Scientists at Cornell University have demonstrated a new mechanism that can “split” electron spins in magnetic material. When the right material is held at the right angle, a strategy to switch the magnetization in thin layers of a ferromagnet was shown, an approach that could lead to more energy-efficient magnetic memory devices.
Much research has gone into attempting to alter the orientation of electron spins in magnetic materials through manipulation by magnetic field. However, as described in Nature Electronics [Bose et al. Nat. Electron. (2022) DOI: 10.1038/s41928-022-00744-8], here spin currents carried by electrons were used, which exist when electrons have spins generally oriented in one direction. On interacting with a thin magnetic layer, these spin currents transfer their angular momentum and produce enough torque to switch the magnetization by 180 degrees.
The team were investigating methods of controlling the direction of the spin in spin currents by generating them with antiferromagnetic materials. To test the method, two different conducting oxides with the rutile structure, IrO2 and RuO2, were examined since they are predicted to have strong spin-momentum coupling that might lead to efficient current-induced torques when incorporated into magnetic devices.
On measuring the differences in the behavior of the materials, it was realized this was due to RuO2 being an antiferromagnet while IrO2 is not. In measuring how its spin currents tilted the magnetization in a thin layer of a ferromagnet, its effects were measured at different magnetic field angles, allowing them to identify “momentum-dependent spin splitting”, a mechanism unique to ruthenium oxide and other antiferromagnets in the same class.
When the crystal structure in the antiferromagnet is oriented appropriately, the mechanism allows the spin current to be tilted at an angle that can enable better magnetic switching than other spin-orbit interactions. As senior author Dan Ralph said, “Essentially, the antiferromagnetic order can lower the symmetries of the samples enough to allow unconventional orientations of spin current to exist. The mechanism of antiferromagnets seems to give a way of actually getting fairly strong spin currents, too.”
The mechanism of the current-induced torque generated by RuO2 is also likely to result from exchange interactions and crystal fields rather than a mechanism based on spin-orbit coupling. As senior author Dan Ralph told Materials Today, “Because exchange interactions and crystal fields are generally stronger than the spin-orbit interactions, this may provide a strategy to develop stronger torques.”
The torques measured are not sufficiently strong to be immediately useful, but the team hope to assess if they can develop processes to produce RuO2 or related antiferromagnets with large enough domains to create single-domain devices. This could lead to very efficient, very dense and non-volatile magnetic memory devices that improve upon silicon memory devices.
“Because exchange interactions and crystal fields are generally stronger than the spin-orbit interactions, this may provide a strategy to develop stronger torques.”Dan Ralph