EXTREMAG: an Exeter-based Time Resolved Magnetism Facility

了解我们日常世界的运作需要我们以更大的放大镜检查其组成部分，直到观察到潜在的基本过程。在许多情况下，在所谓的纳米尺度上检查几十或几百个原子长度的物体就足够了。纳米技术试图驯服这个小人国的世界，创造具有特定功能的人工结构，从而构建具有有用性能的宏观材料和设备。然而，我们必须记住，纳米世界的发展速度非常快，许多过程发生的时间不到一个纳秒（ns），也就是10-9秒。我们的计算机拥有以千兆赫时钟速度运行的中央处理器单元，而光纤电缆以每秒千兆位的速率传输数据。这意味着计算机内的每个逻辑操作，或信息的每个位（二进制代码中的0或1）的发送，在不到1ns的时间内发生。存储信息的设备也是如此，比如我们的计算机中的硬盘驱动器（hdd）和云计算的服务器群，在云计算中，数据的写入或检索是在sub-ns的时间尺度上进行的。人眼能够观察到大约十分之一秒的时间尺度上发生的变化，因此需要不同的方法来观察纳米世界中的变化。有时，在电视屏幕上，直升机旋转的叶片可能是静止的。电视以大约每秒一百张的速度呈现一系列静止图像，这足以使眼睛相信物体在连续移动。然而，如果巧合的是，直升机叶片在同一位置，每次获得图像，叶片反而会出现静止。这是重复现象的频闪成像原理，即被摄体的运动与相机获取的图像被故意同步。通过稍微移动拍摄照片的相对时间，主体可以在其运动周期中的任何一点被冻结。传统相机的快门速度太慢，捕捉不到毫秒级的变化，因此有必要打开快门，使用与拍摄对象同步的闪光灯。如今，超快激光器能够产生一束脉冲，每束脉冲的持续时间不到100飞秒（fs），即10-13秒。在本项目中，我们将建造一个新的超快激光设备来探测和成像磁和自旋电子系统的子动力学。传统的电子学通过电场作用于电子电荷来控制电子的运动，而电子也有磁矩，就好像带电粒子通过其中心绕轴“旋转”一样。自旋电子学试图通过磁矩来控制电子的运动。磁性材料的磁化引起从其表面反射的光的偏振发生微小但可测量的变化。因此，我们将分析反射的超快激光脉冲的偏振变化，以获得磁性和自旋电子材料和器件中磁化的时间分辨图像。埃克塞特大学在使用时间分辨磁光成像方面有着悠久的历史，现在将与EXTREMAG设施的外部用户分享其专业知识。用户的兴趣范围非常广泛，从磁矩的切换到下一代hdd中的存储信息，到了解在磁化范围内可能形成的漩涡和天空等物体的形成和动力学，到通过与超导材料的相互作用来操纵磁态。虽然这项研究将是基础性的，但它将推动信息技术的未来发展，这将对我们的生活和工作方式产生深远的影响。

Understanding the workings of our every day world requires us to examine its constituent components under increased magnification until the underlying fundamental processes are observed. In many cases it is sufficient to examine objects that are tens or hundreds of atoms in length, at what is known as the nanoscale. Nanotechnology seeks to tame this Lilliputian world, creating artificial structures that deliver a specific function, leading to the construction of macroscopic materials and devices with useful properties. However we must remember that the nanoscale world moves at very high speed, with many processes occurring in times of less than a single nanosecond (ns), or 10-9s. Our computers have central processor units that run at GHz clock speeds, while fiber optic cables transfer data at rates of Gbits per second. This means that each logical operation within the computer, or the sending of each bit (a 0 or 1 in binary code) of information, occurs in less than 1 ns. The same is true of devices for storage of information such as the hard disk drives (HDDs) that are found within our computers and the server farms for cloud computing, where data is written or retrieved on sub-ns timescales.The human eye is capable of observing changes that occur on timescales of about one tenth of a second, so different methods are required to observe changes within the nanoworld. Sometimes the rotating blades of a helicopter may appear stationary on a TV screen. The TV presents a series of still images at a rate of about one hundred per second, which is sufficient to convince the eye that the subject is moving continuously. However, if by coincidence the helicopter blades are in the same position each time an image is acquired, the blades will instead appear stationary. This is the principle of stroboscopic imaging of repetitive phenomena, whereby the motion of the subject is deliberately synchronized with the acquisition of images by the camera. By slightly shifting the relative time at which the pictures are taken, the subject can be frozen at any point in its cycle of motion. The shutter speed of a conventional camera is too slow to capture sub-ns changes, so it is instead necessary to leave the shutter open and use a flash-gun that is synchronized with the subject. Today an ultrafast laser is able to generate a beam of pulses that each has duration of less than 100 femtosconds (fs), or 10-13s.In this project we will construct a new ultrafast laser facility to probe and image the sub-ns dynamics of magnetic and spintronic systems. While conventional electronics control the movement of electrons by means of electric fields acting upon the electron charge, the electron also has a magnetic moment, as if the charged particle were "spinning" about an axis through its centre. Spintronics seeks to control the movement of the electron via its magnetic moment. The magnetization of a magnetic material causes a small but measurable change in the polarization of light reflected from its surface. We will therefore analyze the polarization change of reflected ultrafast laser pulses to obtain time resolved images of magnetization in magnetic and spintronic materials and devices.The University of Exeter has a long track record in the use of time-resolved magneto-optical imaging and will now share its expertise with external users of the EXTREMAG facility. The users have a very broad range of interests ranging from switching of magnetic moments to store information in the next generation of HDDs, to understanding the formation and dynamics of objects such as vortices and skyrmions that may form within the magnetization, to manipulating the magnetic state through interaction with superconducting materials. While the research will be of a fundamental nature, it will power the future development of information technology that will have a profound impact on the way we live and work.