Spin Current Phenomena in Non-Collinear Antiferromagnets:From Fundamental Physics to Device Concepts

Colorado State University, Fort Collins CO

Investigators

Abstract

Recent years have witnessed a rapidly growing interest in antiferromagnetic spintronics. Up to now, however, most of the research has focused on collinear antiferromagnetic systems. Spin current phenomena in non-collinear antiferromagnetic systems remain largely unexplored. Very recent theoretical work has uncovered new phenomena in non-collinear antiferromagnets Mn3X, where X is Sn, Ga, or Ge. These include an unusual spin Hall effect, a strong spin Nernst effect, and electric manipulation of antiferromagnetic ordering. The heart of this program is the timely experimental elucidation of these phenomena and their use in prototype devices. The studies will largely promote the understanding of spin-related phenomena in non-colinear antiferromagnets and will thereby significantly advance the research field of antiferromagnetic spintronics. From a technological perspective, the proposed research will have significant implications for the future development of magnetic memory, spin-torque nano-oscillators, and spin-wave logic and will therefore have a major impact on future advanced electronic systems. Two undergraduate students and two graduate students per year will participate in film growth, nanofabrication, electrical transport and high-frequency measurements, and data analysis. Outreach to high schools will be accomplished through the Colorado State University 'Little Shop of Physics' and the 'summer Research and Engineering Apprenticeship" Programs. The program consists of three main research thrusts. Thrust I will focus on a rather unusual spin Hall effect in Mn3Sn. A normal spin Hall effect can convert a longitudinal charge current to a transverse spin current whose polarization is orthogonal to the spin flow direction. In stark contrast, the transverse spin current produced by the unusual spin Hall effect has a polarization that is along the spin flow direction. This unusual phenomenon will be demonstrated through two distinct approaches. Further, the effect will be used to induce magnetization switching, drive magnetization precession, and excite spin waves in ferromagnetic thin films with perpendicular anisotropy. None of these effects can be achieved, in principle, with spin currents produced by the normal spin Hall effect, the Rashba effect, or topological surface states. Thrust II will focus on the spin Nernst effect, a process in which one uses a thermal gradient to produce pure spin currents. Recent experiments have revealed this effect in Pt. Very recent theoretical work indicates that the effect can also occur in Mn3X and can be significantly stronger than in Pt. Thrust II will elucidate these predicted strong spin Nernst effect for Mn3X films. It is also planned to use this effect for spin-orbit torque-induced magnetization switching in ferromagnetic films with perpendicular anisotropy. Thrust III will focus on the control of antiferromagnetic ordering in Mn3X. Work is planned to manipulate antiferromagnetic states using damping-like and field-like spin-orbit torques and move domain walls through spin transfer torque. While theory suggests that such ordering control is feasible, this has not yet been demonstrated experimentally. 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.