UCLA

Spin torque can toggle magnetization state of nanomagnets in magnetic tunnel junctions. In heavy metals, giant Spin Hall Effect gives rise to a large spin-orbit torque (SOT) enabling magnetization switching of adjacent ferromagnets. Spin torque ratio is the key parameter in determining energy efficiency. Recently, a giant SOT was demonstrated in topological insulators (TIs), which are a class of materials owing insulating bulk and metallic surface states. Although several theoretical studies show that spin-momentum locking of surface states generates this giant SOT, experimental clarification is still lacking. In this thesis, we use the second-harmonic anomalous Hall resistance to access the current-induced spin-orbit fields (and thus SOTs). Heavy metal/ferromagnet hetoerstructure, Ta/CoFeB/MgO, is used to verify the validation of this method. Then, we quantitatively estimate the SOT in different TI/chromium-doped TI heterostructures with different magnetic dopant positions, which shows the evidence of surface states originated SOTs. At last, we show that the magnitude of SOTs can be controlled by the gate voltage due to its semiconducting nature. All these studies may contribute to the future low power spintronics.