Topological Surface States-Caused Large Damping Enhancement in Dirac Semimetal-Ferromagnetic Metal Layered Structures

Abstract

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 a damping enhancement in the ferromagnetic layer and a spin current in the non-magnetic layer. This effect can be strong if the non-magnetic layer is a heavy metal thin film, because such a film can work as a spin sink and produce a very weak spin backflow. The effect can be even stronger if the non-magnetic component is a topological insulator, because it can efficiently convert a spin current to a charge current and thereby serve as a very efficient spin absorber, due to the spin-momentum locking of the topological surface states (TSS). In principle, TSS should also be present in other topological materials, such as Dirac or Weyl semimetals. These TSS are also expected to exhibit spin-momentum locking, but their interplays with magnetism are largely unexplored. This presentation reports strong damping enhancement in a ferromagnetic NiFe thin film due to a neighboring α-Sn thin film. Multiple evidences suggest that the damping enhancement is associated with TSS in the topological Dirac semimetal (TDS) phase of α-Sn. The TDS α-Sn films are realized on InSb substrates. The measurements used α-Sn(6nm)/Ag(2nm)/NiFe(20nm) layered structures where Ag works as a spacer to physically separate α-Sn and NiFe. Ferromagnetic resonance studies show that the damping in these structures is ~4.8 bigger than in the bare NiFe film. Control measurements indicate that this damping enhancement is mostly due to the TSS in the α-Sn, rather than the bulk of the α-Sn film or the Ag spacer; it is absent in β-Sn(6nm)/Ag(2nm)/NiFe(20nm) where the Sn film is topologically trivial. This work demonstrates that TDS may be as promising as topological insulators in terms of spintronics applications.