Anomalous Hall Effect sensor based on CoFeB with ultrahigh magnetic field detectability*

The anomalous Hall Effect (AHE) in ferromagnetic metals and alloys have been widely studied as a potential candidate for magnetic sensing applications, due to the relative easiness of fabrication, broad frequency response and lower intrinsic noise. Nevertheless, AHE sensor suffers from relatively low sensitivity in comparison with semiconductor Hall sensor. In this work, we have demonstrated that sensitivity of AHE sensor based on Ta/CoFeB/MgO/Ta multi-layer structure can be greatly enhanced, through engineering of magnetic anisotropy. By tuning thickness of CoFeB layer as well as the annealing temperature, ultra-high sensitivity (1000 V/A/T, comparable to that of semiconductor Hall sensor) and zero hysteresis can be achieved. We have also performed noise measurement over a broad frequency range. The low-frequency 1/f noise is found to strongly depend on input current and bias magnetic field, while the high-frequency white noise shows much weaker dependence. The best magnetic field detectability of our AHE sensor reaches 70 nT/rtHz at 1 Hz and 10 nT/rtHz at 1 kHz. Our work shows that AHE sensors are suitable for ultra-small magnetic field sensing applications.