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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 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 epitaxial growth of sub-20 nm Mn4N films with PMA at 400 °C–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 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. [ABSTRACT FROM AUTHOR]

Magnetic properties in ferrimagnetic GdCo alloy films with bulk perpendicular magnetic anisotropy (PMA) are investigated as a function of annealing temperature (Tanneal) and annealing time for several capping layers. Magnetic properties in films capped by TaOx vary markedly with Tanneal; the saturation magnetization and coercivity vary progressively with increasing Tanneal up to 300°C, and above that temperature, PMA is lost abruptly. By comparing the annealing temperature dependence for Co-dominated and Gd-dominated compositions close to the magnetization compensation point, the data are readily explained by preferential oxidation of Gd during annealing. When films are capped by a Ta/Pt bilayer, the film properties are stable up Tanneal = 300 °C, indicating that oxidation at high temperatures is effectively blocked, but the abrupt loss of PMA for Tanneal > 300 °C is still observed. X-ray diffraction measurement reveals that the amorphous structure of the films remains the same after high-temperature annealing that is sufficient to remove PMA, indicating that crystallization from the amorphous phase is not responsible for the lack of PMA. Instead, our results suggest that high annealing temperatures may cause segregation of Co and Gd atoms in the films, which reduces anisotropic pair-pair correlations responsible for the observed bulk PMA in the as-grown state. [ABSTRACT FROM AUTHOR]