Skyrmions can be stabilized in magnetic systems with broken inversion symmetry and chiral interactions, such as Dzyaloshinskii-Moriya interactions (DMi). further, compensation of magnetic moments in ferrimagnetic materials can significantly reduce magnetic dipolar interactions, which tend to favor large skyrmions. tuning DMi is essential to control skyrmion properties, with symmetry breaking at interfaces offering the greatest flexibility. However, in contrast to the ferromagnet case, few studies have investigated interfacial DMi in ferrimagnets. Here we present a systematic study of DMI in ferrimagnetic CoGd films by Brillouin light scattering. We demonstrate the ability to control DMI by the coGd cap layer composition, the stack symmetry and the ferrimagnetic layer thickness. the DMi thickness dependence confirms its interfacial nature. In addition, magnetic force microscopy reveals the ability to tune DMI in a range that stabilizes sub-100 nm skyrmions at room temperature in zero field. our work opens new paths for controlling interfacial DMi in ferrimagnets to nucleate and manipulate skyrmions. Magnetic skyrmions due to their non-trivial topology have interesting properties 1-3 that make them attractive for spintronic applications, such as racetrack memory and logic devices 4-6. A magnetic skyrmion designates a chiral spin texture with a whirling spin configuration 7. Skyrmions can be stabilized by broken inversion symmetry and chiral interactions, such as the Dzyaloshinskii-Moriya interactions (DMI) 8,9 , which is an antisymmetric exchange interaction that favors non-collinear neighboring spins. Ultrathin magnetic materials with interfaces to heavy non-magnetic metals with large spin-orbit coupling exhibit interfacial DMI that stabilizes skyrmions and chiral domain walls 10-13. The interfacial DMI and the nucleation of skyrmions have been extensively investigated in ferromagnetic materials 10,14-18. Very recently, magnetic skyrmions and chiral domains were reported in ferrimagnetic systems 19-21. Nearly compensated thin ferrimagnetic films with interfacial DMI are interesting materials due to their low stray fields, reduced sensitivity to external magnetic fields, and fast spin dynamics, which are predicted to lead to ultrasmall and ultrafast skyrmions 19,22. Unlike in ferromagnets where fast current-induced motion of chiral textures is impeded by the Walker breakdown and domain wall pinning 13,23-25 , high domain wall velocities-reaching 1000 m s-1-have been observed in ferrimagnetic CoGd films near the angular momentum compensation temperature 19. In addition, the large dipolar fields in ferromagnets are obstacles to the formation of ultrasmall skyrmion 22. Hence, ferrimagnetic thin films are promising candidates for ultrafast skyrmion-based spintronics. Recently, bulk DMI was reported in an amorphous ferrimagnetic GdFeCo alloy 26. However, the significant advantages of interfacial DMI are that it can be controlled by the nature of the interfaces and widely tuned to stabilize skyrmions. Yet, interfacial DM...

Ferrimagnetic alloy thin films that exhibit perpendicular (out-of-plane) magnetic anisotropy (PMA) with low saturation magnetization, such as GdCo and Mn4N, were predicted to be favorable for hosting small Néel skyrmions for room temperature applications. Due to the exponential decay of interfacial Dzyaloshinskii-Moriya interaction (DMI) and the limited range of spin-orbit-torques, which can be used to drive skyrmion motion, the thickness of the ferrimagnetic layer has to be small, preferably under 20 nm. While there are examples of sub-20 nm, rare earth-transition metal (RE-TM), ferrimagnetic thin films fabricated by sputter deposition, to date rare-earth-free sub-20 nm Mn4N films with PMA have only been reported to be achieved by molecular beam epitaxy, which is not suitable for massive production. Here we report the successful thermal growth of sub-20 nm Mn4N films with PMA at 400-450 °C substrate temperatures on MgO substrates by reactive sputtering. The Mn4N films were achieved by reducing the surface roughness of MgO substrate through a high-temperature vacuum annealing process. The optimal films showed low saturation magnetization (Ms = 43 emu/cc), low magnetic anisotropy energy (0.7 Merg/cc), and a remanent magnetization to saturation magnetization ratio (Mr/Ms) near 1 at room temperature. Preliminary ab-initio density functional theory (DFT) calculations have confirmed the ferrimagnetic ground state of Mn4N grown on MgO. The magnetic properties, along with the high thermal stability of Mn4N thin films in comparison with RE-TM thin films, provide the platform for future studies of practical skyrmion-based spintronic materials.