Spin-Orbit effect in ferrimagnetic thin film

The influence of spin-orbit coupling on transport properties has been a topic of strong and growing interest in the last ten years. In order to use of spin-orbit torque for applications in the field of spin electronics, it is necessary to reduce the critical current necessary for the reversal and to decrease or eliminate the planar external magnetic field applied. My thesis work concerns the experimental study of heavy metal / ferrimagnetic bilayer model systems (W / CoxTb1-x or Pt / CoxTb1 - x). In such ferrimagnetic alloys, the magnetization of the Cobalt sub-lattice is coupled antiparallel to the magnetization of the Terbium sub-lattice. These alloys are particularly interesting because for certain concentration, there is a temperature for which the magnetization of the two sub-networks are equal resulting in zero magnetization. This is the magnetization compensation temperature. At first I characterized these systems using magnetometry and Hall cross measurements for temperatures ranging from 10 to 350 K. The experiments of magnetization reversal of magnetization induced by the current were carried out in a Spin- orbit torque” (SOT) geometry where the current pulses are injected into the plane and the reversal of the magnetization is detected by measuring the Hall resistance. The complete magnetization reversal was observed in all the samples. The current reversal was found to vary continuously with the alloy composition and we did not observe any reduction at the compensation point despite the large increase in the SOT efficiency. A model based on the coupled Landau-Lifschitz-Gilbert equations shows that the reversal current density is proportional to the effective perpendicular anisotropy, which does not decrease at the compensation point. Although TbCo has a strong perpendicular magnetic anisotropy, the reversal occurs for a weak planar magnetic field. We were able to show that the heating caused by the current plays an important role in the switching. Indeed the reversal seems to occur at a characteristic switching temperature (Tswitch) induced by Joule heating. Tswitch is larger than the magnetic and angular compensation temperatures, but lower than the Curie temperature. Everything happens as if it was necessary to reach a temperature close to the order temperature for the reversal to take place.