  Magnetostrictive optomechanical cavities provide a new optically-readout
approach to room temperature magnetometry. Here we report ultrasensitive and
ultrahigh bandwidth cavity optomechanical magnetometers constructed by
embedding a grain of the magnetostrictive material Terfenol-D within a high
quality (Q) optical microcavity on a silicon chip. By engineering their
physical structure, we achieve a peak sensitivity of 26 pT/Hz^1/2 comparable to
the best cryogenic microscale magnetometers, along with a 3~dB bandwidth as
high as 11.3 MHz. Two classes of magnetic response are observed, which we
postulate arise from the crystallinity of the Terfenol-D. This allows single-
and poly-crystalline grains to be distinguished at the level of a single
particle. Our results may enable applications such as lab-on-chip nuclear
magnetic spectroscopy and magnetic navigation.
