  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 per sqrHz and 10 nT per sqrHz 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 per sqrHz. 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.
