电场调控CoFeB垂直磁化膜各向异性及自旋转移矩的研究

The high spin polarization CoFeB-based perpendicular magnetic tunnel junctions was used to fabricate the novel spin transfer torque magnetic random access memory (STT-MRAM). This kind of STT-MRAM has a lot of advantages, such as small critical switching current density, low power consumption, small device size, ultra dense data storage, large magnetoresistace effect, high stability and so on. What’s more, the electrical field control of perpendicular magnetic films has made it possible to effectively manipulate the current induced magnetic switching. So, the electrical control of STT-MRAM is quite promising in the magnetic storage. However, for the future application of this new STT-MRAM, some basic problems cannot be ignored, such as, what is the relationship between electrical field and spin polarized current as well as spin polarization in STT effect, how to improve the sensitivity of electrical field control effect in perpendicular magnetic thin films and what effect does electrical field have on the magnetic properties and anti-ferromagnetic coupling in magnetic tunnel junctions, These are all problems that are urged to be solved. In this project, magnetron sputtering will be used to fabricate CoFeB perpendicular thin films. Further, perpendicular magnetic tunnel junctions will be fabricated using micromachining, such as e-beam and so on. The effect of electrical field on magnetic properties of CoFeB perpendicular thin films and perpendicular magnetic tunnel junctions will be studied. Also, we plan to carry out first principle calculations to provide model and theoretical analysis. The research achievements can promote the design, fabrication and future application of electrical field control STT-MRAM.

采用高自旋极化率的CoFeB基垂直磁性隧道结（PMTJ）制备自旋转移矩磁随机存储器（STT-MRAM），具有临界翻转电流密度小、功耗低、器件尺寸小、存储密度大、磁阻效应大、存储稳定性高等特点。通过电场调控垂直磁化膜的磁特性可以提高PMTJ的极化电流致磁性翻转可控性，因此电场调控STT-MRAM具有很好的应用前景。尚有一系列基础问题亟待解决，例如，调控电场与产生STT效应的自旋极化电流及自旋极化率的关系、如何提高电场对垂直磁化膜各向异性调控的灵敏度、电场调控对PMTJ磁特性及反铁磁耦合特性的影响等。本课题拟采用磁控溅射的方法制备高质量的CoFeB基垂直磁化膜，在此基础上采用电子束光刻等微加工工艺制备垂直磁性隧道结，研究电场对CoFeB基垂直磁化膜各向异性及PMTJ 中STT效应的影响，并通过第一性原理进行建模计算。研究成果将有效促进电场调控型STT-MRAM的设计、制备和应用。

自旋流是自旋电子学前沿课题，相关研究对新型自旋器件的研发具有突出意义。然而，外加电流所带来的较大热耗是限制器件应用的瓶颈。通过电场调控磁性体系的磁各向异性能有望极大地减小该热耗，因而受到较多关注。本项目针对各种自旋流器件性能电场调控开展实验和计算研究，主要取得了以下成果：.1. 实验研究了重金属/铁磁金属体系自旋霍尔磁电阻的电场调控。通过磁控溅射在PMN-PT压电衬底上生长了W/CoFeB/Pt等重金属/铁磁金属异质结薄膜。外加电场极化衬底可以显著影响磁性薄膜磁各向异性场，从而改变磁电阻磁场响应特性曲线。但体系的磁电阻大小无明显改变。说明界面自旋-磁矩耦合对外电场引发的应力具有较强鲁棒性。另外，我们在PMN-PT压电衬底上生长了YIG/Pt，并研究其自旋塞贝克效应的电场调控，结果和自旋霍尔磁电阻效应的电场调控类似，外加电场可显著改变自旋塞贝克效应的磁场响应特性，但对热电电压大小无明显影响。.2. 实验研究了铁磁金属/PMN-PT异质结构各向异性磁电阻效应的电场调控。包括FeCoSiB和NiCo两种合金体系的各向异性磁电阻效应。对于FeCoSiB，外加电场极化衬底实现了电阻条磁各向异性取向的非90度旋转，从而有望实现对磁场测试方向的电场调控。而对于NiCo，研究发现无外加电场作用下，该合金表现四重对称磁各向异性，外加足够强的电场可实现从四重各向异性向非易失单轴各向异性的过渡，并显著拓宽此单轴各向异性难轴方向的磁场响应范围，从而为研发新型宽磁场测试范围的各向异性磁电阻传感器奠定基础。.3. 基于微磁学理论和仿真，开展了电场调控磁性和自旋流的计算研究：(1).在考虑磁畴壁宽度变化基础上，严格推导了外电场辅助下自旋流驱动手性畴壁运动的方程，并进行数值求解。结果显示外加电场辅助可导致畴壁速度增大一倍，并从匀速运动变为加速运动；(2). 研究了外加电场驱动人工反铁磁结构畴壁的运动，发现相比单一铁磁层畴壁，人工反铁磁耦合的畴壁可以具有更大的运动速度，并且畴壁倾斜和Walker崩溃均得以抑制；(3). 研究了外加电场驱动下人工反铁磁结构中skyrmion半径的改变，发现可以在很小的电场下实现半径的显著改变，从而为研发新型低功耗神经计算器件奠定基础；(4). 研究了外加电场对四重对称和交换弹簧等特殊磁各向异性体系磁性的调控，以及对自旋阀传感器核心电桥单位输出特性的调控。