Perpendicular Magnetization from Crystallite Boundaries in Polycrystalline Antiferromagnetic $γ$-FeMn
Single-crystalline noncollinear antiferromagnets, such as Mn$_3$Sn, are actively explored as memory media, but they generally require high-temperature growth that is incompatible with industrial-scale device fabrication. Here, we show that polycrystalline films of antiferromagnetic $γ$-FeMn, grown at room temperature, develop a perpendicular magnetization and an anomalous Hall effect, opening an industrially compatible route to antiferromagnetic memory. While conventional noncollinear antiferromagnets demand near-perfect crystal order, $γ$-FeMn films derive their memory-enabling properties from crystal defects, particularly boundaries separating nanoscale crystallites. When the crystallite size $D$ falls below $\approx$ 30 nm, a field-switchable magnetization emerges and increases with the boundary density $\propto 1/D$. This magnetization is accompanied by a room-temperature anomalous Hall conductivity exceeding 10 S/cm, comparable to that of single-crystalline Mn$_3$Sn. Our systematic experimental results, including scanning magnetometry, indicate uncompensated spin moments in the boundaries coupled to the compensated antiferromagnetism within the crystallites. We further find that the perpendicular easy axis of the magnetization is stabilized by residual strain within the $γ$-FeMn film. Our findings in manufacturing-friendly polycrystalline $γ$-FeMn redefine crystal defects as enablers, rather than impediments, for scalable antiferromagnetic memory.