Spin Torque Switching in Nanopillars With Antiferromagnetic Reference Layer

Spin-transfer-torque-induced switching is investigated in 200 nm diameter circularly shaped, perpendicularly magnetized nanopillars. A synthetic antiferromagnet, consisting of two Co/Ni multilayers coupled antiferromagnetically 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 4 $\times$ [Co/Ni] multilayer. The use of Pt and Pd was avoided to lower the spin-orbit scattering in magnetic layers and intrinsic damping in the free layer, and therefore, reduce the critical current required for spin-transfer-torque switching. The intrinsic Gilbert damping constant of a continuous 4 $\times$ [Co/Ni] multilayer film was measured by ferromagnetic resonance to be $\alpha = 0.022$ , which is significantly lower than in Pt- or Pd-based magnetic multilayers. In zero magnetic field, the critical current required to switch the free layer from parallel to antiparallel alignment is 5.2 mA, and from antiparallel to parallel alignment is 4.9 mA. Given the volume of the free layer, $V_{FL} = 1.01 \times 10^{-22}$ m $^{3}$ , the switching efficiency, $I_{c}/(V_{FL}\times\mu_{0}H_{c}$ ), is $5.28 \times 10^{20}$  A/T $\cdot$ m $^{3}$ , twice as efficient as any previously reported device with a similar structure.