Tunable Magnetic Skyrmions in Ferrimagnetic Mn$_4$N

Abstract

Thin films of ferrimagnetic Mn 4 N are candidate materials to host magnetic skyrmions that have demonstrated thermal stability up to 450 • C. However, there are no experimental reports observing skyrmions in this system. Here, we discuss the results of sputter grown 15-17 nm Mn 4 N thin films on MgO substrate capped with Pt 1−x Cu x layers. Vibrating sample magnetometry measurement of out-of-plane hysteresis loops confirmed that magnetic properties are insensitive to the cap layer composition. Imaging based on magnetic force microscopy measurements observed 300 to 50 nm sized skyrmions, as the Cu concentration was increased from x = 0 to 0.9. We performed density functional theory calculations and found that the interfacial Dzyaloshinskii-Moriya interactions (iDMI) follow a trend: Mn 4 N/MgO(001) < Cu/Mn 4 N(001) < Pt/Mn 4 N(001). We infer from these calculations that x in Pt 1−x Cu x capping layer can serve as a robust tuning knob to tailor the iDMI and control the skyrmion size. This work provides guidance to achieve smaller Néel-type skyrmions in Mn 4 N thin films, which is an important step forward for building thermally stable skyrmionic devices.