A step on the road to spintronics

A Japanese team might have found a route to low-power spintronics, and even analogue spintronics, by lifting the lid on anti-ferromagnetic materials and studying spin inside.

Anti-ferromagnet Tohoku University

It is investigating spin-orbit torque induced switching in an anti-ferromagnet-ferromagnet bilayer system.

“Until now, the motion of electron spin in anti-ferromagnetic materials has not yet been studied well, said Tohoku University.

The research group of Professor Hideo Ohno and Shunsuke Fukami fabricated switching devices from a stack with an anti-ferromagnetic platinum-manganese and a ferromagnetic chromium-nickel multilayer, and electrically evaluated the switching properties at room temperature.

They found that the current flowing in the anti-ferromagnet generates a spin-orbit torque large enough to induce the magnetisation switching in the adjacent ferromagnet.

“It sheds light on a new physics of anti-ferromagnet and also open various pathways toward ultra-low-power integrated circuits that can store information via the magnetisation direction under no power supply, and other novel applications such as neuromorphic computing,” said the University.

To realise low-power spintronic ICs, a key issue is achieving fast and reliable magnetisation switching simply, and without using much energy.

Switching using the flow of electron spin (spin current, from spin-orbit interaction) has attracted attention as a way of doing this, typically in heterostructures consisting of ferromagnet and non-magnetic heavy metal layer. This is spin-orbit torque induced magnetisation switching.

“It is notable that whereas the spin-orbit torque switching in non-magnet-ferromagnet bilayer systems studied previously requires in-plane external field, the present [Tohoku] system allows field-free switching owing to a unique property arising at the anti-ferromagnet-ferromagnet interface,” said the University. “Furthermore, they found that in specific stack structures, the reversed portion of magnetisation can be controlled in an analogue manner by the magnitude of the applied current, and this feature can also be attributed to the nature of the anti-ferromagnet.”

For physics, results allow for deeper understanding of antiferromagnet and spin transport phenomena, such as the topological Hall effect.

For applications, external-field-free switching shows promise for implementing low-power spin-orbit torque-based devices, and the analogue-like behaviour resembles the way synapses operate in the brain, said Tohoku – hence the potential link to neuromorphic computing.

Hall resistance represents the perpendicular component of magnetization. The reversed component of magnetisation depends on the magnitude of applied current.

Image: Hall resistance versus applied current measured at zero magnetic field.

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