Efficiency of Spin-Transfer-Torque Switching and Thermal-Stability Factor in a Spin-Valve Nanopillar with First- and Second-Order Uniaxial Magnetic Anisotropies

The efficiency of spin-transfer-torque (STT) switching and the thermal-stability factor are important figures of merit in STT-based magnetoresistive random-access memory. We derive analytical expressions of the STT-switching efficiency and the thermal-stability factor for a perpendicularly magnetized spinvalve nanopillar with the first-and the second-order uniaxial magnetic anisotropy. It is shown that the STT-switching efficiency is maximized when the effective first-order anisotropy constant (K-u1,K- eff) is equal to the second-order anisotropy constant (K-u2). It is also shown that the thermal-stability factor is most (least) sensitive to a variation of the applied current when K-u2 = -0.41 (0.70) K-u1,K- eff.