Emergence of spin-orbit coupled ferromagnetic surface state derived from Zak phase in a nonmagnetic insulator FeSi

TL;DR

This study reveals a spin-orbit coupled ferromagnetic surface state in nonmagnetic FeSi, driven by topological properties and Zak phase, with potential for spintronic applications.

Contribution

It demonstrates the existence of a topologically derived ferromagnetic surface state in FeSi thin films with strong SOC and spin-momentum locking, a novel finding in nonmagnetic insulators.

Findings

Inherent ferromagnetic surface state in FeSi thin films.

Presence of Rashba-type spin splitting and spin textures.

Surface magnetization switching driven by spin-momentum locking.

Abstract

A chiral compound FeSi is a nonmagnetic narrow-gap insulator, exhibiting peculiar charge and spin dynamics beyond a simple band-structure picture. Those unusual features have been attracting renewed attention from topological aspects. Although a signature of surface conduction was indicated according to size-dependent resistivity in bulk crystals, its existence and topological properties remain elusive. Here we demonstrate an inherent surface ferromagnetic-metal state of FeSi thin films and its strong spin-orbit-coupling (SOC) properties through multiple characterizations of the two-dimensional (2D) conductance, magnetization and spintronic functionality. Terminated covalent-bonding orbitals constitute the polar surface state with momentum-dependent spin textures due to Rashba-type spin splitting, as corroborated by unidirectional magnetoresistance measurements and first-principles calculations. As a consequence of the spin-momentum locking, non-equilibrium spin accumulation causes magnetization switching. These surface properties are closely related to the Zak phase of the bulk band topology. Our findings propose another route to explore noble-metal-free materials for SOC-based spin manipulation.