具有中心对称和高垂直各向异性的铁磁单层薄膜的自旋转矩及其导致的磁矩反转

High density and low power consumption are the primary goals of spintronic research. High density demands materials with high anisotropy. Recently, the magnetization switching induced by strong spin-orbit interaction (SOI) has been shown to effectively reduce energy dissipation and improve the performance. However, it requires systems with inversion asymmetry: either in a non-magnetic (NM)/ferromagnetic (FM) bi-layer or multi-layer structure; or in a single FM (AFM) layer without centro-symmetry. Recent studies have shown that the spin reflection at the NM/FM interface and the electron band-structure matching of NM-FM layers played a central role in determining the charge-spin conversion efficiency. Furthermore, the switching current is proportional to the FM layer thickness, which makes it unfavorable towards high density and low power. The frequently used FM materials, such as FeCo, FeCoB and L10 FePt/CoPt with high perpendicular anisotropy have centro-symmetry. The spin torque in those single FM layer depends on the quadratic of the SOI parameter, which are normally too small to be observable. However, the magnetic anisotropy also scales with SOI parameter in the second order and is very large in L10 FePt / CoPt/MnAl. Therefore, it is expected the spin torque of single layer L10 films should not be small. Our preliminary results showed that this spin torque is sufficient to switch the moment of L10 thin film. The project focuses on the spin torque (ST) and such ST-induced magnetization switching in single FM layer with high perpendicular anisotropy and centro-symmetry (L10 FeP/CoPt/MnAl). The main contents include in-plane current induced magnetization switching, magnitude and efficiency of spin torque (effective field and spin Hall angle, etc.), physical origin of spin torque, possible approaches and geometry on field-free switching. This study not only physically opens up a new kind of spin torque, but also has potential applications in low-energy and high-density spin-memory chips.

现有的自旋-轨道转矩只可以在没有反转对称性的系统中实现：一是非磁/铁磁双层或多层结构；二是不具有中心对称的铁磁或反铁磁单层薄膜。在非磁/铁磁结构中，自旋流的界面反射及能带的失配性降低其效率。而常用的铁磁材料（Fe，Co，FeCoB及L10 FePt）等具有中心对称性，其单层膜的自旋转矩取决于自旋-轨道耦合(SOI)参数的二次方，一般认为太小而观察不到。但磁晶各向异性也是依赖于SOI参数的二次方，且在L10相中，非常大。所以单层L10 薄膜的自旋转矩也许不小。我们的初步结果显示这种自旋转矩足以反转其磁矩。本项目将对这个具有挑战性的未知领域进行探索：研究具有中心对称和高垂直各向异性(L10合金)的铁磁单层薄膜的自旋转矩及其导致的磁矩反转。集中探讨自旋转矩的物理起源和无磁场反转的方法和几何结构。这项研究不仅在物理上开拓了一种新的自旋转矩，且在低能耗、高密度的自旋记忆芯片方面有着广泛的应用前景。

本项目将对这个具有挑战性的未知领域进行探索：研究具有中心对称和高垂直各向异性(L10合金)的铁磁单层薄膜的自旋转矩及其导致的磁矩反转。通过系统研究L10-FePt薄膜厚度和生长温度的依赖关系，证明无场翻转有很大的调控窗口。为电流驱动磁矩翻转的无磁场化提供了一种新的实现方案，不同于之前无磁场翻转研究采用的方案，我们在实验中发现，将L10-FePt生长在斜切的MgO(001)衬底上，不需要添加其他功能层，也不需要设计薄膜形状的非对称性，适用于工业化生产。这种技术路线有望在其他磁性甚至反铁磁体系得到应用。本项目集中探讨自旋转矩的物理起源和无磁场反转的方法和几何结构。另外本项目还重点研究的SOT的效率和自旋Hall角的提升方法。这项研究不仅在物理上开拓了一种新的自旋转矩，且在低能耗、高密度的自旋记忆芯片方面有着广泛的应用前景。