Spin-Axis-Layer Locking for Intrinsic Bipolar Altermagnetic Semiconductors: Proof-of-Concept in Bilayer CuBr2

Abstract

Electrical control of spin and magnetic sublattice degrees of freedom is essential for multifunctional and low-power spintronic devices. Bipolar altermagnetic semiconductors (BAMSs)-characterized by opposite spin polarizations at the valence and conduction band edges-offer such control, yet known systems require external strain and sizable valley polarization for gate-tunable switching. Here, we propose a universal spin-axis-layer locking (SALL) paradigm to overcome these limitations. By stacking two quasi-1D ferromagnetic monolayers with a 90 degrees twist, the bilayer reconstructs altermagnetic symmetry, yielding an intrinsic BAMS state where carrier spin is locked to specific layers and transport directions. Using synthesized CuBr2 monolayers as proof-of-concept, we demonstrate via first-principles calculations a robust BAMS state. Electrostatic gating enables simultaneous, reversible switching of carrier type, spin, and active layer, generating fully spin-polarized axial charge currents and directionally controllable pure spin currents with near-unity charge-to-spin conversion efficiency. This SALL model establishes a versatile, strain-independent strategy for advanced all-electrical altermagnetic devices.