热氧化硅衬底上超薄Cu $_2$ Sb型（Mn-Cr）AlGe薄膜的垂直磁各向异性
Perpendicular magnetic anisotropy in ultra-thin Cu$_2$Sb-type (Mn-Cr)AlGe films onto thermally oxidized silicon substrates
摘要：
表现出小饱和磁化强度（$M_\mathrm{s}$）的垂直磁化膜对于自旋转移矩写入型磁阻随机存取存储器（STT-MRAM）是必不可少的。在本研究中，研究了Cu $_2$ Sb型晶体结构的金属间化合物{（Mn-Cr）AlGe}作为显示低$M_\mathrm{s}$（$\sim 300$ kA/m）和高垂直磁各向异性$K_\mathrm{u}$的材料。在热氧化硅衬底上制备的薄膜样品中，研究了$K_\mathrm{u}$的层厚度依赖性以及在顶部和底部（Mn-Cr）AlGe $|$ MgO界面处的Mg插入层的影响，以在几纳米的厚度范围内实现高$K_\mathrm{u}$。最佳镁插入厚度分别为1.4和3.0 nm的底部和顶部的接口，这是相对较厚的结果相比，在以前的研究中报道的磁性隧道结类似的插入效应调查。横截面的透射电子显微镜图像显示，镁插入层作为障碍的相互扩散的Al原子，以及从MgO层的氧化。对于5 nm和3 nm厚的（Mn-Cr）AlGe薄膜，在最佳Mg插入厚度下，室温下$K_\mathrm{u}$的值分别约为$7 \times 10^5$和$2 \times 10^5$ J/m $^3$。在几个纳米厚度的$K_\mathrm{u}$是可比的或高于那些报道的垂直磁化的CoFeB薄膜，这是传统上使用的MRAM，而$M_\mathrm{s}$值是三分之一或更小的CoFeB薄膜。所开发的（Mn-Cr）AlGe薄膜不仅具有良好的磁性能，而且在薄膜制备中与硅工艺的兼容性也是很有前途的。
英文摘要：
Perpendicularly magnetized films showing small saturation magnetization, $M_\mathrm{s}$, are essential for spin-transfer-torque writing type magnetoresistive random access memories, STT-MRAMs. An intermetallic compound, {(Mn-Cr)AlGe} of the Cu$_2$Sb-type crystal structure was investigated, in this study, as a material showing the low $M_\mathrm{s}$ ($\sim 300$ kA/m) and high-perpendicular magnetic anisotropy, $K_\mathrm{u}$. The layer thickness dependence of $K_\mathrm{u}$ and effects of Mg-insertion layers at top and bottom (Mn-Cr)AlGe$|$MgO interfaces were studied in film samples fabricated onto thermally oxidized silicon substrates to realize high-$K_\mathrm{u}$ in the thickness range of a few nanometer. Optimum Mg-insertion thicknesses were 1.4 and 3.0 nm for the bottom and the top interfaces, respectively, which were relatively thick compared to results in similar insertion effect investigations on magnetic tunnel junctions reported in previous studies. The cross-sectional transmission electron microscope images revealed that the Mg-insertion layers acted as barriers to interdiffusion of Al-atoms as well as oxidization from the MgO layers. The values of $K_\mathrm{u}$ were about $7 \times 10^5$ and $2 \times 10^5$ J/m$^3$ at room temperature for 5 and 3 nm-thick (Mn-Cr)AlGe films, respectively, with the optimum Mg-insertion thicknesses. The $K_\mathrm{u}$ at a few nanometer thicknesses is comparable or higher than those reported in perpendicularly magnetized CoFeB films which are conventionally used in MRAMs, while the $M_\mathrm{s}$ value is one third or less smaller than those of the CoFeB films. The developed (Mn-Cr)AlGe films are promising from the viewpoint of not only the magnetic properties, but also the compatibility to the silicon process in the film fabrication.
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