Magnetic X-ray Transmission Microscopy of Domain Walls in Magnetic Nanowires

在平面硅芯片上制造的磁性“纳米线”网络最近作为一种系统出现，在这种系统中，磁化行为可以非常精确地进行控制。这些纳米线可能的技术应用包括传感器、存储元件和信息处理。这些导线通常由镍和铁等常见的铁磁性材料制成，通常厚度为5/20纳米，宽度为100/500纳米，长度为几微米。导线几何形状至关重要，因为它控制磁化作用沿导线长度分布。这可以被视作导线两端的磁北极和磁南极，或者更有用的是，被视作带有代表磁北极的箭头的箭头。然而，相反的磁化区域可以在这些导线内部相遇，这可以通过两个指向彼此的箭头来显示。这里，出现了大约100纳米长的小过渡区，其中磁化旋转了180度。科学家们把这种现象称为“磁区墙”。利用电磁铁将磁场施加到磁线上，就会导致磁区壁沿着磁线移动。理解这一动议对域名墙技术的未来发展至关重要，因为它可能为如何控制域名墙提供新的见解，并对可能性设置一些限制。此外，人们对域壁如何响应外加磁场有基本的科学兴趣，有关域壁如何运动的几个理论预测还有待实验验证。然而，域壁不是完全相同的，而是具有取决于其环境的磁结构，例如导线尺寸或施加的磁场。磁畴壁运动的性质将取决于磁化壁磁结构，一个关键的问题是如何测量磁化壁运动并对磁化壁结构成像。我们已经知道，磁区壁可以以每秒1000米以上的速度移动，因此测量必须相对较快，并且必须与成像磁区壁的磁性结构的能力相结合。今天使用的大多数测量技术无法满足这两个标准中的一个或两个，但X射线成像可以提供解决方案。X射线圆二向色性使用入射到样品上的圆偏振X射线。在磁性材料中，如果正确选择X射线，在一个方向磁化的区域将比在相反方向磁化的区域吸收更大比例的X射线。因此，产生的图像对比度与磁性特征小至15纳米，这是理想的研究磁纳米线的磁畴壁。不幸的是，标准实验室X射线源的低强度需要较长的曝光时间才能获得图像，不适合观察磁区壁上的运动。然而，“同步加速器”源使用高度加速的电子来产生极强的X射线源，从而在几分之一秒内获得磁化图像。此外，美国加利福尼亚州劳伦斯伯克利国家实验室的高级光源同步加速器设施可以提供仅持续80皮秒的X射线爆发。这已经被用来研究微观方形磁性元件中的磁化变化，并有可能转向纳米线中的磁化壁运动。这项研究提案的总体目标是测试使用同步辐射X射线成像磁性纳米线中磁化磁壁的可行性。有几个问题需要解决，例如在X射线透明材料上制造纳米线，测量技术的灵敏度，以及磁区壁运动的重复性。解决这些问题将导致一种新的、强大的成像域壁运动的方法，这将使重要的科学和技术问题得到解答。

Networks of magnetic 'nanowires' fabricated on a flat silicon chip have recently emerged as a system in which the magnetisation behaviour can be controlled with a great deal of precision. Possible technological applications of these nanowires include sensors, memory elements and information processing. The wires are typically made of commonly occurring ferromagnetic materials, such as nickel and iron, and are typically 5 / 20 nanometres thick, 100 / 500 nanometres wide and several micrometres long. Wire geometry is of vital importance since this controls the magnetisation to lie along the wire length. This may be visualised as magnetic north and south poles at opposite ends of a wire or, more usefully, as an arrow with the arrow head representing magnetic north. However, opposite regions of magnetisation can meet inside these wires, which can be visualised by two arrows pointing towards each other. Here, a small transition region approximately 100 nanometres in length occurs in which the magnetisation rotates by 180 degrees. Scientists call this a 'domain wall'.Magnetic fields can be applied to magnetic wires using an electromagnet and can result in domain walls moving along the wires. Understanding this motion is crucial to the future development of domain wall technologies as it is likely to provide new insight on how domain walls can be controlled as well as setting some limits to the possibilities. Furthermore, there is a fundamental scientific interest in how domain walls respond to applied magnetic fields and several theoretical predictions of how domain walls move have yet to be tested experimentally. However, domain walls are not identical but have a magnetic structure that will depend on its environment, e.g. wire dimensions or applied magnetic field. The nature of domain wall motion will depend on the magnetic structure of the domain wall.A key issue is how to measure the domain wall motion and image the domain wall structure. We already know that domain walls can travel at velocities over 1000 metres per second, so the measurement must be relatively fast and must be coupled with a capability to image the magnetic structure of a domain wall. Most measurement techniques used today fail to meet one or both of these criteria but X-ray imaging could provide a solution. X-ray circular dichroism uses circularly polarised X-rays incident on a sample. In a magnetic material, with the correct choice of X-rays, regions with magnetisation in one direction will absorb a greater proportion of X-rays than regions with magnetisation in the opposite direction. Hence, image contrast is generated with magnetic features as small as 15 nanometres, which is ideal for studying domain walls in magnetic nanowires. Unfortunately, the low intensity of standard laboratory X-ray sources requires long exposure times to obtain an image and is not suited for observing domain wall motion. However, 'synchrotron' sources use highly accelerated electrons to produce an extremely intense X-ray source that allows magnetisation images in a fraction of a second. Furthermore, the Advanced Light Source synchrotron facility at Lawrence Berkeley National Laboratories in California, USA can be operated to provide X-ray bursts that last for just eighty picoseconds. This has been used to study magnetisation changes in microscopic square-shaped magnetic elements and has the potential to be turned towards domain wall motion in nanowires.The overall aim of this research proposal is to test the feasibility of using synchrotron X-rays to image domain walls in magnetic nanowires. There are several issues to address, such as fabricating nanowires on top of an X-ray transparent material, how sensitive the measurement technique is, and how repeatable domain wall motion is. Resolving these issues will result in a new and powerful way of imaging domain wall motion that will allow important scientific and technological questions to be answered.