Micromagnetic study of single-domain FePt nanocrystals overcoated with silica

Abstract

Chemically-synthesized FePt nanocrystals must be annealed at a high temperature(>550 °C) to induce thehard ferromagnetic L 10 phase. Unfortunately, the organic stabilizer covering these nanocrystals degradesat these temperatures and the nanocrystals sinter, resulting in the loss ofcontrol over nanocrystal size and separation in the film. We have developed asilica overcoating strategy to prevent nanocrystal sintering. In this study,6 nm diameter FePt nanocrystals were coated with 17 nm thick shells of silicausing an inverse micelle process. Magnetization measurements of the annealedFePt@SiO2 nanocrystals indicate ferromagnetism with a high coercivity at room temperature.Magnetic force microscopy (MFM) results show that the film composed of nanocrystalsbehaves as a dipole after magnetization by an 8 T external field. The individualnanocrystals are modelled as single-domain particles with random crystallographicorientations. We propose that the interparticle magnetic dipole interaction isweaker than the magnetocrystalline energy in the remanent state, leading to anunusual material with no magnetic anisotropy and no domains. Films of thesenanoparticles are promising candidates for magnetic media with a data storage density of∼Tb/in2.