Micromagnetic study of single-domain FePt nanocrystals overcoated with silica

Abstract

Chemically-synthesized FePt nanocrystals must be annealed at a high temperature (> 550 degrees C) to induce the hard ferromagnetic L1(0) phase. Unfortunately, the organic stabilizer covering these nanocrystals degrades at these temperatures and the nanocrystals sinter, resulting in the loss of control over nanocrystal size and separation in the film. We have developed a silica overcoating strategy to prevent nanocrystal sintering. In this study, 6 nm diameter FePt nanocrystals were coated with 17 nm thick shells of silica using an inverse micelle process. Magnetization measurements of the annealed FePt@SiO2 nanocrystals indicate ferromagnetism with a high coercivity at room temperature. Magnetic force microscopy ( MFM) results show that the film composed of nanocrystals behaves as a dipole after magnetization by an 8 T external field. The individual nanocrystals are modelled as single-domain particles with random crystallographic orientations. We propose that the interparticle magnetic dipole interaction is weaker than the magnetocrystalline energy in the remanent state, leading to an unusual material with no magnetic anisotropy and no domains. Films of these nanoparticles are promising candidates for magnetic media with a data storage density of similar to Tb/in(2).