Symmetry Enhanced Unconventional Spin Current Anisotropy in a Collinear Antiferromagnet

Abstract Spin‐orbit torque (SOT) presents a promising avenue for energy‐efficient spintronics devices, surpassing the limitations of spin transfer torque. While extensively studied in heavy metals, SOT in antiferromagnetic quantum materials remains largely unexplored. Here, SOT is investigated in epitaxial FeSn, a collinear antiferromagnet with a kagome lattice. FeSn exhibits intriguing topological quantum features, including 2D flat bands and Dirac‐like surface states, making it an ideal platform for investigating emergent SOT properties. Using spin‐torque ferromagnetic resonance, a six‐fold symmetric damping‐like SOT is uncovered in epitaxial‐FeSn/Py heterostructures, reflecting the six‐fold symmetry of the epitaxial [0001]‐oriented FeSn films. Additionally, a substantial unconventional field‐like torque is observed, originating from spin currents with out‐of‐plane spin polarization. This torque exhibits a unique angular dependence–a superposition of six‐fold crystalline symmetry and uniaxial symmetry associated with the antiferromagnetic spin Hall effect. The theoretical calculations, employing density functional theory and tight‐binding methods, reveal a previously unreported nodal line with large Berry curvature near the Fermi level (along the A − L path) in bulk FeSn, which the observed experimental results are attributed. The findings demonstrate an unconventional spin current anisotropy, tunable by crystalline and magnetic symmetry, offering a novel pathway for controlling SOT in antiferromagnetic spintronics.