Expanded access to the Exeter time resolved magnetism (EXTREMAG) facility

理解我们日常世界的运作需要我们在更大的放大倍数下检查其组成部分，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数十或数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，创造出提供特定功能的人工结构，从而构建出具有有用特性的宏观材料和设备。然而，我们必须记住，纳米世界以非常高的速度移动，许多过程发生在不到一纳秒（ns）或10 e-9 s的时间内。我们的计算机具有以GHz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，或发送信息的每个位（二进制代码中的0或1），发生在不到1 ns的时间内。存储信息的设备也是如此，例如我们的计算机中的硬盘驱动器（HDD）和用于云计算的服务器群，其中数据的写入或检索都是在ns以下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世界内的变化。有时，直升机的旋转桨叶可能在电视屏幕上看起来是静止的。电视以每秒一百幅的速度呈现一系列静止图像，这足以让眼睛相信物体正在连续移动。然而，如果每次获取图像时直升机桨叶都恰好处于相同的位置，则桨叶反而会看起来是静止的。这是重复现象的频闪成像的原理，由此对象的运动被有意地与相机的图像采集同步。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点被冻结。传统相机的快门速度太慢，无法捕捉纳秒以下的变化，因此需要将快门打开，并使用与拍摄对象同步的闪光灯。今天，超快激光器能够产生一束脉冲，每个脉冲的持续时间小于100飞秒（fs），或10 e-13 s。埃克塞特时间分辨磁性（EXTREMAG）设施使用超快激光源探测和成像的磁性和自旋电子系统的亚ns动力学。虽然传统的电子学通过作用在电子电荷上的电场来控制电子的运动，但电子也具有磁矩，就好像带电粒子围绕通过其中心的轴“旋转”一样。自旋电子学试图通过磁矩控制电子的运动。磁性材料的磁化引起从其表面反射的光的偏振的微小但可测量的变化。因此，对反射超快激光脉冲的偏振变化的分析可用于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，并将通过EXTREMAG设施与外部用户分享其专业知识。用户有非常广泛的兴趣，从磁矩的切换到在下一代HDD中存储信息，到理解对象的形成和动态，例如可能在磁化中形成的漩涡和skyrmions，到通过与超导材料的相互作用来操纵磁状态。虽然这项研究是基础性的，但它将推动信息技术的未来发展，对我们的生活和工作方式产生深远的影响。

Understanding the workings of our everyday 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 10e-9 s. 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 10e-13 s.The Exeter time resolved magnetism (EXTREMAG) facility uses ultrafast laser sources 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. Analysis of the polarization changes of reflected ultrafast laser pulses can therefore be used 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 share its expertise with external users through 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 is 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.