Origin and measurement of quartz resonator magnetic sensitivity

The magnetic sensitivity of quartz crystal resonators is a consequence of the ferromagnetic properties of the metal used as support for the vibrating plate. Various magneto-mechanic interactions can contribute to the overall sensitivity, the most important of them is shown to be magnetostriction: when submitted to a magnetic field, most of the ferromagnetic materials undergo an expansion or contraction which induces a stress in the sustained plate and then a change in the crystal resonant frequency. The striking likeness between nickel magnetostriction curve and the frequency-vs.-magnetic field curve of resonators is at the origin of this assumption. Other experiments have been performed to check on this assumption, in particular an experimental setup using a heterodyne interferometric laser probe with enhanced sensitivity has been used to investigate in situ the magnetostriction of the plate supports. Also, a comparison between magnetic behaviour of identical resonators mounted with different materials definitely proves the responsibility of the supports in the magnetic sensitivity of resonators and gives interesting information on its reduction