Novel Transverse Spin Hall Effect Induced Phenomena in Single Ferromagnet and Magnetic Heterostructures

University Of Delaware, Newark DE

Investigators

Abstract

PART 1: NON-TECHNICAL SUMMARY Next generation memory chips demand ultralow energy consumption, which is currently mostly wasted in 'writing' information into these chips. This project aims to develop an efficient 'writing' method by discovering novel properties in magnetic materials. It is well-known that when an electrical current passes through a magnetic thin film, a voltage develops at two sides of the magnetic film transverse to the electrical current. This so-called anomalous Hall effect is broadly used in magnetic field sensors. By asking the question of what happens at the top and bottom surfaces of the magnetic films, it was discovered that the magnetization directions, like the direction from south Pole to north pole of a refrigerator magnet, of the magnetic film are tilted out of the film plane at both surfaces. The principle behind this effect will be investigated and the effect will be used to develop a much more efficient and less energy consuming 'write' process for next generation memory chips. Research based education and outreach are also an integral part of this project. Leveraging newly established state-of-the-art Nanofabrication Facility at University of Delaware, several experiment modules will be developed in order to teach about magnetic device fabrication. This project is not only interesting in scientific and technical developments, but also will have long lasting impacts on work force training for maintaining the United States' technological edge in the global economy. PART 2: TECHNICAL SUMMARY Spin-orbit coupling (SOC) can convert a charge current into a spin current, enabling electrical control of magnetization. A quintessential example of SOC-induced transport in a ferromagnetic conductor (FM) is the anomalous Hall effect (AHE), in which an electric current perpendicular to the magnetization generates a transverse spin current and charge accumulations on the surface. Applying similar considerations to the configuration of a current parallel to the magnetization, SOC should also give rise to a transverse spin current with spins orthogonal to both the magnetization and spin current. The transverse spins precess rapidly about the magnetization direction and exert torque on the magnetization as they dephase, in analogy with the spin transfer torque. This transverse spin Hall effect (TSHE), named to distinguish from the SHE in a heavy metal, is experimentally confirmed and it leads to anomalous spin-orbit torque (ASOT) on the top and bottom surfaces of a FM. This project aims to understand (1) the mechanisms for ASOT, (2) the TSHE-induced spin orbit torque (SOT) behavior on a ferrimagnet (FiM) with perpendicular anisotropy (PMA), particularly at the angular moment compensation temperature at which the spin dynamics are governed by the antiferromagnetic coupling, and (3) the spin dynamics induced by ASOT in a single FM and SOT in a PMA FiM. This proposal is built on recent experimental breakthroughs in revealing ASOT behaviors in a single FM as well as a MOKE-based SOT characterization technique that was upgraded with time-resolved and temperature capability. 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.