• Ghosh, Sambit

Abstract

The boundaries between magnetic domains are known as domain walls (DW). The motion of these DWs using magnetic fields or spin polarized currents has been one of the main focus of spintronics research in the last two decades. One mechanism leading to current induced domain wall motion is the spin transfer torque, where the spin polarized current is generated within the ferromagnetic layer and exerts a torque on the local magnetic moments of the domain walls. The other mechanism is the spin orbit torque, which is now widely used for the domain wall motion experiments, and where the spin polarized current is generated by an adjacent heavy metal layer.Recently, current-induced magnetization dynamics in ferrimagnets has become an active field of research. The magnetic and/or the angular momentum compensation can be achieved by either changing the temperature or by changing the composition of the materials. As the magnetization or the angular momentum that has to be reversed is small close to these points, previous reports on ferrimagnets have evidenced large domain wall velocities under action of spin orbit torques. In this manuscript we will focus on current-driven domain wall dynamics in an epitaxial rare-earth free ferrimagnetic nitride, Manganese Nitride (Mn4N), using spin transfer torques.We show that epitaxially grown Mn4N thin films grown on SrTiO3 substrate have a very low magnetization and mm-scale domains with very low pinning. On this system, DW motion was studied by fabricating nanowires by e-beam lithography. Using magneto optic kerr measurements, we measured a high domain wall velocity of more than 900 m/s in Mn4N at J = 1.3 TA/sq.m, at room temperature, with only spin transfer torque.In order to reach the compensation point, different samples were grown epitaxially while increasing the doping Ni concentration. X-ray magnetic circular dichroism (XMCD) measurements showed that the Ni atoms preferentially occupy the corner site in Mn4N. Since the magnetic moment carried by the Ni atoms is anti-parallel to that of Mn corner atoms , increasing the Ni content decreases the net magnetic moment. Beyond a critical Ni concentration, the net magnetization is then expected to be reversed. Using the values from neutron diffraction measurements, the expected magnetic compensation point lies around Ni atomic concentration of x = 0.18 which corresponds to 3.6% of Ni. The presence of the magnetic compensation point around this concentration is confirmed by XMCD and Anomalous Hall effect measurements. The DW velocity is found to increase as the Ni concentration gets closer to the angular momentum compensation point, with a velocity up to 2000 m/s before the compensation point and approaching 3000 m/s after crossing the compensation point. Interestingly it was also observed that the DW motion direction is reversed beyond the compensation point. In order to explain these results, we used the q–ϕ model, expanded to a ferrimagnetic system consisting of two sub-lattices, and using effective magnetic parameters for the two sub-lattices. If one assumes that the spin polarization does not change after the angular momentum compensation point, the DW motion reversal is therefore due to a relative change of orientation of the net spin polarization with respect to global magnetization.To confirm the validity of these assumptions, ab-initio calculations were performed, showing that the net magnetization is reversed at the Ni concentration x = 0.15, which match well with our experimental results. The simulations confirms that the conduction occurs through the Mn face centered sites, and that the spin polarization remains in the same direction (given by corner sites) while the net magnetization direction is reversed.The studied materials, composed of abundant elements, and free of critical elements such as rare-earths and heavy metals, are thus promising candidates for sustainable spintronics applications.

Topics

• perovskite
• impedance spectroscopy
• experiment
• thin film
• simulation
• nitride
• neutron diffraction
• Manganese
• magnetization
• lithography
• spin polarization