Sensitivity enhancement of magnetoelectric sensors through frequency-conversion

Thin film magnetoelectric (ME) sensors show sensitivity levels as low as 7.1 pT root Hz over narrow bandwidths at bandwidths of some Hz around their mechanical resonance frequency. The high sensitivity is making the sensors - in principle - suitable for biomagnetic measurements like magnetoencephalography (MEG) and magnetocardiography (MCG). Biomagnetic measurements, however, usually require high sensitivity over a wide frequency band of 0.1-100 Hz. Unfortunately, at such low frequencies far from resonance the ME coefficient decreases dramatically and the noise level increases: this leads to a significant reduction in sensitivity. This work proposes and demonstrates a novel frequency-conversion-approach, which represents a remedy to the sensitivity decay. It allows wideband measurements at low frequencies by utilizing the non-linear characteristics of the magnetostriction curve. The new technique offers the possibility to achieve resonance enhanced sensitivities at virtually arbitrary frequencies outside and therefore also far below resonance. Measurements show that sensitivity at 1 Hz can be enhanced by a factor of similar to 1000 compared to the non-resonant case using the proposed modulation technique. The new technique also offers advantages for the increase of the sensor slew rate, the suppression of mechanical noise and for the operation of such sensors in arrays. (C) 2012 Elsevier B.V. All rights reserved.