Very fast current driven and reverse domain wall motion in a rare-earth free compensated ferrimagnetic Mn4-xNixN.

Abstract

Current induced domain wall motion is a crucial part of spintronics that has received large attention in the last 15 years and has resulted in memory as well as logic applications. Spin tranfer torques(STT) and Spin orbit torques(SOT) are the two mechanism through which the current induced domain wall motion takes place. In the case of STT, the electron spins get polarized through exchange interaction within the ferromagnetic layer and then transfers its angular momentum into the local magnetic moment of the domain walls thereby resulting in its motion. On the other hand, in the case of spin orbit torque, the spin current is generated by Spin Hall effect and Rashba effect in an adjacent heavy metal layer or interface and then diffuses into the magnetic layer applying a torque on the domain wall magnetic moments. Here, we will focus on spin transfer torque driven domain wall motion in ferrimagnetic Ni doped Mn4N thin films.Recent work on domain wall motion has focused on ferrimagnets in which angular or spin moment compensation can be obtained by either changing the temperature of the material or the composition. Very fast domain wall velocities has been shown at the angular momentum compensation point where the precessional motion of the local spin is neglible thus resulting in faster domain wall motion. Mn4N is a class of rare-earth free ferrimagnetic material with a low magnetization and a high perpendicular magnetic anisotropy. It has an anti-perovskite crystal structure with antiferromagnetically coupled magnetic Mn atoms at the corner sites Mn(I) and at the face centered sites Mn(II). Very high current driven domain wall velocities has already been demostrated in thin Mn4N films epitaxially grown on SrTiO3 [1]. Since in these system domain walls have Bloch internal structure, the driving mechanism is the classical spin-transfer torque. The compensation point is not achieved by varying the temperature of the material. However, by doping of Ni in Mn4N the magnetic compensation can be achieved at room temperature [2]. The Ni atoms replace the Mn(I) atom at the corner sites and their moments align parallel to that of Mn(II) leading to a decrease in the net magnetization and a magnetic compensation or around 3.6% of Ni concentration [3].On these epitaxially grown Mn4-xNixN thin films on SrTiO3 substrates, due to the reduction of the angular momentum originating from the Ni doping and the large spin-polarization of conduction electrons, very high velocities approaching 3000 m/s have been achieved for compositions close to the magnetic compensation point. Moreover, a reversal of the domain wall motion direction was observed after crossing the angular momentum (and magnetization) compensation point where the domain walls move in the opposite to the electron flow. This unique phenomenon is in agreement with the analytical 1D model applied to ferrimagnetic systems and is due to the switch of the sign of the angular moment with respect to that of the spin polarization after the compensation point. This is supported by the results of ab-initio calculations. **