Large Current Driven Domain Wall Mobility and Gate Tuning of Coercivity in Ferrimagnetic Mn₄N Thin Films

Abstract

Spintronics, which is the basis of a low-power, beyond-CMOS technology for computational and memory devices, remains up to now entirely based on critical materials such as Co, heavy metals and rare-earths. Here, we show that Mn4N, a rare-earth free ferrimagnet made of abundant elements, is an exciting candidate for the development of sustainable spintronics devices. Mn4N thin films grown epitaxially on SrTiO3 substrates possess remarkable properties, such as a perpendicular magnetisation, a very high extraordinary Hall angle (2%) and smooth domain walls, at the millimeter scale. Moreover, domain walls can be moved at record speeds by spin polarised currents, in absence of spin-orbit torques. This can be explained by the large efficiency of the adiabatic spin transfer torque, due to the conjunction of a reduced magnetisation and a large spin polarisation. Finally, we show that the application of gate voltages through the SrTiO3 substrates allows modulating the Mn4N coercive field with a large efficiency.

Magnetic and magneto-transport properties of Mn4N thin films by Ni substitution and their possibility of magnetic compensation

“…Recent experiments have shown that epitaxial Mn 4 N thin films exhibit a perpendicular magnetization, remarkable magnetic properties within particular millimetric and smooth DWs, 18 and record DW velocities owing to spin transfer torque. 19 As such, we have focused on ferrimagnetic Mn 4 N doped with Ni to obtain a material close to magnetic compensation at room temperature (RT). Using neutron diffraction, the magnetic moment of the Mn atoms in bulk cubic Mn 4 N was measured and it was found to be 3.53 μ B at the corner sites (I sites) and −0.89 μ B at the face-centered sites (II sites), 20 as shown in Fig.…”

“…Indeed, we achieved a v DW of 900 m/s at j = 1.2 × 10 12 A/m 2 for a Mn 4 N nanowire grown on STO(001) without applying an external magnetic field. 19 Substituting other transition metal atoms for Mn atoms in Mn 4 N is an effective way of modifying the material's magnetic properties. In previous studies, we grew Co x Mn 4−x N 23 and Fe 4−x Mn x N 26 epitaxial thin films by molecular beam epitaxy (MBE) and investigated their crystalline qualities and magnetic properties.…”

Tunable Magnetic Skyrmions in Ferrimagnetic Mn$_4$N

“…Total simulated space is 300 nm x 300 nm x 15 nm in micromagnetic simulations, and each cell size is 5 nm x 5 nm x 5 nm. For these simulations, the exchange stiffness constant (A) is 1.5 x 10 − 11 J/m, estimated from Curie temperature of 745 K 24 . Saturation magnetization Ms is 50 kA/m and anisotropy K u is between 0.7 and 1.1 x 10 5 J/m 3 , based on the previous measurements 27 .…”

“…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 .…”

“…Furthermore, recent experiment reports current-driven domain motion of up to 900 m/s for current density of 1.3 x 10 12 J/m 2 . 24 Such high mobility is crucial for high speed skyrmions motion. Although Mn 4 N thin films appear to possess many favorable characteristics for hosting skyrmions, to date there is no experimental report in the open literature of observing skyrmions in Mn 4 N thin films.…”