We report very large spin transfer torque driven domain wall velocities, approaching 3000 m/s, in rare-earth free ferrimagnetic Mn4-xNixN thin films close to the angular momentum compensation point. We also observe a reversal of the domain wall motion direction after the angular momentum compensation point.
The interest towards ferrimagnets has recently grown because of their ability to reach the vicinity of the angular momentum compensation point, by varying the temperature or the composition. The smaller magnetization allows obtaining faster domain wall motions than in ferromagnets. This has also propelled studies on ferrimagnets for domain wall based devices logic and more generally for spintronics memory applications. Among the ferrimagnets, an interesting rare-earth and critical elements-free compound is Mn4N, which can be doped with Ni to reach the compensation point.
(left) TEM image of Mn3.75Ni0.25N on SrTiO3 substrate showing epitaxial growth. (right) Current density vs velocity curves for Mn4-xNixN, positive velocities shown for before and negative velocities after the angular momentum compensation point.
Mn4-xNixN is grown epitaxially with Mn and Ni target source with an RF-nitrogen plasma on SrTiO3 substrates, on which good quality films with large mm-scale and smooth domain have already been demonstrated. From the TEM characterization, it is shown that Ni is well distributed in the whole system with a very nice epitaxial growth. As the Ni concentration is increased, the velocity increases as the system reaches closer to the angular momentum compensation point approaching 3000m/s. As the angular momentum compensation point is crossed, the direction of the domain wall motion is reversed : the domain walls propagate in opposite direction as that of the electron flow. This phenomenon is explained by DFT calculations, which show that the spin polarization is carried out by the Mn II sub-lattice, which always remains positive in this system while the magnetization is reversed.
These results provide an insight on the effect of spin transfer torques on the domain walls before and after the compensation points. They are of great interest from a technological point of view, as the velocities obtained here are comparable to domain wall velocities obtained in spin-orbit torque driven systems, whereas there is no additional heavy metal layers here, nor external in-plane fields or Dzyaloshinskii-Moriya interaction.
Teams: Topological Spintronics, Theory and Simulation
Collaboration: University of Tsukuba (Japan), Institut Néel
Funding: IDEX-DOMINO, JSPS KAKENHI (No. 19KK0104 and 19K21954), Marie Sklodowska-Curie Grant Agreement No. 754303, Laboratoire d’excellence LANEF in Grenoble (ANR-10-LABX-0051)
Further reading: Current-Driven Domain Wall Dynamics in Ferrimagnetic Nickel-Doped Mn4N Films: Very Large Domain Wall Velocities and Reversal of Motion Direction across the Magnetic Compensation Point, S. Ghosh, T. Komori, A. Hallal, J. Peña Garcia, T. Gushi, T. Hirose, H. Mitarai, H. Okuno, J. Vogel, M. Chshiev, J.-Ph. Attané, L. Vila✉, T. Suemasu, S. Pizzini✉, Nano Lett. 21, 2580 (2021).
Contacts: Laurent Vila, Jean-Philippe Attané
