Synthesis and Magnetic Properties of L1$_{0}$FePt / Silica Core Shell Nanoparticles

FePt nanocrystals were coated with silica (SiO$_{2})$ shells in an inverse micelle microemulsion by tetraethyl orthosilicate (TEOS) hydrolysis and condensation. The shell thickness can be varied (from 6 to 25 nm), along with the FePt loading (per silica shell) to a limited extent, by changing the FePt:TEOS ratio. The silica-coated FePt nanocrystals can be heated up to $\sim $850$^{o}$C without shell layer decomposition or FePt sintering. Annealing under forming gas (7{\%}H2/ 93{\%}N2) at 700\r{ }C for 2 hours transforms the as-synthesized fcc FePt nanocrystals to the L1$_{0}$ phase with at least 90{\%} conversion. Magnetic measurements of annealed FePt nanocrystals confirm their phase transformation, with blocking temperatures exceeding room temperature. However, the hysteresis loops exhibit a constriction at low fields and zero field cooled (ZFC) magnetization scans show an intermediate plateau at temperatures between 50K$\sim $200K. Temperature and time-dependent remanance relaxation measurements reveal a short and fast, $\sim $10$^{4}$ and 10$^{5}$ seconds, relaxation of the remanance, which might be due to the presence of an additional ``soft''magnetic phase in the sample. The possible origins of the soft magnetic component will be presented and discussed.