Termination-Preserved Ultrahigh Tunneling Magnetoresistance in Altermagnetic KV₂Se₂O

Altermagnets exhibit nonrelativistic spin splitting without net magnetization, offering a promising platform for next-generation spintronic devices. Although altermagnetic tunnel junctions (AMTJs) represent promising realizations, their practical applications are hindered by low tunneling magnetoresistance (TMR) ratios and strong sensitivity to interfacial configurations. Here, we systematically explore the transport properties and microscopic mechanisms of AMTJs based on the d-wave altermagnet KV₂Se₂O. Using first-principles calculations and orbital-resolved analysis, we demonstrate that the synergy between compressed nodal-point-like spin-degenerate channels and coplanar interfacial magnetic order yields an ultrahigh intrinsic TMR above 10⁵% for all interfacial terminations. More importantly, K-termination effectively preserves bulk spin polarization through its passivation characteristics, leading to an ultrahigh TMR up to 10¹²%. These results highlight the coupling between momentum-space topology and interfacial passivation as a reliable strategy for realizing giant magnetoresistive responses in altermagnetic spintronic devices.