隧穿磁电阻效应磁场传感器中低频噪声的测量与研究

基于隧穿磁电阻效应(TMR)的磁场传感器具有很高的磁场灵敏度, 但同时噪声也较大,有效抑制TMR磁场传感器的噪声, 尤其是低频噪声的抑制对于其在高灵敏度要求场合的应用具有重要的意义. 本文采用高精度数据采集卡搭建了噪声测量系统, 测量了全桥结构TMR磁场传感器的噪声频谱图, 发现TMR传感器的噪声在低频段表现为1/f特性, 同时噪声功率谱密度与工作电流平方成正比关系; 低频噪声在自由层翻转区间内噪声急剧增大, 证明了1/f噪声主要来源于磁噪声, 这一结果为TMR磁场传感器的噪声特性优化指明了方向.

Measurement and study of low-frequency noise in TMR magnetic field sensor

The magnetic field sensor based on tunnel magnetoresistance (TMR) effect has potential applications in various fields due to its very high field sensitivity and low power comsuption. However, the resolution of magnetic sensor depends on not only field sensitivity, but also intrinsic noise level. The intrinsic noise of an electronic device is normally frequency-dependent and increases in low frequency range. In a magnetic tunneling system, thermal magnetization fluctuation in the magnetic layer can couple to the resistance through the spin-dependent tunneling effect and create low-frequency magnetic noise. In addition, the charge trapping effect in the oxide barrier may also contribute to the external low-frequency noise. Therefore, the depression of the noise in TMR magnetic field sensor, especially the low-frequency noise, is extremely important for the application with high resolution requirement. In this work, a low-frequency noise measurement system for TMR magnetic sensor is built by using a highaccuracy data acquisition card and a low noise preamplifier. After subtracting the circuit noise from the measured noise, the noise spectral patterns of TMR magnetic field sensor with a full Wheatstone bridge structure are obtained under various bias currents and external magnetic fields. It is found that the noise spectra of the TMR sensor exhibit a clear 1/f character in the low frequency region and the noise power spectral intensity is proportional to the square of the bias current. By fitting the power spectral density of the noise versus frequency in the TMR sensor, the Hooge parameters are obtained, which remain unchanged in the measurement. The noise intensity increases abruptly in the magnetization switching region of the free layer in magnetic tunnel junction, suggesting that the 1/f noise mostly comes from the magnetic noise. In a magnetic hysteresis loop, this noise power is strongly field-dependent, which is due to thermal magnetization fluctuations in magnetic layers. We attribute this magnetic fluctuation to thermally excited hopping of the magnetic domain wall between the pinning sites. Finally, according to the R-H transfer curves and the measured noise spectra of the TMR sensor, the detectable minimum magnetic fields of the sensor are 9 nT and 1.3 nT at 100 Hz and 4 kHz with 1 V input voltage, respectively. These results pave a way for optimizing the noise properties of TMR magnetic sensors.