fan Shi

Easy axis alignment of chemically partially ordered FePt nanoparticles

Partially ordered Fe53Pt47 nanoparticles with size around 8nm were prepared by the simultaneous decomposition of iron pentacarbonyl and platinum acetylacetonate. The high boiling point chemical, hexadecylamine, was used as a solvent, and 1-adamantanecarboxylic acid was used as a stabilizer. X-ray diffraction measurements reveal that as-made FePt particles were partially transformed into the ordered L10 phase with some weak superlattice peaks. The room-temperature hysteresis loop and remanence curve suggest a broad distribution of anisotropies in the partially ordered particles. By coating the partially ordered FePt nanoparticles with a polyvinylchloride polymer binder, the particles could be re-dispersed in cyclohexanone. Furthermore, the easy axis of the particles coated with the polyvinylchloride polymer binder could be aligned under an external field. Easy axis alignment was confirmed from both alternating gradient magnetometer and x-ray diffraction measurements.

Size effect on L10 ordering and magnetic properties of chemically synthesized FePt and FePtAu nanoparticles

There is growing evidence that FePt nanoparticles become increasingly difficult to chemically order as the size approaches a few nanometers. We have studied the chemical ordering of FePt and FePtAu nanoparticle arrays as a function of particle size. Monodisperse Fe49Pt51 and Fe48Pt44Au8 nanoparticles with a size about 6nm were synthesized by the simultaneous decomposition of iron pentacarbonyl and reduction of platinum acetylacetonate and gold (III) acetate in a mixture of phenyl ether and hexadecylamine (HDA), with 1-adamantanecarboxylic acid and HDA as stabilizers. The nanoparticles were dispersed in toluene, films of the particles were cast onto silicon wafers from the dispersion, and the films were annealed in a tube furnace with flowing Ar+5%H2. The magnetic anisotropy and switching volumes were determined from time- and temperature-dependent coercivity measurements. By comparing with 3‐nm FePt and FePtAu nanoparticles of comparable composition, the phase transformation is easier for the larger parti...

Microstructures and magnetic alignment of L10 FePt nanoparticles

Department of Physics, University of Texas at Arlington. Center for Materials for Information Technology, The University of Alabama

Direct synthesis and easy axis alignment of L10‐FePt nanoparticles

Partially ordered Fe53Pt47 nanoparticles with size around 8nm were prepared by the simultaneous decomposition of iron pentacarbonyl and platinum acetylacetonate. The high boiling point chemical, hexadecylamine, was used as a solvent, and 1-adamantanecarboxylic acid was used as a stabilizer. The reflux temperature of the solution could exceed 360°C, where disordered FePt particles could be partially transformed into the ordered L10 phase. A nonmagnetic mechanical stirrer was used in order to avoid agglomeration of the fct-FePt particles during synthesis. The particles were dispersed in toluene and films of the particles were cast onto silicon wafers from the solution. X-ray diffraction patterns of as-made samples showed weak superlattice peaks, indicating partial chemical ordering of the Fe53Pt47 particles. The room-temperature hysteresis loop of the as-made sample reveals a small coercivity (∼600Oe) because of thermal fluctuations; however, the loop is wide open and hard to saturate. The remanence coerciv...

Easy control of the size and composition of FePt nanoparticles with Improved synthesis

A new synthesis of FePt nanoparticles with tunable size and composition has been developed. Unlike conventional synthesis methods with which it is difficult to simultaneously control the size and chemical composition of FePt nanoparticles, the new synthesis offers a convenient way to tune FePt nanoparticles with different sizes and compositions. The synthesis involves the simultaneous decomposition of Fe3(CO)12 and reduction of Pt(acac)2 in diethylene glycol. Fe3(CO)12 is a powder (nonvolatile) which is critical in governing the FePt particle size and composition. By varying the amount of surfactants and precursors ratio [Fe3(CO)12∕Pt(acac)2], FePt particles with tunable composition and particle size (2–8nm) can be obtained. After high temperature annealing (∼600°C), Fe50Pt50 nanoparticles are partially transformed to the L10 phase as indicated by the soft and hard components in the hysteresis loops. The coercivity ranges from a few kilo-Oersted to more than 15kOe, depending on the particle size. Dynamic ...

Improved synthesis and easy-axis alignment of L10-FePt nanoparticles

A synthesis of partially ordered FePt nanoparticles has been developed. It involves the simultaneous reduction of iron acetate (or iron chloride) and platinum acetylacetonate. The high boiling point chemical hexadecylamine or trioctylamine was used as a solvent, and oleic acid or 1-adamantanecarboxylic acid was used as a surfactant. The reflux temperature of the mixture solutions ranged from 330to360°C, where disordered FePt particles can be partially transformed into the ordered L10 phase. Compared with previous results, x-ray-diffraction patterns of as-made samples prepared with the synthesis show a higher degree of chemical ordering. The composition of the FePt nanoparticles with the synthesis can be easily tuned. The room-temperature coercivity of as-made samples ranged from 1to4kOe, depending on the particle composition as well as the refluxing temperature during synthesis. The as-made particles were aligned in a 10kOe magnetic field, giving a parallel to perpendicular remanence ratio of about 1.6.

Crystal structure and compressibility of FePt nanoparticles under high pressures and high temperatures

FePt nanoparticles with an average grain size of 4 nm and equiatomic composition of Fe and Pt was studied under high pressures in a diamond anvil cell to investigate its structural stability and compressibility under high compression. The ambient pressure disordered face-centered-cubic (fcc) phase was found to be stable to the highest pressure of 61 GPa (compression of 15%) at room temperature. The compression of Fe50Pt50 nanoparticles is closer to the compression curve for pure Pt and shows lower compressibility than what would be expected for a bulk Fe50Pt50 alloy. The nanoparticle character of Fe50Pt50 sample is maintained to the highest pressure without any observable grain coarsening effects at ambient temperature. Laser heating of disordered fcc phase at 32 GPa to a temperature of 2000 K resulted in a phase transformation to a microcrystalline phase with the distorted fcc structure.

FePt and Related Nanoparticles

This chapter reviews recent studies of chemically synthesized FePt and related nanoparticles. Various methods for synthesizing the nanoparticles and controlling their shape are described. Thermal effects in nanoparticles near the superparamagnetic limit are discussed. Some of the methods for reducing sintered grain growth during annealing to obtain the L10 phase are described, including the use of a hard shell, annealing in a salt matrix , and flash annealing . The effect of metal additives on the ordering temperature and on sintered grain growth is discussed. Additive Ag and Au significantly not only reduce the ordering temperature but also the grain growth temperature in close-packed 3-D arrays. Preliminary experiments that show additive Ag also reduces the ordering temperature when sintering is prevented. Easy-axis alignment of L10 FePt nanoparticles can be achieved by drying a nanoparticle dispersion in a magnetic field, and the effect of thermal fluctuations on orientation is discussed. Large particle-to-particle compositional distributions in chemically synthesized FePt nanoparticles have been measured. A method of determining the anisotropy distribution is described. Theoretical and experimental works showing the size effect on chemical ordering of FePt nanoparticles are discussed.

Self-Assembly and Magnetic Properties of MnO Coated FePt Nanoparticles

This paper reports on the synthesis and self-assembly of FePt/MnO core/shell nanoparticles. The enhanced magnetic properties of FePt nanoparticles with MnO coating is shown. Results also demonstrated the suppression of aggregation of MnO coated FePt nanoparticles during the annealing process. The magnetic moment of the FePt nanoparticles is significantly enhanced with the MnO shell. Also, the room temperature hysteresis loops curves reveal that the annealed FePt nanoparticles without the MnO shell have a large coercivity around 14 kOe, while FePt nanoparticles with the MnO shell show a sheared loop with a small coercivity about 3 kOe.