TOPOLOGICAL INSULATOR/MAGNETIC INSULATOR HETEROSTRUCTURES: SYNTHESIS AND CHARACTERIZATION

Open Access
Author:
Kally, James Cameron
Graduate Program:
Materials Science and Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
October 09, 2017
Committee Members:
  • Nitin Samarth, Dissertation Advisor
  • Nitin Samarth, Committee Chair
  • Chaoxing Liu, Committee Member
  • Suzanne E Mohney, Committee Member
  • Roman Engel-Herbert, Outside Member
Keywords:
  • Topological insulator
  • ferromagnetic insulator
  • MnAl
Abstract:
This dissertation focuses on the applications of topological insulators for spintronics. Bismuth-chalcogenide topological insulators have large spin orbit coupling leading to surface states defined by a helical Dirac cone. These surface states have potential for spintronics due to the “spin-momentum locking” of the surface electrons, and could be efficient for the generation or detection of spin currents. To study the efficiency of topological insulators for spin to charge conversion, heterostructures of topological insulator/ferromagnetic insulators were studied by ferromagnetic resonance spin pumping. To facilitate this work different topological insulators were grown by molecular beam epitaxy on the ferromagnetic insulator yttrium iron garnet. The first challenge in this work was to grow crystalline topological insulator thin films on the yttrium iron garnet substrate. This involved utilizing multiple temperature step growth via molecular beam epitaxy. While the Bi-chalcogenide topological insulators are reasonably well latticed match to InP(111)A (a=0.415 nm, <0.2% with Bi2Se3), yttrium iron garnet has a larger cubic unit cell with 1.24 nm lattice constant. Since the Bi-chalcogenide interlayer bonds are van der Waals forces, it is possible to overcome the lattice mismatch by lowering the surface energy to promote nucleation of the topological insulator film. Next, these topological insulator/yttrium iron garnet heterostructures were used for spin pumping studies of a pure spin current sourced from yttrium iron garnet to the topological insulator. In the topological insulator, this pure spin current is converted into a charge current. In this work, we explore the mechanism for the spin to charge conversion and determine it to be from the inverse Rashba-Eldelstein effect. The spin-charge conversion in the topological insulator is determined to be dominated by the surface states at the interface. While the spin pumping measurements are all performed at room temperature, magnetotransport measurements were performed on the topological insulator/ferromagnetic insulator heterostructures. At temperatures below ~2K, we observed features in the magnetoresistance that are not observed in topological insulators on non-magnetic substrates. These features have a long relaxation time and the mechanism is still being explored. We hypothesize that it originates in the relaxation of paramagnetic impurities in the yttrium iron garnet substrate, resulting in a phonon bottleneck that results in a magnetocaloric effect. Lastly, we present work on a ferromagnet metal τ-MnAl with strong out of plane magnetization. This material has a strong out of plane magnetic anisotropy and could be used for spintronics applications. We demonstrated successful growth by molecular beam epitaxy on GaAs(001)B to stabilize the meta-stable tau phase. The magnetization of the film was characterized by SQUID and magneto-transport and shown to be robust with ~1T coercivity. The long term goal is to integrate this ferromagnetic layer with a topological insulator for spin-orbit torque switching devices.