Engineering magnetic heterostructures to obtain large spin Hall efficiency for spin-orbit torque devices

Abstract

By sandwiching a CoFeB ferromagnetic layer between Ta and Pt heavy metals with an opposite spin Hall angle, spin currents of the same polarity are transmitted from both interfaces of the Ta/CoFeB/Pt trilayer to the CoFeB layer simultaneously. Here, we investigated the spin-orbit torque, magnetization dynamics, and interface spin transmission efficiency of the trilayer heterostructure by spin-torque ferromagnetic resonance. A large effective spin Hall angle, substantially larger than both Ta and Pt, was obtained in the Ta/CoFeB/Pt stack. The thickness-dependence study showed that with the reducing of CoFeB thickness, Gilbert damping enhances by spin pumping and spin Hall angle increases by the spin Hall effect and the Rashba effect. Furthermore, the spin transparency derived from effective spin mixing conductance was 0.63 ± 0.07 and 0.48 ± 0.02 at the CoFeB/Pt and Ta/CoFeB interfaces, respectively. Hence, the spin Hall angle could be further enhanced by improving the spin transmission efficiency at the interface. Our method of increasing spin-orbit torque through stack engineering would have potential applications in domain wall racetrack memory, logic gates, and magnetic tunnel junctions.