Spin-valley-mismatched altermagnet for giant tunneling magnetoresistance

Altermagnet-based heterojunctions have demonstrated magnetoresistive effects in experiments, however, a predictive theoretical model for non-ferromagnetic structures has remained elusive. In this work, we develop a tunneling-based spin-transport theory that explicitly incorporates the transverse-wavevector (k_∥)-dependent spin polarization of an altermagnet’s transport channels, enabling the prediction of giant tunneling magnetoresistance (TMR). Based on the theory, we predict that the altermagnet KV₂Se₂O can reach the extreme limit of magnetoresistance. By performing first-principles transport calculations, we verify that magnetic tunnel junctions using the metallic KV₂Se₂O as the electrodes and few-layer MgO as the spacer exhibit zero-bias magnetoresistance larger than 7.57 × 10⁷%, which is robust against the bias and thickness of the spacer. Our research provides a quantitative design principle for next-generation spin-electronic devices and establishes KV₂Se₂O/MgO/KV₂Se₂O as a leading candidate material system for room-temperature ultra-high-density non-volatile memory.