Spin-Orbit Torques in Various Multilayers

Spintronics has been a field of great interest due to its promising future for applications, especially for memory devices. In spintronics, the efficient manipulation of magnetic objects, i.e. domain wall motion and magnetization switching, has been an important task and widely studied for the past few decades. In this context, spin transfer torque had been considered as the most promising means to control the spin states. However, it has been also faced practical issues, particularly, concerning its efficiency. Most recently, a new concept of current induced magnetization manipulation, so-called spin-orbit torque (SOT), has been experimentally and theoretically demonstrated. This phenomena is attributed to the electron scattering by spin-orbit coupling and its efficiency is known to be advantageous over the conventional spin transfer torque. Up to now, the spin-orbit torques have been mostly studied on a multilayer system consisting of a heavy metal/ferromagnet/oxide. However, very recently, the spin-orbit torques have been also demonstrated in ferri- and antiferro-magnets contacted with various materials with a large spin-orbit coupling.

In this project, we are aiming at investigating physical origin of the spin-orbit torques in various multilayers and developing external means to manipulate them. From the scientific point of view, the spin-orbit torques are known to be attributed to two distinct effects; spin Hall effect and inverse spin galvanic effect (or Rashba effect). However, due to the difficulty in disentangling these two effects experimentally, it is still not so clear which effect plays a more dominant role. For this purpose, we are investigating the spin-orbit torques under various conditions such as at different temperatures, strained, and gated situation by using our mains skills of the second harmonic measurement, the depinning measurement, and the spin-torque ferromagnetic resonance measurement (ST-FMR). On the basis of our understanding on the effect, ultimately, we will develop the method to enhance a spin-to charge conversion efficiency and manipulate the spin-orbit torques, which in turn provide new functionality for manipulating spin states in magnetic materials.

Figure 1: (a) schematics of spin Hall effect. A longitudinal electron current, Jc, in heavy metal (HM) is converted into a transverse spin current by spin-orbit scattering. The spin current leads to a spin accumulation at the HM/FM interface that diffuses across the interface into the FM and exerts torques. (b) The effective field generated by the inverse spin galvanic effect at the interface, which can lead to a switching of the magnetization.


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