|Institution:||University of Cambridge|
|Department:||Department of Physics|
|Full text PDF:||http://www.repository.cam.ac.uk/handle/1810/247392
This thesis describes the control of magnetisation dynamics using electricity. Direct electrical control of magnetisation is desirable for the development of efficient scalable magnetic memories. In materials with broken inversion symmetry and spin-orbit coupling, electrical current can exert a torque on a local magnetisation. In the studies presented here, a microwave current-induced ferromagnetic resonance (spin-orbit FMR) technique is used to characterise the dynamical magnetic properties and to determine the symmetries of the spin-orbit torques of ferromagnetic layers with broken inversion symmetry. Ultra-thin ferromagnetic/heavy metal bilayers have recently become an important area of study in spintronics. Magnetic torques originating from the spin-Hall effect and a Rashba spin-orbit field have both been reported in these materials. These spin-orbit torques may allow commercialisation of magnetic random access memories with higher efficiency than previous technologies. However, the exact origin of the torques is still not well understood. In the first study of this thesis, dynamic pumping of spin current induced by an external waveguide is used to investigate the dependence of the inverse spin-Hall effect in Co/Pt on the magnet thickness. An enhancement of the inverse-spin Hall effect is seen in devices with the thinnest cobalt layers which can not be explained by a conventional understanding of the spin-Hall effect. In the second study, spin-orbit FMR is used to identify the symmetries of the current-induced torques in the same Co/Pt layers. Anti-damping and field-like torque symmetries are identified, consistent in thicker cobalt layers with origins from the spin-Hall effect and an Oersted field respectively. In thinner cobalt layers, an additional field-like torque opposing the Oersted torque appears, consistent with a Rashba origin. (Ga,Mn)As is a dilute magnetic semiconductor with a record highest Curie temperature of around 180 K. At low temperatures, large spin-orbit torques with a Dresselhaus symmetry are known to exist in the material. In the final study of the thesis, spin-orbit FMR measurements demonstrate that the broken symmetry of unannealed Ga(0.03)Mn(0.97)As can generate torques with the same Dresselhaus symmetry in an adjacent iron layer at room temperature. This enables the spin-orbit torque to be distinguished from the torque due to the spin-Hall effect by symmetry.