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

We have investigated the spin and orbital magnetic moments of Fe in FePt nanoparticles in the $L$1${}_0$-ordered phase coated with $\mathrm{Si}{\mathrm{O}}_2$ 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 effective spin magnetic moment ($M_{\mathrm{spin}}^{\mathrm{eff}}$) of Fe to be $M_{\mathrm{orb}}/M_{\mathrm{spin}}^{\mathrm{eff}}=0.08$. This $M_{\mathrm{orb}}/M_{\mathrm{spin}}^{\mathrm{eff}}$ 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 ($H_c$) 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 noninteracting single-domain nanoparticles with the $H_c$ distributed from 1 to 5 T.

Figure 1

XAS and XMCD spectra at the Fe $L_{2,3}$ edges of the $\mathrm{Si}{\mathrm{O}}_2$-coated FePt nanoparticles under the applied magnetic field of 9 T. All the spectra have been normalized to the $L_3$ XAS peak intensity. Inset shows a schematic figure of the FePt nanoparticle coated by $\mathrm{Si}{\mathrm{O}}_2$.

Figure 2

Decomposition of the measured XAS and XMCD spectra into those of FePt and a Fe${}^{3+}$ oxide. (a) The experimental XAS spectrum and that of gas-phase-condensed FePt nanoparticles [8] are shown by crosses and a solid curve, respectively, at the top. Spectra after subtraction of the FePt ones from the measured ones by different ratios are indicated below by dot-dashed curves. The XAS spectrum of BiFeO${}_3$ by Singh et al. [23] is shown at the bottom. (b) The experimental XMCD spectrum (crosses) and the FePt one (Ref. [8], dotted curve) are shown at the top. The XMCD spectrum of BiFeO${}_3$ by Singh et al. [23] is shown at the bottom.

Figure 3

Hysteresis loops of the XMCD intensities (normalized to the saturation magnetization) at the Fe $L_3$ edge of the $\mathrm{Si}{\mathrm{O}}_2$-coated FePt nanoparticles. (a) Hysteresis loops at $h\nu =707.7$ eV (open circles) and 709.2 eV (open triangles) corresponding to a peak due to FePt and the Fe${}^{3+}$ oxides, respectively. A ferromagnetic component extracted from the hysteresis loop at $h\nu =707.7$ eV (open squares) is also plotted. (b) The ferromagnetic component of the XMCD hysteresis curve compared with a calculated Stoner-Wohlfarth hysteresis loop [27] for a single $H_c$ (=2.5 T) (dashed curve) and for distributed $H_c$ (solid curve). Inset shows the log-normal distribution of $H_c$ normalized to the area.