Damping and switching in thin films and hetero-structures of magnetic materials and topological materials
Date
2020
Authors
Ding, Jinjun, author
Wu, Mingzhong, advisor
Camley, Robert, committee member
Field, Stuart, committee member
Roberts, Jacob, committee member
Shores, Matthew, committee member
Journal Title
Journal ISSN
Volume Title
Abstract
Yttrium iron garnet (Y3Fe5O12, YIG) materials have been widely used in microwave devices and have also shown high potential for magnonics and spintronics applications. This is because the fact that YIG materials have very low intrinsic damping and is electric insulating. The development of YIG-based spintronics demands YIG films that have a thickness in the nanometer (nm) range and at the same time exhibit low damping comparable to single-crystal YIG bulk materials. In this dissertation, the demonstration of using magnetron sputtering to grow high-quality polycrystalline nm-thick YIG films on gadolinium gallium garnet (Gd3Ga5O12, GGG) substrates is discussed in detail, which is of great technological significance as well as scientific research. The damping constant of the YIG films is the lowest among all the previous reports of nm-thick YIG films grown. Such demonstration of high-quality nm-thick YIG films proves the possibility of nanoscale patterning of YIG films and the future development of YIG-based nanoscale devices. Further, YIG thin films having a thickness of several nanometers and showing both strong perpendicular magnetic anisotropy (PMA) and low magnetic damping are realized in this dissertation. The phenomenon of spin pumping refers to the transfer of spins from precessional moments in a ferromagnet to a non-magnetic material. In a ferromagnetic/non-magnetic bi-layered system, spin pumping manifests itself as two distinct effects: (1) an enhancement in the damping in the ferromagnetic layer and (2) a pure spin current in the non-magnetic layer. This dissertation studies spin pumping effects in a ferromagnetic NiFe thin film associated with topological surface states (TSS) in a neighboring topological Dirac semimetal α-Sn thin film. Large damping enhancement due to the TSS of the Dirac semimetal alpha-Sn thin film is observed. Moreover, the spin current generated in the alpha-Sn film was utilized to switch a magnet through spin-orbit torque (SOT). The switching efficiency is comparable to that in topological insulators, which paves the way for the application of alpha-Sn thin films in future SOT-based magnetic memory. When a topological insulator (TI) is interfaced with a magnetic insulator (MI), it may host the anomalous Hall effect (AHE) and the quantum AHE associated with Berry-phase curvature in momentum space. This dissertation reports a bona fide topological Hall effect (THE) in a single magnetic phase TI/MI heterostructure (Bi2Se3/BaFe12O19) where the electrical transport is exclusively confined to the TI layer. Experimental observations are consistent with a THE originating from skyrmions in BaFe12O19 that are formed due to interfacial Dzyaloshinskii–Moriya interaction.
Description
Rights Access
Subject
topological Dirac semimetal (TDS)
topological insulator (TI)
magnetic insulator (MI)
heterostructure
yttrium iron garnet (YIG)
topological Hall effect (THE)
alpha-Sn thin film
topological surface states (TSS)