Electrons not only have charges but also have spin. By utilizing the electron spin, the energy consumption of electronic devices can be reduced, their size can be scaled down and the efficiency of `read' and `write' in memory devices can be significantly improved. Hence, the manipulation of electron spin in electronic devices becomes more and more appealing for the advancement of microelectronics. In spinbased devices, the manipulation of ferromagnetic order parameter using electrical currents is a very useful means for currentdriven operation. Nowadays, most of magnetic memory devices are based on the socalled spin transfer torque, which stems from the spin angular momentum transfer between a spinpolarized current and the magnetic order parameter. Recently, a novel spin torque effect, exploiting spinorbit coupling in noncentrosymmetric magnets, has attracted a massive amount of attention. This thesis addresses the nature of spinorbit coupled transport and torques in noncentrosymmetric magnetic semiconductors.
We start with the theoretical study of spin orbit torque in three dimensional ferromagnetic GaMnAs. Using the Kubo formula, we calculate both the currentdriven fieldlike torque and antidampinglike torque. We compare the numerical results with the analytical expressions in the model case of a magnetic Rashba twodimensional electron gas. Parametric dependencies of the different torque components and similarities to the analytical results of the Rashba twodimensional electron gas in the weak disorder limit are described. Subsequently we study spinorbit torques in two dimensional hexagonal crystals such as graphene, silicene, germanene and stanene. In the presence of staggered potential and exchange field, the valley degeneracy can be lifted and we obtain a valleydependent Berry curvature, leading to a tunable antidamping torque by controlling the valley degree of freedom.
This thesis then addresses the influence of the quantum spin Hall effect on spin orbit torque in nanoribbons with a hexagonal lattice. We find a dramatic modification of the nature of the torque (field like and dampinglike component) when crossing the topological phase transition. The relative agnitude of the two torque components can be significantly modifies by changing the magnetization direction.
Finally, motivated by recent experimental results, we conclude by investigating the features of spinorbit torque in magnetic transition metal dichalcogenides. We find the torque is associated with the valley polarization. By changing the magnetization direction, the torque can be changed from a finite value to zero when the valley polarization decreases from a finite value to zero.
Date of Award  Jun 21 2016 

Original language  English (US) 

Awarding Institution   Physical Sciences and Engineering


Supervisor  Aurelien Manchon (Supervisor) 

 Spin Orbit Torque
 Spin Orbit Coupling
 Magnetization