Field-free switching of perpendicular magnetization through spin-orbit torque in FePt/[TiN/NiFe]₅ multilayers

In order to maintain the thermal stability of SOT devices with nanoscale size, it is desirable to achieve current induced magnetic switching in magnetic materials with high perpendicular anisotropy. In the present paper, current induced field-free switching of FePt/[TiN/NiFe]₅ is achieved by interlayer exchange coupling, in which in-plane magnetized NiFe serves as a coupling layer through a TiN space layer. The large K_u (1.03 × 10⁷ erg cc^-1) and low critical current density values (0.17 × 10⁷ A cm^-2) show great advantages in thermal stability and energy consumption. Interestingly, it is found that the rotation directions of the current-induced magnetic switching loops under different applied magnetic fields are dependent on the sputtering temperature of [TiN/NiFe]₅ multilayers: once sign change for FePt/[TiN/NiFe]₅ RT and three sign changes for FePt/[TiN/NiFe]₅ HT. Simultaneously, when the magnetization direction of NiFe changes from the H direction to -H direction, the switching polarities at H = 0 always remain unchanged, which is different from other groups' reports. These phenomena may be attributed to the combined effect of TiN layer thickness induced ferromagnetic or antiferromagnetic coupling and the inherent H_in. Furthermore, gradual tuning of resistance states through the trains of current pulses has also been realized, showing potential applications in artificial synaptic networks. These results will put forward the applications of L1₀-FePt in current controlled MRAM and neuromorphic computing.