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).