Strong correlation between uniaxial magnetic anisotropic constant and in-plane tensile strain in Mn4N epitaxial films

Ferrimagnetic Mn4N is a promising candidate for current-induced domain wall motion assisted by spin-transfer and spin-orbit torques. Mn4N can be doped to have perpendicular magnetic anisotropy (PMA) and a small spontaneous magnetization. However, the origin of the PMA of Mn4N has yet to be fully understood. Here, we investigated the relationship between the ratios of the perpendicular lattice constant c to the in-plane lattice constant a of Mn4N epitaxial thin films (c/a) and the uniaxial magnetic anisotropic constant (K-u) in Mn4N thin films grown on MgO(001), SrTiO3(001), and LaAlO3(001) substrates. The lattice mismatches between Mn4N and these substrates are approximately -6%, -0.1%, and +2%, respectively. All the Mn4N thin films had PMA and in-plane tensile distortion (c/a < 1) regardless of the Mn4N thickness and substrate. Although the magnitude of c/a depended on several factors, such as the Mn4N layer thickness and substrate, we found a strong correlation between c/a and K-u; K-u increased markedly when c/a deviated from 1. This result indicates that the origin of PMA is tensile distortion in Mn4N films; hence, it might be possible to control the magnitude of K-u by tuning c/a through the Mn4N layer thickness and the substrate.