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

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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.