Very large domain wall velocities driven by spin transfer torque in ferrimagnetic Mn4N compounds INVITED

Abstract

Spin-transfer torque (STT) and spin-orbit torque (SOT) are spintronic mechanisms allowing magnetization manipulation using electrical currents. Beyond their fundamental interest, they allow envisaging applications to new classes of magnetic memories and logic devices, in particular based on domain wall motion. In the last 10 years the interest of the spintronic community has focused on thin ferromagnetic films deposited on a heavy metal, in which the interfacial Dzyaloshinskii-Moriya interaction (DMI) stabilises chiral Néel walls which can be moved by spin orbit torques associated to the spin Hall effect (SHE-SOT). Studies on systems in which domain walls are moved by STT are now rare, mainly because there are practically no reports of efficient STT in thin films with perpendicular magnetisation.A growing interest towards ferrimagnets appeared recently, as the efficiency of the torques is enhanced in the vicinity of magnetization compensation point. In this work, we report the study of current-driven domain wall (DW) dynamics in ferrimagnetic manganese nickel nitride (Mn4-xNixN) films with perpendicular magnetic anisotropy (PMA). The films are deposited on SrTiO3(001) substrates and crystallize in an anti-perovskite structure with two types of Mn atoms: Mn(I) located at the corners and Mn(II) located at the face centered sites. The two magnetic sublattices are antiferromagnetically coupled, with the net magnetization parallel to the Mn(I) moment.In the Mn4N films, neither bulk nor interfacial DMI are present; domain walls have then Bloch internal structure and are driven by the “classical” spin-transfer torque (STT). Our Kerr microscopy measurements showed that, thanks to the large spin polarization of conduction electrons and to the ferrimagnetic structure leading to low spontaneous magnetisation, domain walls in Mn4N can be moved by STT with an unprecedented efficiency [1]. More recently, DW dynamics was also studied in Ni-doped Mn4N films (Mn4-xNixN). Since the Ni moment aligns antiparallel to the net magnetization in Mn4N, the film composition can be finely adjusted to give a magnetisation compensation close to room temperature [2,3]. Films with compositions above and below the compensation composition and net magnetization parallel and antiparallel to that of Mn4N were grown. Close to the compensation point, the reduced angular moment strongly enhances the spin-transfer torque so that domain wall velocities approaching 3000 m/s were measured for a current density of 1.2 x 1012 A/m2. These speeds are comparable with the largest reported for ferrimagnetic films deposited on a heavy metal, where SHE-SOT is the driving mechanism. In addition, while below the compensation point the DWs move in the direction of the electron flow, a reversal of the domain wall motion direction is observed when the magnetic compensation composition is crossed (Figure 1). This striking feature, related to the change of direction of the spin polarization with respect to that of the angular moment, is in agreement with 1D analytical model and explained using ab initio calculations. Our material, composed of abundant elements, and free of critical elements such as cobalt, rare earths and heavy metals, is a promising candidate for the development of sustainable spintronics applications. **