Magneto-optical and spin-transfer switching properties of current-perpendicular-to plane spin valves with perpendicular magnetic anisotropy

We evaluated the magneto-optical properties of spin-valve (SV) stacks with perpendicular magnets comprised of a Gd-Fe (4-30 nm) free layer and a Tb-Fe-Co/CoFe pinned layer, and the spin-transfer switching properties of current-perpendicular-to-plane (CPP) SVs with the perpendicular magnets. The Gd-Fe free layer had polar Kerr rotation θ_k of 0.12° at a thickness of 10 nm. The θ_k reduced with a decrease in GdFe thickness and the sign of θ_k switched from positive to negative at a Gd-Fe thickness of 6 nm or thinner. Coercivity (H_c) was maximum and saturation magnetization (M_s) was minimum at a thickness of 8 nm. These phenomena may be explained by an inhomogeneous Gd-Fe composition. The CPP spin valves with the Gd-Fe (10 nm) free layer had a magneto-resistance (MR) of 0.038%. The free layer of a CPP SV device was switched by a pulsed current, which exhibited intrinsic switching current densities of J_c0-P-AP = -3.3 × 10⁷ A/cm² and J_c0-AP-P = 4.3 × 10⁷ A/cm² with a thermal stability of 75, which is above the required value of 40. Perpendicular magnets are very useful for obtaining large magneto-optical signals from nano-magnets driven by spin-transfer switching.