(346d) Synthesis and Magnetic Properties of Silica-Coated Fept Nanocrystals

6 nm diameter colloidal FePt nanocrystals were synthesized and then coated with silica (SiO2) shells. The silica shell thickness could be varied from 10 to 25 nm. As-made FePt@SiO2 nanocrystals have low magnetocrystalline anisotropy due to a compositionally disordered FePt core. By annealing films of FePt@SiO2 particles under hydrogen at 650°C or above, the FePt core transform to the compositionally ordered L10 phase and superparamagnetic blocking temperatures exceeding room temperature are obtained. The SiO2 shell prevents FePt coalescence at annealing temperatures up to ~850oC. Annealing under air or nitrogen does not induce the FePt phase transition. The silica shell limits magnetic dipole coupling between the FePt nanocrystals; however, low temperature (5 K) and room temperature magnetization scans show slightly constricted hysteresis loops with coercivities that decrease systematically with decreased shell thickness, possibly resulting from differences in magnetic dipole coupling between particles.