N as a Skyrmion Host*

Skyrmion  devices  have  potential  to  cause  significant  disruption  inthe high-density storage industry.  To maximize performance, nucleated skyrmions should be small and stable around room temperature.  Thinfilms of ferrimagnetic Mn4N show promise with improved stability be-cause of the high N ́eel temperature, low saturation magnetization, and presence  of  perpendicular  magnetic  anisotropy  in  the  thin  film  geometry.   However,  conditions  that  govern  the  skyrmion  size  in  ultra-thin Mn₄N films are not known.  Here, density functional theory calculations are used to investigate the salient magnetic properties that govern theskyrmion size in Mn4N thin films.  We calculate the impact of changing both the interfacial composition as well as orientation on the interfacial Dzyaloshinskii-Moriya interaction (iDMI). We predict that Cu substitution in the Pt capping layer reduces the iDMI, which reveals the promise of capping layer composition engineering for tuning the skyrmion size. Results from DFT are validated by growth of a < 20 nm Mn₄N thin film on MgO (001) substrate with CuxPt1−xcapping layer.  Imaging using magnetic force  microscopy showed that in CuxPt1−x can serve as a knob to tune the skyrmion size (50–300 nm) at room temperature.