Collaborative Research: Spin Currents and Spin-orbit Torques in Single Layer Magnetic Systems

Non-technical Abstract:

Ferromagnetic materials are widely used in digital information storage devices such as hard disk drives, which boast long-term memory, high storage density, and cost-effectiveness. Compared to other semiconductor-based memory devices, the operation speed of hard disk drives is relatively slow, prompting the development of magnetic memories without moving parts.

To make such memory technologies competitive requires efficient control of magnetism using electrical currents rather than magnetic fields. In recent years, a novel mechanism called spin-orbit torque has been shown to electrically control magnetism in both ferromagnetic and antiferromagnetic materials—the latter of which are far more prevalent in nature but currently underutilized, despite offering several advantages over their ferromagnetic cousins like potentially higher speed and density.

However, the spin-orbit torques generated in multilayer magnetic systems arise from several competing mechanisms that are difficult to disentangle, widening the gap between experiment and theory and preventing device optimization. This collaborative project aims to determine the microscopic origins and behavior of spin-orbit torques generated within single ferromagnetic and antiferromagnetic layers using both theoretical and experimental methods.

Success in this research will help optimize spin-orbit torques, thus expediting the development of faster magnetic memory devices used for traditional information storage and for artificial intelligence applications. This project trains graduate and undergraduate students in a variety of research techniques and prepares them for the workforce in science and technology.

Planned outreach activities include establishing an online seminar series inviting researchers from underrepresented groups in physics and engineering to give talks combining their frontier research with their personal experience in pursuit of their scientific career. Technical Abstract:

Achieving efficient electrical control of magnetic order is crucial for the development of memory technology. Spin-orbit torque—which is a transfer of angular momentum from the atomic lattice of crystals to magnetic order under an applied electric field—promises faster, more reliable, and more energy-efficient switching than previous write mechanisms in magnetic memories.

While control of magnetism using spin-orbit torque has been demonstrated in various devices, the role of the ferromagnetic and antiferromagnetic layers in generating spin-orbit torques remains unclear, creating inconsistencies between experiment and theory and preventing device optimization. This collaborative project experimentally and theoretically characterizes boundary spin-orbit torques generated within single-layer magnetic materials, eliminating competing mechanisms from adjacent layers.

The single-layer magnetic systems include ferromagnets, non-collinear and collinear antiferromagnets. The boundary spin-orbit torques are measured using the magneto-optic-Kerr-effect and the results are theoretically interpreted using both semiclassical models and the first-principles transport calculations. This collaborative research aims to disentangle the spin torque contributions that must also occur in multilayers while paving the way for single layer magnetic memories.

It also undertakes the first characterization of spin torques in single antiferromagnetic layers with non-collinear and collinear magnetic order, broadening the role of antiferromagnets in spin-orbit coupled nanostructures. This project trains graduate and undergraduate students in a variety of research techniques such as thin film growth, micro-fabrication, optical detection, first-principles calculations, and semiclassical modeling, preparing them for the workforce in science and technology.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.