Chemical synthesis of monodisperse and magnetic alloy...

Details Chemical synthesis of monodisperse and magnetic alloy... Chemical synthesis of monodisperse and magnetic alloy...

2001-03-05

2001-10-16

King, Roy (Department: 1742)

Specialized metallurgical processes, compositions for use therei

Processes

Producing or purifying free metal powder or producing or...

C075S348000, C075S362000, C075S710000, C427S128000, C427S130000, C427S132000, C427S217000, C427S374100

Reexamination Certificate

active

06302940

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
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.
2. Description of the Related Art
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.
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.
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 7×10e7 erg/cm
3
have been obtained, compared to 5×10e6 erg/cm
3
for hcp cobalt-based recording media. Typical Co based recording media today have anisotropies of order 1-2×10e6 erg/cm
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
10
Cr
22
B
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.
A common procedure leading to CoPt or FePt alloy materials is co-sputtering 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.
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
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)
2
(acac=acetylactonate, CH
3
COCHCOCH
3
anion) by long chain diol and decomposition of Fe(CO)
5
at a high temperature (260° C.-300° C.) solution phase yields high quality nanoparticles.
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.
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
2
, Si, Si
3
N
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° C. to 650° 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.
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° and 300° 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
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
3
COCHCOCH
3
)
2
, Pt(CF
3
COCHCOCF
3
)
2
, Tetrakis (triphenylphosphine) Platinum(O), or Pt(O) (triphenylphosphine)
4−x
(CO)
x
where x is at least 1 and no greater than 3. The organometallic compound includes one of Fe(CO)
5
, Co
2
(CO)
8
, Co
4
(CO)
12
, Fe
2
(CO)
9
, Fe
3
(CO)
12
, Fe(CNR)
5
, and (Diene) Fe(CO)
5
(e.g., Cyclopentadiene, Cyclooctadiene, etc.).
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° and 300° 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° 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
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
2
or Ar. The annealing temperature is between 400° C. and 650° C. The annealing forms a layer of amorphous carbon around particles in the precipitate. The organometallic compound includes a solven

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