Effects of Substrates on the Self-Assembling of FePt Nanocrystals
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Effects of Substrates on the Self-Assembling of FePt Nanocrystals Min Chen and David E. Nikles* The University of Alabama, Tuscaloosa, Center for Materials for Information Technology, Box 870209, Tuscaloosa, Alabama, 35487-0209, US *[email protected] ABSTRACT Fe48Pt52 nanoparticles were synthesized by the simultaneous chemical reduction of platinum acetylacetonate and thermal decomposition of iron pentacarbonyl. As-prepared the particles were spherical with an average diameter of 3 nm and a polydispersity of less than 5%. The particles were superparamagnetic and had a fcc structure. Highly ordered self-assembled supercrystals of particles were formed in TEM grids by deposition from dispersions in hydrocarbon solvents. Nanoparticles deposited on amorphous carbon-coated and SiO2-coated Cu grids tend to assemble into small domains of hexagonal arrays. Larger domains of hexagonal arrays formed on Si3N4 membrane TEM grids. For thin multilayers, the FePt nanoparticles tends to assemble into hexagonal close-packed lattices (ABABAB stacking). For the thicker multilayers, ABCABC stacking was observed. Small angle X-ray reflectivity of the particles on a Si (100) substrate show highly ordered multiplanar structure with d-spacing of 6.2 nm. The coercivity of self-assembled FePt films strongly depended on the annealing temperature. After annealing at 700°C for 30 minutes, the particles transformed from FCC to “FCT” phase and the coercivity of film increased up to 11570 Oe. However, the particle size increased to 16 nm due to sintering. INTRODUCTION The "face-centered tetragonal" phase FePt is emerging as an important material in ultrahigh density magnetic data storage media. This phase has large uniaxial magnetocrystalline anisotropies (Ku) due to its tetragonal structure. The study of nanoscale magnetic domains are of both fundamental and pressing technical interest. The grain size of advanced recording media is rapidly being reduced to dimensions, where the magnetic materials approach the superparamagnetic limit. Development of a detailed understanding of the properties of magnetic nanocrystals is essential to the development of future magnetic recording technology. It is expected that if an ordered monolayer is formed by magnetic nanocrystals with sizes down to ~3 nm, the storage density can be up to 100-1000 Gbits/in2 [1-2]. There are three conventional methods for magnetic material processing: vacuum sputtering, physical vacuum evaporation and molecular bean epitaxy. Chemical vacuum evaporation and electrochemical deposition are also used in processing the magnetic thin film. Progress in magnetic recording density is due in part to the development of media with finer and finer grain magnetic films [3-9]. Processing of ordered 2D or 3D structure of magnetic nanocrystalls becomes an area of great interest for the thin granular films. The synthesis of nanoparticles, characterized by a narrow size distribution, is a new challenge in solid-state chemistry. Due to their small size, nanoparticles exhibit novel material properti
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