Fe contribution to the magnetic anisotropy of $L{1_0}$-ordered FePt thin films studied by angle-dependent x-ray magnetic circular dichroism

Abstract

Among magnetic thin films with perpendicular magnetic anisotropy (PMA), $L1_0$-ordered FePt has attracted significant attention because of its exceptionally strong PMA. However, the microscopic origin of its strong PMA has not been elucidated experimentally. We have investigated the contribution of the Fe $3d$ electrons to its magnetic anisotropy energy by angle-dependent x-ray magnetic circular dichroism at the Fe $L_{2,3}$ edge. By this technique, one can deduce the magnetic dipole moment $m_\text{T}$, which represents the anisotropic spatial distribution of spin-polarized electrons, and the orbital moment anisotropy (OMA) of Fe $3d$ electrons. Detected finite $m_\text{T}$ indicates that the spin-polarized Fe $3d$ electrons are distributed preferentially in the out-of-plane direction of the films. This $m_\text{T}$ of Fe overwhelms the positive contribution of OMA to PMA, and reduces the PMA of $L1_0$-ordered FePt thin films, consistent with a previous first-principles calculation. The present result implies that a large positive contribution of the non-magnetic element Pt rather than Fe governs the PMA of $L1_0$-ordered FePt thin films.

Figures (6)

• Figure 1: Fe $L_{2,3}$-edge x-ray absorption spectroscopy (XAS) and angle-dependent x-ray magnetic circular dichroism (AD-XMCD) measurements. (a) Schematic illustration of the experimental geometry. The angles of the incident x ray ($\theta_\text{inc}$), the external magnetic field ($\theta_{\bm{H}}$) and the magnetization ($\theta_{\bm{M}}$) are defined with respect to the sample normal. $\theta_\text{inc}$ is fixed at $60^\circ$ in the present measurements. $\theta_{\bm{H}}$ can be varied in the range of $-{90}^\circ \leq \theta_{\bm{H}} \leq {90}^\circ$. Note that the external magnetic field $\bm{H}$ and the magnetization $\bm{M}$ are not always parallel in the presence of magnetic anisotropy. (b--d) XAS and (e--g) AD-XMCD spectra of the FePt thin films with $L1_0$ order degrees $S=0$ (b,e), $S=0.3$ (c,f) and $S=0.4$ (d,g). Linear and smoothed two-step backgrounds have been subtracted from the raw XAS spectra. All the XAS spectra have been normalized to the height of the $L_3$ peak at $\sim 708\ \text{eV}$. $\theta_{\bm{H}}=60^\circ$ corresponds to the geometry where the incident x ray and the magnetic field are parallel [See Panel (a)]. XMCD intensity at the $L_3$ edge is proportional to the projected magnetic moment $\hat{\bm{P}}\cdot\bm{M}$ shown in Panel (a).
• Figure 2: Magnetic field angle dependence of effective spin magnetic moment projected on the light axis ($\hat{\bm{P}}\cdot\bm{m}_\text{spin}^\text{eff}$). (a) Experimental data for the film with $S=0.4$ (red square) and simulated angular dependencies based on the Stoner-Wohlfarth model without magnetocrystalline anisotropy (MCA) and $\braket{ Q_{zz} }$ (blue dotted curve), with MCA but without $\braket{ Q_{zz} }$ (green dotted curve), and with both MCA and $\braket{ Q_{zz} }$ (black curve). (b) Experimental and simulated angular dependencies of $\hat{\bm{P}}\cdot\bm{m}_\text{spin}^\text{eff}$ for various $L1_0$ order degrees $S$. Both $K_\text{u}$ and $\braket{ Q_{zz} }$ are incorporated in the simulation.
• Figure 3: Electric quadrupole moment $(7/2)\braket{ Q_{zz} }$ of Fe as a function of $L1_0$ order degree $S$. Error bars represent the standard deviation deduced from the fit in Fig. \ref{['fig:AD_spin']}. The ellipsoids schematically describe the charge densities for $\braket{ Q_{zz} }>0$ and $\braket{ Q_{zz} }<0$.
• Figure 4: Contribution to the MCA energy originating from the orbital moment anisotropy (spin-conservation term, blue) and the charge-density anisotropy (spin-flip term, red). The sum of the two terms (total MCA energy) is also shown (green).
• Figure S1: X-ray diffraction (XRD) patterns of the grown $L1_0$ FePt films with $L1_0$ order degrees $S=0$, $S \sim 0.3$, and $S \sim 0.4$. $T_\text{S}$ and $T_\text{A}$ represent the substrate temperature during the growth and the post-annealing temperature, respectively.