Magnetic and electrical transport signatures of uncompensated moments in epitaxial thin films of the noncollinear antiferromagnet Mn₃Ir

Noncollinear antiferromagnets, with either an L1₂ cubic crystal lattice (e.g., Mn₃Ir and Mn₃Pt) or a D0₁₉ hexagonal structure (e.g., Mn₃Sn and Mn₃Ge), exhibit a number of phenomena of interest to topological spintronics. Among the cubic systems, for example, tetragonally distorted Mn₃Pt exhibits an intrinsic anomalous Hall effect (AHE). However, Mn₃Pt only enters a noncollinear magnetic phase close to the stoichiometric composition and at suitably large thicknesses. Therefore, we turn our attention to Mn₃Ir, the material of choice for use in exchange bias heterostructures. In this letter, we investigate the magnetic and electrical transport properties of epitaxially grown, face-centered-cubic γ-Mn₃Ir thin films with (111) crystal orientation. Relaxed films of 10 nm thickness exhibit an ordinary Hall effect, with a hole-type carrier concentration of (1.500 ± 0.002) × 10²³ cm^-3. On the other hand, TEM characterization demonstrates that ultrathin 3 nm films grow with significant in-plane tensile strain. This may explain a small net magnetic moment, observed at low temperatures, shown by X-ray magnetic circular dichroism spectroscopy to arise from uncompensated Mn spins. Being of the order of 0.02 μ_B/atom, this dominates electrical transport behavior, leading to a small AHE and negative magnetoresistance. These results are discussed in terms of crystal microstructure and chiral domain behavior, with spatially resolved XML(C)D-PEEM supporting the conclusion that small antiferromagnetic domains, <20 nm in size, with differing chirality account for the absence of observed Berry curvature driven magnetotransport effects.