Layer-mediated tuning of spin and valley physics in stacked tetragonal altermagnetic bilayers

TL;DR

This study demonstrates how stacking and external electric fields can tune spin and valley properties in altermagnetic bilayers, revealing potential for advanced spintronic and valleytronic applications.

Contribution

It introduces a symmetry-based framework for controlling spin and valley degrees of freedom in altermagnetic bilayers through stacking and electric fields.

Findings

Symmetry constraints govern spin degeneracy in AM bilayers.

Interlayer sliding induces valley splitting and ferrimagnetic transition.

Enhanced tunneling magnetoresistance achieved via valley splitting.

Abstract

As an emerging magnetic phase, altermagnets (AMs) with collinear compensated magnetism in real space and alternating spin splitting in the band structure have attracted widespread attention. Here, based on first-principles calculations, we demonstrate that the layer stacking imposes symmetry constraints on the spin and valley degrees of freedom (DOFs) in an AM bilayer composed of two tetragonal altermagnetic monolayers, thereby enabling the tuning of these DOFs through interlayer sliding as well as by an external electric field. Using several representative AM bilayers, we reveal that the [C2||P] and [C2||Mz] symmetries intrinsically enforce spin degeneracy, while the coupling between spin and layer DOFs establishes a general framework for achieving electric field control of spin states. Appropriate interlayer sliding breaks the [C2||Md] symmetry of AM bilayers, thereby giving rise to a spontaneous valley splitting and driving a transition to a fully compensated ferrimagnetic state. Furthermore, owing to the tunable valley splitting induced by interlayer sliding, enhanced tunneling magnetoresistance (TMR) can be realized by AM bilayers. This work highlights the intrinsic correlation among spin, valley, and layer DOFs, offering symmetry-based design principles for layer-based spintronic and valleytronic devices.