Theory of cluster-octupole spin-transfer torque in all-antiferromagnetic tunnel junctions

All-antiferromagnetic tunnel junctions (AFMTJs) based on noncollinear antiferromagnets have recently emerged as promising building blocks for next-generation ultrafast, field-free spintronic memory, owing to their large tunneling magnetoresistance (TMR) and efficient current-driven switching [1]. In a recent study, we reported the experimental observation of octupole-driven spin-transfer torque (OTT) in PtMn₃|MgO|PtMn₃ AFMTJs, achieving a record-high TMR of 363% at room temperature [2]. Here, we develop a first-principles analytical framework to describe spin-dependent tunneling and current-induced torques in such AFMTJs. Sublattice- and spin-projected tunneling amplitudes reveal an intrinsic imbalance between intra- and inter-sublattice tunneling—a direct manifestation of the anisotropic distribution of sublattice-resolved conduction channels and spin polarizations in reciprocal space arising from the octupolar order of PtMn₃. We derive the intra- and inter-sublattice torque efficiencies, β₁ and β₂, whose imbalance β_eff=β₁−β₂ governs the observed octupolar spin-transfer torque. Finally, by mapping these torques onto the multipole symmetry basis, we identify symmetry-allowed damping-like and field-like multipole torques. Our results reveal the microscopic origin of OTT and establish a rigorous foundation for multipole spin dynamics and switching analyses in noncollinear antiferromagnets.