Picosecond magnetization dynamics of nanomagnets: time resolved XMCD and XPEEM

纳米磁体是未来数据存储技术和自旋电子器件的基石，可应用于电信技术和量子计算。到目前为止，最成功的自旋电子器件是用于硬盘记录头的自旋和隧道阀传感器。该阀包括两个铁磁层，固定层和自由层，由非磁性金属或氧化物隧道势垒分开。当向器件施加磁场时，自由层磁化从反平行重定向到平行于固定层磁化，从而产生磁阻（MR），该磁阻提供来自传感器的信号。具有双稳态磁状态的阀阵列为磁性随机存取存储器（MRAM）提供了基础。最近观察到，当电流垂直于器件平面通过时，纳米级横向尺寸的自旋阀也表现出自旋转移矩（STT）效应。由于角动量守恒，将自旋极化电流注入铁磁层可以在局部磁化上产生扭矩，其产生类似于由脉冲磁场引起的响应。由于STT的磁开关被提出用于第二代MRAM，并且被认为是缩放到更小的单元尺寸所必需的。STT还可用于维持阀中自由层的稳定进动，从而产生微波。自旋阀已经处于纳米技术领域内，在研究水平的记录头中，层厚度仅为几纳米，横向尺寸小至50纳米。然而，较小的特征尺寸和因此增加的存储容量在没有相应地减少读取和写入每个信息位所花费的时间的情况下具有有限的用途。这些纳米级阀装置的动态响应是本提案的主题。在埃克塞特，我们有十年的经验，使用飞秒激光器来刺激和检测亚纳秒磁过程。我们已经开发了一种时间分辨扫描克尔显微镜（TRSKM），允许响应的样品脉冲磁场频闪成像。最近，我们与日立全球存储技术公司合作，研究了自由层材料纳米级元素阵列的皮秒动力学。我们已经推断，对于小于200 nm的元件尺寸，磁化强度对脉冲场的响应在空间上是不均匀的，并且由定位在元件边缘的自旋波模式主导。这种不均匀性可能导致未来记录传感器中信噪比的降低。我们相信，这一点，也将成为一种必然。我们现在建议使用时间分辨的X射线测量技术，以获得独特的信息，皮秒的磁化动力学内纳米级磁体，特别是自旋阀。我们将使用时间分辨的X射线磁圆二色性（TRXMCD），使自旋阀结构内的进动动力学的元素特定的测量，而时间分辨的X射线光电子显微镜（TRXPEEM）将被用来获得时间分辨的图像的磁性状态。因此，我们将确认，边缘模式占主导地位的深纳米级制度内的元素的磁响应，并测试策略的选择性激发一个更空间均匀的响应。在英国同步加速器源的时间分辨磁测量的发展将打开这个相当新的领域，英国磁性研究人员的大社区，而该项目将受益于我们的时间分辨测量技术的长期经验。最后，我们相信这个项目为博士生提供了一个很好的培训机会，他们将成为测量技术发展的一部分，这些技术可能会在未来几年得到更广泛的应用。

Nanomagnets are the building blocks for future data storage technology and spintronic devices that have applications in telecommunications technology and quantum computation. By far the most successful spintronic devices today are the spin and tunnel valve sensors used in hard disk recording heads. The valve consists of two ferromagnetic layers, the fixed layer and the free layer , separated by either a non-magnetic metal or an ultrathin oxide tunnel barrier. When a magnetic field is applied to the device, the free layer magnetization reorients from anti-parallel to parallel to the fixed layer magnetization, giving rise to a magnetoresistance (MR) that provides the signal from the sensor. Arrays of valves with bi-stable magnetic states provide the basis for a magnetic random access memory (MRAM). It was recently observed that spin-valves of nano-scale lateral size also exhibit spin-transfer-torque (STT) effects when current is passed perpendicular to the plane of the device. Due to conservation of angular momentum, injection of spin-polarized current into a ferromagnetic layer can produce a torque upon the local magnetization that generates a response similar to that induced by a pulsed magnetic field. Magnetic switching due to STT is proposed for 2nd generation MRAM and is thought to be essential for scaling to smaller cell sizes. STT may also be used to maintain a steady precession of the free layer in a valve leading to microwave generation. The spin-valve already lies well within the realm of nanotechnology with layer thicknesses of just a few nanometers and lateral sizes as small as 50 nm in research level recording heads. However, smaller feature sizes and hence increased storage capacity are of limited use without a commensurate reduction in the time taken to read and write each bit of information. The dynamic response of these nanoscale valve devices is the subject of this proposal. In Exeter we have ten years experience of using femtosecond lasers to stimulate and detect sub-nanosecond magnetic processes. We have developed a time resolved scanning Kerr microscope (TRSKM) that allows the response of a sample to a pulsed magnetic field to be imaged stroboscopically. Recently, in collaboration with Hitachi Global Storage Technologies, we have studied the picosecond dynamics of arrays of nano-scale elements of free layer material. We have inferred that, for element sizes less than 200 nm, the response of the magnetization to a pulsed field is spatially non-uniform and dominated by spin wave modes localised at the element's edges. This non-uniformity may result in a degradation of the signal to noise ratio in future recording sensors. Indeed we believe that STT devices will be similarly blighted. We now propose to use time resolved X-ray measurement techniques to obtain unique information about the picosecond magnetization dynamics within nanoscale magnets and specifically spin-valves. We will use time resolved x-ray magnetic circular dichroism (TRXMCD) to make element specific measurements of precessional dynamics within spin-valve structures, while time resolved x-ray photoemission electron microscopy (TRXPEEM) will be used to obtain time resolved images of the magnetic state. We will hence confirm that edge modes dominate the magnetic response of elements within the deep nanoscale regime, and test strategies for the selective excitation of a more spatially uniform response. The development of time resolved magnetic measurements at UK synchrotron sources will open this rather new field to a large community of UK magnetism researchers, while the project will benefit from our longstanding experience of time resolved measurement techniques. Finally, we believe that this project presents an excellent training opportunity for a PhD student who will be integral to the development of measurement techniques that are likely to find much wider application in years to come.