Magnetic and electrical transport signatures of uncompensated moments in epitaxial thin films of the non-collinear antiferromagnet Mn${}_3$Ir

Abstract

Non-collinear antiferromagnets, with either an L1${}_2$ cubic crystal lattice (e.g. Mn${}_3$Ir and Mn${}_3$Pt) or a D0${}_{19}$ hexagonal structure (e.g. Mn${}_3$Sn and Mn${}_3$Ge), exhibit a number of novel phenomena of interest to topological spintronics. Amongst the cubic systems, for example, tetragonally distorted Mn${}_3$Pt exhibits an intrinsic anomalous Hall effect (AHE). However, Mn${}_3$Pt only enters a non-collinear magnetic phase close to the stoichiometric composition and at suitably large thicknesses. Therefore, we turn our attention to Mn${}_3$Ir, the material of choice for use in exchange bias heterostructures. In this paper, we investigate the magnetic and electrical transport properties of epitaxially grown, face-centered-cubic ${\hat{I}}^3$-Mn${}_3$Ir thin films with (111) crystal orientation. Relaxed films of 10 nm thickness exhibit an ordinary Hall effect, with a hole-type carrier concentration of (2.24 $\pm$ 0.08) $\times$ 10${}^{23}$ 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 remanent moment, observed at low temperatures, shown by XMCD spectroscopy to arise from uncompensated Mn spins. Of the order 0.02 $\hat{I}{¼}_B$ / 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, of differing chirality account for the absence of observed Berry curvature driven magnetotransport effects.