Title of Invention	A PROCESS FOR MANUFACTURE OF HALF METALLIC FERROMAGNET WITH CHROMIUM DIOXIDE OR COMPOSITE OF CHROMIUM DIOXIDE AND CHROMIUM SESQUIOXIDE
Abstract	A process for manufacture of half metallic ferromagnet, substantially pure chromium dioxide (Cr02) or composite of chromium dioxide and chromium sesquioxide (Cr02/Cr203) comprising heating Cr8021 to a temperature in the range between 350 and 480°C for a period between 1-5 hours whereby substantially pure chromium dioxide (Cr02) or composite of chromium dioxide and chromium sesquioxide (Cr02/Cr203) are formed such that Cr8021 is converted to said half metallic ferromagnet substantially pure chromium dioxide (Cr02) when temperature is maintained between 350-400°C or to composite of chromium dioxide and chromium sesquioxide (Cr02/Cr203) when temperature is maintained between 400 - 480°C

Title of Invention

A PROCESS FOR MANUFACTURE OF HALF METALLIC FERROMAGNET WITH CHROMIUM DIOXIDE OR COMPOSITE OF CHROMIUM DIOXIDE AND CHROMIUM SESQUIOXIDE

Abstract

A process for manufacture of half metallic ferromagnet, substantially pure chromium dioxide (Cr02) or composite of chromium dioxide and chromium sesquioxide (Cr02/Cr203) comprising heating Cr8021 to a temperature in the range between 350 and 480°C for a period between 1-5 hours whereby substantially pure chromium dioxide (Cr02) or composite of chromium dioxide and chromium sesquioxide (Cr02/Cr203) are formed such that Cr8021 is converted to said half metallic ferromagnet substantially pure chromium dioxide (Cr02) when temperature is maintained between 350-400°C or to composite of chromium dioxide and chromium sesquioxide (Cr02/Cr203) when temperature is maintained between 400 - 480°C

Full Text	FORM 2 THE PATENTS ACT 1970 COMPLETE SPECIFICATION (See Section 10) With chraniun dixide or complsite of chronium dioxide and chronium sesquioxide APPLICANT Tata Institute of Fundamental Research, Homi Bhabha Road, Colaba, Mumbai 400 005, Maharashtra, India. An aided autonomous institution under the administrative purview of the Department of Atomic Energy, Government of India, Anushakti Bhavan, Chatrapati Shivaji Maharaj Marg, Mumbai 400 005, Maharashtra, India. The following specification particularly describes and ascertains the nature of this invention and the manner in which it is to be performed. Field of invention: The present invention relates to a process for manufacture of half-metallic ferromagnet either substantially pure chromium dioxide (Cr02), or composites of chromium dioxide and chromium sesquioxide (Cr02/Cr02O3). The present invention particularly relates to a simple process for manufacture of ferromagnetic chromium dioxide (Cr02) from chromium trioxide (Cr03), as well as a composite of chromium dioxide and chromium sesquioxide (CrO2/Cr2CO3) via an intermediate oxide Cr8C21. Background and Prior Art: B.L Chamberland has reviewed in 1977 "The chemical and physical properties of Cr02 and tetravalent chromium oxide derivatives" in CRC Critical Rev in Solid Stale and Maler. Sci. 7, 1 (1977). Chromium dioxide Cr02) is a metallic, room temperature ferromagnet with Curie temperature (Tc) around 385 K. It stabilizes in tetragonal Rutile structure, Space Group P42/mnm. In this compound, Cr is in 4+ state contributing 2 u.n per Cr atom in the system. Its known to show the metallic conductivity in single crystals and epitaxially grown thin films. However, activated behavior is seen in polycrystalline CrC»2 , that is believed to be arising from hopping of the charge carrier across grain boundaries in polycrystals. JMD Coey et.al, have related this mechanism of conduction to the origin of large magnetoresistance exhibited by Cr02 in their paper entitled "Magnetoresistance of Chromium dioxide powder compacts, Phy. Rev. Lett. 80, 3815, 1998". Recently in 2001, Ji et.al, have observed, through Andreev reflection measurements, the maximum spin polarization close to 100%.in CrOi, vis a vis other materials showing half metallicity Phy. Rev. Lett. 86, 5585, 2001. The ferromagnetic sample is characterized by its saturation magnetization Ms at 0 K and Curie temperature Tc. The theoretical values for saturation magnetization for C1O2 is about 135 emu/gm. The best reported value for the satuaration magnetization for polycrystalline samples range from 75 -87 emu/gm (Table 2). The single crystals have shown value of the order of 108 emu/gm whereas epitaxial thin films of high quality have 2 shown Ms close to the theoretical values. For instance. Li et.al have reported in Appl. Phy. Led, 75, 713, 1999. Ms value of the order of 135 cmu/gm for a 4000A thick Cr02 films. It is known in literature that sintering of C'rOj is difficult since it is a metastable phase and easily converts to Cr02 even at modest temperatures of 200°C (F.Y. Yang et.al, Appl. Phys. Lett.,77,286,2000). The ferromagneticCr02 is used as a particulate media in recording industries. As mentioned earlier, Cr02 has been found to be a half metallic ferrmagnet with nearly full spin polarization. Recently, the half-metallic ferromagnets. as a magnetoresistive material have been employed in various magneto-electronic applications. One area in which half metallic ferromagnets have tremendous device application is Magnetic Tunnel Junctions. (Warren E Picket and Jagdeesh S Moodera, "Half-metallic Magnets" Physics Today, May 2001). The presence of even a small amount of impurity phase not only affects the ferromagnetic properties but also interferes with the phenomenon of half metallicity / spin polarization severely. This results in the loss of spin polarization and thus bringing down the efficiency of the concerned device, based on spin polarized current. Since Cr02 is a material of industrial importance, there have been a large number of patents and papers on various preparation methods and intricacies involved for preparing Cr02 The related prior art is presented in Table 1. Since the saturation magnetization value (Ms) is an important criteria for a pure ferromagnetic material, and is a test for comparing various processes, some Ms values from literature for Cr02 are given in Table 2. 3 Tabic 1. US Patent Starting Material Temp. Pressure Product Remarks 2923685 (1960) GO,, H20, Na2SO., 450°C, 1000 Aim CrO, * 3423320 (1969) KCr,Os, H20, 2600 Aim Cr02 needle like FM 3449073 (1969) CrO,, Cr203, Na2Cr20, 850 Atm Cr02 fine grain particles 4428852 (1984) preheated hydrated chromium oxide elevated pressure/ continuous process CrO, * 3117093 (1964) CrxOy 2y/x is 4 - 5.5 50 : 3000 Aim Cr02 * 3493338(1970)) Cr03? NO, 02 225, 325 flC no pressure CrO, 94%+ CrO,, 5% * 5856008(1999) CrO, 520 nC 35000 bar Cr02 CrO, coated * with Cr20, ** • relevant to production of Cr02; * * relevant to production of Cr02 / Cr203 Composite. Table 2: Saturation Magnetization Values for CrC>2 Reference Saturation Magnetisation (Ms) (emu/gm) Theoretical Value of Ms -135 US Patent 4747974 (polycrystal) 75-78 US Patent 3493338 (polycrystal) 24-84 (including modifier) US Patent 3486851 (polycrystal) 78-87 US Patent 3451771 (polycrystal) 21-35 US Patent 2923683 (polycrystal) 38-66 (with modifier) Poly crystal Chamberland(1977) 73-75 Single crystal and Epitaxial films (Chamberlaitdl977) 100 4000 A thick film (Li, Gupta and Xiao 1999) -133 4 Table 1 shows that (a) Cr03 (chromium VI oxide) has been used as a starting material. Cr02 can be prepared by thermal decomposition ofCrO3 and mixed chromium oxides. (b) There exist a very narrow window of temperature and pressure in which many other oxides of chromium stabilize, including Cr02 and the phase boundary between these oxides is very fuzzy. Consequently a little variation in preparation condition results in mixed phase or impure compounds Chamberland has discussed this aspect by showing it in Figs.l and 2 on page 3 of his review (ibid.). (c) The difficulties of accurately measuring and controlling pressure, along with the fact that it requires expensive high-pressure assemblies leading to high production cost are the main drawbacks of above mentioned preparative methods. It is desirable to have a preparative method, which does not need pressure as controlling parameter. (d) The last three US patents in the above table form relevant prior art to the present invention and will be discussed after describing the present invention in detail. Object The main object of the present invention is to provide a method for manufacture of either half-metallic ferromagnet chromium dioxide (C1O2), or composites of chromium dioxide and chromium sesquioxide Cr02/Cr02O3) that does not require pressure as a control parameter and that does not result in any additives in the product. Summary of the Present Invention: Thus according to the present invention there is provided a process for manufacture of substantially pure half metallic ferromagnet chromium dioxide (Cr02), or composites of chromium dioxide and chromium sesquioxide (Cr02/Cr02O3) comprising heating Cr8C21 to a temperature of between 350 and 480°C for a period of between 1-5 hours whereby substantially pure chromium dioxide (CCr02), or composites of chromium dioxide and chromium sesquioxide Cr02/Cr02O3) are formed. 5 Detailed Description of the Present Invention. Thus the process of present invention comprises the following reactions A Cr8021-> Cr02 or composite Cr02 / Cr203 by heating at 350-480 °C. Al Cr8021-» Cr02 by heating at 350-400X.; A2 Cr8021 -> composite Cr02 / Cr203 by heating at 400-480 °C]. According to the above process, when temperature is maintained between 350 C and 400°C Cr02 is formed as the product (Step Al) and when the temperature is between 400°C to 480°C it gives Cr02 / Cr203 composite (Step A2). In step A it is preferred to maintain the temperature of Cr8021 in the specified range for 2-3 hours. To convert Cr8021 to substantially pure Cr02 according to step Al it is preferred to maintain the temperature range between 390-400°C and to convert Cr8021 to the said composite of chromium dioxide and chromium sesquioxide (Cr02 / Cr2O3) according to step A2 the temperature range should be preferably maintained between 400-450°C. According to a preferred aspect Cr8021 is prepared from CrO3 by heating the latter and maintaining the temperature from 220-330°C for 6 to 14 hours, preferably for 8-12 hours. It is further preferred to maintain a temperature in the range of 250-280°C. Cr03 is taken in an inert container and heated slowly to raise the temperature to about 250°C and thereafter maintained in the temperature range from 250-280°C, for about 8-12 hours in dry oxygen/air at about atmospheric pressure, till it is converted to Cr8021 as characterized by X ray diffraction. Thereafter it is cooled to room temperature at the same rate as that used for raising the temperature initially. The product so formed is crushed in any inert vessel to form a powder and then the powder is sealed in a glass tube of inert material. The powdered can also be palletized before sealing it in the inert tube. Thereafter the tube is placed in a preheated furnace maintained at a temperature in the range of about 390-450°C for about 2-3 hours till it is converted to said half-metallic ferromagnet viz. said substantially pure chromium 6 dioxide CrO2, when at 390 - 400°C or a composite of chromium dioxide and chromium sesquioxide CrO2,/Cr2O3) when at 400 - 450°C, as characterized by XRD. The sealed tube is then opened after cooling it to ambient temperature and the contents are stored in any dry container. After lot of experimentation we have found that the stoichiometry of the final product at a certain temperature depends on the quality of precursor. It is found that stoichiometric CrO2, can be prepared even if oxygen flow was not maintained while making Cr8C21 It is preferable to maintain the oxygen flow else a slight variation in preparation temperature leads to the formation of other oxides of chromium, which may adversely effects, the quality of CrO2,. US patent 3,493,338 there is only one step reaction: C1O3 -> CrO2,. There is no isolation of the intermediate product. Heat treatment cycle is as follows: RT -> 175 °C in 10 min. -> 225 °C in 1 hour -> 325 °C in 20 min. and maintained at 325 for 2 hrs. All the while there is gas (- a mixture of 3.5 % NO and O2) flow at normal atmospheric pressure is run over the boat carrying C1O3 at about 200 cc /min. The Ms value of the CrO2, obtained is ~ 84 emu/g; where as that in the present process product is ~ 125 emu/g. The US patent 3,117,093 describes two step process, the second step startmg with CrO2, with average chromium valance ratio (2y/x) of 4.2 - 4.3 which is 5.25 for Cr8O21 used in the process of present invention*-This is mixed with water and other modifiers such as antimony sesqui oxide inside a platinum tube and reaction carried out under a pressure of 50 - 3000 Atm. There are, however 3 examples (No. 14, 15 and 16) in this patent, which describe the process without any modifier. The first step starts with chromic nitrate nonahydrate; it is heated at 330 °C for 2 hours to obtain an intermediate chromium oxide CrxOy .not a well-defined compound as said above. In these three examples the reaction CrxOj, ->CrO2, was carried out at 400 °C for 1 to 2 hours. This starting material CrxOy was either moist 3 - 9 % moisture or taken water for milling before heat treatment. Though in these examples there is no mention about pressure, the reaction has been claimed to be done at from 50 - 3000 atmosphere pressure. It will be 7 clearly seen that the Step A of the present invention is different from this prior art. The starting material in step B of the present invention is different from this prior art. Step B in the present invention is in dry state, that in these examples without modifier in the US patent 3,117.093 is in the moist state. The temperature of the reaction at step IB is 400°C at 750 atmosphere and 17% water, in the US patent 3.117,093. Conditions are therefore different in the present invention for production of CrO2. Thus the process of present invention is different from this prior art process. In US patent 5,856,008 process, the reaction CrO3 -> CrO2, is done in a gold capsule at 35 k bar pressure at 520 C for 2 hours in a piston cylinder type high pressure furnace; on cooling to room temperature the pressure is removed. Thus this is a one step reaction different from the process of present invention. The CrO2, thus obtained is annealed for 30 minutes in air at 380 C, and another sample at 420 C. And it is believed that in this process CrO2, gets a surface coating of CT2O3, based on XRD measurements. In the process of present invention, CrO3 -> Cr8O21 -> composite CrO2, / CrO2, by heating at 400 - 450 °C, and is clearly different from US patent 5,856,008. The process of present invention for manufacture of composites is simpler and does not need pressure as a control parameter. EXAMPLES The invention will now be illustrated with the help of examples. The examples are by way of illustration only and in no way restrict the scope of the invention. Equipment Used: The equipment used in these examples is a tubular furnace (Carbolite model CTF 12/65 furnace). The furnace has a temperature range up' to 1200°C and has the arrangement for flow of inert or oxygen flow. Inert containers used for reaction are chosen from quartz, Pyrex glassware. 8 Chemicals used: CrO3 granules with 99.9% purity were obtained from Aldrich Chemical company, USA. Analytical Facilities used: Siemens Diffractometer (Model D-500): for powder X-ray diffraction measurements (XRD). Wave Dispersive Electron Probe Microanalyser (CAMECA Model EPMA SX-100): for scanning electron microscopy (SEM). Vibrating Sample Magnetometer (Oxford MagLab VSM) and SQUID magnetometer (Quantum Design MPMS-XL7): for magnetization measurements. Description of Figures in the accompanying drawings: Fig. 1 (a) - (d) : X-Ray diffraction pattern (20 vs Intensity) for the product of Examples I, II, III and IV respectively; the solid lines are fit to the experimental curve (circles). Fig. 2: Scanning Electron Micrograph of the product of Example III. The long Needle shaped structures show the regular shape of CrC»2 particles. Fig. 3. Magnetisation (M) as a function of magnetic field (H) for the product of Examples 1,1L, III and IV at 5 K. Example I: Chromium trioxide was taken in a quartz tube and it was heated slowly to raise the temperature to about 250 °C, and thereafter the temperature was maintained at 250°C, for 10 hours under oxygen flow at about atmospheric pressure. The end product, Cr8C>2i after this reaction was a hard bar, which was crushed and powdered using agate mortar. This powder of Cr8O21 (approx. 500 mg) obtained as above, was placed inside a Pyrex glass tube of 12 cm length and 1.5 cm diameter. The glass tube was sealed at atmospheric pressure and the ampoule was kept in a tube furnace at 392°C for 2 hours. 9 X-ray diffraction. Ritveld refinement of powder XRD (R.A. Young et al Program DBWS-941 1. 1994) was performed on tliis sample. The results are shown in Fig. 1(a). This product was therefore CrO2 as evident from XRD pattern. The powder was examined for the measurement of saturation magnetization on a SQUID magnetometer (Model MPMS-XL7 of Quantum Design, USA). At a temperature of 5K, this powder shows saturation magnetization (Ms) as large as 132 emu/gm, which is very close to the theoretically predicted value 135 emu/gm (E. P. Wohlfarth, Ferromagnetic Materials Vol.2, 471, 1980) [See Fig.3]. This proved that the CrO2 product obtained at the end of Example I was a substantially pure material. For comparison, reported values of different ferromagnetic materials see Table 2. It will be seen that this is the only compound -the product of Example I - has the highest Ms value and it is very close to theoretical value. Example II The procedure of Example I was repeated, the only change being Cr8O21 (1.5 g) was sealed inside a test tube of 1 cm diameter and 10 cm length. It was placed in the furnace at 392 °C for 2 hrs. The final product was stoichiometric CrO2, and was identical with product of Example I in the above tests. The product of present invention is CrO2, of polycrystalline type as confirmed by X-ray diffraction measurements, where no impurity peaks are observed (Fig 1(b)). Example III The procedure of Example 1 was repeated, the only change being Cr8O21 (1.5 g) was ground for few hours and palletized in the form of a pellet of about 1.2 cm diameter and 0.2 cm thickness. This pellet was sintered at 250° C for few hours in tubular furnace. It was cooled to room temperature and was then sealed in a glass ampule at atomospheric pressure. This ampule was kept in the oven at 392°C for 2 hrs. The final product was a fairly hard pellet of stoichimetric Cr02. 10 I he product of Example III was examined at room temperature under Scanning Election Microscope (SEM). The SEM analysis was done on electron probe microanalysis (EPMA) system (model SX-100 from M/s CAM AC. France). The SEM picture is shown in Fig. 2, where long needle shaped grain are seen. I he X-ray diffracJion measurement was done as m Example J and results are shown In Fig. 1(c). The product of Example III was examined for the measurement of saturation magnetization (Ms) as in Example I. The results are shown in Fig. 3. The saturation magnetization is of the order of 125 emu/gm for this sample at 5 K (Table 1). Example IV: The procedure of Example I was repeated, with the only change of Cr8O21 (1.5 g) sealed in a glass tube was kept in the oven maintained at temperature at around 425°C. The X-ray diffraction examination was done as in Example I and the results are shown in Fig. 1(d). The product formed was found to be a composite of Cr02/Cr2C3 from XRD. The fraction of insulating Cr203 was also determined to be 45% (molar). The product of Example IV was examined for Ms value, as in Example I. (See Fig. 3). The Ms value found to be 75 emu/gm at 5 K. The Ms values obtained for Half-metallic FM materials obtained in these examples are given in Table 3. Table 3 : Ms Values Polycrystal example I, II 132 emu/g Polycrystal example III 125 emu/g Composite Example IV 75 emu/g at 5 K 11 Advantage of (he present invention: The process of present invention for manufacture of half metallic ferromagnet. high purity chromium dioxide (Cr()2). or composites of chromium dioxide and chromium sesquioxide (CrO2/Cr2O3), i. does not need high pressure equipment, is simple to operate and is cost effective; ii.gives a polycrystalline CrO2 in bulk, having saturation magnetization value nearer to that in epitaxial thin films and at a temperature of 5 K close to the theoretical value; iii. gives the final product in desired shape of fairly good hardness, required for all practical purposes such as for measuring electrical resistivity etc.; iv. provides C1O2 of substantially high purity for spintronic devices. 12 WE CLAIM 1. A process for manufacture of half metallic ferromagnet, substantially pure chromium dioxide (Cr02) or composite of chromium dioxide and chromium sesquioxide (Cr02/Cr203) comprising heating Cr8021 to a temperature in the range between 350 and 480°C for a period between 1-5 hours whereby substantially pure chromium dioxide (Cr02) or composite of chromium dioxide and chromium sesquioxide (Cr02/Cr203) are formed such that Cr8021 is converted to said half metallic ferromagnet substantially pure chromium dioxide (Cr02) when temperature is maintained between 350-400°C or to composite of chromium dioxide and chromium sesquioxide (Cr02/Cr203) when temperature is maintained between 400 - 480°C 2. A process as claimed in claim 1, wherein Cr8021 used in the process of the invention is prepared by heating Cr03 and maintaining the temperature in the range of 230-320°C, preferably in the range 250-280°C; 3. A process as claimed in claim 1 to 2, wherein said Cr03 is heated and maintained in the said temperature range for 6-14 hours, preferably 8-12 hours; 4. A process as claimed in any one of the preceding claims, wherein Cr03 is heated in dry oxygen/air; 5. A process as claimed in any one of the preceding claims, wherein Cr03 is heated at about atmospheric pressure; 6. A process as claimed in any one of the preceding claims, wherein Cr03 is heated slowly to raise the temperature to about 230°C and then maintained in the said temperature range; 7. A process as claimed in claim 1, wherein Cr8021 formed is cooled slowly to room temperature substantially at the same rate as it was heated. 8. A process as claimed in claim 1, wherein Cr8021 is crushed powder form; 9. A process as claimed in any of the preceding claims, wherein the said Cr8021 powder is sealed in a tube or an be palletized and sintered before sealing in a tube; 13 10. A process as claimed in any one of the preceding claims, wherein the temperature of Cr802i is maintained in the said range for 2-3 hrs; 11. A process as claimed in any one of the preceding claims, wherein Cr802i is converted to said substantially pure chromium dioxide Cr02 when the temperature is maintained between 390-400°C; 12. A process as claimed in any one of the preceding claims, wherein Cr8021 is converted to said composite of chromium dioxide and chromium sesquioxide (Cr02/Cr203) when the temperature is maintained between 400-450°C; 13. A process for manufacture of substantially pure half metallic ferromagnet chromium dioxide (Cr02), or composites of chromium dioxide and chromium sesquioxide (Cr02/Cr203), substantially as herein described in the text, examples and accompanying drawings. Dated this 28th day of August 2002 Dr. SanchitarGanguli Of S.MAJUMDAR & CO Applicant's Agent 14

Full Text

FORM 2
THE PATENTS ACT 1970 COMPLETE SPECIFICATION
(See Section 10)
With chraniun dixide or complsite of chronium
dioxide and chronium sesquioxide
APPLICANT
Tata Institute of Fundamental Research, Homi Bhabha Road, Colaba, Mumbai 400 005, Maharashtra, India.
An aided autonomous institution under the administrative purview of the
Department of Atomic Energy, Government of India,
Anushakti Bhavan, Chatrapati Shivaji Maharaj Marg,
Mumbai 400 005, Maharashtra, India.
The following specification particularly describes and ascertains the nature of this invention and the manner in which it is to be performed.

Field of invention:
The present invention relates to a process for manufacture of half-metallic ferromagnet either substantially pure chromium dioxide (Cr02), or composites of chromium dioxide and chromium sesquioxide (Cr02/Cr02O3).
The present invention particularly relates to a simple process for manufacture of ferromagnetic chromium dioxide (Cr02) from chromium trioxide (Cr03), as well as a composite of chromium dioxide and chromium sesquioxide (CrO2/Cr2CO3) via an intermediate oxide Cr8C21.
Background and Prior Art:
B.L Chamberland has reviewed in 1977 "The chemical and physical properties of Cr02 and tetravalent chromium oxide derivatives" in CRC Critical Rev in Solid Stale and Maler. Sci. 7, 1 (1977). Chromium dioxide Cr02) is a metallic, room temperature ferromagnet with Curie temperature (Tc) around 385 K. It stabilizes in tetragonal Rutile structure, Space Group P42/mnm. In this compound, Cr is in 4+ state contributing 2 u.n per Cr atom in the system. Its known to show the metallic conductivity in single crystals and epitaxially grown thin films. However, activated behavior is seen in polycrystalline CrC»2 , that is believed to be arising from hopping of the charge carrier across grain boundaries in polycrystals. JMD Coey et.al, have related this mechanism of conduction to the origin of large magnetoresistance exhibited by Cr02 in their paper entitled "Magnetoresistance of Chromium dioxide powder compacts, Phy. Rev. Lett. 80, 3815, 1998". Recently in 2001, Ji et.al, have observed, through Andreev reflection measurements, the maximum spin polarization close to 100%.in CrOi, vis a vis other materials showing half metallicity Phy. Rev. Lett. 86, 5585, 2001.
The ferromagnetic sample is characterized by its saturation magnetization Ms at 0 K and Curie temperature Tc. The theoretical values for saturation magnetization for C1O2 is about 135 emu/gm. The best reported value for the satuaration magnetization for polycrystalline samples range from 75 -87 emu/gm (Table 2). The single crystals have shown value of the order of 108 emu/gm whereas epitaxial thin films of high quality have
2

shown Ms close to the theoretical values. For instance. Li et.al have reported in Appl. Phy. Led, 75, 713, 1999. Ms value of the order of 135 cmu/gm for a 4000A thick Cr02 films.
It is known in literature that sintering of C'rOj is difficult since it is a metastable phase and easily converts to Cr02 even at modest temperatures of 200°C (F.Y. Yang et.al, Appl. Phys. Lett.,77,286,2000).
The ferromagneticCr02 is used as a particulate media in recording industries. As mentioned earlier, Cr02 has been found to be a half metallic ferrmagnet with nearly full spin polarization. Recently, the half-metallic ferromagnets. as a magnetoresistive material have been employed in various magneto-electronic applications. One area in which half metallic ferromagnets have tremendous device application is Magnetic Tunnel Junctions. (Warren E Picket and Jagdeesh S Moodera, "Half-metallic Magnets" Physics Today, May 2001).
The presence of even a small amount of impurity phase not only affects the ferromagnetic properties but also interferes with the phenomenon of half metallicity / spin polarization severely. This results in the loss of spin polarization and thus bringing down the efficiency of the concerned device, based on spin polarized current.
Since Cr02 is a material of industrial importance, there have been a large number of patents and papers on various preparation methods and intricacies involved for preparing Cr02 The related prior art is presented in Table 1.
Since the saturation magnetization value (Ms) is an important criteria for a pure ferromagnetic material, and is a test for comparing various processes, some Ms values from literature for Cr02 are given in Table 2.
3

Tabic 1.

US Patent Starting Material Temp. Pressure Product Remarks
2923685 (1960) GO,, H20, Na2SO., 450°C, 1000 Aim CrO, *
3423320 (1969) KCr,Os, H20, 2600 Aim Cr02 needle like FM
3449073 (1969) CrO,, Cr203, Na2Cr20, 850 Atm Cr02 fine grain particles
4428852 (1984) preheated hydrated chromium oxide elevated pressure/ continuous process CrO, *
3117093 (1964) CrxOy 2y/x is 4 - 5.5 50 : 3000 Aim Cr02 *
3493338(1970)) Cr03? NO, 02 225, 325 flC no pressure CrO, 94%+ CrO,, 5% *
5856008(1999) CrO, 520 nC 35000 bar Cr02 CrO, coated * with Cr20, **
• relevant to production of Cr02; * * relevant to production of Cr02 / Cr203 Composite.
Table 2: Saturation Magnetization Values for CrC>2

Reference Saturation Magnetisation (Ms) (emu/gm)
Theoretical Value of
Ms -135
US Patent 4747974 (polycrystal) 75-78
US Patent 3493338 (polycrystal) 24-84 (including modifier)
US Patent 3486851 (polycrystal) 78-87
US Patent 3451771 (polycrystal) 21-35
US Patent 2923683 (polycrystal) 38-66 (with modifier)
Poly crystal Chamberland(1977) 73-75
Single crystal and Epitaxial films (Chamberlaitdl977) 100
4000 A thick film (Li, Gupta and Xiao 1999) -133
4

Table 1 shows that
(a) Cr03 (chromium VI oxide) has been used as a starting material. Cr02 can be prepared by thermal decomposition ofCrO3 and mixed chromium oxides.
(b) There exist a very narrow window of temperature and pressure in which many other oxides of chromium stabilize, including Cr02 and the phase boundary between these oxides is very fuzzy. Consequently a little variation in preparation condition results in mixed phase or impure compounds Chamberland has discussed this aspect by showing it in Figs.l and 2 on page 3 of his review (ibid.).
(c) The difficulties of accurately measuring and controlling pressure, along with the fact that it requires expensive high-pressure assemblies leading to high production cost are the main drawbacks of above mentioned preparative methods. It is desirable to have a preparative method, which does not need pressure as controlling parameter.
(d) The last three US patents in the above table form relevant prior art to the present invention and will be discussed after describing the present invention in detail.
Object
The main object of the present invention is to provide a method for manufacture of either half-metallic ferromagnet chromium dioxide (C1O2), or composites of chromium dioxide and chromium sesquioxide Cr02/Cr02O3) that does not require pressure as a control parameter and that does not result in any additives in the product.
Summary of the Present Invention:
Thus according to the present invention there is provided a process for manufacture of substantially pure half metallic ferromagnet chromium dioxide (Cr02), or composites of chromium dioxide and chromium sesquioxide (Cr02/Cr02O3) comprising heating Cr8C21 to a temperature of between 350 and 480°C for a period of between 1-5 hours whereby substantially pure chromium dioxide (CCr02), or composites of chromium dioxide and chromium sesquioxide Cr02/Cr02O3) are formed.
5

Detailed Description of the Present Invention.
Thus the process of present invention comprises the following reactions A Cr8021-> Cr02 or composite Cr02 / Cr203 by heating at 350-480 °C.
Al Cr8021-» Cr02 by heating at 350-400X.;
A2 Cr8021 -> composite Cr02 / Cr203 by heating at 400-480 °C].
According to the above process, when temperature is maintained between 350 C and 400°C Cr02 is formed as the product (Step Al) and when the temperature is between 400°C to 480°C it gives Cr02 / Cr203 composite (Step A2).
In step A it is preferred to maintain the temperature of Cr8021 in the specified range for 2-3 hours. To convert Cr8021 to substantially pure Cr02 according to step Al it is preferred to maintain the temperature range between 390-400°C and to convert Cr8021 to the said composite of chromium dioxide and chromium sesquioxide (Cr02 / Cr2O3) according to step A2 the temperature range should be preferably maintained between 400-450°C.
According to a preferred aspect Cr8021 is prepared from CrO3 by heating the latter and maintaining the temperature from 220-330°C for 6 to 14 hours, preferably for 8-12 hours. It is further preferred to maintain a temperature in the range of 250-280°C.
Cr03 is taken in an inert container and heated slowly to raise the temperature to about 250°C and thereafter maintained in the temperature range from 250-280°C, for about 8-12 hours in dry oxygen/air at about atmospheric pressure, till it is converted to Cr8021 as characterized by X ray diffraction. Thereafter it is cooled to room temperature at the same rate as that used for raising the temperature initially.
The product so formed is crushed in any inert vessel to form a powder and then the powder is sealed in a glass tube of inert material. The powdered can also be palletized before sealing it in the inert tube. Thereafter the tube is placed in a preheated furnace maintained at a temperature in the range of about 390-450°C for about 2-3 hours till it is converted to said half-metallic ferromagnet viz. said substantially pure chromium
6

dioxide CrO2, when at 390 - 400°C or a composite of chromium dioxide and chromium sesquioxide CrO2,/Cr2O3) when at 400 - 450°C, as characterized by XRD.
The sealed tube is then opened after cooling it to ambient temperature and the contents are stored in any dry container.
After lot of experimentation we have found that the stoichiometry of the final product at a certain temperature depends on the quality of precursor. It is found that stoichiometric CrO2, can be prepared even if oxygen flow was not maintained while making Cr8C21 It is preferable to maintain the oxygen flow else a slight variation in preparation temperature leads to the formation of other oxides of chromium, which may adversely effects, the quality of CrO2,.
US patent 3,493,338 there is only one step reaction: C1O3 -> CrO2,. There is no isolation of the intermediate product. Heat treatment cycle is as follows: RT -> 175 °C in 10 min. -> 225 °C in 1 hour -> 325 °C in 20 min. and maintained at 325 for 2 hrs. All the while there is gas (- a mixture of 3.5 % NO and O2) flow at normal atmospheric pressure is run over the boat carrying C1O3 at about 200 cc /min. The Ms value of the CrO2, obtained is ~ 84 emu/g; where as that in the present process product is ~ 125 emu/g.
The US patent 3,117,093 describes two step process, the second step startmg with CrO2, with average chromium valance ratio (2y/x) of 4.2 - 4.3 which is 5.25 for Cr8O21 used in the process of present invention*-This is mixed with water and other modifiers such as antimony sesqui oxide inside a platinum tube and reaction carried out under a pressure of 50 - 3000 Atm. There are, however 3 examples (No. 14, 15 and 16) in this patent, which describe the process without any modifier. The first step starts with chromic nitrate nonahydrate; it is heated at 330 °C for 2 hours to obtain an intermediate chromium oxide CrxOy .not a well-defined compound as said above. In these three examples the reaction CrxOj, ->CrO2, was carried out at 400 °C for 1 to 2 hours. This starting material CrxOy was either moist 3 - 9 % moisture or taken water for milling before heat treatment. Though in these examples there is no mention about pressure, the reaction has been claimed to be done at from 50 - 3000 atmosphere pressure. It will be
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clearly seen that the Step A of the present invention is different from this prior art. The starting material in step B of the present invention is different from this prior art. Step B in the present invention is in dry state, that in these examples without modifier in the US patent 3,117.093 is in the moist state. The temperature of the reaction at step IB is 400°C at 750 atmosphere and 17% water, in the US patent 3.117,093. Conditions are therefore different in the present invention for production of CrO2.
Thus the process of present invention is different from this prior art process.
In US patent 5,856,008 process, the reaction CrO3 -> CrO2, is done in a gold capsule at 35 k bar pressure at 520 C for 2 hours in a piston cylinder type high pressure furnace; on cooling to room temperature the pressure is removed.
Thus this is a one step reaction different from the process of present invention. The CrO2, thus obtained is annealed for 30 minutes in air at 380 C, and another sample at 420 C. And it is believed that in this process CrO2, gets a surface coating of CT2O3, based on XRD measurements.
In the process of present invention, CrO3 -> Cr8O21 -> composite CrO2, / CrO2, by heating at 400 - 450 °C, and is clearly different from US patent 5,856,008. The process of present invention for manufacture of composites is simpler and does not need pressure as a control parameter.
EXAMPLES
The invention will now be illustrated with the help of examples. The examples are by way of illustration only and in no way restrict the scope of the invention.
Equipment Used:
The equipment used in these examples is a tubular furnace (Carbolite model CTF 12/65 furnace). The furnace has a temperature range up' to 1200°C and has the arrangement for flow of inert or oxygen flow. Inert containers used for reaction are chosen from quartz, Pyrex glassware.
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Chemicals used:
CrO3 granules with 99.9% purity were obtained from Aldrich Chemical company, USA.
Analytical Facilities used:
Siemens Diffractometer (Model D-500): for powder X-ray diffraction measurements (XRD).
Wave Dispersive Electron Probe Microanalyser (CAMECA Model EPMA SX-100): for scanning electron microscopy (SEM).
Vibrating Sample Magnetometer (Oxford MagLab VSM) and SQUID magnetometer (Quantum Design MPMS-XL7): for magnetization measurements.
Description of Figures in the accompanying drawings:
Fig. 1 (a) - (d) : X-Ray diffraction pattern (20 vs Intensity) for the product of
Examples I, II, III and IV respectively; the solid lines are fit to the
experimental curve (circles). Fig. 2: Scanning Electron Micrograph of the product of Example III. The
long Needle shaped structures show the regular shape of CrC»2 particles. Fig. 3. Magnetisation (M) as a function of magnetic field (H) for the product of
Examples 1,1L, III and IV at 5 K.
Example I:
Chromium trioxide was taken in a quartz tube and it was heated slowly to raise the temperature to about 250 °C, and thereafter the temperature was maintained at 250°C, for 10 hours under oxygen flow at about atmospheric pressure. The end product, Cr8C>2i after this reaction was a hard bar, which was crushed and powdered using agate mortar. This powder of Cr8O21 (approx. 500 mg) obtained as above, was placed inside a Pyrex glass tube of 12 cm length and 1.5 cm diameter. The glass tube was sealed at atmospheric pressure and the ampoule was kept in a tube furnace at 392°C for 2 hours.
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X-ray diffraction. Ritveld refinement of powder XRD (R.A. Young et al Program DBWS-941 1. 1994) was performed on tliis sample. The results are shown in Fig. 1(a). This product was therefore CrO2 as evident from XRD pattern.
The powder was examined for the measurement of saturation magnetization on a SQUID magnetometer (Model MPMS-XL7 of Quantum Design, USA). At a temperature of 5K, this powder shows saturation magnetization (Ms) as large as 132 emu/gm, which is very close to the theoretically predicted value 135 emu/gm (E. P. Wohlfarth, Ferromagnetic Materials Vol.2, 471, 1980) [See Fig.3]. This proved that the CrO2 product obtained at the end of Example I was a substantially pure material. For comparison, reported values of different ferromagnetic materials see Table 2. It will be seen that this is the only compound -the product of Example I - has the highest Ms value and it is very close to theoretical value.
Example II
The procedure of Example I was repeated, the only change being Cr8O21 (1.5 g) was sealed inside a test tube of 1 cm diameter and 10 cm length. It was placed in the furnace at 392 °C for 2 hrs. The final product was stoichiometric CrO2, and was identical with product of Example I in the above tests. The product of present invention is CrO2, of polycrystalline type as confirmed by X-ray diffraction measurements, where no impurity peaks are observed (Fig 1(b)).
Example III
The procedure of Example 1 was repeated, the only change being Cr8O21 (1.5 g) was ground for few hours and palletized in the form of a pellet of about 1.2 cm diameter and 0.2 cm thickness. This pellet was sintered at 250° C for few hours in tubular furnace. It was cooled to room temperature and was then sealed in a glass ampule at atomospheric pressure. This ampule was kept in the oven at 392°C for 2 hrs. The final product was a fairly hard pellet of stoichimetric Cr02.
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I he product of Example III was examined at room temperature under Scanning Election Microscope (SEM). The SEM analysis was done on electron probe microanalysis (EPMA) system (model SX-100 from M/s CAM AC. France). The SEM picture is shown in Fig. 2, where long needle shaped grain are seen.
I he X-ray diffracJion measurement was done as m Example J and results are shown In Fig. 1(c).
The product of Example III was examined for the measurement of saturation magnetization (Ms) as in Example I. The results are shown in Fig. 3. The saturation magnetization is of the order of 125 emu/gm for this sample at 5 K (Table 1).
Example IV:
The procedure of Example I was repeated, with the only change of Cr8O21 (1.5 g) sealed in a glass tube was kept in the oven maintained at temperature at around 425°C. The X-ray diffraction examination was done as in Example I and the results are shown in Fig. 1(d). The product formed was found to be a composite of Cr02/Cr2C3 from XRD. The fraction of insulating Cr203 was also determined to be 45% (molar). The product of Example IV was examined for Ms value, as in Example I. (See Fig. 3). The Ms value found to be 75 emu/gm at 5 K.
The Ms values obtained for Half-metallic FM materials obtained in these examples are given in Table 3.
Table 3 : Ms Values

Polycrystal example I, II 132 emu/g
Polycrystal example III 125 emu/g
Composite Example IV 75 emu/g at 5 K
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Advantage of (he present invention:
The process of present invention for manufacture of half metallic ferromagnet. high purity chromium dioxide (Cr()2). or composites of chromium dioxide and chromium sesquioxide (CrO2/Cr2O3),
i. does not need high pressure equipment, is simple to operate and is cost effective; ii.gives a polycrystalline CrO2 in bulk, having saturation magnetization value nearer to that in epitaxial thin films and at a temperature of 5 K close to the theoretical value; iii. gives the final product in desired shape of fairly good hardness, required for all
practical purposes such as for measuring electrical resistivity etc.; iv. provides C1O2 of substantially high purity for spintronic devices.
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WE CLAIM
1. A process for manufacture of half metallic ferromagnet, substantially pure chromium dioxide (Cr02) or composite of chromium dioxide and chromium sesquioxide (Cr02/Cr203) comprising heating Cr8021 to a temperature in the range between 350 and 480°C for a period between 1-5 hours whereby substantially pure chromium dioxide (Cr02) or composite of chromium dioxide and chromium sesquioxide (Cr02/Cr203) are formed such that Cr8021 is converted to said half metallic ferromagnet substantially pure chromium dioxide (Cr02) when temperature is maintained between 350-400°C or to composite of chromium dioxide and chromium sesquioxide (Cr02/Cr203) when temperature is maintained between 400 - 480°C
2. A process as claimed in claim 1, wherein Cr8021 used in the process of the invention is prepared by heating Cr03 and maintaining the temperature in the range of 230-320°C, preferably in the range 250-280°C;
3. A process as claimed in claim 1 to 2, wherein said Cr03 is heated and maintained in the said temperature range for 6-14 hours, preferably 8-12 hours;
4. A process as claimed in any one of the preceding claims, wherein Cr03 is heated in dry oxygen/air;
5. A process as claimed in any one of the preceding claims, wherein Cr03 is heated at about atmospheric pressure;
6. A process as claimed in any one of the preceding claims, wherein Cr03 is heated slowly to raise the temperature to about 230°C and then maintained in the said temperature range;
7. A process as claimed in claim 1, wherein Cr8021 formed is cooled slowly to room temperature substantially at the same rate as it was heated.
8. A process as claimed in claim 1, wherein Cr8021 is crushed powder form;
9. A process as claimed in any of the preceding claims, wherein the said Cr8021 powder is sealed in a tube or an be palletized and sintered before sealing in a tube;
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10. A process as claimed in any one of the preceding claims, wherein the temperature of Cr802i is maintained in the said range for 2-3 hrs;
11. A process as claimed in any one of the preceding claims, wherein Cr802i is converted to said substantially pure chromium dioxide Cr02 when the temperature is maintained between 390-400°C;
12. A process as claimed in any one of the preceding claims, wherein Cr8021 is converted to said composite of chromium dioxide and chromium sesquioxide (Cr02/Cr203) when the temperature is maintained between 400-450°C;
13. A process for manufacture of substantially pure half metallic ferromagnet chromium dioxide (Cr02), or composites of chromium dioxide and chromium sesquioxide (Cr02/Cr203), substantially as herein described in the text, examples and accompanying drawings.
Dated this 28th day of August 2002
Dr. SanchitarGanguli Of S.MAJUMDAR & CO Applicant's Agent
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Documents:

783-mum-2002-cancelled pages(24-5-2004).pdf

783-mum-2002-claims(granted)-(24-5-2004).doc

783-mum-2002-claims(granted)-(24-5-2004).pdf

783-mum-2002-correspondence(27-7-2006).pdf

783-mum-2002-correspondence(ipo)-(16-11-2006).pdf

783-mum-2002-drawing(30-9-2002).pdf

783-mum-2002-form 1(24-5-2004).pdf

783-mum-2002-form 1(29-8-2002).pdf

783-mum-2002-form 19(30-10-2003).pdf

783-mum-2002-form 2(granted)-(24-5-2004).doc

783-mum-2002-form 2(granted)-(24-5-2004).pdf

783-mum-2002-form 3(29-8-2002).pdf

783-mum-2002-power of attorney(24-5-2004).pdf

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Patent Number

204533

Indian Patent Application Number

783/MUM/2002

PG Journal Number

24/2007

Publication Date

15-Jun-2007

Grant Date

23-Feb-2007

Date of Filing

29-Aug-2002

Name of Patentee

TATA INSTITUTE OF FUNDAMENTAL RESEARCH

Applicant Address

HOMI BHABHA ROAD, COLABA, MUMBAI 400 005, MAHARASHTRA, INDIA.

Inventors:

#	Inventor's Name	Inventor's Address
1	DR. ASHNA BAJPAI	TATA INSTITUTE OF FUNDAMENTAL RESEARCH, HOMI BHABHA ROAD, COLABA, MUMBAI 400 005
2	PROF. ARUN KUMAR NIGAM	TATA INSTITUTE OF FUNDAMENTAL RESEARCH, HOMI BHABHA ROAD, MUMBAI 400 005

PCT International Classification Number

C 01 G 37/027

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1			NA