Effect of second-order magnetic anisotropy on nonlinearity of conductance in CoFeB/MgO/CoFeB magnetic tunnel junction for magnetic sensor devices

We studied the effect of second-order magnetic anisotropy on the linear conductance output of magnetic tunnel junctions (MTJs) for magnetic-field-sensor applications. Experimentally, CoFeB/MgO/CoFeB-based MTJs were fabricated, and the nonlinearity, NL was evaluated for different thicknesses, t of the CoFeB free layer from the conductance. As increasing t from 1.5 to 2.0 nm, maximum NL, NL^max was found to decrease from 1.86 to 0.17% within the dynamic range, H_d = 1.0 kOe. For understanding the origin of such NL behavior, a theoretical model based on the Slonczewski model was constructed, wherein the NL was demonstrated to be dependent on both the normalized second-order magnetic anisotropy field of H_k2/|H_k^eff| and the normalized dynamic range of H_d/|H_k^eff|. Here, H_k^eff, H_k2, are the effective and second-order magnetic anisotropy field of the free layer in MTJ. Remarkably, experimental NL^max plotted as a function of H_k2/|H_k^eff| and H_d/|H_k^eff|, which were measured from FMR technique coincided with the predictions of our model. Based on these experiment and calculation, we conclude that H_k2 is the origin of NL and strongly influences its magnitude. This finding gives us a guideline for understanding NL and pioneers a new prospective for linear-output MTJ sensors to control sensing properties by H_k2.