Materials World Network: Spin dynamics of the ferromagnet/antiferromagnet interface studied by time-resolved x-ray magnetic dichroism

自旋电子学领域的目标是通过磁矩或“自旋”以及电荷控制电子的运动来提供新的设备功能。自旋电子学的杰出成就是在用于从硬盘驱动器读取数据的自旋阀传感器中使用巨磁阻（GMR）。自旋阀由两个由非磁性层分隔的铁磁（F）层组成。每一个F层都有一个磁矩，它就像一个指南针，并根据施加的磁场（比如存储在硬盘上的数据位产生的磁场）重新定向。如果两个罗盘针改变它们的相对方向，就会发生GMR。如果其中一个罗盘针保持固定，而另一个罗盘针在磁场中自由地重新定向，那么数据就可以作为传感器电阻的变化被读出。本项目关注的是其中一个罗盘针的固定方法。已建立的方法是在F层之一的外侧上存款反铁磁（AF）层。F/AF界面产生固定F层磁化方向的强有效磁场。这种效应被称为“交换偏差”，被广泛使用，但对其细节了解甚少。在F材料中，每个原子都有一个磁矩，并且每个这样的磁矩，或罗盘针，都被强大的“交换相互作用”迫使向同一方向排列。在AF材料内，相邻磁矩相反地彼此反平行地对准。在F/AF界面处，F的磁矩相对于界面AF层中的磁矩变得固定。AF材料没有净磁矩，因此在很大程度上不受外加磁场的影响，其磁性更难以观察。关于F/AF界面处AF的磁矩（自旋）知之甚少，特别是当存在结构缺陷时。自旋阀被要求改变他们的磁性排列在亚纳秒的时间尺度上，在AF内的磁矩的运动和它们对F的影响是完全unexplored.We将使用同步加速器X射线辐射，使在F/AF接口在GHz频率的磁矩的运动的第一次测量。特别是，我们将利用X射线磁性圆二色性和线性二色性效应，分别称为XMCD和XMLD。F/AF样品由原子组成，其中原子核被填充和部分填充的电子壳层包围。将电子从充满的壳层激发到部分充满的壳层所需的能量具有特定于特定原子的能量，而来自同步加速器的X射线的能量可以调整，以便只研究该原子。X射线以亚纳秒持续时间的脉冲产生。通过使X射线与具有正弦时间变化的磁场同步，可以在其振荡周期中的给定点处确定样品的瞬时状态。具体地，XMCD和XMLD效应允许独立地确定F和AF层的磁状态。我们将使用伯克利的高级光源（ALS）和英国的钻石光源，将这种测量技术应用于最高结构质量的样品，由加州大学伯克利分校的分子束外延制造。F层的GHz频率动态将首先在埃克塞特通过时间分辨的磁光测量来表征。X射线和磁光测量都将作为温度的函数进行，以便比较AF层具有不同程度的反铁磁序时的响应。因此，我们将更深入地了解AF层如何控制F层对高频磁场的响应。

The field of spintronics aims to deliver new device function by controlling the motion of an electron through its magnetic moment, or "spin", as well as through its electric charge. The outstanding success of spintronics has been the use of Giant Magnetoresistance (GMR) in the spin-valve sensors used to read data from hard disk drives. The spin-valve consists of two ferromagnetic (F) layers separated by a non-magnetic layer. Each F layer has a magnetic moment that acts as a compass needle and reorients in response to an applied magnetic field, such as that generated by the bits of data stored on a hard disk. GMR occurs if the two compass needles change their relative orientation. If one of the compass needles is kept fixed while the other is free to reorient in the magnetic field then data can be read out as a change in the electrical resistance of the sensor.This project is concerned with the means by which one of the compass needles is fixed. The established method is to deposit an antiferromagnetic (AF) layer on the outside of one of the F layers. The F/AF interface generates a strong effective magnetic field that fixes the orientation of the F layer magnetization. This effect, known as "exchange bias", is widely used but poorly understood in detail. Within a F material each atom has a magnetic moment, and every such magnetic moment, or compass needle, is forced to align in the same direction by the powerful "exchange interaction". Within an AF material adjacent magnetic moments instead align anti-parallel to each other. At the F/AF interface the magnetic moments of the F become fixed relative to those in the interfacial AF layer. The AF material has no net magnetic moment, so is largely unaffected by applied magnetic fields, and its magnetism is more difficult to observe. Little is known about the magnetic moments (spins) of the AF at an F/AF interface, particularly when structural imperfections are present. Spin-valves are required to change their magnetic alignment on sub-nanosecond timescales, where the motion of the magnetic moments within the AF and their influence upon the F are completely unexplored.We will use synchrotron x-ray radiation to make the first measurements of the motion of the magnetic moments at the F/AF interface at GHz frequencies. In particular we will make use of the x-ray magnetic circular and linear dichroism effects, known as XMCD and XMLD respectively. The F/AF samples consist of atoms in which a nucleus is surrounded by filled and partially filled shells of electrons. The energy required to excite an electron from a filled to a partially filled shell has an energy that is specific to a particular atom, while the energy of the x-rays from the synchrotron can be tuned so as to study only that atom. The x-rays are produced in pulses of sub-nanosecond duration. By synchronizing the x-rays with a magnetic field that has a sinusoidal time variation, the instantaneous state of the sample may be determined at a given point in its cycle of oscillation. Specifically the XMCD and XMLD effects allow the magnetic state of the F and AF layers to be determined independently.We will use the Advanced Light Source (ALS) in Berkeley and the Diamond Light Source in the UK to apply this measurement technique to samples of the highest structural quality, fabricated by molecular beam epitaxy at the University of California, Berkeley. The GHz frequency dynamics of the F layer will first be characterized by time resolved magneto-optical measurements at Exeter. Both x-ray and magneto-optical measurements will be performed as a function of temperature so as to compare the response when the AF layer has different degrees of antiferromagnetic order. We will hence obtain much deeper insight into how the AF layer controls the response of the F layer to a high frequency magnetic field.