Spin Torque Switching in Nanopillars With Antiferromagnetic Reference Layer

Spin-transfer-torque-induced switching is investigated in 200 nm diameter circularly shaped, perpendicularly 10 magnetized nanopillars. A synthetic antiferromagnet, consisting of two Co/Ni multilayers coupled antiferromagnetically 11 across a Ru layer, is used as a reference layer to minimize the dipolar field on the free layer. The free layer is a single 12 4 × [Co/Ni] multilayer. The use of Pt and Pd was avoided to lower the spin-orbit scattering in magnetic layers and intrinsic 13 damping in the free layer, and therefore, reduce the critical current required for spin-transfer-torque switching. The intrinsic 14 Gilbert damping constant of a continuous 4 × [Co/Ni] multilayer film was measured by ferromagnetic resonance to be 15 α = 0.022, which is significantly lower than in Ptor Pd-based magnetic multilayers. In zero magnetic field, the critical 16 current required to switch the free layer from parallel to antiparallel alignment is 5.2 mA, and from antiparallel to parallel 17 alignment is 4.9 mA. Given the volume of the free layer, V
_FL
= 1.01 × 10
^-22
m
³
, the switching efficiency, I
_c
/(V
_FL
× μ0Hc), 18 is 5.28 × 1020 A/T·m
³
, twice as efficient as any previously reported device with a similar structure.