Systematic investigation on correlation between sensitivity and nonlinearity in magnetic tunnel junction for magnetic sensor

Summary form only given. Magnetic sensor-application of magnetic tunnel junctions (MTJs) has been proposed and demonstrated by many groups[1][2]. In order to achieve a linear magnetoresistance (MR) behavior to magnetic field, it is one popular method to align an easy axis of a sensing layer and a reference layer to be perpendicular to each other. This crossed magnetization-configuration can be realized by utilizing perpendicular magnetic anisotropy materials[3][4]. We previously reported the linear response in the MTJs with the perpendicularly magnetized CoFeB-sensing layer[5]. The improvement of the sensing properties, both of sensitivity and nonlinearity, is necessary for practical use so far. However, the Stoner-Wohlfarth (SW) model predicts the trade-off relationship between the sensitivity and the nonlinearity in this system. In this study, we systematically investigated the correlation between the sensitivity and the nonlinearity by varying the composition and the thickness of the CoFeB-sensing layer. The stacking structure was Si-substrate/SyF pinned layers/MgO barrier/CoFeB sensing layer /capping layers, deposited by DC/RF magnetron sputtering. We employed Co₂₀Fe₆₀B₂₀ and Co₄₀Fe₄₀B₂₀ as the sensing layer, and varied its thickness t_CoFeB from 1.20 nm to 2.20 nm. The MTJs were microfabricated into the rectangular junctions with the size of 80 × 40 μm² by photolithography process. Post-annealing process was performed in a vacuum for 1 h at varying temperatures T_a under the in-plane magnetic field of 1 T. We measured transport properties by DC four-probe-method under in-plane magnetic field, and magnetic properties by vibrating sample magnetometer. Fig. 1 shows the MR curves for the MTJs annealed at T_a = 275°C. We successfully obtained the linear response in the MTJs with the t_CoFeB less than 1.80 nm and 1.65 nm for Co- sub>20Fe₆₀B₂₀ and Co₄₀Fe₄₀B₂₀, respectively. This indicates the CoFeB-sensing layers were perpendicularly magnetized and the crossed magnetization-configuration was realized in these MTJs. From the magnetization curves for the CoFeB-sensing layers, the easy axes were determined to be the perpendicular direction, corresponding to the MR curves. Fig. 2 shows the sensitivity as a function of nonlinearity in the MTJs, and its calculation result by the SW model. The sensitivity is determined as the value of TMR ratio divided by the sensing range (400 Oe). The nonlinearity is defined as the normalized difference between the data and its linear fit, expressed as the percentage of full scale (FS) in the sensing range. The calculation was performed on the assumption that the magnitude of TMR ratio = 40%. The trade-off relationship between them agrees with the SW model qualitatively. The slight deviation from the SW model may derive from multiple factors: the change in the magnitude of TMR ratio, another mechanism of the magnetization motion such as domain wall motion, or the nonuniformity of the magnetic anisotropy in the sensing layer.