Tunable Magnetic Skyrmions in Ferrimagnetic Mn$_4$N

TL;DR

This study demonstrates the experimental observation and tunability of magnetic skyrmions in Mn$_4$N$ thin films through capping layer composition, providing a pathway for thermally stable skyrmionic device development.

Contribution

It reports the first experimental observation of skyrmions in Mn$_4$N$ thin films and shows how interfacial Dzyaloshinskii-Moriya interactions can be tuned via capping layer composition.

Findings

Skyrmions observed range from 50 to 300 nm in size.

Skyrmion size can be controlled by Cu concentration in the cap layer.

Interfacial DMI strength varies with different capping layers.

Abstract

Thin films of ferrimagnetic Mn$_4$N are candidate materials to host magnetic skyrmions that have demonstrated thermal stability up to 450$^\circ$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 N\'{e}el-type skyrmions in Mn$_4$N thin films, which is an important step forward for building thermally stable skyrmionic devices.