Composition dependence of crystalline and magnetic properties in Mn_4-xGe_xN epitaxial thin films

We investigated the crystalline and magnetic properties of Mn4-xGexN epitaxial films prepared on MgO(001) and SrTiO3(001) substrates using plasma-assisted molecular beam epitaxy. Reflection high-energy electron diffraction and X-ray diffraction profiles confirmed the epitaxial growth of Mn4-xGexN films with 0.0 <= x <= 0.8 on both substrates, with particularly better crystalline quality observed for the films on SrTiO3(001). The X-ray magnetic circular dichroism (XMCD) spectrum of Mn4N films at the Mn-L-3 absorption edge is characterized by a sharp negative peak (alpha), mainly due to Mn at the corner sites, and a broad positive peak (beta), mainly due to Mn at the face-centered sites. At x = 0.3, the intensity of peak beta was particularly reduced. The anomalous Hall resistivity exhibited a sign reversal between x = 0.4 and x = 0.6, regardless of the substrate. However, in the XMCD spectrum, no sign reversal occurred. This is different from the previously reported Mn4N-based epitaxial films, where the sign reversal of the anomalous Hall resistivity was always accompanied with the sign reversal of peak beta. Therefore, the sign reversal of the anomalous Hall resistivity needs to be explained by another cause. The change in the XMCD spectrum between x = 0.0 and x = 0.3 suggests an increase in noncollinear components such as Gamma(5g) in the magnetic structure.