Structural and Magnetic Properties of FePt/SiO2 Core/Shell Nanoparticles

The disorder-order phase transition of FePt/SiO2 core/shell nanoparticles (NPs) induced by rapid thermal annealing was investigated. The FePt/SiO2 NPs were synthesized using FePt cores as seeds. For as-prepared FePt/SiO2 NPs, the average size of the FePt core and the thickness of the SiO2 shell are about 4.8 and 1.2 nm respectively, and the FePt NPs exhibit a disordered fcc structure. The FePt core NPs show obvious phase transformation from a disordered to an ordered structure after they were annealed at 600 °C for 2 min. Moreover, increasing annealing time can promote effectively the formation of the ordered fct-FePt phase. The relatively large coercivity Hc\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$H_{\mathrm {c}}$\end{document} is obtained for FePt NPs with a thicker SiO2 shell. The Hc\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$H_{\mathrm {c}}$\end{document} decreases firstly with decreasing temperature from 300 to 100 K and then increases from 100 to 10 K, which is ascribed to the coexistence of magnetically hard fct-FePt and magnetically soft fcc-FePt in the sample, and the incomplete exchange coupling between the hard and soft magnetic phases.