Ultrahigh charge-to-spin conversion and tunneling magnetoresistance in quasi-two-dimensional <i>d</i> -wave altermagnet

The emergence of altermagnets has driven groundbreaking advances in spintronics. Notably, d-wave altermagnets support non-relativistic spin transport, efficient charge-to-spin conversion, and T-odd spin currents. In addition, their integration as electrodes in antiferromagnetic tunnel junctions (AFMTJs) enables a tunneling magnetoresistance (TMR) effect, allowing electrical detection of Néel vectors for next-generation memory devices. In this work, we investigate the non-relativistic spin transport properties of the quasi-two-dimensional (quasi-2D) d-wave altermagnet KV2Se2O and the TMR effect in KV2Se2O-based AFMTJs via first-principles calculations. Our results reveal that KV2Se2O exhibits both non-relativistic longitudinal spin polarization and a spin Hall angle exceeding 60% at room temperature, while KV2Se2O-based AFMTJs achieve a giant TMR ratio of 8.20 × 1013%, which remains robust against Fermi-level shifts. These findings highlight the anisotropic spin polarization inherent to d-wave staggered magnetism and underscore the critical role of Fermi surface topology in enhancing T-odd spin transport and the TMR effect in AFMTJs.