A pico-Tesla magnetic sensor with PZT bimorph and permanent magnet proof mass

Ferromagnetic-ferroelectric composites have attracted interests in recent years for use as magnetic field sensors. The sensing is based on magneto-electric (ME) coupling between the electric and magnetic subsystems and is mediated by mechanical strain. Such sensors for AC magnetic fields require a bias magnetic field to achieve pT-sensitivity. Here we discuss measurements and theory for a novel passive, AC magnetic sensor that does not require a bias magnetic field and is based on a PZT bimorph with a permanent magnet proof mass. Mechanical strain on the PZT bimorph in this case is produced by interaction between the applied AC magnetic field and remnant magnetization of the permanent magnet, resulting in an induced voltage across PZT. Our studies have been performed on sensors with a bimorph of oppositely poled PZT platelets and a NdFeB permanent magnet proof mass. Magnetic floor noise N on the order of 100 pT/√Hz and 10 nT/√Hz are measured at 1 Hz and 10 Hz, respectively. When the AC magnetic field is applied at the bending resonance of ~ 40 Hz for the bimorph, the measured N ~ 700 pT/√Hz. We also discuss a theory for the magneto-electro-mechanical coupling at low frequency and bending resonance in the sensor and theoretical estimates of ME voltage coefficients are in very good agreement with the data.