Researchers use a multiferroic magnetoelectric material to electrically control spin currents

A research team, led by the University of California, Berkeley, recently took a step toward a spin-based computer by demonstrating a way to switch spin currents on and off electrically.

The development of devices based on pure spin currents instead of charge currents is the goal of many scientists working in spin electronics, or spintronics. A subfield of spintronics, called magnonics, focuses on devices in which these spin currents are carried specifically by magnons—wave-like disturbances of the aligned spins in a magnetic material. Magnonics researchers face a challenge in that simply exciting magnons in a material is not enough to guarantee the creation of a spin current: when the magnons are uniformly distributed, the spin current is exactly equal to zero. The magnons must be controlled, and controlling magnons in insulating materials—ones that, because of the absence of charge currents, dissipate the least amount of energy—has proven difficult. In previous experiments, researchers have sought to achieve this control using large magnetic fields, but such fields can cause collateral heating, undermining the reason for pursuing magnonics in the first place.

The recent experiment by Eric Parsonnet at the University of California, Berkeley, and his colleagues may change the game. Their result builds on previous work in the field of spintronics that showed how to establish pure spin currents via the Seebeck effect and how to detect them via the inverse spin Hall effect. In their experiment, the Seebeck effect generates a pure spin current when one side of the BiFeO3 film is heated: thermal magnons flow along the temperature gradient from warm to cold, while the spin current flows in the opposite direction. The inverse spin Hall effect reveals the presence and direction of this spin current by turning it into a detectable voltage.

But Parsonnet and his colleagues went beyond this simple creation and detection of spin currents by switching the direction of the BiFeO3 film’s ferroelectric moment P using an electric field pulse. This change in P forces the associated magnetization M to relax into another equilibrium direction perpendicular to P. The researchers show that this linked switching of both P and M connects two states with different spin-current directions, demonstrating an all-electrical way of controlling spin currents in an insulator. Crucially for spintronic applications, these states are nonvolatile in that they remain stable when no electric or magnetic fields are applied; that is, their standby power is zero.

By using BiFeO3 thin films to demonstrate that nonvolatile, all-electrical switching of pure spin currents is possible without magnetic fields, the team made multiferroic materials a strong contender in the spintronics game. Future work should see this switch converted into the spin-current equivalent of a transistor.

Posted: Aug 16,2022 by Roni Peleg