Tunable Magnetic Skyrmions in Ferrimagnetic Mn$_4$N
2021
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.
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