  Spin-transfer torque (STT) and spin-orbit torque (SOT) are spintronic
phenomena allowing magnetization manipulation using electrical currents. Beyond
their fundamental interest, they allow developing new classes of magnetic
memories and logic devices, in particular based on domain wall (DW) motion. In
this work, we report the study of STT driven DW motion in ferrimagnetic
manganese nickel nitride (Mn4-xNixN) films, in which a fine adjustment of the
Ni content allows setting the magnetic compensation at room temperature. The
reduced magnetization, combined with the large spin polarization of conduction
electrons, strongly enhances the STT so that domain wall velocities approaching
3000 m/s can be obtained for Ni compositions close to the compensation point.
In addition, a reversal of the domain wall motion direction is observed when
the magnetic compensation composition is crossed. This striking feature,
related to the change of direction of the spin polarization with respect to
that of the net magnetization, is clarified by ab initio band structure
calculations.
