The field of the invention:
The present invention relates to stable granular GMR materials of Cr02 nanoparticles
especially for use in magnetic memory devices and more particularly in a magnetic
random access memory for writing information with a reduced write-current and other
applications. The invention also relates to the process for preparing stabilized Cr02 in
form of a finely divided loose powder suitable for fabricating small devices and
components in a specific shape or size.
Background art:
Chromium dioxide, Cr02, is a metastable compound of a blackish characteristic colour in
comparison to the Or2O3 (greenish characteristic colour), which is the most stable
compound of chromium oxides. It is a half-metallic ferromagnet with very special
magnetic and electrical properties. Moreover, it offers new properties of granular GMR
{see PRL 80 (1998) 3825}. Commercially, it is extensively used in magnetic memories,
magnetic disc drives, solid state angular and displacement measuring instruments, and
optical sensors (US 20010025978 and 20020065349).
A major limitation in these commercial applications of Cr02 is nonavailability of a viable
method of its synthesis in a mass scale at an economic cost. This is one of the reasons
that this compound is not available for a commercial distribution so far. Although there
are many techniques of synthesis of ceramics in small particles but those are not capable
in producing of Cr02 in a stable product due to its metastable nature.
Objects of the Invention:
It is thus the basic object of the present invention to provide stabilized Cr02 having
excellent GMR properties.
Another object is to provide stabilized Cr02 having excellent and improved GMR
properties than those achieved in conventional Cr02 whereby it would be possible to
extend the use/application of such Cr02 in magnetic memories, magnetic disc drives,
optical sensors, and other devices.
Yet further object of the present invention is directed to a method of manufacturing CrOp
whereby it would be possible to obtain a single phase Cr02 powder by a very convenient and
simple method to stabilize Cr02 phase which would be cost effective and simple involving fewer
steps.
Another object is directed to yield finely divided loose CrO? nanopowder with a narrow size
distribution.
Yet further object is directed to provide a very viable method for processing of stabilized CrO^
ceramics & composites.
Another object is to provide a method of manufacture of stabilized Cr02 which would involve
readily available and cost-effective ingredients/raw materials.
Statement of invention
A process for manufacture of stable CrO? nanoparticles comprising providing aqueous solution
of Cr03 polyvinyl alcohol (PVA) and sucrose wherein the Cr03 content is in the range of 0.2 to
0.5M; dispersing Cr03 in small groups or micelles with said PVA-sucrose polymer molecules;
capping the Cr03 micelles by the said PVA-sucrose polymer molecules as a precursor solution;
aging said precursor solution to provide a stable capping in the form of a film of said PVA-
sucrose polymer molecules; heating the precursor of d) above to a temperature of 400 to 500"C
to thereby obtain the desired stable coated Cr02 nanoparticles.
Summary of the Invention
Thus according to one aspect of the present invention there is provided a stable CrO^
nanoparticles having a value of the saturation magnetization of M, of 35 to 48 preferably 45 8
emu/g, coercivity Hc of 200 to 300 Oe at room temperature, the GMR properties with a value of
MR 20% to 50% preferably 40% at liquid helium temperature.
The above GMR properties appear in a peculiar microstructure with as small crystallites as a few
manometers in size. Such small particles have many distinct features because of a rather large
fraction of atoms present in the grain surface or boundaries.
4
In accordance with a preferred aspect of the invention the stable CrO? nanoparticles comprise a
thin surface coating of individual particles with anyone or more of Cr203 and Ag metal.
The stable CrO? nanoparticles comprise recrystallized Cr02 nanopowder of a tetragonal crystal
structure with preferred lattice parameters of a=0.4391 nm and c=0.2871 nm.The average
crystallite and/or particle size vary from30to50 nm
Also, the stable Cr02 nanoparticles of the invention comprise single phase Cr02 powder.
According to another aspect of the present invention there is provided a process for the
manufacture of stable Cr02 nanoparticles comprising :
a) providing aqueous solutions of CrO-,, PVA and sucrose ;
b) dispersing Cr03 in small groups or micelles with PVA-sucrose polymer molecules in a
mixed solution in the three components ;
c) capping the micelles by the PVA-sucrose polymer molecules into a stable film ;
d) aging of the precursor;
e) heating the precursor of d) above to a temperature of 400 to 500°C to thereby obtain the
desired stable Cr02.
This invention therefore provides a novel method to synthesize CrO?as a stable compound. The
process involves synthesis of a precursor of Cre* cations, with a polymer to cap the cations in
small micelles. A stable Cr02 product appears by a reconstructive decomposition and
recrystallization on heating the precursor at 400 to 500"C in air for 1 to 2 h. The product is a
finely divided loose powder suitable to fabricate the components and devices. A thin surface
coating of Cr?0:. or silver is put to support improved CrO;> stability with stable properties
Detailed Description of the Invention:
The starting reagents/ingredients used are preferably commercial CrO:.. poly-vinyl alcohol (PVA!
and sucrose of 99.9 % purity. Separate solutions are made in three chemicals by dissolving in
distilled water in predetermined amounts. The initial concentration can be varied from 1 0 to 2.0
M for CrO;, while 2 0 to 4 0 g/dl for PVA or 20 to 50 g/dl for sucrose. Freshly prepared solutions
have been used The final reaction was carried out as follows.
As shown hereunder in Scheme 1. the method of the present invention includes the steps of fa)
providing the aqueous solutions of CrO:). PVA and sucrose, (b) dispersing CrO:) in small groups
or micelles with PVA-sucrose polymer molc-c-Jles in a mixed solution in the three components
(c) capping the micelles by the PVA-sucrose polymer molecules in a rather stable film, and then
(d) aging of the precursor in air at room temperature or lower A product of CrO:- occurs in form
of a finely divided loose nanopowder after heating tne precursor so obtained at 400 to 500!C in
air.
Capping :
Here the polymer molecules act as a capping agent. According to it in solution they cap or
enclose the reactant of Cr03 in small groups resulting in these capped or enclosed Cr03 in
small groups are no longer in direct contact to the H20 molecules in the solution. This provides
a controlled coreduction of Cr03 (of black characteristic colour) in reaction with the polymer
molecules, which enclose the Cr03. Otherwise, an extended reaction occurs, leading to
formation of an undesired product of Cr03 (or hydroxide) of a greenish characteristic colour.
Aging :
The term aging refers to formation of a rather stable sample of capped Cr03 in small groups in
polymer molecules. It occurs by a local structural rearrangement of Cr03 via polymer molecules
in small groups with formation of a rather stable film of the polymer. Here it is achieved by
keeping the sample, after the primary reaction which is carried out at high temperature as 50 to
60°C. at a lower temperature of 15 to 25°C for 30 to 50 h in air.
Preferably, a drop wise addition of Cr03 (in 1.0 to 2.0 M solution) to a mixed PVA-sucrose
solution (in 1:10 ratio by mass) in water at 50 to 60-C (controlled through a constant temperature
bath of Julabo model HD -7) results in a homogeneous dispersion of Cr03 in micelles. It is
observed that in the solution the CrOr. has an exothermic reaction with PVA-sucrose polymer
molecules. The reaction proceeds with a successive change in average colour of the resulting
solution from characteristically reddish one in the beginning to a pale yellow to an orange
yellowish and then finally to a blackish one. The change of the colour in the reaction is studied
further with in-situ measurement of the uv-visible spectrum The results infer that the reaction
occurs with a continuous change in the oxidation state of the chromium cations from Cr5, in the
initial CrOs precursor solution to Cr'" in the resulting equilibrium solution after the reaction.
After the reaction, the sample is cooled and aged at 15 to 25 C for 30 to 50 h before drying to a
solid precursor mass by heating at 70 to 80 C in air A fluffy voluminous mass of characteristic
black colour results in dried sample. X-ray diffraction levealed that it is amorphous in structure 1
:o 2 h of heating at 400 to 500 C in air. after pulverizing in a refined powder by grinding in a
mortar with a pestle, results in a recrystallized CrO? nanopowder of a tetragonal crystal structure.
The lattice parameters of a = 0.4391 nrn and c = 0.2871 nm. as determined by X-ray diffraction.
compare the reported values of a = 0.4421 nm and c = 0.2916 nm in the literature. In TEM
micrograph, average crystallite and/or particle size varies from 30 to 50 nm according to the final
temperature used to recrystallized by heating from an amorphous precursor

T'ro. obtained CrO,. nanopowdcr is found to have a value of the saturation magnetization of M, -
-•!¦>.8 cr.u/g, in agreement with the standard M,. - 51.5 emu/g value (Brandle, et al., Phys. Rev.
D, 4G (1992) 13889}, with a value of coercivity hi of 200 to 300 Oe at room temperature. It
showed GMR properties with a value of MR as much as 40% at liquid helium temperature. In
accordance with a preferred aspect, a thin surface coating of individual particles with Cr203 or
Ag-metal improved the GMR properties with no much loss in the final M,; value.
The abew- characteristics of the CrO- were? found to be reproducible and importantly useful for
applications -. product by a precursor of CrO;.
rcciuired a r'i'her controlled reconstructive thermal dnrnmnncinn ~f ;*¦ ;n -> ^^(-r^u^ -i-^,^^,.u —
so mat it boos not reach the equilibrium Cr203 product. Ilvs is achieved with a polymer precursor
of Cr03 in which Cr03 molecules are capped in micelles in rather stable polymer molecules of PVA
and sucrose. It is proposed that on heating, the polymer molecules, which encapsulate the CrOi
in micelles, do not decompose at these temperatures and allow a controlled conversion of CrO.-,
into CrO.; as per the reaction of Scheme III hereunder:

The reaction resulted in CrO;i micelles encapsulated in the polymer subject to 1 to 2 h of heating
in air at 400 to 500 C from a proper precursor (dried and pulverized in a fine powder) after a
proper initial reaction of the encapsulation in micelles. At this temperature, the polymer burns out
leaving behind a desired product of a pure Cr02 nanopowder
A stable polymer CrOr. precursor with PVA and sucrose polymer molecules is synthesized as
described above. According to it. a batch of 150 ml of PVA-sucrose solution is taken at 50 to
60°C and stirred for about 10 min. It had a total of 3.0 g of PVA and 30.0 g of sucrose in 1:10
ratio (mass wise). Then a total of about 50 ml of a CrC\ solution of 1.0 M concentration was
added drop wise by stirring the mixture at this temperature. After the reaction, the sample was
cooled and aged at 15 to 25-C for 30 to 50 h and dried to a solid precursor mass by heating at 70
to 80CC in air.
Example - 3 :
The above process of Example 2 was repeated with 2.0 M solution of Cr03 in water. It produced
a similar Cr02 nanopowder after a controlled reconstructive thermal decomposition by heating
the precursor over 400 to 500°C temperatures in air. Several other batches of the reactions were
carried out by varying the concentrations for the initial solutions of 0.2 and 2.0 M in Cr03 while 2
to 4.0 g/dl in PVA or 20 to 50 g/dl in sucrose. All of the reaction batches result in a finely divided
loose Cr02 nanopowder. However, the reaction was found to occur best with moderate
concentrations of the starting solutions as used in Example 2 or in the present Example 3 with
2.0 M Cr03 solution.
Example - 4 :
Under this Example a thin coating of silver metal (in 3 to 5 nm thickness) was Drovided over
>iiu,.,i,uUi ^,^2 Huiii^io-o. i no ^iu^i>s wcj6 caineu oui Dy uispeising me polymer precursor
powder (after drying and pulverizing in a fine powder) in an aqueous AgNO;. solution (1 to 2 M
concentration) by a vigorous stirring at room temperature. After 10 to 20 min of defluxion in a 5 to
10 g batch of the powder in about 100 ml AgNOi solution, the sample is filtered, dried at room
temperature, and then heated in air at 200 to 400 C to ensure a stable silver coating of CrO-
particles. The Ag-coated CrO? nanoparticles have been found to have a characteristically
different colour of silvery shining surfaces. The process of the reaction is repeated in order to
improve the thickness in multilayers.
Example - 5 :
h 'his Example it was ascertained that part of CrO; at surface converts to Cr,0;> in a thin
amorphous surface layer (2 to 4 nm thickness) on 1 to 2 h of heating of a CrO,> nanopowder at
500 C in O.- gas. In raising the temperature above 600 C the Cr203 enriches and recrystallizes in
R3c hexagonal crystal structure of a = 0.4890 nm and c = 1.3970 nm lattice parameters. A
recrystallized Cr-CX nanopowder occured in 2 h of heating at about 700 C in air. It has a =
0 4890 nm and c = 1.3110 nm lattice parameters in agreement with reported a = 0.4954 nm and
c = 1.3584 nm values in literature (JCPDS file 6.0504).
It is thus possible by way of the present invention to advantageously and beneficially produce a
single-phase CrO^ powder. The process involves simple reaction conditions in water with
commercially available economic raw materials. Thus the process is cost effective and also
provides a finish product finely divided loose CrO? nanopowder suitable for processing of Cr07
devices and components in a desired shape.
. Example - 5 :
In this Example it was ascertained that part of CrO;; at surface converts to Cr?0. in a thin
amorphous surface layer (2 to 4 nm thickness) on 1 to 2 h of heating of a CrO; nanopowder at
500 C in O, gas. In raising the temperature above 600 C. the Cr?0:; enriches and recrystalhzes in
R3c hexagonal crystal structure of a = 0.4890 nm and c = 1.3970 nm lattice parameters A
recrystallized Cr20;; nanopowder occured in 2 h of heating at about 700' C in air It has ,-i --
0.4890 nm and c = 1.3110 nm lattice parameters in agreement with reported a - 0.4954 nm and
c = 1.3584 nm values in literature (JCPDS file 6.0504)
It is thus possible by way of the present invention to advantageously and beneficially produce a
single-phase CrO? powder. The process involves simple reaction conditions in water with
commercially available economic raw materials. Thus the process is cost effective and also
provides a finish product finely divided loose CrO? nanopowder suitable for processing of CrO;
devices and components in a desired shape.
WE CLAIM:
A process for manufacture of stable CrO; nanoparticles comprising:
a) providing aqueous solution of CrO; polyvinyl alcohol (PVA) and sucrose wherein the
Cr03content is in the range of 0.2 to 0.5M;
b) dispersing QO3 in small groups or micelles with said PVA-sucrose polymer molecules;
c) capping the Cr03 micelles by the said PVA-sucrose polymer molecules as a precursor
solution;
d) aging said precursor solution to provide a stable capping in the form of a film of said
PVA-sucrose polymer molecules;
e) heating the precursor of d) above to a temperature of 400 to 500::'C to thereby
obtain the desired stable coated CrO?. nanoparticles.
2. A process for the manufacture of stable CrO? nanoparticles as claimed in claim 1
comprising optionally providing a thin surface coating of anyone or more of CrpO:i and Ag
(silver) to thereby provide coated CrO? with improved stability and properties.
3. A process as claimed in anyone of claims 1 or 2 wherein the initial cone, of Cr03 used varies
between 1.0 to 2.0 M, PVA varies between 2.0 to 4.0 g/dl and sucrose varies between 20 to
50 g/dl.
4. A process as claimed in anyone of claims 1 to 3 comprising :
dropwise addition of CrO-i (in 1.0 to 2.0 M solution) to a mixed PVA-sucrose solution
prereraDly in 1:10 ratio by mass in water ai b0 to 60°C to obtain a homogeneous
dispersion of CrO, in micelles ;
cooling the reaction mix above and ageing at 15 to 25°C for 30 to 50 h ;
drying to a solid precursor mass by heating at 70 to 80°C ;
pulverizing and heating in the temperature range of 400 to 500°C for a period of 1 to
2 h to thereby provide rccrystallined CrO. nanopowcler preferably of a tetragonal
crystal structure.
5. A process as claimed in claim 4 wherein the lattice parameters of the tetragonal
crystal struciure obtained is a : 0.4391 nm and c - 0.2871 nm.
6. A process as claimed in anyone of claims 1 to 5 wherein the average crystallite
and/or particle size obtained vary from 30 to 50 nm
7. A process as claimed in anyone of claims 1 to 6 comprising optionally providing a
thin coating of silver metal over said OO2 panicles comprising the steps of
dispersing the polymer precursor powder after said drying and pulverizing in an
aqueous AgNOj solution;
filtering, drying and heating to a temperature of 200 to 400C t" nrovide => s»»blf»
snvei coateu L1U2 powder.
8. A process as claimed in claim 7 wherein the aqueous AgNCb solution used is of a
cone, in the range of 1 to 2 M
9. A process as claimed in anyone of claims 1 to 8 comprising optionally providing a
thin amorphous surface layer 2 to 4 nm thickness of C1O3 following the steps of a)
heating the OO2 nanopowdcr at 400 to 500 preferably 500°C in O2 gas for a
period of 1 to 2 h. b) raising the temperature to above 600°C whereby the Cr20;i
enriches and rccrystallization in R3c hexagonal crystal struciure.
10. A process as claimed in claim 9 comprising heating at 700CC in air to
rccrystallization CT2O3 having a =- 0,4890 and c - 1.3110 nm lattice parameteres.
11. A process Tor manufacture a stable CrC^ nanoparlicles substantially as
hereindescribed and illustrated with reference to the accompanying examples.

A stable granular GMR materials of CrO2 nanoparticles especially for use in
magnetic memory devices and more particularly in a magnetic random access
memory for writing information with a reduced write-current and other
applications Also disclosed is the process for preparing stabilized single phase
CrO2 in form of a finely divided loose powder suitable for fabricating small devices
and components in a specific shape or size.
The stable Cr02 nanoparticles have a value of the saturation magnetization of M,
of 35 to 48 preferably 45.8 emu/g. coercivity H