A team of scientists led by the Max Born Institute (MBI), Berlin, and the Massachusetts Institute of Technology (MIT), USA, has discovered how magnetization patterns called skyrmions can be generated in a ferromagnet faster than known so far.
The researchers have clarified how the topology of the magnetic system changes in the process. As they report in the journal Nature Materials, the results offer fundamental insights into topological phase transitions and inspire new ideas on how magnetic skyrmions can be used for information technology.
Magnetic skyrmions refer to small eddies in the magnetization of thin magnetic layers, where the magnetization points in different directions, as shown in the first picture. It turns out that a certain magnetization pattern can be characterized by its topology - a mathematical concept to describe the shape or geometry of a body, a quantity or - as in this case - a physical field (see info box on topology). What is important is that the topology of a skyrmion differs from that of a state in which the magnetization is oriented in the same direction everywhere. So if the magnetization pattern changes, the topology of the system must also change. This fact contributes to the stability of the skymion vortices and makes it difficult to.
For its work, the team first used X-ray and electron microscopy to make the nanometer-sized skyrmions visible. It was shown that a single light pulse from a laser with a sufficiently high intensity is sufficient to generate skyrmions with a fixed topology - i.e. a certain vortex shape of magnetization.
In a second step, the researchers investigated the question of how the laser pulse causes the change in topology and how exactly the transition from uniform magnetization to skyrmions takes place. To do this, they carried out scattering experiments with X-rays at the X-ray laser in Hamburg (European XFEL), in which the deflection of the X-rays by the skymions is measured. By first bombarding the magnetic layer with an optical laser and then with an X-ray laser, the physicists were able to show how the size and spacing of the skyrmions change over time.
Surprisingly, the topology change was completed after just 300 picoseconds. Thus, the generation of the skyrmions proceeded faster than observed for other ferromagnetic systems. By comparing the experimental data with theoretical simulations, the science team was also able to explain how the topological transformation comes about. The laser heats the magnetic layer to a state in which the magnetization breaks up into small, independently fluctuating areas in which the direction of magnetization changes rapidly. In this state of topological fluctuations, the energy barrier to be overcome for the generation of skyrmions is greatly reduced and skyrmions are constantly being created and disappearing. When the system cools down again after being heated by the laser, some of these skyrmion nuclei solidify and subsequently grow to the size observed in the microscopic images.
In view of the fact that skyrmions can have a size of only ten nanometers and are still stable at room temperature, these results open up interesting perspectives for future concepts of magnetically based data processing and storage. Even today, the size of the bits on a hard disk is limited by whether a magnet is able to rewrite these very small but also very permanent bits, i.e. to re-magnetize them. The technology of locally heating the bits with a laser and thus making them magnetically "soft" is already being developed in order to achieve even higher storage densities. Generating skyrmions with lasers could give this concept a new twist.