Controlling magnetic vortices in an antiferromagnet

A study published in Nature Nanotechnology has shown how electrical creation and control of magnetic vortices in an antiferromagnet can be achieved, a discovery that could increase the data storage capacity and speed of next-generation devices. Researchers from the University of Nottingham’s School of Physics and Astronomy have used magnetic imaging techniques to map the structure of newly formed magnetic vortices and demonstrate their back-and-forth movement due to alternating electrical pulses.

Oliver Amin, the lead author of the study, said that although these magnetic vortices have been proposed as information carriers in next-generation memory devices, evidence of their existence in antiferromagnets has been scarce. “Now, we have not only generated them, but also moved them in a controllable way. It’s another success for our material, CuMnAs, which has been at the centre of several breakthroughs in antiferromagnetic spintronics over the last few years,” Amin said.

CuMnAs has a specific crystal structure, grown in an almost complete vacuum, atomic layer by atomic layer. It has been shown to behave like a switch when pulsed with electrical currents, and the research group in Nottingham, led by Dr Peter Wadley, have ‘zoomed in’ on the magnetic textures being controlled; first with the demonstration of moving domain walls and now with the generation and control of magnetic vortices.

Key to this research is a magnetic imaging technique called photoemission electron microscopy, which was carried out at the UK’s synchrotron facility, Diamond Light Source. The synchrotron produces a collimated beam of polarised x-rays, which is shone onto the sample to probe the magnetic state. This allows for the spatial resolution of micromagnetic textures as small as 20 nanometres in size.

Magnetic materials have been technologically important for centuries, from the compass to hard disks. However, almost all of these materials have belonged to ferromagnetism, producing an external magnetic field that can be ‘felt’ because all of the tiny atomic magnetic moments that constitute them like to align in the same direction. It is this field that causes fridge magnets to stick and that is sometimes mapped out with iron fillings. Because they lack an external magnetic field, antiferromagnets are hard to detect and hard to control. For this reason, they have found almost no applications.

Antiferromagnets produce no external magnetic field because all of the neighbouring constituent tiny atomic moments point in exactly opposite directions from each other. In doing so they cancel each other out and no external magnetic field is produced: they won’t stick to fridges or deflect a compass needle. However, antiferromagnets are magnetically more robust and movement of their tiny atomic moments happens approximately 1000 times faster than a ferromagnet. This could create computer memory which operates faster than current memory technology.

“Antiferromagnets have the potential to out-compete other forms of memory which would lead to a redesign of computing architecture, huge speed increases and energy savings. The additional computing power could have large societal impact. These findings are really exciting as they bring us closer to realising the potential of antiferromagnet materials to transform the digital landscape,” Wadley said.

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