Harmonic response of GMR sensor under sinusoidal excitation field

In this work, we report the readout technique of the GMR magnetometer by detecting the odd-harmonic response under an sinusoidal excitation field. The time domain output waveform of the sensor is measured, and an analytical model is used to simulate its behaviour. In the frequency domain, it is found that the major Fourier components of the sensor response are the even harmonics of excitation frequency. The even harmonics are insensitive to the applied field, whereas the odd-harmonic amplitudes change significantly with the increasing applied field. To maximize the field-to-voltage transfer coefficient, the mixing circuit with a square wave reference is used to detect the odd-harmonic components of the output signal. In a small signal mode, the measured field-to-voltage coefficient of the sensor is 138 mV/mT under a 9 V bias voltage, achieving the noise level of 17 nT/Hz^1/2 at 1 Hz. The noise level can be further reduced by utilizing the multiple-harmonic reference. The proposed device is suitable for measuring the E arth's dc magnetic field.