  We demonstrate a centimeter-scale optomechanical magnetometer based on a
crystalline whispering gallery mode resonator. The large size of the resonator
allows high magnetic field sensitivity to be achieved in the hertz to kilohertz
frequency range. A peak sensitivity of 131 pT per root Hz is reported, in a
magnetically unshielded non-cryogenic environment and using optical power
levels beneath 100 microWatt. Femtotesla range sensitivity may be possible in
future devices with further optimization of laser noise and the physical
structure of the resonator, allowing applications in high-performance
magnetometry.
