Heusler型Mn $_{2-δ}$ CoGa $_{1+δ}$薄膜的磁弹性各向异性
Magnetoelastic anisotropy in Heusler-type Mn$_{2-δ}$CoGa$_{1+δ}$ films
摘要：
对于使用磁性材料的高密度存储器应用，永磁磁化是必不可少的。传导电子的高自旋极化也需要从自旋相关的传输现象实现大的电信号。Heusler合金是一种具有半金属电子结构的材料。然而，其立方晶格性质有利于面内磁化，因此通常使垂直磁各向异性（PMA）最小化。本研究的重点是一个反型Heusler合金，Mn $_{2-δ}$ CoGa $_{1+δ}$（MCG）与一个小的偏离化学计量比（$δ$），这是一个半金属材料。我们观察到相对较大的单轴磁晶各向异性常数（$K_\mathrm{u}$）的顺序为10 $^5$ J/m $^3$在室温下在MCG薄膜的几个百分点的小四角畸变。证实了$c/a$晶格常数比与$K_\mathrm{u}$之间的正相关性。使用克尔显微镜的磁畴成像清楚地表明，随着$K_\mathrm{u}$的域模式的变化。利用同步辐射软X射线的X射线磁性圆二色性（XMCD）研究了PMA的来源。可忽略的角度变化的轨道磁矩（$Δm_\mathrm{orb}$）使用XMCD光谱评估建议一个次要的角色，所谓的布鲁诺的长期$K_\mathrm{u}$。我们的第一性原理计算合理地解释了小的$Δm_\mathrm{orb}$和$c/a$比率与$K_\mathrm{u}$之间的正相关性。基于二阶微扰理论，从自旋-轨道耦合的角度讨论了磁晶各向异性的起源，认为占据的$\uparrow$ -和未占据的$\downarrow$ -自旋态的混合是导致MCG薄膜的PMA的原因.
英文摘要：
Perpendicular magnetization is essential for high-density memory application using magnetic materials. High-spin polarization of conduction electrons is also required for realizing large electric signals from spin-dependent transport phenomena. Heusler alloy is a well-known material class showing the half-metallic electronic structure. However, its cubic lattice nature favors in-plane magnetization and thus minimizes the perpendicular magnetic anisotropy (PMA), in general. This study focuses on an inverse-type Heusler alloy, Mn$_{2-δ}$CoGa$_{1+δ}$ (MCG) with a small off-stoichiometry ($δ$) , which is expected to be a half-metallic material. We observed relatively large uniaxial magnetocrystalline anisotropy constant ($K_\mathrm{u}$) of the order of 10$^5$ J/m$^3$ at room temperature in MCG films with a small tetragonal distortion of a few percent. A positive correlation was confirmed between the $c/a$ ratio of lattice constants and $K_\mathrm{u}$. Imaging of magnetic domains using Kerr microscopy clearly demonstrated a change in the domain patterns along with $K_\mathrm{u}$. X-ray magnetic circular dichroism (XMCD) was employed using synchrotron radiation soft x-ray beam to get insight into the origin for PMA. Negligible angular variation of orbital magnetic moment ($Δm_\mathrm{orb}$) evaluated using the XMCD spectra suggested a minor role of the so-called Bruno's term to $K_\mathrm{u}$. Our first principles calculation reasonably explained the small $Δm_\mathrm{orb}$ and the positive correlation between the $c/a$ ratio and $K_\mathrm{u}$. The origin of the magnetocrystalline anisotropy was discussed based on the second-order perturbation theory in terms of the spin--orbit coupling, claiming that the mixing of the occupied $\uparrow$- and the unoccupied $\downarrow$-spin states is responsible for the PMA of the MCG films.
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