U.S. patent application number 09/797605 was filed with the patent office on 2001-07-26 for chemical synthesis of monodisperse and magnetic alloy nanocrystal containing thin films.
This patent application is currently assigned to INTERNATIONAL BUSINESS MACHINES CORPORATION. Invention is credited to Murray, Christopher Bruce, Sun, Shouheng, Weller, Dieter K..
Application Number | 20010009119 09/797605 |
Document ID | / |
Family ID | 23414694 |
Filed Date | 2001-07-26 |
United States Patent
Application |
20010009119 |
Kind Code |
A1 |
Murray, Christopher Bruce ;
et al. |
July 26, 2001 |
Chemical synthesis of monodisperse and magnetic alloy nanocrystal
containing thin films
Abstract
A method and structure for forming magnetic alloy nanoparticles
includes forming a metal salt solution with a reducing agent and
stabilizing ligands, introducing an organometallic compound into
the metal salt solution to form a mixture, heating the mixture to a
temperature between 260.degree. and 300.degree. C., and adding a
flocculent to cause the magnetic alloy nanoparticles to precipitate
out of the mixture without permanent agglomeration. The deposition
of the alkane dispersion of FePt alloy particles, followed by the
annealing results in the formation of a shiny FePt nanocrystalline
thin film with coercivity ranging from 500 Oe to 6500 Oe.
Inventors: |
Murray, Christopher Bruce;
(Ossining, NY) ; Sun, Shouheng; (Ossining, NY)
; Weller, Dieter K.; (San Jose, CA) |
Correspondence
Address: |
Christopher N. Sears, Esq.
Suite 200
8321 Old Courthouse Road
Vienna
VA
22182-3817
US
|
Assignee: |
INTERNATIONAL BUSINESS MACHINES
CORPORATION
Armonk
NY
|
Family ID: |
23414694 |
Appl. No.: |
09/797605 |
Filed: |
March 5, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09797605 |
Mar 5, 2001 |
|
|
|
09359638 |
Jul 26, 1999 |
|
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|
Current U.S.
Class: |
75/348 ;
G9B/5.295 |
Current CPC
Class: |
C21B 15/00 20130101;
B82Y 25/00 20130101; G11B 5/84 20130101; B22F 9/24 20130101; H01F
10/123 20130101; H01F 1/0063 20130101; B22F 2998/00 20130101; B22F
2998/00 20130101; B22F 1/054 20220101; B22F 2998/00 20130101; B22F
1/054 20220101 |
Class at
Publication: |
75/348 |
International
Class: |
B22F 009/00 |
Claims
What is claimed is:
1. A method of forming magnetic alloy nanoparticles, comprising:
forming a metal salt solution with a reducing agent and stabilizing
ligands; introducing an organometallic compound into said metal
salt solution to form a mixture; heating said mixture to a
temperature between 260.degree. and 300.degree. C.; and adding a
flocculent to cause said magnetic alloy nanoparticles to
precipitate out of said mixture without permanent
agglomeration.
2. The method in claim 1, wherein said organometallic compound
includes a solvent comprising one of phenyl ether, dioctyl ether,
polyphenyl ether, and polyethylene glycol.
3. The method in claim 1, wherein the reducing agent comprises a
long chain diol.
4. The method in claim 3, wherein said long chain diol comprises
one of 1,2-hexadecanediol, 1,2-dodecanediol and 1,2-octanediol.
5. The method in claim 1, wherein said stabilizing ligands include
RCOOH and RNH.sub.2, where R comprises an alkyl, alkenyl
hydrocarbon chain (C6 or longer).
6. The method in claim 1, wherein said flocculent comprises an
alcohol including one of methanol, ethanol, propanol and
butanol.
7. The method in claim 1, wherein said metal salt comprises one of
Pt salt, Pt(CH.sub.3COCHCOCH.sub.3).sub.2,
Pt(CF.sub.3COCHCOCF.sub.3).sub.2, Tetrakis (triphenylphosphine)
Platinum(O), or Pt (triphenylphosphine).sub- .4-x(CO).sub.x where x
is at least 1 and no greater than 3.
8. The method in claim 1, wherein said organometallic compound
comprises one of Fe(CO).sub.5, Co.sub.2(CO).sub.8,
Co.sub.4(CO).sub.12, Fe.sub.2(CO).sub.9, Fe.sub.3(CO).sub.12,
Fe(CNR).sub.5, and (Diene) Fe(CO).sub.5 (e.g., Cyclopentadiene,
Cyclooctadiene).
9. A method of forming a magnetic alloy nanoparticle film,
comprising: forming a metal salt solution with a reducing agent and
stabilizing ligands; introducing an organometallic compound into
said metal salt solution to form a mixture; heating said mixture to
a temperature between 260.degree. and 300.degree. C.; adding a
flocculent to cause said magnetic alloy nanoparticles to produce a
precipitate out of said mixture without permanent agglomeration;
forming a dispersion with said precipitate; depositing said
dispersion on a solid surface; annealing said dispersion in an
inert atmosphere at temperature up to 650.degree. C.; and cooling
said dispersion under an inert atmosphere.
10. The method in claim 9, wherein said dispersion comprises one
of: an alkane dispersion, including pentane, hexane, heptane,
octane and dodecane; a chlorinated solvent dispersion, including
dichloromethane and chloroform; and an aromatic solvent dispersion,
including benzene, toluene and xylene.
11. The method in claim 9, wherein said dispersion comprises an
RNH.sub.2 (where R comprises an alkyl or alkenyl chain of C12 or
longer) formed by the addition of alcohol.
12. The method in claim 9, wherein said inert atmosphere comprises
one of N.sub.2 or Ar.
13. The method in claim 9, wherein said annealing temperature is
between 400.degree. C. and 650.degree. C.
14. The method in claim 9, wherein said annealing forms a layer of
amorphous carbon around particles in said precipitate.
15. The method in claim 9, wherein said organometallic compound
includes a solvent comprising one of phenyl ether and dioctyl
ether.
16. The method in claim 9, wherein the reducing agent comprises a
long chain diol.
17. The method in claim 16, wherein said long chain diol comprises
one of 1,2-hexadecanediol, 1,2-dodecanediol and 1,2-octanediol.
18. The method in claim 9, wherein said stabilizing ligands include
RCOOH and RNH.sub.2, where R comprises an alkyl, alkenyl
hydrocarbon chain (C6 or longer).
19. The method in claim 9, wherein said flocculent comprises an
alcohol including one of methanol, ethanol, propanol and
butanol.
20. The method in claim 9, wherein said metal salt comprises one of
Pt salt, Pt(CH.sub.3COCHCOCH.sub.3).sub.2,
Pt(CF.sub.3COCHCOCF.sub.3).sub.2, Tetrakis (triphenylphosphine)
Platinum(O), or Pt (triphenylphosphine).sub- .4-x(CO).sub.x where x
is at least 1 and no greater than 3.
21. The method in claim 9, wherein said organometallic compound
comprises one of Fe(CO).sub.5, Co.sub.2(CO).sub.8,
Co.sub.4(CO).sub.12, Fe.sub.2(CO).sub.9, Fe.sub.3(CO).sub.12,
Fe(CNR).sub.5, and (Diene) Fe(CO).sub.5 (e.g., Cyclopentadiene,
Cyclooctadiene).
22. A method of forming a magnetic storage device having magnetic
alloy nanoparticles, comprising: forming a metal salt solution with
a reducing agent and stabilizing ligands; introducing an
organometallic compound into said metal salt solution to form a
mixture; heating said mixture to a temperature between 260.degree.
and 300.degree. C.; and adding a flocculent to cause said magnetic
alloy nanoparticles to precipitate out of said mixture without
permanent agglomeration.
23. The method in claim 22, wherein said organometallic compound
includes a solvent comprising one of phenyl ether and dioctyl
ether.
24. The method in claim 22, wherein the reducing agent comprises a
long chain diol.
25. The method in claim 24, wherein said long chain diol comprises
one of 1,2-hexadecanediol, 1,2-dodecanediol and 1,2-octanediol.
26. The method in claim 22, wherein said stabilizing ligands
include RCOOH and RNH.sub.2, where R comprises an alkyl, alkenyl
hydrocarbon chain (C6 or longer).
27. The method in claim 22, wherein said flocculent comprises an
alcohol including one of methanol, ethanol, propanol and
butanol.
28. The method in claim 22, wherein said metal salt comprises one
of Pt salt, Pt(CH.sub.3COCHCOCH.sub.3).sub.2,
Pt(CF.sub.3COCHCOCF.sub.3).sub.2, Tetrakis (triphenylphosphine)
Platinum(O), or Pt (triphenylphosphine).sub- .4-x(CO).sub.x where x
is at least 1 and no greater than 3.
29. The method in claim 22, wherein said organometallic compound
comprises one of Fe(CO).sub.5, Co.sub.2(CO).sub.8,
Co.sub.4(CO).sub.12, Fe.sub.2(CO).sub.9, Fe.sub.3(CO).sub.12,
Fe(CNR).sub.5, and (Diene) Fe(CO).sub.5 (e.g., Cyclopentadiene,
Cyclooctadiene).
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention generally relates to convenient
chemical syntheses of stable, nearly monodisperse alloy magnetic
nanoparticles and an economic chemical approach to magnetic alloy
nanocrystalline thin film production on a solid surface. The
coercivity of the thin film can be controlled in the range between
500 Oe and 6500 Oe. This synthesis route offers a viable approach
to the production of ultra-high density recording media.
[0003] 2. Description of the Related Art
[0004] The storage density of commercial magnetic recording media
is increasing at a compound rate of 60% per annum. As the storage
density increases the individual storage bit size decreases
proportionally. To control signal to noise ratios and other
recording parameters, it is advantageous to maintain a large number
of ferromagnetic grains per bit. Thus, the development of new
magnetic media with smaller grains, high coercivity and high
magnetization is required. Furthermore, to maximize the signal to
noise ratio, the grains should be well isolated from each other to
prevent exchange coupling between the grains, and possess a narrow
size distribution. However, this scaling approach is limited by the
onset of superparamagnetic behavior when the grain size falls below
some material-dependent characteristic dimension.
[0005] One approach to reduce particle dimensions but still
maintain sufficiently high coercivity is to take advantage of the
high magnetocrystalline anisotropy found in some metal alloy
systems. It is known that an assembly of very small non-interacting
magnetic particles with high anisotropy possesses high coercivity.
The coercivity arises because the small particles can support only
a single domain state and irreversible magnetization rotation is
the only possible mechanism of flux reversal.
[0006] The CoPt or FePt binary alloys are excellent candidates for
this approach because of their chemical stability, and high
magnetocrystalline anisotropy (thus high coercivity) arising from
the existence of ordered intermetallic phases. For these alloys,
magnetocrystalline anisotropies of around 7.times.10e7 erg/cm.sup.3
have been obtained, compared to 5.times.10e6 erg/cm.sup.3 for hcp
cobalt-based recording media. Typical Co based recording media
today have anisotropies of order 1-2.times.10e6 erg/cm.sup.3. Since
the stored magnetic energy scales with the anisotropy constant and
the particle volume, KV, smaller particles of high K materials like
FePt and CoPt can potentially be used in future media applications.
The advantage is narrower transitions and reduced read back noise.
Hcp cobalt based granular thin films doped with Pt are being used
today in ultra-high density recording applications, a typical
composition being CoP.sub.10Cr.sub.22B.sub.6. Accordingly,
tremendous research efforts have been focused on synthesis and
characterization of near equiatomic CoPt and FePt alloys. These
alloys may also have great potential for use as magnetic bias films
of magneto-resistive elements, and magnetic tips for magnetic force
microscopy.
[0007] A common procedure leading to CoPt or FePt alloy materials
is cosputtering of Co (or Fe) and Pt. This procedure allows little
control over particle size or size distribution. The following
description discloses that stable, monodisperse magnetic alloy
nanoparticles and related nanocrystalline thin film can be easily
synthesized by convenient chemical procedures.
[0008] The invention includes a convenient chemical way to prepare
stable monodisperse Fe/Pt alloy magnetic nanoscale materials, an
approach to form smooth nanocrystalline films on a variety of
substrates, and exploring the possibility of using the developed
materials as ultra-high density recording media.
SUMMARY OF THE INVENTION
[0009] The principal object of synthesizing magnetic Fe/Pt alloy
nanoparticles and nanocrystalline thin films has been achieved in
this invention. A combination of reduction of metal salt and
decomposition of neutral organometallic precursor has been
developed for the formation of the magnetic alloy nanoparticles.
For example, in situ reduction of
Pt(acac).sub.2(acac=acetylactonate, CH.sub.3COCHCOCH.sub.3 anion)
by long chain diol and decomposition of Fe(CO).sub.5 at a high
temperature (260.degree. C.-300.degree. C.) solution phase yields
high quality nanoparticles.
[0010] The particles are protected from agglomeration by a
combination of long chain carboxylic acid, such as oleic acid, and
long chain primary amine, such as oleyl amine. This stabilization
is so effective that the particles can be handled easily either in
solution phase or as solid form under air.
[0011] The particles are easily dispersed in alkane and chlorinated
solvent and purified by precipitation through the addition of
alcohol. Deposition of the alkane solution of the alloy particles
on SiO.sub.2, Si, Si.sub.3N.sub.4, or glass leads to the formation
of a smooth particulate thin film, offering an economic route for
the production of thin film media. The as-synthesized magnetic
alloy shows a single-phase FCC and is magnetically soft. Under
thermal conditions in the range of 500.degree. C. to 650.degree.
C., the precipitated particles undergo long-range ordering to a
structure of the CuAu-I type. This structure is tetragonal with the
(002) planes, normal to the c axis, occupied alternately by Fe and
Pt atoms, giving high magnetocrystalline anisotropy at room
temperature. Coercivities between 500 Oe to 6500 Oe have been
achieved at room temperature.
[0012] It is, therefore, an object of the present invention to
provide a structure and method forming magnetic alloy
nanoparticles, which includes forming a metal salt solution with a
reducing agent and stabilizing ligands, introducing an
organometallic compound into the metal salt solution to form a
mixture, heating the mixture to a temperature between 260.degree.
and 300.degree. C., and adding a flocculent to cause the magnetic
alloy nanoparticles to precipitate out of the mixture without
permanent agglomeration. The organometallic compound includes a
solvent including one of phenyl ether and dioctyl ether. The
reducing agent includes a long chain diol, which includes one of
1,2-hexadecanediol, 1,2-dodecanediol and 1,2-octanediol. The
stabilizing ligands include RCOOH and RNH.sub.2, where R includes
an alkyl, alkenyl hydrocarbon chain (C6 or longer). The flocculent
includes an alcohol including one of methanol, ethanol, propanol
and butanol. The metal salt includes one of Pt salt,
Pt(CH.sub.3COCHCOCH.sub.3).sub.2, Pt(CF.sub.3COCHCOCF.sub.3).sub-
.2, Tetrakis (triphenylphosphine) Platinum(O), or Pt(O)
(triphenylphosphine).sub.4-x(CO).sub.x where x is at least 1 and no
greater than 3. The organometallic compound includes one of
Fe(CO).sub.5, Co.sub.2(CO).sub.8, Co.sub.4(CO).sub.12,
Fe.sub.2(CO).sub.9, Fe.sub.3(CO).sub.12, Fe(CNR).sub.5, and (Diene)
Fe(CO).sub.5 (e.g., Cyclopentadiene, Cyclooctadiene, etc.).
[0013] Another embodiment of the invention is a method of forming a
magnetic alloy nanoparticle film, which includes forming a metal
salt solution with a reducing agent and stabilizing ligands,
introducing an organometallic compound into the metal salt solution
to form a mixture, heating the mixture to a temperature between
260.degree. and 300.degree. C., adding a flocculent to cause the
magnetic alloy nanoparticles to produce a precipitate out of the
mixture without permanent agglomeration, forming a dispersion with
the precipitate, depositing the dispersion on a solid surface,
annealing the dispersion in an inert atmosphere at temperature up
to 650.degree. C., and cooling the dispersion under an inert
atmosphere. The dispersion includes either an alkane dispersion,
including pentane, hexane, heptane, octane and dodecane, a
chlorinated solvent dispersion, including dichloromethane and
chloroform, or an aromatic solvent dispersion, including benzene,
toluene and xylene. The dispersion includes an alkane RNH.sub.2
(where R includes an alkyl or alkenyl chain of C12 or longer)
formed by the addition of alcohol. The inert atmosphere includes
one of N.sub.2 or Ar. The annealing temperature is between
400.degree. C. and 650.degree. C. The annealing forms a layer of
amorphous carbon around particles in the precipitate. The
organometallic compound includes a solvent including one of phenyl
ether and dioctyl ether. The reducing agent includes a long chain
diol, which includes one of 1,2-hexadecanediol, 1,2-dodecanediol
and 1,2-octanediol. The stabilizing ligands include RCOOH and RNH2,
where R includes an alkyl, alkenyl hydrocarbon chain (C6 or
longer). The flocculent includes an alcohol including one of
methanol, ethanol, propanol and butanol. The metal salt includes
one of Pt salt, Pt(CH.sub.3COCHCOCH.sub.3).sub.2,
Pt(CF.sub.3COCHCOCF.sub.3).sub.2, Tetrakis (triphenylphosphine),
Platinum(O), or Pt(O) (triphenylphosphine).sub.4-x(CO).sub.x. The
organometallic compound includes one of Fe(CO).sub.5,
Co.sub.2(CO).sub.8, Co.sub.4(CO).sub.12, Fe.sub.2(CO).sub.9,
Fe.sub.3(CO).sub.12, Fe(CNR).sub.5, and (Diene) Fe(CO).sub.5 (e.g.,
Cyclopentadiene, Cyclooctadiene, etc.).
[0014] Yet another embodiment is a method of forming a magnetic
storage device having magnetic alloy nanoparticles, which includes
forming a metal salt solution with a reducing agent and stabilizing
ligands, introducing an organometallic compound into the metal salt
solution to form a mixture, heating the mixture to a temperature
between 260.degree. and 300.degree. C., and adding a flocculent to
cause the magnetic alloy nanoparticles to precipitate out of the
mixture without permanent agglomeration. The organometallic
compound includes a solvent including one of phenyl ether and
dioctyl ether. The reducing agent includes a long chain diol, which
includes one of 1,2-hexadecanediol, 1,2-dodecanediol and
1,2-octanediol. The stabilizing ligands include RCOOH and
RNH.sub.2, where R includes an alkyl, alkenyl hydrocarbon chain (C6
or longer). The flocculent includes an alcohol including one of
methanol, ethanol, propanol and butanol. The metal salt includes
one of Pt salt, Pt(CH.sub.3COCHCOCH.sub.3).sub.2,
Pt(CF.sub.3COCHCOCF.sub.3).sub.2, Tetrakis (triphenylphosphine),
Platinum(O), or Pt(O) (triphenylphosphine).sub.4-x(CO).sub.x. The
organometallic compound includes one of Fe(CO).sub.5,
Co.sub.2(CO).sub.8, Co.sub.4(CO).sub.12, Fe.sub.2(CO).sub.9,
Fe.sub.3(CO).sub.12, Fe(CNR).sub.5, and (Diene) Fe(CO).sub.5 (e.g.,
Cyclopentadiene, Cyclooctadiene, etc.).
[0015] The invention's synthetic method leads to nearly
monodisperse particle thin films with controllable coercivity,
providing a viable approach to such ultrahigh density recording
media.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] The foregoing and other objects, aspects and advantages will
be better understood from the following detailed description of a
preferred embodiment of the invention with reference to the
drawings, in which:
[0017] FIG. 1 shows a TEM image of as-synthesized FePt alloy
particles where the sample was deposited on SiO-coated copper grid
from its hexane dispersion and the particles are nearly
monodisperse;
[0018] FIG. 2 shows the coercivity change of one set of samples
(48% at. Pt) as a function of annealing temperature after annealing
for 30 min. where coercivity data were collected on 9500 Vibrating
Sample Magnetometer at room temperature with thin film paralleling
to magnetic field;
[0019] FIG. 3 shows the X-ray diffraction patterns of (a)
as-synthesized FePt particles, (b)-(e) the thin film formed after
annealing for 30 minutes under N.sub.2 and (f) 45 nm amorphous
carbon thin film where the samples were deposited on glass
substrate and the diffraction pattern was collected on a Siemens
D-S00 diffractometer with Cu K(alpha) radiation (lamda=1.54056
.ANG.);
[0020] FIG. 4 shows the coercivity change of thin film with various
Pt concentration where the film was annealed at 600.degree. C.
under N.sub.2 for 30 minutes;
[0021] FIG. 5 shows (a) an atomic force microscopy image of the
topography of a FePt sample annealed to 600.degree. C., and (b) a
magnetic force microscopy image of the same area of the sample in
which the as-deposited magnetic domain structure is evident where
the topography of the sample has a z-range of 9.3 nm over this 3
.mu.m region, and RMS roughness of 0.999 nm and the magnetic
structure shows a fairly broad length scale distribution, as taken
from a 2D power spectral density calculation, which peaks at 188
nm;
[0022] FIGS. 6a and 6b show in plane and out of plane coercivities
of 3 FePt nanoparticle samples measured with a Kerr magnetometer
where coercivities of 1800 Oe, 2000 Oe and 5000 Oe for the three
different annealing conditions indicated at the top of the figure
are found and there is little difference between in-plane and
out-of-plane coercivities, indicating 3D-random orientation of the
magnetic axes;
[0023] FIG. 7 shows magneto-optical Kerr spectra taken in the
energy range 0.8-5.3 eV where characteristic shifts in peak
positions and changes in the Kerr angle are observed, consistent
with the higher ordering for the higher coercivity sample;
[0024] FIG. 8 shows an actual write/read experiment on the 1800 Oe
(30 min 600.degree. C.) sample using a static write/read test
apparatus;
[0025] FIG. 9 shows a so called dynamic coercivity measurement,
from which a thermal stability ratio of
C.sup.-1.congruent.E.sub.B/k.sub.BT.congruen- t.48 has been
extracted; and
[0026] FIG. 10 is a flow diagram showing the process of forming the
magnetic alloy nanoparticle film.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION
[0027] Long chain diols such as 1,2-octanediol, 1,2-dodecanediol
and 1,2-hexadecanediol, etc. have been used previously to reduce
metal salts (such as Pt salt, Fe(OCOCH.sub.3).sub.2,
Fe(CH.sub.3COCHCOCH.sub.3).sub.2- , Co(OCOCH.sub.3).sub.2,
Co(CH.sub.3COCHCOCH.sub.3).sub.2, etc. ) to metal nanoparticles,
(e.g., see Murray et al., copending U.S. patent application Ser.
No. 09/127,005, which is fully incorporated herein by reference).
Thermal decomposition of an organometallic compound (such as
Fe(CO).sub.5, Co.sub.2(CO).sub.8, Fe.sub.2(CO).sub.8,
[C.sub.5H.sub.5Fe(CO).sub.2].sub.2, and (C.sub.5H.sub.5).sub.2Fe)
is a known procedure to separate the Fe and its oxide particles
(e.g., M. O. Bentzon, et al., Phil. Mag. B., 1989, V. 60(2), 169,
fully incorporated herein by reference). The invention combines
these two chemical reactions (e.g., the reduction of the metal salt
and the decomposition of the organometallic compound) in situ to
form magnetic alloy particles that have a very controlled grain
size and are nearly monodisperse.
[0028] In our synthetic conditions, the reduction of the metal salt
(e.g., Pt(acac).sub.2) by diol and the decomposition of the
organometallic compound (e.g., Fe(CO).sub.5) do not occur until the
temperature is greater than 180.degree. C. These slow reduction and
decomposition processes are useful in forming the high quality
magnetic alloy particles.
[0029] More specifically, the invention forms such high quality
magnetic alloy particles by mixing metal salt, diol, stabilizing
ligands, organometallic compound and solvent under an inert
atmosphere. The mixture is then heated to reflux. The reduction of
the metal salt and the decomposition of the organometallic compound
lead to homogeneous nucleation for the formation of the magnetic
alloy particles. The growth of the magnetic alloy particles is
completed in about 30 minutes. The hot black product dispersion is
cooled to room temperature. A flocculent (e.g.,alcohol, methanol,
ethanol, propanol, ethylene, glycol, etc.) is added to precipitate
the product. Indeed, the magnetic alloy particles precipitated out
are nearly monodisperse.
[0030] The supernatant is discarded and the precipitated magnetic
alloy particles are dispersed, in alkane solvent in the presence of
oleic acid and amine. The dispersion can include an alkane
dispersion, such as pentane, hexane, heptane, octane or dodecane; a
chlorinated solvent dispersion, such as dichloromethane or
chloroform; or an aromatic solvent dispersion, including benzene,
toluene or xylene. Any unsolved impurity can be removed by
centrifugation of the hexane dispersion. Purification of the crude
product is performed by adding ethanol to the hexane dispersion of
the particles, providing nearly monodisperse magnetic alloy
nanoparticles which can be easily redispersed in an alkane
solvent.
[0031] Oleic acid is preferred as a stabilizing ligand in the above
process because it is known to stabilize cobalt and iron
nanoparticles. Long aliphatic chains of oleic acid present a
significant steric barrier for strong interactions between the
particles, and magnetic exchange coupling between the particles is
eliminated completely by the physical separation induced. Similar
long chain carboxylic acids such as erucic acid and linoleic acid
can also be used instead of oleic acid. Oleic acid is preferred
because it is readily available.
[0032] Further, amine-based or other similar ligands, such as
RNH.sub.2 (R>C.sub.12H.sub.25) R.sub.2NH when R.dbd.C.sub.1 to
C.sub.22, are included in a preferred embodiment to protect
platinum particles. In the invention, long chain primary amine,
such as oleyl amine, or alkyl amine RNH.sub.2 with
R>C.sub.12H.sub.25, etc., is used to protect the surface Pt
component of the magnetic alloy particles. The combination of
acid/amine provides good control over the magnetic alloy particles'
growth and stabilization.
[0033] Phenylether or n-dioctylether is preferably used as the
solvent. However, as would be known by one ordinarily skilled in
the art given this disclosure, any suitable solvent with high
boiling point and good solubility of Fe and Pt compounds could be
used. Other solvents include mineral oil, paraffin wax and
polyethylene glycol. The reaction can be carried out at
temperatures ranging from 260.degree. C. to 300.degree. C.,
depending on the solvent used.
[0034] Referring now to the drawings, one example of the invention
which forms FePt nanoparticles using the above process is
illustrated. More specifically, in this example, Pt salt, diol,
stabilizing ligands, Fe(CO).sub.5 and solvent are combined under an
inert atmosphere. The mixture is then heated to between 260.degree.
C. and 300.degree. C. for 30 minutes and allowed to cool to room
temperature. Alcohol is added to precipitate the FePt
nanoparticles.
[0035] In FIG. 1, the particle size and distribution are determined
by TEM image analysis. To produce the image shown in FIG. 1,
carbon- or SiO-coated copper grids were dipped into the hexane
solution of the FePt nanoparticles and dried at room temperature.
The TEM image (from Philips EM 420, 120 KV) in FIG. 1 shows that
the FePt nanoparticles have an average diameter of 4 nm. The FePt
nanoparticles are uniformly dispersed in the film and have a narrow
size distribution.
[0036] The magnetic alloy particles can be processed into well
formed particle thin films. As mentioned above, the magnetic alloy
particles are well isolated from each other by the protecting
ligands. Oleic acid and oleylamine ligands around the particles can
be replaced by other aliphatic acids and primary amines. Thus,
purification by adding ethanol to hexane dispersion in the presence
of 1 octadecylamine, leads to octadecylamine and oleic acid
protected magnetic alloy particles.
[0037] Depositing the hexane dispersion of magnetic alloy particles
on a flat substrate, such as SiO.sub.2, Si, glass, or carbon and
drying the same at room temperature leads to well formed particle
thin films. Magnetic measurements of FePt films shows that the
magnetic alloy films are magnetically soft with no coercivity at
room temperature. Annealing the magnetic alloy films at
temperatures between 500.degree. C. and 650.degree. C. produces
mirror-like thin films with controlled coercivities between 500 Oe
and 6500 Oe.
[0038] Using the same FePt example as discussed above with respect
to FIG. 1, a typical annealing temperature dependent Hc is shown in
FIG. 2. The hard magnetic behavior of a FePt alloy is related to
the crystalline phase transition. The inventive magnetic alloy
usually has a disordered fcc structure which transforms into an
ordered fct structure with c/a=0.98 after annealing. This
crystalline phase change in the film can be easily detected by
X-ray diffraction.
[0039] The predominant features in the XRD pattern of the exemplary
FePt particle thin film are characteristic of a disordered fcc
structure, as shown in FIG. 3(a). Annealing to higher temperatures
leads to pronounced structure changes. FIG. 3(b)-(e) show a series
of diffraction patterns at different annealing temperatures. These
demonstrates that, with respect to the FePt example being
discussed, the crystal phase change for FePt (at 48% Pt) particles
occurs at about 450.degree. C. and is complete at around
580.degree. C.
[0040] During the alloying phase change, in the FePt example being
discussed, the particle diameter remained about constant at
approximately 4 nm, as confirmed by both TEM and high resolution
SEM analyses. The strong peaks that appear at 2 .phi..sup.0=24, 33,
41, 47, and 70 correspond to reported (JCODS) tetragonal FePt
crystal phase with a=3.8525 angstrom and c=3.7133 angstrom.
[0041] Further, annealing at high temperatures (greater than
500.degree. C.) under N.sub.2 did not result in the loss of the
stabilizing ligand. Rather, the stabilizing ligand decomposed to
give amorphous carbon that covered around the particles. The bumps
in FIG. 3(b)-(e) are due to the existence of such amorphous carbon.
This can be easily verified by comparing Figures (b)-(e) with the
X-ray diffraction pattern of 45 nm amorphous carbon film on a glass
substrate shown in FIG. 3(f).
[0042] This carbon coated FePt nanocrystalline thin film is very
stable towards oxidation. Experimentally exposing the annealed FePt
samples to the ambient environment did not result in the formation
of layers of oxides, as confirmed by X-ray analysis. However, it
should be mentioned that N.sub.2 purity is important for successful
alloy phase change during the annealing. The above annealing
experiment was performed in a N.sub.2 Box with O.sub.2 content
lower than 10 ppm. Otherwise, oxides formation was observed.
[0043] The component of FePt particle materials has been studied
either by ICPAtomic Emission Spectrometer for as-synthesized
particles or by Rutherford Backscattering for the thin film. The
FePt ratio can be readily controlled by changing the ratio of
starting molecules of Fe(CO).sub.5 and Pt(acac).sub.2 from 40% at.
Pt to about 20% at. Pt. However, magnetic measurements show that
the film containing a little Fe rich component gives high
coercivity, as shown in FIG. 4.
[0044] The roughness of the deposited films in this example has
been investigated by atomic force microscopy (AFM), as shown in
FIG. 5(a), and RMS roughness values of approximately 1 nm have been
achieved over areas of 3.times.3 .mu.m. The magnetic domain
structures which evolve in the as-deposited films have been
investigated by magnetic force microscopy (MFM) and it has been
found that there is a stable magnetic domain structure with length
scales on the order of hundreds of nanometers, as shown in FIG.
5(b). This long range ordering is attributed to the dipolar
coupling between the particles.
[0045] FIG. 6 shows in plane and out of plane coercivities of 3
FePt nanoparticle samples that were measured with a Kerr
magnetometer. Coercivities of 1800 Oe, 2000 Oe and 5000 Oe for the
three different annealing conditions were found as indicated at the
top of FIG. 6. There is little difference between the in-plane and
the out-of-plane coercivities, indicating that a 3D-random
orientation of the magnetic axes exists.
[0046] Further evidence for the presence of chemical ordering and
the formation of the high anisotropy FePt L1.sub.0 phase is gained
from magneto-optical Kerr spectroscopy measurements, shown in FIG.
7. Magneto-optical Kerr spectroscopy measurements have been
intensively used in the past to establish magneto-structural
correlations. The observed shift in the Kerr peak to higher
energies upon further annealing is consistent with earlier work on
thin FePt films [D. Weller, Spin Orbit Influenced Spectroscopies,
1995].
[0047] FIG. 8 shows an actual write/read experiment on the 1800 Oe
(30 min 600.degree. C.) FePt sample using a static write/read test
apparatus. More specifically, FIG. 8 shows a two-dimensional stray
field image taken with the MR element of a standard record head.
Twenty parallel 100 .mu.m long tracks of written bit transitions
varying in linear density from 500 to 5000 flux changes per mm were
written and imaged with the record head in physical contact with
the sample, indicating the abrasion resistance of these
nanoparticle films. The same apparatus has been used to assess the
thermal stability of the sample, which is characterized by the
ratio of reversal barrier height and thermal energy
E.sub.B/k.sub.BT.
[0048] FIG. 9 shows a so called dynamic coercivity measurement,
from which this number can be extracted as a slope parameter
C.sup.-1, H.sub.CR=H.sub.0(1-{C In
(t.sub.pf.sub.0/1n2)}.sup.2/3).
[0049] H.sub.0 is an intrinsic switching field reached at short
write field pulses t.sub.p.congruent.1 ns and
C.sup.-1.congruent.E.sub.B/k.sub.- BT. We find C.sup.-1=48, which
indicates sufficient thermal stability for this FePt sample.
Improved stability occurs in the higher coercivity samples. This is
just a demonstration that these kinds of measurements can be
carried out on the present FePt nanoparticle samples.
[0050] The process of forming the magnetic alloy nanoparticle film
includes forming a metal salt solution with a reducing agent and
stabilizing ligands 100, introducing an organometallic compound
101, heating the mixture 102, adding a flocculent to produce a
precipitate 103, forming a dispersion 104, depositing the
dispersion on a solid surface 105, and annealing the dispersion
106, as shown in FIG. 10.
[0051] Thus, the deposition of the alkane dispersion of FePt alloy
particles, followed by the annealing results in the formation of a
shiny FePt nanocrystalline thin film with coercivity ranging from
500 Oe to 6500 Oe.
[0052] The following examples demonstrate some of the specific uses
of the invention. The first example is for the synthesis of FePt
particles. Platinum acetylacetonate/1,2-hexadecanediol/dioctylether
in the ratio of 1.0 mmol:1.5 mmol/40 mL were mixed in a glass
vessel under nitrogen and heated to 100.degree. C. to give a yellow
solution. Oleic acid (1 mmol) and oleylamine (1 mmol) were added
and the mixing process was continued with N.sub.2 flushing for 20
minutes. The N.sub.2 flushing was stopped and the N.sub.2 was
adjusted to by-pass the reaction mixture slowly to insure full
protection of the reaction product from oxidation. Fe(CO).sub.5 (2
mmol) was added and the mixture was heated to reflux (293.degree.
C.) in a period of about 14 minutes. The refluxing was continued
for 30 minutes. The heat source was removed and the black reaction
mixture was cooled to room temperature.
[0053] Ethanol was then added. The black product was precipitated
and separated by centrifugation. Yellow-brown supernatant was
discarded and the black product was dispersed in hexane in the
presence of oleic acid and 1-octadecylamine. The product was
precipitated out by adding ethanol and centrifugation. The
precipitate was once again dispersed in hexane in the presence of
only 1-octadecylamine. Any unsolved precipitation was removed by
centrifugation. The particle materials were precipitated out by
adding ethanol and centrifugation, re-dispersed in hexane, and
stored under N.sub.2. Although the product can be easily handled
without any inert gas protection, it is preferable to store the
hexane dispersion under N.sub.2 for long term protection.
[0054] A second example involved the synthesis of nanocrystalline
thin films. Hexane dispersion of FePt nanoparticles (0.5 mg/mL) was
deposited on a SiO.sub.2/Si substrate. The solvent hexane
evaporated at room temperature. The thickness of particle thin film
can be easily controlled by controlling the volume of hexane
dispersion deposited on the solid surface. The as-deposited thin
film was transferred to a N.sub.2 glove box with 0.sub.2 content
lower than 10 ppm and annealed in a oven. The temperature was
raised to a desired level (e.g., 550.degree. C.) from room
temperature in a course of around 13 minutes and kept at that
temperature level. After a certain time (e.g., 30 minutes), the
thin film sample was taken out of the oven and cooled down to room
temperature. The film produced was stable towards oxidation and
could be used for further measurement. The roughness of the
deposited films has been investigated by atomic force microscopy
(AFM), as shown in FIG. 4, and RMS roughness values of
approximately 1 nm have been achieved over areas of 3.times.3
.mu.m.
[0055] Another example involves the magnetic domain structure of
the nanocrystalline thin films. The magnetic domain structures
which evolve in the as deposited films have been investigated by
magnetic force microscopy (MFM) and it has been found that there is
a stable magnetic domain structure with length scales of order of
hundreds of nanometers, as shown in Figure. 4(b). This long range
ordering is attributed to the dipolar coupling between the
particles.
[0056] High density recording media uniform particles with an
average diameter of 8-10 nm or less and a high Hc of 2500 Oe will
soon be required. The invention's synthetic method leads to nearly
monodisperse particle thin films with controllable coercivity,
providing a viable approach to such ultra-high density recording
media.
[0057] This alloy may also have great potential for use as magnetic
bias films of magneto-resistive elements and magnetic tips for
magnetic force microscopy. Specifically, nonvolatile magnetic
memory devices and magneto-resistive sensory for magnetic recording
systems "Read Head".
[0058] While the invention has been described in terms of preferred
embodiments, those skilled in the art will recognize that the
invention can be practiced with modification within the spirit and
scope of the appended claims.
* * * * *