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

最近，在一个扁平的硅芯片上制造的磁性“纳米线”网络已经成为一个系统，在这个系统中，磁化行为可以被非常精确地控制。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。线通常由常见的铁磁材料制成，例如镍和铁，并且通常为5 / 20纳米厚、100 / 500纳米宽和几微米长。导线几何形状至关重要，因为它控制着沿着导线长度的磁化强度。这可以被可视化为在导线的相对端处的磁北极和南极，或者更有用地，被可视化为具有表示磁北极的箭头的箭头。然而，相反的磁化区域可以在这些线内相遇，这可以通过两个指向彼此的箭头来可视化。在此，出现长度约为100纳米的小过渡区域，其中磁化旋转180度。科学家们称之为“畴壁”，可以用电磁铁将磁场施加到磁性导线上，从而导致畴壁沿着导线移动。理解这种运动对于畴壁技术的未来发展至关重要，因为它可能提供关于如何控制畴壁的新见解，并为可能性设置一些限制。此外，有一个基本的科学兴趣，在畴壁如何响应所施加的磁场和畴壁如何移动的几个理论预测还有待实验测试。然而，磁畴壁并不相同，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。畴壁运动的性质取决于畴壁的磁结构，如何测量畴壁运动并对畴壁结构成像是一个关键问题。我们已经知道，磁畴壁可以以每秒1000米以上的速度移动，因此测量必须相对快速，并且必须具有对磁畴壁的磁结构进行成像的能力。今天使用的大多数测量技术都不能满足这些标准中的一个或两个，但X射线成像可以提供一个解决方案。X射线圆二色性使用入射在样品上的圆偏振X射线。在磁性材料中，通过正确选择X射线，在一个方向上磁化的区域将比在相反方向上磁化的区域吸收更大比例的X射线。因此，图像对比度产生的磁性特征小至15纳米，这是理想的研究磁纳米线的畴壁。不幸的是，标准实验室X射线源的低强度需要长的曝光时间来获得图像，并且不适合于观察畴壁运动。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，从而可以在几分之一秒内获得磁化图像。此外，位于美国加州劳伦斯伯克利国家实验室的先进光源同步加速器设备可以提供持续时间仅为80皮秒的X射线爆发。这已被用来研究磁化强度的变化，在微观方形磁性元件，并有可能转向畴壁运动nanowires.The总体研究建议的目的是测试使用同步加速器X射线成像磁纳米线畴壁的可行性。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畴壁运动的可重复性。解决这些问题将导致一种新的和强大的成像畴壁运动的方式，将允许重要的科学和技术问题得到回答。

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.