UC San Diego

The rise of neuromorphic computing and iteration of new memory call for energy-efficient magnetization manipulation by spin and a better understanding of fundamental magnetotransport properties. Over the past decades, most works on the spin source focused on polycrystalline and amorphous materials. In the first part of this dissertation, we investigate the spin Hall effect in epitaxial Pt (200), (220), and (111). A tunable spin current generation can be achieved in Pt (220). The charge-to-spin conversion efficiency can be significantly enhanced along specific crystallographic orientation by utilizing the anisotropic resistivity Our work highlights the future work on enhanced SHE arising from crystalline orientation dependence which will boost the energy-efficient spintronic devices.

On the other hand, tunable charge-to-spin conversion can be achieved by utilizing the materials with magnetic phase transition, which motivated us to study the 3d transition metal-Pt3 alloy and the rare earth material (Ho). Prior to further spintronics study, the fundamental magnetotransport properties in those materials need to be investigated. We successfully fabricate the chemically ordered epitaxial CrPt3 and FePt3. The large anomalous Hall effect measured on CrPt3 is in good agreement with the theoretical prediction. Moreover, by probing the exchange bias via electrical transport, it helps us quantitatively investigate the stability of antiferromagnetic nature in FePt3 under external perturbations.

Furthermore, there are still intriguing properties remaining in "old" materials which lead to the magnetotransport study on hcp Ho and hcp Co. Our discovery of six-fold angular dependent magnetoresistance and step-like planar Hall effect in Ho gives rise to the strong magnetostriction coupled magnetoresistance. Remarkably, although predicted years ago, the strong c-axis dependent anomalous Hall conductivity in hcp Co lacks experimental support, which is demonstrated in this dissertation. The magnetotransport study on hcp Co offers a comprehensive picture of anisotropic anomalous Hall effect and magnetoresistance in ferromagnets with in-plane uniaxial magnetic anisotropy.