Reduction of intrinsic critical current density under a magnetic field along the hard axis of a free layer in a magnetic tunnel junction

We investigated the effect of a magnetic field along a hard in-plane axis H_hard on a current-induced magnetization switching (CIMS) in magnetic tunnel junctions (MTJs). Since H_hard causes the effective field to tilt away from the easy axis, we evaluated the H_hard dependence of two contributing factors in CIMS [the intrinsic critical current density (J _c0) and the thermal stability factor (E/k_BT)] as functions of the tilting angle (θ_H). Both measurements and numerical simulations showed that the presence of H_hard can reduce J _c0 by more than the amount estimated by Slonczewski's polarization function g(θ) by an order of magnitude and that E/k_BT is independent of θ_H. These findings suggest that the effect of H_hard mainly appears in the dynamic properties due to the nonconservative force of the spin-transfer torque based on the Slonczewski's model. A simple stability analysis demonstrated that the tilt of the magnetization direction away from the easy axis caused by the presence of H _hard induces an imbalance between the spin-transfer and damping torques and that applying a current achieves the further tilted stable state. Achievement of this stable state can be interpreted as the suppression of the effect of the effective demagnetization field (Hd*). Therefore the major reduction in J_c0 is due to the suppression of Hd* caused by the presence of H_hard.