Coexistence of ferroelectricity and altermagnetism in wurtzite vanadium oxide: a first-principles study.

Abstract

Multiferroic and altermagnetic materials that simultaneously exhibit coupled electric and magnetic functionalities have emerged as promising candidates for next-generation spintronic and memory devices. In this work, we present a comprehensive first-principles investigation of bulk wurtzite vanadium oxide (w-VO), revealing its unique coexistence of robust ferroelectricity and altermagnetism. The w-VO compound crystallizes in a non-centrosymmetric hexagonal wurtzite phase (space group <i>P</i>6₃ <i>mc</i>), confirmed to be both thermodynamically and dynamically stable. Magnetic energy analysis identifies a layered antiferromagnetic (AFM) ground state with a local V magnetic moment of 2.42 µB, yielding a fully compensated magnetization. The electronic structure displays momentum-dependent spin splitting of ∼50 meV in the valence band along non-symmetric <i>k</i>-paths, a defining characteristic of altermagnetism arising from crystal symmetry and magnetic ordering rather than spin-orbit coupling. Furthermore, the system exhibits an in-plane magnetic easy axis and a substantial spin Hall conductivity (SHC), peaking at -182 (<i>ħ</i>/<i>e</i>) S cm^-1 under hole doping, surpassing values reported for known spin Hall materials. Remarkably, w-VO also demonstrates a strong out-of-plane spontaneous polarization of 113.12 µC cm^-2 along the [001] direction, substantially higher than conventional perovskite ferroelectrics. The polarization switching process, with a moderate energy barrier of 0.71 eV per f.u., confirms its ferroelectric reversibility. Strikingly, reversing the ferroelectric polarization induces a complete reversal of spin character near the Fermi level, thereby electrically toggling the spin-resolved electronic structure without altering the total magnetization. These findings establish w-VO as a rare multiferroic altermagnet in which ferroelectric polarization and compensated spin order are intrinsically coupled. The ability to control spin polarization and spin Hall response through electric-field-driven polarization switching offers a new paradigm for non-volatile, field-free spintronic devices based on voltage-controlled spin functionality.