the range of from 400 to 470°C, preferably from 430 to 460°C It is considered that the two Curie temperature mean that the constructional structure of the ferrite crystal of the invention has a two-phase structure of an M type ferrite different in magnetic properties due to the production process described later, provided that a single phase of an M phase is observed by ordinary X-ray diffractiometry. The squareness Hk/HcJ is preferably 90% or more, more preferably 92% or more, which becomes 95% at most. The magnet of the invention preferably has a degree of orientation Ir/Is of 96.5% or more, more preferably 97% or more, which becomes about 98% at most. A high Br value can be obtained by increasing the degree of orientation. Since the degree of magnetic orientation of a molded body is influenced by the density of the molded body, the crystallographic degree of orientation (X-ray degree of -26- orientation) is important, which is obtained from the plane index and the intensity of the peaks appearing in the X-ray diffractiometry measurement result of the surface of the molded body. The X-ray degree of orientation of the molded body controls the degree of magnetic orientation of a sintered body to a certain extent. Preferably, ??(00L)/??I(hkL) is used as the X-ray degree of orientation. (00L) is an expression of the generic name of the c planes such as (004) and (006), and ??(00L) is a total intensity of the peaks of all (00L) planes. (hkL) means all the peaks detected, and ??(hkL) is a total intensity thereof. Thus, ??(00L)/??(hkL) means the degree of c. plane orientation. ??I(00L)/??(hkL) is preferably 0.85 or more, more preferably 0.9 or more, the upper limit of which is about 1.0. In the following examples, this is expressed by ??(001)/??(hkl) in some cases. The magnet of the invention comprises a primary phase of a hexagonal magnetoplumbite ferrite containing Sr or Ba, Co and R, where R represents at least one element selected from the group consisting of rare earth elements including Y, and Bi, wherein the primary phase is preferably at least one element selected from the group consisting of Sr, Ba, Ca and Pb, and when A represents an element essentially including Sr or Ba, R represents at least one element selected from the group consisting of rare earth elements -27- including Y, and Bi, and M represents Co, or Co and Zn, the proportions of the elements A, R, Fe and M with respect to the total amount of the metallic elements are from 1 to 13 atomic% for A, from 0.05 to 10 atomic% for R, from 80 to 95 atomic% for Fe, and from 0.1 to 5 atomic% for M. These are more preferably from 3 to 11 atomic% for A, from 0.2 to 6 atomic% for R, from 83 to 94 atomic% for Fe, and from 0.3 to 4 atomic% for M. These are particularly preferably from 3 to 9 atomic% for A, from 0.5 to 4 atomic% for R, from 86 to 93 atomic% for Fe, and from 0.5 to 3 atomic% for M. In the constitutional elements, A is at least one element selected from the group consisting of Sr, Ba, Ca and Pb. When the amount of A is too small, the M type ferrite is not formed, or the amount of a non-magnetic phase, such as ?-Fe2O3, is increased. When the amount of A is too large, the M type ferrite is not formed, or the amount of a non-magnetic phase, such as SrFeO3-x, is increased. The proportion of Sr in A is preferably 51 -28- atomic% or more, more preferably 70 atomic% or more, and especially preferably 100 atomic%. When the proportion of Sr in A is too small, the improvement in saturation magnetization and the considerable improvement in coercive force is difficult to be obtained at the same time. R is at least one element selected from the group consisting of rare earth elements including Y, and Bi. It is preferred that R essentially contains La, Nd and Pr, particularly La. When the amount of R is too small, the amount of M forming a solid solution becomes small, and thus the effect of the invention is difficult to be obtained. The amount of R is too large, the amount of a non-magnetic foreign phase, such as ortho-ferrite, becomes large. The proportion of La in R is preferably 40 atomic% or more, more preferably 70 atomic% or more, and it is most preferred to use only La as R from for the improvement in saturation magnetization. This is because La exhibits the largest limiting amount forming a solid solution with a hexagonal M type ferrite. Therefore, when the proportion of La in R is too small, the amount of R forming a solid solution cannot become large, and as a result, the amount of the element M forming a solid solution also cannot become large, which reduces the effect of the invention. The combination use of Bi lowers the calcination temperature and the sintering temperature, and is -29- advantageous from the standpoint of productivity. The element M is Co, or Co and Zn. When the amount of M is too small, the effect of the invention is difficult to be obtained. When the amount of M is too large, Br and HcJ are reduced, and the effect of the invention is difficult to be obtained. The proportion of Co in M is preferably 10 atomic% or more, more preferably 20 atomic% or more. When the proportion of Co is too small, the improvement in coercive force becomes insufficient. The hexagonal magnetoplumbite ferrlte is preferably represented by the following formula: Al-xRx(Fe12-yMy)2O19 wherein 0.04 ? x ? 0.9, particularly 0.04 ? x ? 0.6, 0.04 ? y ? 0.5, 0.8? x/y ? 5, and 0.7 ? z ? 1.2. It is more preferably 0.04 ? x ? 0.5, 0.04 ? y ? 0.5, 0.8? x/y ? 5, and 0.7 ? z ? 1.2. It is particularly preferably -30- 0.1 ? x ? 0.4, 0.1 ? y ? 0.4, and 0,8 ? z ? 1.1, and especially preferably 0.9 ? z ? 1.05. In the above formula, when x is too small, i.e., the amount of the element R is too small, the amount of the element M forming a solid solution with the hexagonal ferrite cannot be large, and thus the improving effect of the saturation magnetization and/or the improving effect of the anisotropic magnetic field become insufficient. When x is too large, the element R cannot substitute in the hexagonal ferrite to form a solid solution, and the saturation magnetization is reduced due to the formation of an ortho-ferrite containing the element H. When y is too small, the improving effect of the saturation magnetization and/or the improving effect of the anisotropic magnetic filed becomes insufficient. When y is too large, the element M is difficult to substitute in the hexagonal ferrite to form a solid solution. Even in the range where the element M can substitute to form a solid solution, deterioration of the anisotropic constant (K1) and the anisotropic magnetic field (HA) becomes large. When z is too small, the saturation magnetization is reduced since the amounts of non-magnetic phases containing Sr and the -31- element R are increased. When z is too large, the saturation magnetization is reduced since the amount of an ?-Fe2O3 phase or a non-magnetic spinel ferrite phase containing the element M is increased. The above formula assumes that no impurity is contained. In the formula above, when x/y is either too small or too large, the valences of the element R and the element M cannot be balanced, and a foreign phase, such as a W type ferrite, is liable to be formed. As the element M is divalent, when the element R is trivalent, x/y is ideally 1, The reason why the permissible range of x/y is largely extended to the direction of more than 1 is that even if y is small, the valences can be balanced by the reduction from Fe3+ to Fe2+. In the above formula showing the composition, the number of oxygen atoms of 19 means the stoichiometrlc compositional ratio when all the elements R are trivalent, and x = y and z = 1. Thus, the number of oxygen atoms changes depending on the kind of the element R and the values of x, y and z. In the case where the sintering atmosphere is a reducing atmosphere, there is a possibility of forming lack of oxygen (vacancy). Furthermore, while Fe is generally present as trivalent in the M type ferrite, there is a possibility of changing it to divalent. There is a possibility that the valence of the element -32- represented by M, such as Co, is changed, and the proportion of oxygen to the metallic elements is also changed according thereto. While the number of oxygen atoms is shown as 19 irrespective to the kind of R and the values of x, y and z in the specification, the actual number of oxygen atoms may be somewhat deviated from the stoichiometric compositional ratio. The composition of a ferrite can be measured by fluorescent X-ray quantitative analysis. The presence of the primary phase described above is confirmed by X-ray diffraction and electron beam diffraction. The magnet powder may contain B2O3. The calcination temperature and the sintering temperature can be lowered by the addition of B2O3, which is advantageous from the standpoint of productivity. The content of B2O3 is preferably 0.5% by weight or less based on the total amount of the magnet powder. When the content of B2O3 is too large, the saturation magnetization becomes low. At least one of Na, K and Rb may be contained in the magnet powder. The total content of these elements, as converted into Na20, K2O and Rb2O, is preferably 3% by weight or less based on the total amount of the magnet powder. When the content of these element is too large, the saturation magnetization becomes low. As these elements are represented by MI, MI are contained in the -33- ferrite in the form of the following formula In this case, it is preferred that 0.3 In addition to these impurities, Si, Al Ga, In, Li, Mg, Mn, Ni, Cr, Cu, Ti, Zr, Ge, Sn, V, Nb, Ta, Sb, As, W and Mo may be contained in the form of oxides in an amount of 1% by weight or less for silicon oxide, 5% by weight or less for aluminum oxide, 5% by weight or less for gallium Oxide, 3% by weight or less for indium oxide, 1% by weight or less for lithium oxide, 3% by weight or less for magnesium oxide, 3% by weight or less for manganese oxide, 3% by weight or less for nickel oxide, 5% by weight or less chromium oxide, 3% by weight or less for copper oxide, 3% by weight or less for titanium oxide, 3% by weight or less for zirconium oxide, 3% by weight or less for germanium oxide, 3% by weight or less for tin oxide, 3% by weight or less for vanadium oxide, 3% by weight or less for niobium oxide, 3% by weight or less for tantalum oxide, 3% by weight or less for antimony oxide, 3% by weight or less for arsenic oxide, 3% by weight; or less for tungsten oxide, and -34- 3% by weight or less for molybdenum oxide. The process for producing the sintered magnet is described below. In the process for producing the sintered magnet containing the above-described ferrite, iron oxide powder and powder of compounds containing Fe, Sr or Ba, Co, R (wherein R represents at least one element selected from the group consisting of rare earth elements including Y) and Bi are used, and one kind of these raw material powder or a mixture of two or more kinds thereof is calcined. After calcination, one kind or two or more kinds of compounds containing Fe, Sr, Co, R (wherein R represents at least one element selected from the group consisting of rare earth elements including Y) and Bi are added and mixed, and then sintered. The powder of compounds containing Fe, Sr, Co, R (wherein R represents at least one element selected from the group consisting of rare earth elements including Y) and Bi may be an oxide or a compound that is converted into the oxide on heating, for example, a carbonate, a hydroxide and nitrate. While the average particle size of the raw material powder is not particularly limited, iron oxide is preferably in a form of fine powder, and more preferably has an average size of the primary particle of 1 ?m or less, especially preferably 0.5 ?m or less. The raw material powder may further contain -35- depending on necessity, in addition to the above-described components, B2O3 and other compounds, such as compounds containing Si, Al, Ga, In, Li, Mg, Mn, Ni, Cr, Cu, Ti, Zr, Ge, Sn, V, Nb, Ta, Sb, As, W and Mo, as well as unavoidable impurities. The calcination may be conducted in the air at a temperature of from 1,000 to l,350°C for from 1 second to 10 hours, particularly from 1 second to 3 hours. The resulting calcined body substantially has a magnetoplumblte ferrite structure, and the average particle size of the primary particle is preferably 2 ?m or less, more preferably 1 ?m or less, particularly preferably from 0.1 to 1 ?m, especially preferably from 0.1 to 0.5 ?m. The average particle size can be measured by a scanning electron microscope. After pulverizing the calcined body, the sintered magnet is produced by adding one kind or a mixture of two or more kinds of compounds containing Fe, Sr or Ba, Co, R (wherein R represents at least one element selected from the group consisting of rare earth elements including Y) and Bi, and molded, followed by sintering. Specifically, it is preferably produced according to the following procedures. The addition amount of the compound powder is from 1 to 100% by volume of the calcined body, preferably from 5 to 70% by volume, and particularly from 10 to 50% by -36- volume. The time at which the compounds are added is not particularly limited if it is after the calcining and before the sintering, but it is preferred to add on pulverizing described later. The kind and amount of the raw material powder to be added are arbitrary, and the same raw material may be added separately before and after the calcination, provided that 30% or more, preferably 50% or more of the total amount thereof is preferably added on a step after the calcination. The average powder size of the compound to be added is generally from 0.1 to 2 ?m. In the invention, wet molding is conducted by using a slurry for molding containing oxide magnetic material powder, water as a dispersing medium, and a dispersant. In order to enhance the effect of the dispersant, a wet pulverizing step is provided before the wet molding step. In the case where the calcined material powder is used as the oxide magnetic material powder, since the calcined material powder is generally in a granule form, a dry coarse pulverizing step is preferably provided before the wet pulverizing step for coarse pulverization or deflocculation of the calcined material powder. In the case where the oxide magnetic material powder is produced by a coprecipitation method or a hydrothermal synthesis method, the dry coarse pulverizing step is generally not 34 provided, and the wet pulverizing step is not necessary, but in order to further enhance the degree of orientation, it is preferred to conduct the wet pulverizing step. In the following, the case is described, in which the calcined material powder is used as the oxide magnetic material powder, and the dry coarse pulverizing step and the wet pulverizing step are conducted. In the dry coarse pulverizing step, pulverization is conducted until the BET specific surface area becomes 2 times to 10 times the initial value. After pulverization, the average particle diameter is preferably about from 0.1 to 1 ?m, the BET specific surface area is preferably about from 4 to 10m2/g, and the CV [Coefficient of Variation = Deviation/Expected value] of the particle diameter is preferably maintained at 80% or less, more preferably from 10 to 70%. This means for pulverization is not particularly limited, and a dry vibration mill, a dry attritor (medium stirring mill) and a dry ball mill can be used. It is preferred to use a dry vibration mill. The pulverizing time is appropriately determined depending on the pulverizing means employed. It is preferred that a part of the raw material powder is added on the dry pulverizing step. The dry coarse pulverization also has a function of reducing the coercive force HcB by introducing crystal distortion to the calcined material powder. Agglomeration -37- is suppressed by the reduction of the coercive force, and the dispersibility is improved. The soft magnetization improves the degree of orientation. The soft-magnetized particles are restored into the inherent hard magnetization through the subsequent sintering step to make a permanent magnet. On the dry pulverization, SiO2 and CaCO3 converted into CaO on heating are generally added. A part of SiO2 and CaCO3 may be added before the calcination, and in that case, improvement in characteristics is observed. After the dry coarse pulverization, a slurry for pulverization containing the pulverized powder and water is prepared, and the wet pulverization is conducted using the same. The content of the calcined material powder in the slurry for pulverization is preferably about from 10 to 70% by weight. The means for pulverizing used in the wet pulverization is not particularly limited, a ball mill, an attritor and a vibration mill are generally preferably used The pulverizing time is appropriately determined depending on the pulverizing means employed. After the wet pulverization, a slurry for molding is prepared by condensing the slurry for pulverization. The condensation can be conducted by centrifugation. The content of the calcined material powder in the slurry for molding is preferably about from 60 to 90% by weight. -38- In the wet molding step, molding is conducted in the presence of a magnetic field by using the slurry for molding. The pressure for molding can be about from 0.1 to 0.5 ton/cm2, and the applied magnetic field can be about from 5 to 15 kOe. The use of a non-aqueous dispersing medium in the slurry for molding is preferred since a high degree of orientation is obtained. In the invention, however, the slurry for molding using an aqueous dispersing medium containing a dispersant is employed. Examples of the dispersant that is preferably used in the invention include an organic compound having a hydroxyl group or a carboxyl group, its neutralized salt, its lactone, an organic compound having a hydroxymethylcarbonyl group, an organic compound having an enol type hydroxyl group that can be dissociated as an acid, and its neutralized salt. In the case where a non-aqueous dispersing medium is used, as described in e.g., US Patent Application Serial No 08/072,967, a surface active agent, such as oleic acid, is added to an organic solvent, such as toluene and xylene, to form a dispersing medium. By using such a dispersing medium, a high degree of magnetic orientation of 98% at the highest even when ferrite grains of a submicron size, which are hard to be dispersed, are employed. The above-described organic compounds have a carbon -39- number of from 3 to 20, preferably from 4 to 12, in which hydroxyl groups are bonded to 50% or more of the carbon atoms except for the carbon atoms attached to oxygen atoms via a double bond. When the carbon number is 2 or less, the effect of the invention cannot be obtained. Even when the carbon number is 3 or more, if the ratio of carbon atoms, to which hydroxyl groups are attached, except for the carbon atoms attached to oxygen atoms via a double bond is less than 50%, the effect cannot be obtained. The ratio of the carbon atoms, to which hydroxyl groups are attached* is limited to the above-described organic compounds, and there is no limitation for the dispersants themselves. For example, when a lactone of an organic compound having a hydroxyl group and a carboxyl group (hydroxycarboxylic acid) is used as the dispersant, the ratio of the carbon atoms, to which hydroxyl groups are attached, is applied to the hydroxycarboxylic acid itself but not to the lactone. The basic skeleton of the above-described organic compounds may be a chainlike structure or a cyclic structure, and may be saturated or may contain an unsaturated bond. A hydroxycarboxylic acid and its neutralized salt or lactone are preferred as the dispersant. Particularly, gluconic acid (C=6, OH=5, COOH=1) and its neutralized salt or lactone, lactobionic acid (C=12, OH=8, COOH=l) and its -40- neutralized salt or lactone, tartaric acid (C=4, OH=2, COOH=2) and its neutralized salt or lactone, and glucoheptonic acid-?-lactone (C=7, 0H=5) are preferred. Among these, giuconic acid and its neutralized salt or lactone are particularly preferred since they provide a high effect of improving the degree of orientation and are not expensive. Sorbose is preferred as the organic compound containing a hydroxymethylcabonyl group. Ascorbic acid is preferred as the organic compound having an enol type hydroxyl group that can be dissociated as an acid. In the invention, citric acid and its neutralized salt can be used as the dispersant. While citric acid has a hydroxyl group and a carboxyl group, it does not satisfy the condition in that hydroxyl groups are bonded to 50% or more of the carbon atoms except for the carbon atoms attached to oxygen atoms via a double bond. However, citric acid provides an effect of improving the degree of orientation. The structures of a part of the preferred dispersants described above are shown below. -41- D-gluconic acid ?-D-glucoheptonic acid ?-lactone Lactobionic acid (R,R)-tartaric acid -42- Ascorbic acid L-(-)-sorbose The degree of orientation by the magnetic field orientation is influenced by the pH of the supernatant liquid of the slurry. Specifically, when the pH is too low, the degree of orientation is decreased, and the residual magnetic flux density after sintering is influenced therefrom. In the case where a compound exhibiting an acidic nature in an aqueous solution, such as hydroxycarboxylic acid, is used as the dispersant, the pH of the supernatant liquid of the slurry becomes low. Therefore, it is preferred that the pH of the supernatant liquid of the slurry is adjusted, for example, by adding a basic compound along with the dispersant. As the basic compound, ammonia and sodium hydroxide are preferred. Ammonia may be added as aqueous ammonia. The lowering of the pH can also be prevented by using a sodium salt of a hydroxycarboxylic acid. In the case where SiO2 and CaCO3 are added as auxiliary components as in a ferrite magnet, when a -43- hydroxycarboxylic acid or its lactone is used as the dispersant, SiO2 and CaCO3 effuse along with the supernatant liquid of the slurry mainly on the preparation of the slurry for molding, and the desired performance cannot be obtained, for example, HcJ is decreased. When the pH is heightened by adding the basic compound, the effusing amount of SiO2 and CaCO3 becomes larger. On the other hand, the use of a calcium salt of a hydroxycarboxylic acid can suppress the effusion of SiO2 and CaCO3. By adding an excess amount of SiO2 and CaCO3 on adding the basic compound or on using the sodium salt as the dispersant, the shortage of the amounts of SiO2 and CaCO3 in the magnet can be prevented. When the ascorbic acid is used, there is substantially no effusion of SiO2 and CaCO3. Because of the above-described reasons, the pH of the supernatant liquid of the slurry is preferably 7 or more, more preferably from 8 to 11. The kind of the neutralized salt used as the dispersant is not particularly limited, and may be any of a calcium salt, a sodium salt, etc. Because of the above-described reasons, a calcium salt is preferably used. When a sodium salt is used as the dispersant, or aqueous ammonia is added, a problem arises in that cracks are liable to be formed in the molded body or the sintered body, in addition -44- to the effusion of the auxiliary components. The dispersant may be used in combination of two or more kinds thereof. The addition amount of the dispersant is preferably from 0.05 to 3.0% by weight, more preferably from 0.10 to 2.0% by weight, based on the calcined material powder as the oxide magnetic material powder When the amount of the dispersant is too small, the improvement in degree of orientation becomes insufficient. When the amount of the dispersant is too large, cracks are liable to be formed in the molded body and the sintered body. In the case where the dispersant is one that can be ionized in an aqueous solution, such as an acid or a metallic salt, the addition amount of the dispersant is the ion-converted value, i.e., the addition amount is obtained by converting to only the organic component except for a hydrogen ion and a metallic ion. In the case where the dispersant is a hydrate, the addition amount is obtained with excluding crystallization water. For example, when the dispersant is calcium gluconate monohydrate, the addition amount is obtained by converting into gluconic ion In the case where the dispersant is a lactone or contains a lactone, the addition amount is obtained by converting into a hydroxycarboxylic ion with assuming that the whole lactone are split into a hydroxycarboxylic acid. -45- The time at which the dispersant is added is not particularly limited. The dispersant may be added on the dry coarse pulverizing step or the preparation of the slurry for pulverization for the wet pulverizing step. A part of the dispersant may be added on the dry coarse pulverizing step and the balance may be added on the wet pulverizing step. Alternatively, it may be added after the wet pulverizing step with stirring. In any case, the dispersant is present in the slurry for molding, and thus the effect can be obtained. The addition on the pulverizing step, particularly on the dry coarse pulverizing step, provides higher effect of improving the degree of orientation. It is considered that this is because in the vibration mill used in the dry coarse pulverization, a larger energy is applied to the particles, and the temperature of the particle is increased, in comparison to the ball mill used in the wet pulverization, and thus the conditions in which chemical reactions are liable to occur is realized. It is considered that by adding the dispersant on the dry coarse pulverizing step, the amount of the dispersant adsorbed on the surface of the particles becomes larger, and consequently a higher degree of orientation can be obtained. When the residual amount of the dispersant in the slurry for molding (which is substantially the same as the adsorbed amount) is actually -46- measured, the ratio of the residual amount to the addition amount becomes higher in the case where the dispersant is added on the dry coarse pulverizing step than the case where the dispersant is added on the wet pulverizing step. In the case where the addition of the dispersant is conducted by separating to plural addition operations, the addition amounts of each of the addition operations, are determined in such a manner that the total addition amount is in the preferred range as described above. After the molding step, the molded body is heat treated in the air or nitrogen at a temperature of from 100 to 5000C to sufficiently remove the dispersant added. The molded body is sintered in the subsequent sintering step, for example, in the air at a temperature of from 1,150 to l,250°C, preferably from 1,160 to l,220°C, for about from 0.5 to 3 hours, to obtain an anisotropic ferrite magnet. The average crystal grain diameter of the magnet of the invention is preferably 2 ?m or less, more preferably 1 ?m or less, and especially preferably from 0.5 to 1.0 ?m. Even if the average crystal grain diameter exceeds 1 ?m in the invention, a sufficiently high coercive force can be obtained. The crystal grain diameter can be measured with a scanning electron microscope. The specific resistivity is about 100 ?m or more. The sintered magnet can also be obtained in such a -47- manner that the molded body is pulverized by using a crusher and classified to have the average grain diameter of about from 100 to 700 ?m by a sieve to obtain a magnetic orientation granules, which is then subjected to a dry molding in the presence of a magnetic field, and the resulting molded body is sintered. The magnet powder can be obtained in such a manner that after the pulverization using the slurry of the calcined material, the slurry is dried and sintered. The invention involves a magnetic recording medium comprising a thin film magnetic layer. The thin film magnetic layer has a hexagonal magnetoplumbite ferrite phase as similar to the magnet powder of the invention. The content of impurities is equivalent to the above-described embodiments. The sputtering method is generally preferred for providing the thin film magnetic layer. In the case where the sputtering method is employed, the sintered magnet can be used as a target, or a multi-sputtering method using at least two kinds of targets may be employed. After the film formation by the sputtering method, it is generally subjected to a heat treatment to form the hexagonal magnetoplumbite structure. By using the magnet of the invention, the following effects can generally obtained and superior application -48- products can be obtained. That is, in the case where the magnet of the invention has the same dimension as the conventional ferrite products, because the magnetic flux density generated from the magnet can be increased, it contributes to the provision of application products having higher performance, for example, a high torque can be obtained in the case of a motor, and a good sound quality with high linearity can be obtained due to the reinforcement of the magnetic circuit in the case of a speaker or a headphone. In the case where the same performance as the conventional magnet is enough, the size (thickness) of the magnet can be small (thin), and it contributes to make application products small-sized and lightweight (thin). Furthermore, in the motor using a wound type electromagnet as a magnet for a field system, the electromagnet can be replaced by the ferrite magnet to contribute to provision of the motor of lightweight and low cost, and the reduction in production process thereof. Furthermore, because the magnet of the invention is excellent in temperature characteristics of the coercive force (HcJ), it can be used under the low temperature conditions, under which the conventional ferrite magnet involves a danger of low temperature demagnetization (permanent demagnetization), and thus the reliability of products used in cold areas and areas highly above the sea -49- level can be considerably increased. The magnet material of the invention is worked into prescribed shapes and is used in the wide range of applications described below. The magnet material of the invention can be preferably used as a motor for an automobile, such as for a fuel pump, a power window, an antilock brake system, a fan, a windshield wiper, a power steering, an active suspension system, a starter, a door lock system and an electric side mirror; a motor for an office automation and audio-visual apparatus, such as for an Floppy Disk Drive (FDD) spindie, a Video Tape Recorder {VTR) capstan, a VTR rotation head, a VTR reel, a VTR loading system, a camcorder capstan, a camcorder rotation head, a camcorder zooming system, a camcorder focusing system, a capstan for a combination tape recorder and radio* a spindle for a compact disk player, a laser disk player and a minidisk player, a loading system for a compact disk player, a laser disk player and a minidisk player, and an optical pickup for a compact disk player and a laser disk player; a motor for a home electric apparatus, such as for an air compressor for a air conditioner, a compressor for a refrigerator, driving an electric tool, an electric fan, a fan for a microwave oven, a rotation system for a plate of a microwave oven, driving a mixer, a fan for a hair dryer, driving a shaver and an electric toothbrush; a motor for a -50- factory automation, such as for driving an axis and a joint of an industrial robot, a main driver of an industrial robot, driving a table of a working apparatus, and driving a belt of a working apparatus; and a motor for other applications, such as for a generator of a motor bike, a magnet for a speaker and a headphone, a magnetron tube, a magnetic field generator for an MRI system, a clamper for a CD-ROM, a sensor of a distributor, a sensor of an antilock brake system, a level sensor for a fuel and an oil, and a magnet clutch. EXAMPLE 1 Sintered magnet of Sample Nos. 1 and 2 were prepared by using an aqueous system with the additive compounds added after calcination. As raw materials, the following materials were used. Fe2O3 powder (primary particle size: 0.3 ?m) 1,000,0 g (containing Mn, Cr, Si and Cl as impurities) SrCO3 powder (primary particle size: 2 ?m) 161.2 g (containing Ba and Ca as impurities) As additives, the following materials were used. SiO2 powder (primary particle size: 0.01 ?m) 2.30 g CaCO3 powder (primary particle size: 1 ?m) 1.72 g -51- The raw materials and the additives were pulverized in a wet attrltor, followed by drying and rectification of granules, and baked in the air at l,250°C for 3 hours, to obtain a calcined material in the form of granules* To the resulting calcined material, SiO2, CaOO3, lanthanum carbonate (La2(CO3)3'8H2O) and cofealt oxids (CoO) were added in the amounts shown in Table 1, and calcium gluconate was further added in the amount shown in Table 1. followed by dry coarse pulverization for 20 minutes by a batch vibration rod mill. At this time, distortion due to pulverization was introduced, and the HcJ of the calcined material grains was lowered to 1.7 kOe. Next, 177 g of the coarse pulverized material produced in the same manner as above was added thereto, and 400 cc of water was further added thereto as a dispersing medium, to prepare a slurry for pulverization. By lasing the slurry for pulverization, wet pulverization was conducted in a ball mill for 40 hours. The specific surface area after the wet pulverization was 0.5 m2/g (average grain diametar: o.5 ?m). The pH of the pulverization was 9.5. After the wet pulverization, the slurry for pulverization was subjected to centrifugation to adjust the concentration of the calcined material in the slurry to 78%, so as to prepare a slurry for molding. Compression molding was conducted by using the slurry for molding with removing water frost the slurry. The molding was conducted while applying a magnetic field 1B the direction of compression of about 13 kOe. The resulting molded body had a cylindrical shape having a diameter of 30 mm and a height of 18 mm. The molding pressure was 0.4 ton/cm2, A part of the slurry was dried and fired at 1,000°C to convert the whole contents thereof to oxides, and it was subjected to the fluorescent X-ray quantitative analysis to obtain the contents of the components. The results obtained are shown in Tables 2 and 3, The molded body was subjected to a heat treatment at a temperature of from 100 to 300°C to sufficiently remove gluconic acid, and then sintered in the air with a temperature increasing rate of 5°c/min, followed by maintained at 1,2200C for 1 hour, to obtain a sintered body. The upper and lower surfaces of the resulting sintered body were worked, and was measured for the residual magnetic flux density (Br), the coercive force (HcJ and Hcb), the maximum energy product ((BH)max)) the saturation magnetization (4?Is), the degree of magnetic orientation (Ir/Is), and the squareness (Hlc/HcJ). The sample was then -53- worked into a shape of 5 mm in diameter and 6.5 mm in height. The Curie temperature Tc was obtained by measuring the temperature dependency of the magnetization in the c. axis by VSM. The results obtained are shown in Figures 5 and 6. The SEM photographs of the structures in a axis and c axis of the samples are shown in Figures 1 and 2. It is clear from Figures 5 and 6 that Sample Nos. 1 and 2 of the invention each has two Curie temperatures of 440°C and 456° C for Sample No. 1 and 434°C and 454°C for Sample No. 2. It is considered therefrom that the crystal grains of the samples of the invention have a two-phase structure in which the phases have magnetic characteristics different from each other. The samples were subjected to X-ray diffractiometry, and as a result the samples were of a monophase of an M type ferrite No great difference in lattice index therebetween. COMPARATIVE EXAMPLE 1 Sintered magnet of Sample No. 3 was prepared by using an aqueous system with the additive compounds added before calcination. As raw materials, the following materials were used. Fe2O3 powder (primary particle size: 0.3 ?m) 1,000.0 g (containing Mn, Cr, Si and Cl as impurities) SrCO3 powder (primary particle size: 2 ?m) 130.3 g -54- (containing Ba and Ca as impurities) Cobalt oxide 17.56 g La2O3 35.67 g As additives, the following materials were used. SiO2 powder (primary particle size: 0.01 ?m) 2.30 g CaCO3 powder (primary particle size: 1 ?m) 1.72 g The raw materials and the additives were pulverized in a wet attritor, followed by drying and rectification of granules, and baked in the air at l,250°C for 3 hours, to obtain a calcined material in the form of granules. The magnetic characteristics of the resulting calcined material were measured with a vibration sample magnetomator (VSM), and as a result, the saturation magnetization as was 68 emu/g and the coercive force HcJ was 4.6 kOe. To the resulting calcined material, SiO2 and CaCO3 were added in the amounts shown in Table 1, and calcium gluconate was further added in the amount shown in Table 1, followed by dry coarse pulverization for 20 minutes by a batch vibration rod mill. At this time, distortion due to pulverization was introduced, and the HcJ of the calcined material grains was lowered to 1.7 kOe. Next, 210 g of the coarse pulverized material thus- -55- produced was collected, and 400 cc of water was further added thereto as a dispersing medium, to prepare a slurry for pulverization. By using the slurry for pulverization, wet pulverization was conducted in a ball mill for 40 hours. The specific surface area after the wet pulverization was 8.5 m2/g (average particle diameter: 0.5 ?m) . The pH of the supernatant liquid of the slurry after the wet pulverization was from 9 to 10. After the wet pulverization, the slurry for pulverization was subjected to centrifugation to adjust the concentration of the calcined material in the slurry to 78%, so as to prepare a slurry for molding. Compression molding was conducted by using the slurry for molding with removing water from the slurry. The molding was conducted while applying a magnetic field in the direction of compression of about 13 kOe. The resulting molded body had a cylindrical shape having a diameter of 30 mm and a height of 18 mm. The molding pressure was 0.4 ton/cm2. A part of the slurry was dried and fired at l,000°C to convert the whole contents thereof to oxides, and it was subjected to the fluorescent X-ray quantitative analysis to obtain the contents of the components. The results obtained are shown in Tables 2 and 3. The molded body was subjected to a heat treatment at -56- a temperature of from 100 to 360°C to sufficiently remove gluconic acid, and then sintered in the air with a temperature increasing rate of 5°C/min, followed by-maintained at l,220°C for 1 hour, to obtain a sintered body. The upper and lower surfaces of the resulting sintered body were worked, and was measured for the residual magnetic flux density (Br), the coercive force (HcJ and Hcb), the maximum energy product ((BH)max), the saturation magnetization (4?Is), the degree of magnetic orientation (Ir/Is), and the squareness (Hk/HcJ). The results are shown in Table 4. The sample was then worked into a shape of 5 mm in diameter and 6.5 mm in height. The Curie temperature Tc was obtained by measuring the temperature dependency of the magnetization in the c axis by VSM. The results obtained are shown in Figure 7. It is clear from Figure 7 that the sample has one Curie temperature of 444°C. The specific resistivity in the a axis direction and the c axis direction of the sintered body samples Nos. 1 to 3 were measured. The results are shown in Table 5. The SEM photographs of the structures observed from the a axis direction and the c axis direction for the samples were shown in Figures 3 and 4. It is clear from Figures 1 to 4 that the ferrite of the invention has a larger grain size in comparison to the conventional ferrite shown in Figures 3 and 4. -57- TABLE 1 Sample Calcined material (g) Lanthanum oxide (g) Cobalt oxide (g) Calcium gluconate (g) SiO2 (g) CaCO3 (g) 1 87.26 6.12 1.63 1.13 0.44 1.38 2 87.26 7.23 1.63 1.13 0.44 1.38 3 110 - 1.13 0.44 1.38 *) Comparative Example TABLE 2 Sample Pe2O3 (mol%) MnO (mol%) SrO (mol%) BaO (mol%) SiO2 (mol%) CaO (mol%) La2O3 (mol%) CoO (mol%) (Fe+Co)/ (Sr+La) (molar ratio) La/Co (molar ratio) 1 80.90 0.64 10.72 0.10 1.25 2.32 1.37 2.55 12.2 1.1 2 80.91 0.64 10.59 0.10 1.24 2.31 1.55 2.53 12.0 1.2 * 3 80.71 0.64 10.95 0.10 1.26 2.23 1.36 2.64 12.0 1.0 *) Comparative Example TABLE 3 Sample Fe Mn Sr Ba Si Ca Zn La Co Al Cr Cu 1 88.7 0.4 5.9 0.0 0.7 1.3 0.0 1.5 1.4 0.0 0.1 0.0 2 88.6 0.3 5.8 0.0 0.7 1.3 0.0 1.7 1.4 0.0 0.1 0.0 88.7 0.3 6.0 0.1 0.7 1.2 0.0 1.5 1.5 0.0 0.0 0.0 Note: All numerals are in terms of atomic%. *) Comparative Example TABLE 4 Sample 4??s (KG) Br (KG) HcJ (kOe) Ir/Is (%) Hk/HcJ (%) (BH)max (MGOe) Sintered density (g/cm ) 1 4.47 4.34 4.60 97.2 91.7 4.6 5.02 2 4.49 4.37 4.75 97.3 93.4 4.7 5.02 • 4.52 4.33 4.61 95.8 89.5 4.5 5.06 *) Comparative Example TABLE 5 Sample Conditions p-a (?m) p-c (?m) 1 Addition after calcination (stoichiometric composition) 4.79 x 101 1.47 x 102 2 Addition after calcination (La-rich composition) 1.00 x 101 6.13 x 101 3* Addition before calcination (stoichiometric composition) 2.00 x 103 6.44 x 103 *) Comparative Example It is clear from Table 4 that the cores of the sintered bodies within the scope of the invention exhibited extremely excellent characteristics. It is clear from Table 5 that Sample Nos. 1 and 2 of the invention obtained by the process of addition after the calcination exhibited smaller specific resistivities of 1/10 to 1/100 of that of the comparative sample obtained by the process of addition before the calcination. It is considered therefrom that the sample obtained by the process of addition before the calcination and the samples obtained by the process of addition after the calcination are different in fine structures of the sintered bodies. Among the samples according to the invention. Sample No. 2 having the La-rich composition exhibited a smaller value of 1/4 to 1/2 of that of Sample No. 1. In all the samples, the values of the a axis direction were smaller than the values of the c axis direction. EXAMPLE 2 A comparison was made for the addition of Fe, La and Co after the calcination. The composition as in Example 1 (SrFe12O19 + SiO2: 0.2% by weight + CaCO3: 0.15% by weight) was calcined in the same manner as in Example 1, to obtain a calcined material. To the resulting calcined material in the form of granules, La2(CO3)3'8H2O, CoOx(CoO+Co304) , the iron oxide -61- (?-Fe2O3) and SiO2 (0.4% by weight), which were the same as those used as the raw materials, CaCO3 (1.25% by weight), and calcium gluconate (0.6% by weight) were added in such a manner that the composition after the addition became the following formula: Sr1-xLaxFe12-xCox019 wherein x = y = 0, 0.1, 0.2 or 0.3, followed by subjecting to the coarse pulverization using a small-sized vibration mill. The composition was then subjected to the wet pulverization in the same manner as in Example 1 for 40 hours, followed by baking. Separately, a sample wherein no calcium gluconate was used but only water was used, and a sample wherein xylene was used as the dispersing medium and oleic acid was used as the dispersant were prepared. The degrees of orientation of the molded bodies depending on the addition amounts of La and Co for the sintered body samples are shown in Figure 8, and the HcJ-Br characteristics thereof are shown in Figure 9. The addition amounts of Fe, La and Co after the calcination were expressed by x, with the composition after the addition being represented by the following formula: Sr1-xLaxFe12-xCoxo19 -62- In the case where calcium gluconate was used as the aqueous dispersant, the clear increase in degree of orientation was observed with the increase in the addition amount after the calcination, and in the case of x (substitution degree) of 0.4, it was closed to the value obtained when xylene was used as the non-aqueous solvent and oleic acid was used as the surface active agent. On the other hand, no improvement in degree of orientation was observed when no gluconic acid was added to water. With respect to the characteristics of the sintered bodies, in many cases, Hk/Hcj > 90%, and it was the maximum that x = 0.2. When the addition amount became large (x > 0.3), the moldability was lowered. EXAMPLE 3 A comparison was made for the addition of Fe before the calcination, and La and Co after the calcination, and was also made for the calcination temperatures. To the composition as in Example 1 (SrFe12O19 + SiO2: 0.2% by weight + CaCO3: 0.15% by weight), the iron oxide (??-Fe2O3), which was the same as that used as the raw material was added, in such a manner that the composition was represented by the following formula: Sr1-xLaxFel2-yCoyO19 -63- wherein x = y = 0.2, and calcined materials were obtained in the same manner as in Example 1 except that the composition was calcined at l,150°C, l,200°C, l,250°C and l,300°C. The resulting calcined material samples were subjected to X-ray diffraction analysis, and the presence of the M phase and the hematite phase (?-Fe2O3) was observed. The as value of the Sr M phase in the calcined powder was then calculated under the assumption that in the calcined material obtained by the process where Fe was added before the calcination, the whole incremental amount of Fe was converted into the ?-Fe2O3 phase, and the balance became the Sr M phase. As a result, it was substantially equal to the as value of the Sr M calcined material at the same calcination temperature, and therefore it was considered that the assumption was reasonable. To the resulting calcined material in the form of granules, La2(CO3)3'8H2O, CoOx(CoO+Co3O4) + SiO2 (0.4% by weight), CaCO3 (1.25% by weight), and calcium gluconate (0.6% by weight) were added in such a manner that the composition was represented by the following formula: Sr1-xLaxFe12-yCoyO19 wherein x = y = 0.2, followed by subjecting to coarse -64- pulverization using a small-sized vibration mill. The composition was then subjected to the wet pulverization in the same manner as in Example 1 for 40 hours, followed by-baking. The resulting molded body was measured for the degree of orientation. The results obtained are shown in Figure 10. It is clear for Figure 10 that the degree of orientation of molded body was high in the sample using the calcined body at l,250°C, which was equivalent to the sample obtained by the process where all the additives were added after the calcination. Figure 11 shows the relationship between the sintered density and the degrees of magnetic orientation (Ir/Is) at a calcination temperature of l,250°C. Although the degrees of orientation of the molded body were the same, the sample obtained by the process where only ?-Fe2O3 was added before the calcination had the higher density and the higher degree of orientation. Figure 12 shows HcJ-Br and Hk/HcJ at a calcination temperature of l,250°C. In the sample obtained by the process where only ?-Fe2O3 was added before the calcination, HcJ was lowered but Br was high due to the high density and degree of orientation, which was in the same level of characteristics as the sample of Example 2. EXAMPLE 4 A comparison was made for the addition of Fe and La -65- before the calcination, and only Co after the calcination. To the composition as in Example 1 (SrFe12O19 + SiO2: 0.2% by weight + CaCO3: 0.15% by weight), the iron oxide (??-Fe2O3), which was the same as that used as the raw material, and La2O3 were added in such a manner that the composition was represented by the following formula: Sr1-xLaxFe12-yCoyO19 wherein x = y - 0.2, and calcined materials were obtained in the same manner as in Example 1 except that the composition was calcined at l,200°C or l,250°C. The resulting calcined material samples were subjected to X-ray diffraction analysis, and the presence of the M phase and the hematite phase (?-Fe2O3) was observed. A peak of ortho-ferrite (FeLaO3) was not confirmed. To the resulting calcined material in the form of granules, CoOx(CoO+Co304) + SiO2 (0.4% by weight), CaCO3 (1.25% by weight), and calcium gluconate (0.6% by weight) were added in such a manner that the composition was represented by the following formula: Sr1-xLaxFe12-yCOyO19 wherein x = y = 0.2, followed by subjecting to coarse -6£ pulverization using a small-sized vibration mill. The composition was then subjected to the wet pulverization in the same manner as in Example 1 for 40 hours, followed by baking in the same manner as in Example 1. Hcj-Br and Hk/HcJ are shown in Table 6. TABLE 6 HcJ (KOe) 4?IS (kG) Br (kG) Ir/Is (%) Hk/HcJ (%) (BH)max (MGOe) Sintered density (g/cm3) 4.62 4.58 4.40 9.59 88.5 4.69 5.05 The characteristics were the substantially same as in Examples 2 and 3. EXAMPLE 5 A comparison was made for the addition of Fe and La before the calcination, and only Co after the calcination. To the composition as in Example 1 (SrFei2Oi9 + SiO2: 0.2% by weight + CaCO3: 0.15% by weight), La2(C03h' 8H2O was added in such a manner that the composition after the addition was represented by the following formula: Sri-xLaxFei2-yCoy01,) wherein x = y = 0.1, 0.2, 0.3 or 0.4, to obtain calcined -€? powder (solvent system with addition before the calcination). Separately, La2(CO3)3'8H2O was added after the calcination in such a manner that the composition after the addition was represented by the above formula wherein x = y = 0, 0.1, 0.2, 0.3 or 0.4, to obtain calcined powder (solvent system with addition after the calcination). Oleic acid was added to these species of calcined powder, followed by coarse pulverization by using a small-sized vibration mill. The solvent pulverization was then conducted for 40 hours, followed by baking, to obtain samples. The squareness Hk/HcJ of the resulting sintered body sample obtained at l,220°C is shown in Figure 13, and the degree of magnetic orientation depending on the addition amounts (Ir/Is) is shown in Figure 14. The degrees of orientation of the samples were in the same level, and Hk/HcJ of the sample of the addition after the calcination was improved in comparison to the another. EXAMPLE 6 Investigation was made for the separate addition of La and Co. As raw materials, the following materials were used. Fe2O3 powder (primary grain size: 0.3 ?m) 1,000.0 g SrCO3 powder (primary grain size: 2 ?m) 161.2 g -68- These raw materials were pulverized in a wet attritor, followed by drying and rectification of granules, and baked in the air at l,250°C for 3 hours, to obtain a calcined material in the form of granules. To the resulting calcined material, SiO2 = 0.6% by weight, CaCO3 = 1.4% by weight, lanthanum carbonate (La2(CO3)3'8H2O), cobalt oxide (CoO), and calcium gluconate (0.9% by weight) were added on the pulverization by a dry vibration mill. At this time, the La/Co ratio was changed by changing the addition amount of La. Iron oxide (Fe2O3) was added on the pulverization by a ball mill. Separately, as the calcined material (referred to as mother material in the Table), those of the addition amounts of lanthanum carbonate (La2(CO3)3'8H2O) and cobalt oxide (CoO) before the calcination x - 0 or 0.1 were prepared. The compositions of the samples and the results of analysis of the pulverized materials are shown in Table 7. -69- TABLE 7 Sample No. La/Co addition amount La/Co Analytical result Mother material (x) Addition after the calcination (x) La2O3 (mol%) CoO (mol%) (Fe+Co)/ (Sr+La) (molar ratio) La/Co (molar ratio) 11 0 0.2 1.1 1.37 2.55 12.21 1.07 12 0 0.2 1.3 1.55 2.53 12.02 1.22 13 0 0.2 1.5 1.76 2.48 11.80 1.42 14 0.1 0.1 1.1 1.42 2.62 11.88 1.08 15 0.1 0.1 1.3 1.36 2.43 12.20 1.14 16 0.1 0.1 1.5 1.47 12.25 1.23 The resulting samples were baked at l,200°C, l,220°C and 1,240°C, and measured for magnetic characteristics. The results obtained are shown in Figure 15. In all the samples, a relatively high HcJ and Hk values were obtained in the case of the La-rich composition (La/Co = 1.14 to 1,23). As a comparison was made at the optimum point of La/Co, in the case of the addition of x = 0.1 after the calcination to the mother material of x = 0.1, Hk had a tendency of deteriorated, and in the case of the addition of x = 0.2 after the calcination to the mother material of x = 0, high sintered magnetic characteristics were obtained It has been known that the addition of La and Co on pulverization (after the calcination) provides a higher Hk than the case of the addition to the raw material (before the calcination). In this example, the intermediate behavior was observed between these two cases, and no characteristic result was obtained. While the ferrites containing Sr have been considered in the foregoing examples, it has been confirmed that the equivalent results have been obtained for ferrites containing Ba. The shape of the samples of the invention obtained in the foregoing examples was changed from the cylindrical form to a shape of a filed magnet of a C type motor, to produce a sintered magnet having a shape of a C type motor. -71- The resulting core materials were installed in a motor to replace the sintered magnet of the conventional material. The motor was operated at the rated conditions, and thus good characteristics were exhibited. The torque of the motor was observed, and thus the torque was increased in comparison to the motor using the conventional core material. The same results were obtained by using a bonded magnet. the effect of the invention is clear from the foregoing examples. -72- WE CLAIM : 1. Magnet powder comprising a primary phase of a hexagonal ferrite containing A, M and R, where A represents at least one element selected from the group consisting of Sr, Ba, Ca and Pb, M represents Co, or Co and Zn, and R represents at lest one element selected from the group consisting of rare earth elements comprising Y, and Bi, wherein said magnet powder has at least two different Curie temperatures, said two different Curie temperatures are present within a range of from 400 to 470°C, and an absolute value of a difference therebetween is 5°C or more. 2. Magnet powder as claimed in claim 1, wherein R represents at least La. 3. Magnet powder as claimed in claim 1 or 2, wherein said hexagonal ferrite is a magnetoplumbite ferrite. 4. Magnet powder as claimed in claims 1 to 3, wherein said hexagonal ferrite comprises A, R, Fe, and M, wherein A represents at least one element selected from the group consisting of Sr, Ba, Ca and Pb, provided that Sr or Ba are essentially included in A, R represents at least one element selected from the group consisting of rare earth elements comprising Y, and Bi, and M represents Co, or Co and Zn, and proportions of said elements with respect to the total amount of said metallic elements are from 1 to 13 atomic% for A, from 0.05 to 10 atomic% for R, from 80 to 95 atomic% for Fe, and from 0.1 to 5 atomic% for M. -73- 5. Magnet powder as claimed in any of claims 1 to 4, wherein the proportion of CO in M is 10 atomic% or more. 6. Magnet powder as claimed in any of claims 1 to 5, wherein said magnet powder has an absolute value of a temperature coefficient of a coercive force within a range of from -50 to 50°C of 0.1 %/°C or less. 7. A bonded magnet comprising magnet powder as claimed in any one of claims 1 to 6. 8. A motor comprising a bonded magnet as claimed in claim 7. 9. A magnetic recording medium comprising magnet powder as claimed in any one of claims 1 to 6. 10. A sintered magnet comprising a primary phase of a hexagonal ferrite containing A, M and R, where A represents at least one element selected from the group consisting of Sr, Ba, Ca and Pb, M represents Co, or Co and Zn, and R represents at least one element selected from the group consisting of rare earth elements comprising Y, and Bi, wherein said sintered magnet has at least two different Curie temperatures, said two different Curie temperatures are present within a range of from 400 to 470°C, and an absolute value of a difference therebetween is 5°C or more. 11. A sintered magnet as claimed in claim 10, wherein R represents at least La. 12. A sintered magnet as claimed in claim 10 or 11, wherein said hexagonal ferrite is a magnetoplumbite ferrite. -74- 13. A sintered magnet as claimed in any one of claims 10 to 12, wherein said hexagonal ferrite comprises A, R, Fe, and M, wherein A represents at least one element selected from the group consisting of Sr, Ba, Ca and Pb, provided that Sr or Ba are essentially included in A, R represents at least one element selected from the group consisting of rare earth elements comprising Y, and Bi, and M represents Co, or Co and Zn, and proportions of said elements with respect to the total amount of said metallic elements are from 1 to 13 atomic% for A, from 0.05 to 10 atomic% for R, from 80 to 95 atomic% for Fe, and from 0.1 to 5 atomic% for M. 14. A sintered magnet as claimed in any of claims 10 to 13, wherein the proportion of CO in M is 10 atomic% or more. 15. A sintered magnet as claimed in any of claims 10 to 14, wherein said sintered has a squareness Hk/HcJ of 90% or more. 16. A sintered magnet as claimed in any of claims 10 to 15, wherein said sintered has a degree of orientation Ir/ls of 96% or more. 17. A sintered magnet as claimed in any of claims 10 to 16, wherein said sintered has a ratio of total X-ray diffraction intensity from c plane (??(00L)) to a total X-ray diffraction intensity from all planes (??(hkL)) of 0.85 or more. 18. A sintered magnet as claimed in any of claims 10 to 17, wherein said sintered magnet has an absolute value of a temperature coefficient of a coercive force within a range of from -50 to 50°C of 0.25%/°C or less. 19. A motor comprising a sintered magnet as claimed in any of claims 10 to 18. -75- 20. A magnetic recording medium comprising a thin film magnetic layer comprising a primary phase of a hexagonal ferrite containing A, M and R, where A represents at least one element selected from the group consisting of Sr, Ba, Ca and Pb, M represents Co, or Co and Zn, and R represents at least one element selected from the group consisting of rare earth elements comprising Y, and Bi, wherein said thin film magnetic layer has at least two different Curie temperatures, said two different Curie temperatures are present within a range of from 400 to 470°C, and an absolute value of a difference therebetween is 5°C or more. -76- 21. Magnet powder, substantially as herein described, particularly with reference to the foregoing examples and as illustrated in the accompanying drawings. 22. A sintered magnet, substantially as herein described, particularly with reference to the foregoing examples and as illustrated in the accompanying drawings. 23. A magnetic recording medium, substantially as herein described, particularly with reference to the foregoing examples and as illustrated in the accompanying drawings. Magnet powder comprises a primary phase of a hexagonal ferrite containing A, M and R, where A represents at least one element selected from the group consisting of Sr, Ba, Ca and Pb, M represents Co, or Co and Zn, and R represents at lest one element selected from the group consisting of rare earth elements comprising Y, and Bi, wherein said magnet powder has at least two different Curie temperatures, said two different Curie temperatures are present within a range of from 400 to 470°C, and an absolute value of a difference therebetween is 5°C or more.

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