A fundamental study on the movement of skyrmions, very small and very stable magnetic objects that can occur in certain thin crystals, may help to build smaller and more energy efficient electronic devices, such as memory storage. University of Groningen theoretical physicist Maxim Mostovoy was part of the team that solved the riddle of the spinning skyrmion. The results were published on Sunday 26 January in Nature Materials.
The paper tells you how to move skyrmions around using magnon radiation. If you understand both these terms, you are the exception. So before Mostovoy can explain his work and his passions, these two items need to be explained. Hold on to your seats, here we go!
A skyrmion is a complex magnetic configuration, which roughly resembles a vortex. It can be envisaged as a round object where the magnetic moment on the outer rim is in the opposite direction to the magnetic moment in the centre. Between rim and centre there are concentric rings in which magnetization is rotating from up to down.
The magnon current is also an issue in this article. Basically, this is a spin wave, which may not clarify things very much. But the spin wave is in a sense similar to the ‘wave’ seen in sports stadiums. Spin is a quantum-mechanical phenomenon of particles that defines their angular momentum and has two states: up or down. This spin can be flipped. Each particle in the material responds to such a flip by their neighbour by mimicking it, just as a spectator in a stadium will rise during a wave when his neighbour rises. Next, the spins flip back, like the spectators who sit down once the wave has passed.
So where’s the fun in all this? Well, for one thing, the presence (or absence) of a skyrmion can be detected. As they are very stable, skyrmions can be used to store information if you can move them to any required position. Read the presence of a skyrmion as 1 and the absence as 0, and you have binary storage!
‘Skyrmions are very stable structures which can measure from a few nanometres up to a micrometre in diameter’, explains Mostovoy. ‘And scientists have been looking for ways to use them in information storage and other electronic devices.’
Several years ago, a team of Japanese scientists investigated skyrmions using an electron beam. They noticed the structures began to rotate through the material in which they were embedded. The movement was unidirectional, always clockwise. Mostovoy got involved as a theoretician to explain this movement.
It was ruled out that the electrons caused the movement, as it also occurred in insulating materials. This suggested a thermal effect, as the electron beam heats the material and creates a temperature gradient between the centre spot and the periphery. ‘And a more intense beam made the skyrmions move faster.’
Different theoretical models were proposed to explain the movement, and tested in computer simulations. It took Mostovoy and his colleagues several years to solve the riddle. What they concluded is that the movement is driven by the heat as it moves from the centre outwards in the form of a magnon current (which is, as has just been explained, a ‘spin wave’).
‘When a spin wave reaches a skyrmion, it bounces off’, explains Mostovoy. ‘And it always bounces off in one direction. The impact will send the skyrmion off in the opposite direction.’ What is special about this collision is that it is a quantum-mechanical phenomenon (the spin wave) which sets a macroscopic 'real world’ object, the skyrmion, in motion.
‘As a result of this study, we now know how we can move skyrmions’, says Mostovoy. ‘This was possible before, but only by using a current. Our method, using a spin wave, is more energy efficient.’ This is good news for scientists working on ‘spintronics’, electronic devices that depend on the spin of particles, rather than the charge of electrons (as in ordinary electronic devices).
Mostovoy is not involved in making such devices. ‘My drive is to understand the properties of matter on a fundamental level. Everything we see is made of matter. But how does matter self-organize? There are endless possibilities, and I want to understand how matter works.’ However, Mostovoy is part of the Zernike Institute of Advanced Materials at the University of Groningen, where many of his colleagues are trying to put this sort of fundamental knowledge to use.
The current publication is the third Nature Materials paper to which Mostovoy has contributed in the last 4 months. In 2012, Mostovoy was ranked as one of the ten most influential Dutch scientists, based on citations, by the Centre for Science and Technology Studies (Leiden University).
Reference: Thermally driven ratchet motion of a skyrmion microcrystal and topological magnon Hall effect, Nature Materials AOP 26 januari 2014
See also this article on M ostovoy: New technique helps quest for energy efficient memory
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