Growth of ultrathin Mn4N epitaxial films on SrTiO3(001) and their thickness-dependent magnetic structures

Mn4N thin films meet the requirements for efficient current-driven magnetic domain wall motion, such as perpendicular magnetic anisotropy and small magnetization. To demonstrate efficient field-free spin-orbit torque (SOT)-driven domain wall motion, the thickness of the Mn4N layer must be reduced. In this study, we focus on the fabrication of Mn4N ultrathin films on SrTiO3(001) substrates and demonstrate the epitaxial growth of Mn4N films as thin as around 4 nm. Surprisingly, the sign of the anomalous Hall resistivity of Mn4N reverses when the thickness of Mn4N decreases from approximately 8 to 4 nm. X-ray magnetic circular dichroism measurements suggest that the magnetic structure of Mn4N with a thickness of around 4 nm is different from that of conventional ferrimagnetic Mn4N films. The results obtained in this study are of great importance when considering the use of SOT and the interfacial Dzyaloshinskii-Moriya interaction in Mn4N ultrathin films.