Title of Invention

METHOD FOR THE SYNTHESIS OF γ-FE2O3

Abstract

The present invention relates to the synthesis of γ-Fe2O3 by dissolving the precursor mixture comprising at least a ferric salt and organic amides/hydrazide in a solvent and subjecting the solution to microwave radiation at a temperature sufficient to obtain a foamy, X-ray crystalline mass, further exposing the said mass to microwave radiation to selectively isolate the  phase of the iron oxide.

Full Text

Field of the invention The present invention relates to a method for the synthesis of γ-Fe2O3, More particularly, the present invention relates to method for the synthesis of γ-Fe2O3 using microwave radiation. The invention also relates to a method for the synthesis of cobalt doped γ-Fe2O3 for use in magnetic recording tapes. Background of the invention Magnetic recording media is widely used in audio, video and magnetic disk recording for the purpose of information storage. Magnetic recording media are useful for the substantially permanent storage of information, particularly for computers. Magnetic recording media have become generally higher in density. The recording wavelength has become shorter and recording systems from an analog system to a digital system are available in the art. The recording industry requires a continual increase in the density of the recording media to enable storage of larger volumes of information in a more permanent form. Among the requirements of such magnetic recording media is that information retrieval must be optimal with minimum distortion due to external magnetic fields, minimal erasure or corruption, storage of larger volumes of information on one recording medium, and a greater degree of permanence in that the information stored should not be changed by repeated rereading of the information by the reading device. γ-Fe2O3 is the most widely used magnetic recording material since 1937 since it has a high saturation magnetisation (370kA) and is able to provide a large signal range during the reading process. The coercivity is in the range of 20 - 30 kA/m, which ensures that erasure during reading is prevented, but allows reuse of the media for re-recording. Conventionally, γ- Fe2O3- is prepared by needle shaped growth of geothite FeOOH precipitate to form a solution of iron salts (Mee, C. D., and Daniel E. D., Magnetic Recording I: Technology, McGraw Hill Book Company, New York, 1987, p 130). The increased demand for better recording media in audio, video and the computer industry in terms of higher storage capacity, increased degree of permanence, and reduced distortion requires higher degrees of coercivity than can be obtained from pure gamma iron oxide. Currently, audio tapes, VHS video tapes, and disks for computer applications predominantly use co-modified oxide particles. It is known to add cobalt as a dopant to ferric oxide at the last stage of processing before it is coated on to the substrate in the form of a tape. Cobalt accumulates preferentially on the surface of the tape to a depth of about 30°A, thereby increasing the anisotropy of the material leading to higher degrees of coercivity (Kishimoto, M., Kitaoka, S., Andoh H., Amamiya, M., and Hayama. F., IEEE Trans. Magn. 14 (1978), p 649). Magnetic recording particulate medium formed by coating a dispersion of a ferromagnetic powder in a binder on a support has been found to be inferior to a magnetic recording metal-film medium (metal-evaporated) in electromagnetic characteristics due to low packing density of a ferromagnetic powder. However, with the improvement of the performance of ferromagnetic powders and the advancement of the coating technique of an extremely thin layer in recent years, almost the same level of electromagnetic characteristics with those of the metal-film medium have been achieved. Further, a magnetic recording particulate medium is superior in productivity and corrosion resistance. Ferromagnetic iron oxide, Co-modified ferromagnetic iron oxide, CrO2, ferromagnetic metal (including alloys), etc., dispersed in a binder have been generally used as magnetic recording particulate media in the prior art. Various methods for improving magnetic characteristics of ferromagnetic powders or surface smoothness of magnetic layer have been suggested but these are not sufficiently satisfactory in view of higher density recording. In order to reduce the overwriting erasure factor, it has been thought that lowering the coercive force Hc of a magnetic layer would be sufficient. However, while the overwriting erasure factor can be improved by the reduction of the coercive force Hc, it is limited because high frequency output is reduced due to recording demagnetization. Ferromagnetic metal powders may contain a small amount of a hydroxide or an oxide and are prepared in the art by reducing a composite organic acid salt (mainly an oxalate) with a reducing gas, e.g., hydrogen; a method comprising reducing iron oxide with a reducing gas, e.g., hydrogen, to obtain Fe or Fe—Co particles; a method comprising pyrolysis of a metal carbonyl compound; a method comprising adding to an aqueous solution of a ferromagnetic metal a reducing agent, e.g., sodium borohydride, hypophosphite, or hydrazine, to conduct reduction; and a method comprising evaporating a metal in a low pressure inert gas to obtain a fine powder. US Patent 4,178,416 to Hector, et al. discloses a Cobalt doped, acicular, gamma ferric oxide which exhibits reduced print-through without substantial loss in signal output or recording sensitivity. This cobalt doped, acicular, gamma ferric oxide is prepared in the .presence of a water soluble,, acyclic dicarboxylic acid or its precursor, which alters or modifies the usual distribution of the cobalt doping ions in the crystal lattice. The dicarboxylic acid directs the distribution of the cobalt doping ions into the crystal lattice in such a way that the crystals obtained by the process disclosed exhibit an increase of at least 5% in the ratios between the intensities of each of the triplet lines 110, 210 and 211 with respect to the 111 line, as shown upon examination by X-ray diffraction analysis. Typical dicarboxylic acids disclosed are oxalic acid, malonic acid, succinic acid, glutaric acid and maleic acid. Magnetic recording products such as tapes, discs and the like, containing the improved cobalt doped, acicular, gamma ferric oxide is also disclosed. The process of this disclosure comprises (1) forming an aqueous dispersion of ferrous hydroxide particles by adding an aqueous solution of ferrous salt having a pH up to about 5.5 to an aqueous solution of a stoichiometric excess of an alkaline hydroxide, in the presence of cobalt doping ions, while substantially avoiding local excesses of said ferrous salt, (2) introducing oxygen into said dispersion at a temperature below 60.degree. C. for a period of time sufficient to convert said ferrous hydroxide particles to ferric oxide hydrate crystals, and, (3) subjecting said crystals to dehydration, reduction and oxidation to form cobalt doped, acicular, gamma ferric oxide, said process being characterized by the addition of about 0.5 to 20 percent, by weight, based on ferrous hydroxide, of a water soluble acyclic dicarboxylic acid or its precursor prior to said introduction of oxygen. US Patent 4,255,492 to Audran, et al discloses a process for preparing acicular gamma ferric oxide (maghemite) crystals exhibiting excellent magnetic recording properties. These acicular gamma ferric oxide crystals can be used in magnetic media such as magnetic tapes to obtain a low level of white noise without sacrificing other electroacoustic properties. The crystals are basic in character, contain alkali metal ions and have acicularity ratio of at least 15, often 15 to 40. These crystals are prepared under carefully controlled reaction conditions. In general, the process involves (1) reacting ferrous salt with a stoichiometric excess of an alkali hydroxide under non-oxidizing conditions and at a temperature below about 60°C while substantially avoiding local excesses of the ferrous salt, to form an aqueous dispersion of ferrous hydroxide particles, (2) introducing oxygen into the dispersion to convert the ferrous hydroxide particles to crystals of a-ferric oxide hydrate, (3) discontinuing introduction of the oxygen into the dispersion, (4) boiling the dispersion to obtain further crystallization of the a-ferric oxide hydrate crystals and subjecting the crystals to dehydration, reduction and oxidation to form γ-ferric oxide crystals. Additional doping ions such as alkaline earth metal ions and/or other ions of metals such as cobalt, nickel, chromium, zinc, manganese or calcium can be introduced into the y-ferric oxide crystals during preparation to improve its properties. However, as indicated, failure to observe the reaction conditions set out in the specification affects the magnetic recording properties of the y-ferric oxide crystals. US Patent 4,364,988 to Andoh, et al discloses a magnetic recording medium, particularly a magnetic tape suitable for sound recording and video recording. A mixture of Co-containing iron oxide magnetic particles consisting essentially of ferromagnetic iron oxide particles containing cobalt and optionally divalent iron in the outermost layer thereof, and y-ferric oxide particles in the ratio of 10:90 to 50:50 by weight are applied to a substrate (e.g. a polyester film) with a binding agent. It is described that it is preferable to incorporate a ferrous salt (e.g. ferrous sulfate, ferrous chloride, ferrous nitrate) into the surface layer of the cobalt-containing iron oxide in addition to a cobalt salt, by which the cobalt-containing iron oxide particles show a greater coercive force and also improved charging properties in comparison with the particles containing only a cobalt salt. That is, when the cobalt-containing iron oxide magnetic particles are prepared by dispersing y-ferric oxide particles or ferromagnetic iron oxide particles obtained by partial reduction of y-ferric oxide particles into an aqueous solution containing a ferrous salt as well as a cobalt salt and an alkali, and maintaining the mixture at a temperature of higher than room temperature up to the boiling point of the mixture under an atmosphere that the divalent iron is substantially not oxidized to form the most outer layer containing divalent iron and cobalt, the resulting particles showing a greater coercive force than the particles containing only cobalt in the most outer layer. US Patent 4,457,955 of Okamura, et al describes a process for producing cobalt-modified iron oxide particles, which comprises the steps of (1) adding to magnetic iron oxide particles (a) a cobalt compound, (b) an iron component which is either (b1) a ferric compound at an atomic ratio to the cobalt compound in the range of 0.68 to 2 or (b2) a combination of a ferrous compound and a ferric compound, and (c) an alkali at a concentration enough to cause the cobalt and iron to precipitate as hydroxides; and (2) heating the mixture at a temperature of at least 50°C and not higher than the reflux temperature thereof in a non-oxidizing atmosphere. US Patent 4,379,183 to Araki, et al discloses a method of producing cobalt-modified magnetic particles by heating an aqueous suspension of ferric hydroxide at temperatures of 100°-250°C at an alkaline pH in the presence of a water soluble organic or inorganic compound capable of forming complexes with iron as a growth regulating agent, and preferably also in the presence of ex-ferric oxide seed crystals of minor axes not larger than 0.4 microns in average in amounts of 0.1-25 mole % in terms of the Fe content thereof in relation to the ferric hydroxide, converting the obtained a-ferric oxide into -ferric oxide by reduction thereof followed by oxidation, maintaining an aqueous suspension of the -ferric oxide at temperatures of 30°-50°C at an alkaline pH in the presence of ferrous hydroxide and cobaltous hydroxide in amounts of 0.5-50 mole %, respectively, in relation to the Fe content of the y-ferric oxide in the suspension for a length of time sufficient to modify the y-ferric oxide particles, US Patent 4,202,871 of Matsumoto, et al discloses single crystalline, acicular ferric oxide particles produced by maintaining an aqueous suspension of ferric hydroxide at an elevated temperature at an alkaline pH in the presence of an organic phosphonic acid compound or a hydroxycarboxylic acid compound as a growth-regulating agent. The method comprises maintaining an aqueous suspension of amorphous ferric hydroxide at an elevated temperature from 80°C to 300°C at an alkaline pH in the presence of an effective amount of a growth-regulating agent dissolved in said suspension, the agent being selected from an organic phosphonic acid, a salt thereof, an ester thereof, a hydroxycarboxylic acid, a salt thereof, and an ester thereof, the elevated temperature being maintained for a length of time sufficient to convert said amorphous ferric hydroxide into said acicular ferric oxide particles. United States Patent 4,765,920 of Gattuso, et al teaches a high temperature process for producing fine magnetic particles of M-phase structure. The disclosure comprises a method for producing fine magnetic particles having the barium or strontium ferrite M-phase crystal structure. An iron and alkaline earth metal halide feed solution is vaporized to form a precursor and oxidizing vapor phase. The precursor and hydrolyzing or oxidizing vapor phases are held in a reactor at a temperature sufficient to effect vaporization of the feed solution yet which is below the melting point of the desired M-phase crystal structure. Small iron oxide particles precipitate from the vapor phase and alkaline earth oxides thereafter. The alkaline earth oxide particles diffuse into the iron oxide particles to form the desired M-phase structure. When the desired width and thickness of the M-phase crystal platelets is achieved, the M-phase particles are cooled by quenching. Composition of the feed solution may be modified by substituting divalent metal halides for the alkaline earth metal halide or by substituting trivalent halides or a combination of a divalent metal halide and either a penta or quadravalent metal halide for the ferric halide. British Patent No. 888,688 teaches a method of making fine barium ferrites by injecting atomized iron and barium salt solutions into an oxidizing flame at temperatures between 1000°C and 1400°C. The powder produced by this process does not, however, possess the M-phase crystalline form but instead consists of mixed or indeterminate phase particles exhibiting only soft magnetic properties. A subsequent heat treating step at a temperature above 600°C is therefore required to convert the mixed phase particles to the desired M-phase. Because this heat treatment must be applied to powders in bulk, extreme caution must be practiced to ensure that localized sintering of the single powder particles does not take place, otherwise the powder's size, shape and aggregation will be adversely affected. US Patent 4,066,564 to Sasazawa, et al discloses a process for producing cobalt- and iron-containing ferromagnetic powder by heat-treatment in the presence of an oxidizing agent. A Co- and Fe-containing ferromagnetic iron oxide powder having a high coercive force and improved stability to heat and pressure is produced by adding an aqueous alkali solution, an aqueous solution containing Co+2 ions, and an aqueous solution containing not more than 1 equivalent, based on the Co+2 ions, of Fe ions to a suspension of ferromagnetic iron oxide, and heating the mixture at a temperature of at least 80°C in the presence of an oxidizing agent. The process of this disclosure comprises adding to a suspension of an acicular ferromagnetic iron oxide an oxidizing agent, an aqueous alkali solution, an aqueous solution containing Co+2 ions and an aqueous solution of Fe ions and heat treating the resultant mixture, at a temperature of at least about 80°C said CO+2 ions being present in said aqueous solution containing Co+2 ions in an amount of 0.5 to 10 atomic percent based on the amount of iron present in the acicular ferromagnetic iron oxide, said Fe ions being added to the suspension in an amount of at least 0.1 equivalent but not more than 1 equivalent based on the Co+2 ions, said aqueous alkali solution containing an amount of alkali such that after neutralizing the Co+2 ions and the Fe ions, the hydroxyl ion concentration is at least 0.5 mol/liter; and the amount of oxidizing agent, when it is a gas, is at least 0.1 liter/min per liter of the suspension and when it is a liquid or solid, the amount is at least 0.1 equivalent based on the Co+2 ions. US Patent 4,188,302 of Becker, et al discloses a process for the manufacture of an acicular magnetic iron oxide containing from 0.5 to 10 percent by weight of cobalt and up to 24 percent by weight of divalent iron, each based on the amount of iron(III) oxide, wherein an aqueous suspension of gamma-iron(III) oxide, containing cobalt(II) and iron(II) ions, the suspension having a hydroxyl ion concentration of more than 3.6.times.l0-6 moles/liter, is heated from 50°to 100°C for at least 15 minutes, and the resulting precipitate is filtered off, washed until neutral, dried at below 100°C and thereafter heated at from 100° to 200°C. While, the products obtained give a very high coercive force even when their cobalt content is low, the process involves several cumbersome steps. US Patent 4,296,149 Rudolf, et al discloses a process for the manufacture of acicular magnetic iron oxide consisting of a core of gamma-iron(III) oxide and a ferrite shell which contains, based on the total amount of magnetic material, from 0.2 to 12 percent by weight of cobalt(II) ions and from 0.1 to 15 percent by weight of iron(II) ions, wherein a solution containing cobalt(II) hydroxide and iron(II) hydroxide is applied to acicular gamma-iron(III) oxide particles and the latter are heated at 100°-220°C. US Patent 4,218,507 Deffeyes, et al discloses novel powders, and resinous compositions bearing said powders, characterized by good chemical stability, electrical conductivity, and energy-absorbing characteristics which can be used a magnetic recording media. However, these powders include a carbide or silicide source on the surface thereof to improve conductivity. All the prior art methods described above involve several cumbersome or complicated steps and are energy intensive. The processes themselves require high capital investment for conversion to industrial scale. Being multi-step processes, the processes may not be commercially expensive and therefore, there is need for a more commercially viable process. Thus, in view of the above-mentioned drawbacks of the prior art, there is an urgent need for a process for the production of magnetic recording material, which is energy efficient, and wherein the material produced has high coercivity. Objects of the invention The main object of the invention is to provide an improved method for the synthesis of γ-Fe2O3 with improved stability. It is another object of the invention to provide a method for the synthesis of cobalt .doped γ-Fe2O3 with improved stability and higher levels of doping. It is another object of the invention to provide a method for the synthesis of cobalt doped γ-Fe2O3which is economical and efficient. It is a further object of the invention to provide a method for the synthesis of cobalt doped γ-Fe2O3 which is more energy efficient. It is still a further object of the invention to provide a method for synthesising cobalt doped γ-Fe2O3 having excellent magnetic recording properties. These and other objects of the invention are achieved by the method of the invention which involves the synthesis of cobalt doped γ-Fe2O3 by microwave radiation. Summary of the invention The present invention involves the preparation of gamma iron oxide that is optionally cobalt-modified, in a one step, furnace-less method involving microwave radiation using a judicious selection of precursors to form a solution, and exposing the solution of precursors to microwave radiation under controlled conditions. Accordingly, the present invention provides a method for the synthesis of γ-Fe2O3 which comprises dissolving a precursor mixture comprising ferric salt and organic amides/hydrazide in a solvent, subjecting the solution of metal salts and organic amides/hydrazides to microwave at a temperature sufficient to obtain a foamy, X-ray crystalline mass, further exposing the said mass to microwave radiation to selectively isolate the y phase of the iron oxide. In one embodiment of the invention, the metal salt comprises nitrate salt. In another embodiment of the invention, solvent comprises water. In yet another embodiment of the invention, a cobalt salt is added to the precursor mixture. In a further embodiment of the invention, the cobalt salt is cobalt nitrate. Preferably, the metal salts are added in stoichiometric amounts. In yet another embodiment of the invention, cobalt is loaded in an amount of 1 to 10 % by weight of the Fe2O3 Detailed description of the invention Gamma iron oxide is prepared by a one step method without a furnace by employing microwave radiation. The starting precursors are carefully selected with inorganic metal salts and organic amides/hydrazides being preferred. The precursors are dissolved in a solvent to form a solution. The solution is then exposed to microwave radiation at conditions of temperature sufficient to form a foamy, voluminous and X-ray crystalline ultra fine product with improved surface characteristics and magnetic properties. The temperature required is dependant on conditions such as the power of the microwave radiation and the exposure level. The time taken for the process to be completed is usually about 5 minutes. The gamma iron oxide obtained is highly stable due to the exothermic decomposition of the red-ox mixture (precursor solution). If desired, cobalt can be incorporated into the precursor solution in the form of a metal salt such as cobaltous nitrate. The amount of cobalt in the final product is in the range of 1 to 10 % by weight. The process of the invention enables the selective isolation of gamma phase form of magnetic iron oxide in preference to the non-magnetic alpha form. The incorporation of cobalt into the gamma iron oxide in levels of upto 10 % doping modifies the surface of the iron oxide. The cobalt modified gamma iron oxide is highly pure. It is also observed that the cobalt modified gamma iron oxide shows an increase in coercivity with the increase in cobalt doping. The increase in coercivity occurs over a short time period ( Due to high surface energy, the particles produced tend to coalesce and remain as agglomerates. The microwaves comprise electromagnetic radiation in the frequency range of 0.3 to 300 GHz and the corresponding wavelength of 1m to 1mm. The amount of the precursors is determined by the inter se ratio of the two components. The organic amide/hydrazide plays a vital role in deciding the stability of the final product. The choice of the starting precursors and the solvent used are important in the preparation of the γ-Fe2O3 Generally, nitrate salts of iron and cobalt when used are taken as the metal salt precursors. The metal salt precursor is weighed and dissolved in the solvent in stoichiometric quantities. The organic amide/hydrazide precursor is then added to the above solution in predetermined amounts and completely dissolved. On complete dissolution, the solution is kept inside a microwave oven and exposed to radiation for a determined period. The reaction is exothermic accompanied by the evolution of gases. The product formed is foamy, voluminous and X-ray crystalline. The y phase is selectively isolated from the a phase by exposing the reaction mixture to microwave radiation again at a determined power level. Control of the exothermicity of the reaction leads to the selective isolation of the required y phase. The amount of the dopant cobalt can be varied from 1 to 10 % in the final product. The final product obtained is in pure phase for upto 10% doping of cobalt. However, with increase in the levels of cobalt doping, the particle size of the product reduces. The B-H loops clearly indicate that the coercivity and resonance magnetisation values increase with the inclusion of cobalt as a dopant. The coerciviry increases from 600 to 1132 Gauss when the cobalt percentage is increased to 10 %. The cobalt salt does not have to be added at the last stage of processing as in the prior art. An added feature of the process is that cobalt salt can be added along with the ferric salt precursor and dissolved in the solvent along with the organic amide/hydrazide since the process is a one step process. The values in respect of the saturation magnetisation, remanance magnetisation and increase in coercivity of the product are given in Table I below: Table I: (Table Removed) The variation in magnetisation values with increasing Co percentage is evident from Table I. Both saturation magnetisation Ms (emu/g) and resonance magnetisation MR (emu/g) increase with Co doping. However, no linearity is observed in Ms and MR values with Co doping. The coercive field Hc increases systematically from 600 Gauss for 4% Co doping to 1132 Gauss for 10% doping. The invention will be now described in greater detail with reference to the accompanying drawings in which: Figure lis an X - ray diffractogram of Cobalt doped Fe2O3 samples. Figure 2 is a plot of lattice parameter (a/Ǻ) versus cobalt doping. Figure 3 is the particle size distribution histogram of cobalt doped samples. Figure 4 is the hysterisis loop for cobalt doped γ-Fe2O3 samples. Referring now to the figures, figure 1 shows the X ray diffraction patters of γ-Fe2O3 doped with varying Co content indicating the formation of single phase compounds. The sharp nature of the peaks in the X - ray diffractogram shows the substantially crystalline nature of the complex. Figure 2 shows the plot of the lattice parameters, a(A) versus the Co percentage doping. With increasing Co doping, the lattice parameter 'a' decreases. A decrease in the lattice parameter can be correlated with the increase in the reflection angle for the peaks in the X ray diffractogram. The decrease in the range from 4% to 10 % is linear. The lattice parameter 'a' is observed to be 8.3512A, 8.3424A, 8.3332A, 8.3236A for 4, 6, 8 and 10 % co doped γ-Fe2O3 respectively. Figure 3 shows the particle size distribution histograms for Co doped γ-Fe2O3 samples. The average agglomerate size for these samples varies between 10 to 20 mcirometers. Microwave synthesis of γ-Fe2O3, Co doped - γ-Fe2O3 yields fine particles possessing high surface energy. As a result the particles tend to remain as agglomerates. Figure 4 shows the hysterisis loops for the Co doped γ-Fe2O3 samples. With increase in Co doping, the coercivity (Hc) increases from 600 to 1132 Gauss. The addition of cobalt increases the anisotropy of the material leading to higher coercivity. Most video tapes used in the art contain Co surface modified γ-Fe2O3 which has coercivity of 48kA/m (600Oe). The following examples illustrate the process of the invention and should not be construed as limiting the scope of the invention. It will be apparent to a person skilled in the art that modifications and variations of the claimed process are possible without departing from the scope of the invention. Example 1: Stoichiometric amounts of Fe2(NO3)3.9H20 (ferric nitrate) and Co2(NO3)3.6H2O (cobaltous nitrate) were weighed and dissolved in 10 ml of deionized water. To the dissolved salt solution, malonyl dihydrazide (MDH) was added in a predetermined amount dictated by the o/f ratio. The solution thus formed by the mixing of metal nitrates, MDH and deionized water was introduced into a kitchen microwave of 800 Watts and frequency 2.45 GHz. The solution on exposure to radiation boiled with the evolution of oxides of nitrogen and on completion of evaporation ignited with a flame resulting the formation of a product. The process was instantaneous and the product obtained in The samples obtained from example 1 above, were subjected to X-ray diffraction, particle size measurement and magnetic measurement. The X-ray diffraction patterns of y-FeaOs with varying cobalt content indicated the formation of single phase compounds. A decrease in the lattice parameter 'a' is noticed with increasing cobalt doping. Sharp peaks were observed indicating that the microwave synthesised particles are crystalline in nature. Particle size histograms show an average agglomerate size of 5 - 20 µm. Hysteresis loops for the Co doped γ-Fe2O3 using vibrating sample magnetometer shows an increase in coercivity (Hc) from 600 to 1132 Gauss, with increasing cobalt content. It is observed that the remanence magnetisation also increases. The morphology, shape and size of the powders as prepared were studied using Scanning Electron Microscopy and Transmission Electron Microscopy studies. Example 3 Preparation of Co doped γ-Fe2O3: Ferric nitrate [Fe(NO3)3.9H2O] and cobaltous nitrate [Co(NO3)2.6H2O] in stoichiometric amounts were dissolved in 10 ml of deionised water. To the dissolved salt solution, malonyl dihydrazide, was added in an amount dictated by the o/f ratio. The solution thus formed by the mixing of the deionised water, metal salts and the dihydrazide was introduced into a kitchen microwave oven of output power 800 Watts and frequency 2.45 GHz. The solution on exposure to radiation boiled with the evolution of oxides of nitrogen and on complete evaporation ignited with the appearance of a flame resulting in the formation of the product. The time scale of formation was Preparation of Co doped γ-Fe2O3: Ferric nitrate [Fe(NO3)3.9H2O] and cobaltous nitrate [Co(NO3)2.6H2O] in stoichiometric amounts were dissolved in 10 ml of deionised water. To the dissolved salt solution, glycine was added in an amount dictated by the o/f ratio. The solution thus formed by the mixing of the deionised water, metal salts and glycine was introduced into a kitchen microwave oven of output power 800 Watts and frequency 2.45 GHz. The solution on exposure to radiation boiled with the evolution of oxides of nitrogen and on complete evaporation ignited with the appearance of a flame resulting in the formation of the product. The time scale of formation was Preparation of Co doped γ-Fe2O3: Ferric nitrate [Fe(NO3)3.9H2O] and cobaltous nitrate [Co(NO3)2.6H2O] in stoichiometric amounts were dissolved in 10 ml of deionised water. To the dissolved salt solution, urea was added in an amount dictated by the o/f ratio. The solution thus formed by the mixing of the deionised water, metal salts and urea was introduced into a kitchen microwave oven of output power 800 Watts and frequency 2.45 GHz. The solution on exposure to radiation boiled with the evolution of oxides of nitrogen and on complete evaporation ignited with the appearance of a flame resulting in the formation of the product. The time scale of formation was Preparation of Co doped γ-Fe2O3 Ferric nitrate [Fe(NO3)3.9H2O] and cobaltous nitrate [Co(NO3)2.6H2O] in stoichiometric amounts were dissolved in 10 ml of deionised water. To the dissolved salt solution, carbohydrazide was added in an amount dictated by the o/f ratio. The solution thus formed by the mixing of the deionised water, metal salts and the hydrazide was introduced into a kitchen microwave oven of output power 800 Watts and frequency 2.45 GHz. The solution on exposure to radiation boiled with the evolution of oxides of nitrogen and on complete evaporation ignited with the appearance of a flame resulting in the formation of the product. The time scale of formation was Preparation of Co doped γ-Fe2O3: Ferric nitrate [Fe(NO3)3.9H2O] and cobaltous nitrate [Co(NO3)2.6H2O] in stoichiometric amounts were dissolved in 10 ml of deionised water. To the dissolved salt solution, oxalyl dihydrazide was added in an amount dictated by the o/f ratio. The solution thus formed by the mixing of the deionised water, metal salts and the dihydrazide urea was introduced into a kitchen microwave oven of output power 800 Watts and frequency 2.45 GHz. The solution on exposure to radiation boiled with the evolution of oxides of nitrogen and on complete evaporation ignited with the appearance of a flame resulting in the formation of the product. The time scale of formation was Example 8 Preparation of Co doped γ-Fe2O3 Ferric nitrate [Fe(NO3)3.9H2O] and cobaltous nitrate [Co(NO3)2.6H2O] in stoichiometric amounts were dissolved in 10 ml of deionised water. To the dissolved salt solution, tetra formal tris azine was added in an amount dictated by the o/f ratio. The solution thus formed by the mixing of the deionised water, metal salts and tetra formal tris azine was introduced into a kitchen microwave oven of output power 800 Watts and frequency 2.45 GHz. The solution on exposure to radiation boiled with the evolution of oxides of nitrogen and on complete evaporation ignited with the appearance of a flame resulting in the formation of the product. The time scale of formation was In all of examples 3 to 8, the level of Cobalt doping was upto 10% It must be understood that the examples are illustrative of the invention and are not intended to limit the scope of the invention in any manner. We claim: 1. A method for the synthesis of γ-Fe2O3 which comprises dissolving a precursor mixture comprising at least a ferric salt and organic amides/ hydrazide in a solvent such as herein described subjecting the solution of said at least ferric salts and organic amides/ hydrazides to microwave radiation at a temperature sufficient to obtain a foamy, X-ray crystalline mass, further exposing the said mass to microwave radiation to selectivity isolate the γ phase of the iron oxide. 2. A method as claimed in claim 1 wherein said salt comprises nitrate salt. 3. A method as claimed in claim 1 or 2 wherein the solvent comprises water. 4. A method as claimed in any preceding claim wherein said precursor mixture contains a cobalt salt. 5. A method as claimed in claim 4 wherein the cobalt salt is cobalt nitrate. 6. A method as claimed in 4 and 5 wherein said salts are added in stoichiometric amounts, wherein preferably cobalt is loaded in an amount of 1 to 10% by weight of the 7. A method for the synthesis of γ-Fe2O3 substantially as herein described with reference to the foregoing examples and with reference to and as illustrated in the accompanying drawings.

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