Low-Frequency Noise Characterization of CoFeB/MgO/CoFeB MTJ-Based Perpendicular Field Sensor

We report low-frequency electrical resistance noise of Co40Fe40B20/MgO/Co20Fe60B20-based magnetic tunnel junction field sensors with reference and sensing layer magnetization directions along the out-of-plane and in-plane directions, respectively. The devices are fabricated using the sputter deposition and conventional lithography techniques with a short axis of $\sim 6~\mu \text{m}$ long and long axes of $\sim 6$ –12 $\mu \text{m}$ long. Noise power spectra under different bias currents indicate increase in noise power with increasing bias current. A close 1/ $f$ dependence of the noise according to Hooge’s relation, $S_{V}(f)= AV^{2}/f^{\alpha }$ ( $A$ is Hooge’s like constant) is observed with an average value of $\alpha = 1.03 \pm 0.08$ . In smaller devices, the magnetic field dependence of noise amplitude follows similar trend of magnetoresistance (MR) behavior without any distinct peak near antiparallel–parallel states transition of the devices, which is generally observed for magnetic defects-induced resistance fluctuation. The experimental observations infer the resistance noises in those devices that are related to the defects associated with the spin independent charge trapping at structural defects near or in the barrier layer. The device with the smallest lateral size exhibits the Lorentzian contribution, whereas the device with the largest size shows magnetic contribution in total noise. Numerically deduced magnetic fluctuations $\Delta B_{\mathrm{rms}}$ of all the devices shows an average magnetic fluctuation of $\sim 10$ nT except for one device. The devices show the highest MR% of $\sim 27$ % under perpendicular magnetic field with a dynamic range of $\sim \pm 25$ Oe and a sensitivity of $\sim 0.3$ %/Oe. The observed low-frequency noise levels are suitable for the perpendicular field detection application and can be reduced more by improving the crystallinity of the device films stacks and their interfaces.