Untapping the potential of antiferromagnets using light

24 June 2020

Researchers from the University of Oxford’s Department of Physics and the Max Planck Institute for the Structure and Dynamics of Matter (MPSD) have managed to drive a prototypical antiferromagnet into a new magnetic state using terahertz frequency light. Their groundbreaking method produced an effect orders of magnitude larger than previously achieved, and on ultrafast time scales. The team’s work has just been published in Nature Physics.

Controlling the magnetic state

Magnetic materials have been a mainstay in computing technology due to their ability to permanently store information in their magnetic state. Current technologies are based on ferromagnets, whose states can be flipped readily by magnetic fields. Faster, denser, and more robust next-generation devices would be made possible by using a different class of materials, known as antiferromagnets. Their magnetic state, however, is notoriously difficult to control.

In their new paper, the MPSD/Oxford research team attempted to modify the magnetic state of an antiferromagnet via its crystal structure. Instead of applying pressure to the crystal– which is slow and can break the crystal – the team pioneered a novel approach and obtained the same effect using light to excite sound waves, without touching or straining the crystal. The work was based on an experimental idea proposed by co-author Paolo Radaelli from Oxford University while visiting the MPSD in 2018, which is here realised for the first time.

A major step forward

This innovative technique allowed the researchers to create a magnetisation 400 times larger than previously achieved by conventional means. Strikingly, it only took around 100 ps for the magnetisation to develop and the direction of the magnetisation could be reversed by changing the light’s polarisation. The results represent a major advance in the optical control of materials’ properties, and may be employed in future information technologies.

Lead author Ankit Disa from MPSD confirms: ‘This experiment was the first demonstration of “rationally” or “intentionally” engineering a crystal structure with light. We knew what type of structural distortion was needed in order to create a phase transition from an antiferromagnet to a ferromagnet-like state. The trick was to understand how to use light to drive the material into this new crystal structure.’

Paolo Radaelli, who had the original idea for the experiment, said 'It is a rare privilege for a scientist to predict a previously unknown effect and to collaborate to achieve its experimental realisation. Far from being a mere curiosity, this technique highlights the potential of using light to control the properties of materials. For example, opto-magnetic switches based on this effect could be incorporated in memories that will be read and written by light.'

Polarizing an antiferromagnet by optical engineering of the crystal field by A Disa et al, Nature Physics, 22 June 2020

Image © Jörg Harms/MPSD