Study of the Low-Frequency Excess Equivalent Magnetic Noise in GMI-Based Devices

Abstract

Low-frequency equivalent magnetic noise levels of four different giant magneto-impedance (GMI) samples, exhibiting $1/f$ spectral behavior, have been experimentally determined, for several excitation conditions (involving both dc current and ac excitation amplitude). Under appropriate conditions, for which the noise level at 1 Hz is minimal, coherence measurements show that the main noise source responsible for this low frequency noise is the intrinsic noise of the GMI sample. The lowest corresponding magnetic noise performances of the four samples were determined. The lowest noise level observed was $17\,\textrm {pT}/(\textrm {Hz})^{1/2}$ at 1 Hz. It has been proposed that the imaginary part of the magnetic susceptibility, $\chi ^{\prime \prime }$ , a measure of the low frequency magnetization fluctuations, determines the resulting magnetic noise level of the sensor. Its values at 1 Hz have been been evaluated for all four samples. Using these evaluations, expected noise levels at 1 Hz are computed, based on a previously proposed theoretical model, and compared with the measured noise performances. Under conditions for which correlation measurements show that system noise is dominated by sensor noise, the two sets of values are in reasonably good agreement, suggesting that the proposed model for low frequency excess noise of GMI materials is essentially correct.