paper

Altermagnetic Flatband-Driven Fermi Surface Geometry for Giant Tunneling Magnetoresistance

arXiv:2511.17277 · doi:10.1002/adfm.202531921

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 $V_{2} T e_{2} O$ , $Rb V_{2} T e_{2} O$ , and $K V_{2} S e_{2} O$ -- to elucidate how flatband-driven Fermi surfaces minimize spin-channel overlap and boost AMTJ performance. Notably, $Rb V_{2} T e_{2} O$ and $K V_{2} S e_{2} O$ 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 $1 0^{3} %$ in the $K V_{2} S e_{2} O$ -based AMTJ. Incorporating an insulating barrier further enhances the TMR to $\sim 1 0^{6} %$ , surpassing most conventional MTJs. These results not only establish $K V_{2} S e_{2} O$ 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.

Accepted for publication in Advanced Functional Materials (2026)