Rare-earth-free ferrimagnetic Mn4N sub-20 nm thin films as potential high-temperature spintronic material

Abstract

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.

Tunable Magnetic Skyrmions in Ferrimagnetic Mn$_4$N

“…MgO substrates were wet-cleaned and heat-treated before loading into the vacuum chamber. Details of the pre-sputtering cleaning process and sputtering process were reported earlier 27 . Fig.…”

“…Although micromagnetic simulations do not take into account colinear spins in ferrimagnetic Mn 4 N, it has been benchmarked with atomistic simulations for small skyrmions (< 40 nm) to verify its validity for a ferrimagnet. In a previous publication of sub-20 nm Mn 4 N films 27 , X-ray diffraction (XRD) showed only the MgO substrate peak and the Mn 4 N (002) peak in the 2θ -θ XRD profile. In the φ scan, four peaks at 90 • interval confirms the epitaxial growth of Mn 4 N (001)[100] on MgO(001)[100].…”

“…One promising candidate with superior thermal stability compared to the amorphous rare-earth transition metals is rare-earth-free Mn 4 N. There are some key similarities and differences between the rare-earth transition metals and Mn 4 N. Both are ferrimagnetic metals, and there have been successful experimental demonstrations of thin film growth in both systems. Both materials show perpendicular magnetic anisotropy (PMA) in the thin film geometry 16,[20][21][22][23][24][25][26][27] . However, unlike the amorphous structure in the rare-earth transition metals, Mn 4 N is a crystalline compound that forms in the anti-perovskite crystal structure 27 .…”

“…Both materials show perpendicular magnetic anisotropy (PMA) in the thin film geometry 16,[20][21][22][23][24][25][26][27] . However, unlike the amorphous structure in the rare-earth transition metals, Mn 4 N is a crystalline compound that forms in the anti-perovskite crystal structure 27 . Compared to the GdCo ferrimagnets, Mn 4 N thin films offer key materials processing advantages for improving thermal stability due to the following two reasons: First, the Mn 4 N films are deposited at 400-450 • C and no structural transitions or loss of PMA has been reported after annealing or cooling to room temperature.…”

“…We deposited 15±2 nm thick Mn 4 N thin films on MgO(001) 5×5×0.5 mm substrate by reactive radio frequency (rf) sputtering at 450 • C. We also deposited 3 nm thick cap layers of Pt 1−x Cu x (where x = 1, 0.5, 0.1, 0.05) on the sputter grown Mn 4 N layer at room temperature by co-sputtering of Pt and Cu targets to tune the iDMI, and 3 nm Pt film on top as the capping layer to prevent oxidation. Details of the deposition process were reported earlier 27 . X-ray diffraction (XRD) was measured with Rigaku Smart-Lab.…”