Picosecond magnetization dynamics of nanomagnets: time resolved XMCD and XPEEM

纳米磁铁是未来数据存储技术和自旋电子器件的基石，这些设备在电信技术和量子计算中有应用。到目前为止，目前最成功的自旋电子学设备是硬盘记录头中使用的自旋阀和隧道阀传感器。阀门由两个铁磁层组成，固定层和自由层，由非磁性金属或超薄氧化物隧道势垒隔开。当向器件施加磁场时，自由层磁化从反平行重定向到与固定层磁化平行，从而产生提供来自传感器的信号的磁阻(MR)。具有双稳定磁状态的阀阵列提供了磁性随机存取存储器(MRAM)的基础。最近观察到，当电流垂直于器件平面传递时，纳米级横向尺寸的自旋阀也表现出自旋传递扭矩(STT)效应。由于角动量守恒，在铁磁层中注入自旋极化电流可以在局部磁化强度上产生扭矩，从而产生类似于脉冲磁场的响应。基于STT的磁开关被提议用于第二代MRAM，并且被认为是扩展到更小单元尺寸的关键。STT还可用于保持阀门中自由层的稳定进动，从而导致微波产生。自旋阀已经完全属于纳米技术领域，在研究级记录头中，其层厚度仅为几纳米，横向尺寸小至50纳米。然而，如果不相应地减少读取和写入每一位信息所需的时间，则较小的特征尺寸以及因此增加的存储容量的使用是有限的。这些纳米级阀门装置的动态响应是这项提议的主题。在埃克塞特，我们有十年使用飞秒激光来刺激和探测亚纳秒磁过程的经验。我们开发了一种时间分辨扫描克尔显微镜(TRSKM)，它允许以频闪方式成像样品对脉冲磁场的响应。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。我们推断，对于小于200 nm的元素，磁化强度对脉冲场的响应在空间上是不均匀的，并且主要由位于元素边缘的自旋波模式主导。这种不均匀性可能会导致未来记录传感器的信噪比下降。事实上，我们认为STT设备也将受到类似的破坏。我们现在建议使用时间分辨X射线测量技术来获得关于纳米级磁体特别是自旋阀中皮秒磁化动力学的独特信息。我们将使用时间分辨x射线磁性圆二色谱(TRXMCD)来对自旋阀结构中的进动动力学进行元素特定的测量，而时间分辨x射线光电子显微镜(TRXPEEM)将用于获得磁状态的时间分辨图像。因此，我们将确认边缘模式在深纳米尺度范围内主导元素的磁响应，并测试选择性激发更均匀空间响应的策略。英国同步加速器源的时间分辨磁测量的发展将为英国磁学研究人员打开这一相当新的领域，同时该项目将受益于我们在时间分辨测量技术方面的长期经验。最后，我们相信，这个项目为博士生提供了一个极好的培训机会，他们将对测量技术的发展起到不可或缺的作用，这些技术在未来几年可能会得到更广泛的应用。

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.