自旋轨道矩反转可电写电读的人工反铁磁体研究

Spintronics storage devices are expected to meet the requirements of information storage for higher speed, higher density, lower power consumption, non-volatile, and miniaturization, where it is the key step to achieve data writing by using spin-orbit torques to switch magnetization. (1) The project is planning to design and prepare the synthetic antiferromagnets with perpendicular magnetic anisotropy, which can be used for electrical writing and electrical reading. Although the two remanent states show zero magnetization, their anomalous Hall voltages show two different values (one is positive, and another is negative), which can be regarded as reading data. On the other hand, the two remanent states can be controlled by using spin-orbit torques to switch the magnetization of the synthetic antiferromagnets, which can be regarded as writing data. (2) This project will reveal the mechanism of magnetization switching under the spin-orbit torque and external magnetic field as the synthetic antiferromagnets and synthetic ferrimagnets are in the multiple magnetic domain state. (3) By changing the layer thickness, composition of the synthetic antiferromagnets, and the geometry of Hall Bar, we can control the magnetization, perpendicular magnetic anisotropy, interlayer coupling and exchange bias, and finally achieve magnetization switching without external magnetic field by spin-orbit torque. Therefore, our project not only is of great theoretical significance for revealing the spin-orbit torque switching the synthetic antiferromagnets, but also lays the material and device foundation for the practical data storage by electrical writing and electrical reading in synthetic antiferromagnets.

自旋电子存储器件有望满足我们对信息存储的高速度、高密度、低功耗、非易失、微型化的要求，其中利用自旋轨道矩控制磁化反转是实现数据写入的关键步骤。(1)本项目拟设计并制备可电写电读的垂直磁各向异性人工反铁磁体，它的剩磁为零而剩磁态的反常霍尔电压显示一正一负两个状态(可看作读数据），同时这两个状态能够通过自旋轨道矩反转层间反铁磁耦合的磁化强度来实现（可看作写数据）。(2) 本项目将揭示多磁畴的人工反铁磁体和人工亚铁磁体在自旋轨道矩和外磁场作用下的磁化反转机理。(3)通过改变多层膜的厚度、成分，以及Hall Bar的几何尺寸，调控人工反铁磁体的磁化强度、垂直磁各向异性、层间耦合、交换偏置，最终实现不需要外磁场辅助的自旋轨道矩控制的磁化反转。因此，我们的项目不仅对揭示自旋轨道矩反转人工反铁磁体具有重要理论意义，还为人工反铁磁体实际用于电写电读的数据存储奠定材料和器件基础。

具有垂直磁各向异性的强耦合人工反铁磁结构用在自旋轨道矩存储与逻辑器件中有两个难题需要克服：其一，人工反铁磁结构中两个铁磁层的霍尔信号通常相互削弱，不利于用霍尔电压来读出；其二，通常需要施加较大的面内辅助磁场才能获得自旋轨道矩引起的确定的磁化翻转。为了解决这些问题，我们提出构建Pt/CoPt/Ru/CoTb人工反铁磁异质结，通过控制长程层间交换耦合和近邻的亚铁磁耦合，将铁磁CoPt、亚铁磁CoTb、层间反铁磁CoPt/Ru/CoTb自旋构型的优点集成在一个异质结中，在具有宏观层间反铁磁耦合的CoPt/Ru/Co65Tb35异质结中，同时实现室温下磁化的补偿和剩磁态反常霍尔电阻的增强；理论和实验上证实当人工反铁磁体的下磁性层和上磁性层外界面的Dzyaloshinsky-Moriya相互作用有显著差异时，可以实现无磁场的自旋轨道矩引起的磁化翻转。我们还提出利用倾斜溅射制备CoPt成分梯度膜打破中心反演对称性，这样不需要引入任何额外的重金属层就能产生自旋轨道转矩，从而在普通的单层铁磁薄膜中实现无外磁场的自旋轨道转矩翻转磁化强度。为了获得兼具存储与逻辑运算的功能单元，我们构建了IrMn/Co/Ru/CoPt/CoO磁性异质结，用磁场或电流脉冲可以控制IrMn/Co面内交换偏置的方向，再利用自旋轨道矩引起的磁化翻转，控制CoPt/CoO界面的自旋排列方式，可以在单个器件中存储10个甚至更多的信息状态；将输入磁场或电流脉冲作为逻辑输入，磁性异质结的霍尔电压作为输出，可以在单个磁性异质结中实现16中布尔逻辑运算。项目执行期间，发表文章16篇，其中Adv. Mater. 1篇，Nat. Comm. 1篇，Adv. Funct. Mater. 2篇，ACS Nano 2篇。我们的研究工作为非易失多态存储、自旋逻辑以及存算一体器件奠定了材料与器件基础。