Altermagnetic Flatband‐Driven Fermi Surface Geometry for Giant Tunneling Magnetoresistance

ABSTRACT Altermagnetism, characterized by zero net magnetization and symmetry‐protected spin–split band structures, has recently emerged as a promising platform for spintronics. In altermagnetic tunnel junctions (AMTJs), the suppression of tunneling in the antiparallel configuration relies on the mismatch between spin‐polarized conduction channels in momentum space. However, ideal nonoverlapping spin‐polarized Fermi surfaces are rarely found in bulk altermagnets. Motivated by the critical influence of Fermi surface geometry on tunneling magnetoresistance (TMR), we investigate three experimentally synthesized altermagnets – bulk , , and – to elucidate how flatband‐driven Fermi surfaces minimize spin‐channel overlap and boost AMTJ performance. Notably, and host flat altermagnetic Fermi sheets, which confine spin degeneracy to minimal arc‐like or nodal‐like regions. Such Fermi surface geometry drastically reduces spin overlap, resulting in an unprecedented intrinsic TMR well over in the ‐based AMTJ. Incorporating an insulating barrier further enhances the TMR to , surpassing most conventional MTJs. These results not only establish as a compelling candidate AMTJ material, but also highlight the critical role of flatband Fermi surface geometry in achieving high‐performance altermagnetic‐spintronic device technology.