重金属/磁性金属薄膜界面自旋轨道耦合效应的电压调控研究

Magnetic random access memory (RAM) based on spin-orbit coupling effect is a powerful candidate for the next generation of nonvolatile, high density, low energy-consuming and high speed RAM. This spintronic device consists of a magnetic tunnel junction and a heavy metal/magnetic metal bilayer, the latter of which plays a crucial role in determining the magnetization state of the magnetic thin films under applied current. However, there exists sharp divergence regarding the concrete role of the heavy metal layer in community, at least spin Hall effect in the bulk region of heavy metal and Rashba effect in the interface in-between them being involved. Further due to their same dependence on applied current and both exerting the same mathematical form of field-like and damping-like torque to magnetic thin film, their contributions to the total spin-orbit coupling effect are hard to identify experimentally. In this research, a gate voltage is applied to the heavy metal/magnetic metal bilayer to modulate the intensity of interfacial Rashba effect while retaining the bulk spin Hall effect unaffected to distill the contribution of Rashba effect from that of spin Hall effect. Technically, second harmonic AC Hall measurement will be applied in this research to accurately calibrate the effective magnetic field induced by applied current and modulated by gate voltage. This research not only helps to understand the essence of spin-orbit coupling effect in heavy metal/magnetic metal bilayer system but also paves way towards electrically controlling interfacial spin-orbit coupling in this system and helps to further reduce critical current density to reverse magnetization of magnetic film in the future magnetic RAM based on spin-orbit coupling mechanism.

基于自旋轨道耦合效应的磁随机存储器是下一代低功耗、高密度、高速读写的非挥发性随机存储器的主流方案之一。它的核心结构包含一个磁性隧道结和一个重金属/磁性金属双层膜结构，后者是通过电流调控磁性层磁矩状态的关键。对这种结构中的重金属薄膜发挥的作用，学界还存在争议。体区的自旋霍尔效应和界面的Rashba效应均卷入其中。因为这两者对外加电流满足相同的依赖关系，均能产生数学形式相同的有效场和有效力矩，实验上区分两者非常困难。在该项研究中，我们拟通过施加栅极偏压的方式改变Rashba效应的强度，从而分辨该效应在总自旋轨道耦合效应中所作的贡献。具体实验上，我们拟采用二次谐波交流霍尔效应测试技术来精确测量电流引起的有效场的大小。该项研究既为解析重金属/磁性金属薄膜中自旋轨道耦合效应的机制提供实验依据，又能在这类结构中探索一种利用外场调控自旋轨道耦合场的方法，为进一步降低磁性层的临界翻转电流密度做技术储备。

自旋轨道力矩(SOT)能高效实现电流驱动的磁矩翻转，在磁随机存储器和自旋逻辑器件中有广阔应用前景。但是有关不同SOT对磁矩翻转过程的影响、电学手段调控SOT翻转的研究还很匮乏。本项目围绕上述盲点开展工作并获得了以下成果：.(1)类场和类阻尼SOT对磁矩翻转的作用:类阻尼SOT是磁矩翻转的驱动力，决定翻转方向。而类场项的存在，无论极性正负，均能帮助降低临界翻转电流密度和缩短翻转时间。类场和类阻尼SOT的比值，对于Pt和Ta，分别约0.03和1。.(2)偏置电流对磁矩翻转的影响:与翻转电流正交的偏置电流，可调节类场和类阻尼SOT的比例，从而实现对翻转过程的调控。对于类场项极弱的Pt，偏置电流降低了磁矩翻转的难度；对于类场项很显著的Ta，某一极性的偏置电流可降低临界翻转电流密度而相反极性的偏置电流则不能。.(3)零磁场SOT翻转和可编程自旋霍尔逻辑器件的实现:我们利用既具有自旋轨道耦合作用又具有反铁磁性质的IrMn薄膜，实现了磁矩的零磁场SOT翻转。基于翻转极性受外加磁场大小和方向调控的特点，我们还设计了两款可编程自旋霍尔逻辑器件——基于同一单元结构，可实现5种不同的布尔逻辑功能。.(3)重金属和铁磁金属自旋轨道耦合参数的标定:为了测量自旋霍尔角、自旋弛豫长度、自旋弛豫寿命等参数以及研究自旋弛豫机制，除了常用的二次谐波霍尔方法，我们还基于Hanle磁电阻效应、自旋Seebeck效应、二次谐波自旋注入效应开发了新的测试方法，完成重金属和磁性金属自旋轨道耦合参数的测量。.我们有关偏置电流影响SOT翻转的研究清楚表明了类场和类阻尼SOT在磁矩翻转过程中的作用，为后续材料和结构优化提供了依据；提供了一种SOT翻转的电学调控方法，还提供了一种类场项力矩的电学测试方法。我们初步实现了自旋霍尔逻辑原型单元器件，为后续存储逻辑融合的微电子器件开发提供了新思路。我们新开发的自旋轨道耦合参数测量方法也可推广到其他材料，为自旋轨道耦合效应研究提供新的手段和途径。