Current driven domain wall dynamics across the angular momentum compensation point in epitaxially grown ferrimagnetic Ni-doped Mn4N thin films

Abstract

The motion of domain walls by the use of the magnetic fields as well as spin polarized currents has been a major part of spintronics in the last two decades. The two mechanisms of current induced domain wall motion are Spin transfer torque(STT) and Spin orbit torques(SOT). In the recent years, compensated ferrimagnets have been in focus due to their ability to reach the magnetic and the angular momentum compensation points. At the angular mometum compensation point the precessional motion of the local magnetic moment becomes negligible which has shown to have massively increase the domain wall velocity[1,2]. Here we will focus on a rare-earth free ferrimagentic nitride called Manganese Nitride (Mn4N).We have previously shown epitaxially grown Mn4N has a very low magnetization (71 kA/m) and a very high perpedicular magnetic anisotropy (PMA) with mm sized domains[3]. This low magnetization is due to the two different Mn sub-lattices in the anti-perovskite crystal structure. With such a small magnetization and a high spin polarization led us to obtain a very high domain wall velocity of more than 900m/s at J=1.3*1012 A/m2 using pure STT[4].In this work, we have have shown an even larger domain wall velocity approaching 3000 m/s at J=1.26*1012 A/m2 using pure STT near the magnetic and the angular compensation points in epitaxially grown Ni doped Mn4N[5]. The magnetic and angular momentum compensation points are obtained by the adjustment of Ni doping in this system at room temperature. These high domain wall velocities are comparable to the best velocities obtained by SOT. Also, in this system there is no requirement for an external in-plane field as shown in some other systems by SOT. Moreover, we also demonstrate a reversal of the domian wall motion direction after crossing the compensation points where the domain walls move in the same direction as the flow of electrons. This phenomenon will be explained with the help of ab-initio calculations.  Current density vs Velocity curves for Mn4-xNixN thin films. Filled symbols show samples before and the open symbols after the compensation point.  MS vs Velocity plot, measured for J=1*1012 A/m2(blacksquares) is compared with the best fit obtained using the q−φmodel.