Orbital magnetic moment and coercivity of SiO${}_2$-coated FePt nanoparticles studied by x-ray magnetic circular dichroism

Abstract

We have investigated the spin and orbital magnetic moments of Fe in FePt nanoparticles in the $L$1${}_0$-ordered phase coated with SiO${}_2$ by x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) measurements at the Fe $L_{2,3}$ absorption edges. Using XMCD sum rules, we evaluated the ratio of the orbital magnetic moment ($M_{\mathrm{orb}}$) to the spin magnetic moment ($M_{\mathrm{spin}}$) of Fe to be $M_{\mathrm{orb}}/M_{\mathrm{spin}}$ = 0.08. This $M_{\mathrm{orb}}/M_{\mathrm{spin}}$ value is comparable to the value (0.09) obtained for FePt nanoparticles prepared by gas phase condensation, and is larger than the values ($\sim$0.05) obtained for FePt thin films, indicating a high degree of $L$1${}_0$ order. The hysteretic behavior of the FePt component of the magnetization was measured by XMCD. The magnetic coercivity ($H_{\mathrm{c}}$) was found to be as large as 1.8 T at room temperature, $\sim$3 times larger than the thin film value and $\sim$50 times larger than that of the gas phase condensed nanoparticles. The hysteresis curve is well explained by the Stoner-Wohlfarth model for non-interacting single-domain nanoparticles with the $H_{\mathrm{c}}$ distributed from 1 T to 5 T.

5 pages, 3 figures, 1 table