Optical detection of magnetisation dynamics induced by spin-orbit torques

现代世界完全依赖于电子设备，这些设备通过被称为电子即电流的带电粒子流运行。然而，电子也携带“自旋”角动量，并有一个相关的磁矩，就像一个微小的条形磁铁。自旋电子学的目的是利用电子的自旋来控制它的运动以及它如何与磁性材料相互作用。最著名的自旋电子器件是自旋阀，这是一种三层结构，其中两个铁磁(FM)层被一个非磁性隔离层隔开。自旋阀的设计使得一层FM的磁矩是固定的，而另一层的磁矩可以自由地与所施加的磁场对准，就像指南针一样。当两个磁矩的相对取向不同时，观察到三层膜的电阻变化很大。由于电阻很容易测量，自旋阀可以充当磁场传感器。事实上，自旋阀传感器被用来读回今天销售的每个硬盘上的信息。当电流在固定FM层和自由FM层之间传递时，可以观察到相反的效果。电子流将角动量从一个FM传递到另一个FM，并根据牛顿第二定律施加自旋传递扭矩(STT)。这个力矩可以作用于自由层的磁矩，使其改变方向。自旋阀还可以设计成具有两个稳定状态，具有不同的电阻，可用于存储数字信息。在磁随机存取存储器(MRAM)中使用这种器件的阵列。或者，在自旋转移振荡器(STO)中，当施加直流电流时，自由层磁化强度在微波频率上振荡。由于电阻也会振荡，因此会产生微波电压振荡。STO的不同寻常之处在于，它的频率可以通过改变直流电流来通过多个八度进行调节。多个STO可以定义在芯片级、电路元件或阵列中以增加功率输出。近年来，人们已经认识到并证明了自旋-轨道相互作用--一种相对论效应--也可以用来操纵电子自旋。自旋又可以用来产生STT，根据其起源，STT被称为自旋轨道扭矩(SOT)。SOT是由自旋霍尔效应(SHE)和拉什巴效应(Rashba Effect)产生的，但这些力矩之间的分离，以及与电荷流动产生的扭矩(Oersted扭矩)之间的分离，仍然存在争议。用于MRAM的SOT的优化引起了人们的极大兴趣，因为它消除了通过量子力学隧道传导电流通过脆弱的绝缘层的需要。在这个项目中，我们将使用时间分辨扫描克尔显微镜(TRSKM)来探索、了解和优化由HGST、布朗大学和哥德堡大学提供的最高质量器件结构中的SOT，这三家公司都是各自领域的领导者。至关重要的是，我们将改进我们的TRSKM，以便在三维空间中以任何方向施加磁场，同时将高频电探头连接到设备，并使用聚焦的光学探头来确定磁化矢量的瞬时方向。这一国际独一无二的仪器将使我们能够根据施加不同电刺激时磁化的静态和动态响应，而不是电阻来确定SOTS。此外，TRSKM的亚微米空间分辨率将使我们能够通过它们的空间变化来分离不同的扭矩，并了解SOT如何与受限几何中的动态磁模相互作用。最后，我们将使用同样的仪器来了解SOT如何在STO和候选MRAM器件中诱导磁进动。

The modern world is completely dependent upon electronic devices that operate through the flow of charged particles called electrons i.e. electric current. However the electron also carries 'spin' angular momentum, and has an associated magnetic moment, like a tiny bar magnet. The aim of Spintronics is to use the spin of an electron to control its motion and how it interacts with magnetic materials. The most celebrated spintronic device is the 'spin-valve', a trilayer structure in which two ferromagnetic (FM) layers are separated by a non-magnetic spacer layer. The spin-valve is engineered so that the magnetic moment of one FM layer is fixed, while that of the other is free to align with an applied magnetic field, like a compass needle. As the relative orientation of the two magnetic moments varies, a large change in electrical resistance of the trilayer is observed. Since the resistance is easily measured, the spin-valve can act as a magnetic field sensor. In fact a spin-valve sensor is used to read back information in every hard disk that is sold today. When current is passed between the fixed and free FM layers an inverse effect can be observed. The flow of electrons transfers angular momentum from one FM to the other, and, by Newton's 2nd Law, exerts a spin transfer torque (STT). This torque can act upon the magnetic moment of the free layer, causing it to change its orientation. The spin-valve can also be designed to have two stables states, with different electrical resistance, that can be used to store digital information. Arrays of such devices are used in magnetic random access memory (MRAM). Alternatively, in a spin transfer oscillator (STO), the free layer magnetization oscillates at microwave frequency when DC current is applied. Since the resistance also oscillates, microwave voltage oscillations are generated. The STO is unusual in that its frequency can be tuned through multiple octaves by varying the DC current. Multiple STOs can be defined at chip level, as circuit components, or in arrays for increased power output.In recent years it has been realized and demonstrated that the spin-orbit interaction, a relativistic effect, may also be used to manipulate the electron spin. The spin can in turn be used to generate a STT, which has been termed spin-orbit torque (SOT) in light of its origin. SOTs are generated by the spin Hall effect (SHE) and the Rashba effect, but the separation of these torques from each other, and from the torque generated by the flow of charge (Oersted torque), is still being debated. The optimization of SOT for use in MRAM has attracted enormous interest because it removes the need to pass large electric currents through fragile insulating layers that conduct electricity by quantum mechanical tunneling.In this project we will use time resolved scanning Kerr microscopy (TRSKM) to explore, understand and optimize SOTs in device structures of the highest quality supplied by HGST, Brown University and the University of Gothenburg, all of whom are leaders in their respective fields. Crucially we will modify our TRSKM so that a magnetic field can be applied with any orientation in 3 dimensional space, while high frequency electrical probes are connected to the device, and a focused optical probe is used to determine the instantaneous orientation of the magnetization vector. This internationally unique instrument will allow us to determine the SOTs from the static and dynamic response of the magnetization, rather than the electrical resistance, as different electrical stimuli are applied. Furthermore the sub-micron spatial resolution of TRSKM will allow us to separate different torques through their spatial variation, and understand how SOTs interact with dynamic magnetic modes in a confined geometry. Finally, we will use this same instrument to understand how SOTs induce magnetic precession in STOs and switching in candidate MRAM devices.