Proximity band structure and spin textures on both sides of topological-insulator/ferromagnetic-metal interface and their transport probes

TL;DR

This paper combines advanced theoretical methods to analyze the band structure and spin textures at topological insulator/ferromagnetic-metal interfaces, revealing complex hybridized states and proposing a new magnetoresistance probe for spin textures.

Contribution

It introduces a detailed spectral function analysis of TI/FM interfaces using density functional theory and Green's functions, challenging simplified Dirac cone models and highlighting the spin-orbit proximity effect.

Findings

Spectral functions show Rashba ferromagnetic model describes surface states near Fermi level.

Charge transfer pushes Dirac cone remnants below Fermi level with distorted spin textures.

Proposes out-of-plane tunneling magnetoresistance as a probe for interfacial spin textures.

Abstract

The control of recently observed spintronic effects in topological-insulator/ferromagnetic-metal (TI/FM) heterostructures is thwarted by the lack of understanding of band structure and spin texture around their interfaces. Here we combine density functional theory with Green's function techniques to obtain the spectral function at any plane passing through atoms of Bi$_2$Se$_3$ and Co or Cu layers comprising the interface. In contrast to widely assumed but thinly tested Dirac cone gapped by the proximity exchange field, we find that the Rashba ferromagnetic model describes the spectral function on the surface of Bi$_2$Se$_3$ in contact with Co near the Fermi level $E_F^0$, where circular and snowflake-like constant energy contours coexist around which spin locks to momentum. The remnant of the Dirac cone is hybridized with evanescent wave functions injected by metallic layers and pushed, due to charge transfer from Co or Cu layers, few tenths of eV below $E_F^0$ for both Bi$_2$Se$_3$/Co and Bi$_2$Se$_3$/Cu interfaces while hosting distorted helical spin texture wounding around a single circle. These features explain recent observation [K. Kondou {\em et al.}, Nat. Phys. {\bf 12}, 1027 (2016)] of sensitivity of spin-to-charge conversion signal at TI/Cu interface to tuning of $E_F^0$. Interestingly, three monolayers of Co adjacent to Bi$_2$Se$_3$ host spectral functions very different from the bulk metal, as well as in-plane spin textures signifying the spin-orbit proximity effect. We predict that out-of-plane tunneling anisotropic magnetoresistance in vertical heterostructure Cu/Bi$_2$Se$_3$/Co, where current flowing perpendicular to its interfaces is modulated by rotating magnetization from parallel to orthogonal to current flow, can serve as a sensitive probe of spin texture residing at $E_F^0$.