  Cavity optomechanical systems have enabled precision sensing of magnetic
fields, by leveraging the optical resonance-enhanced readout and mechanical
resonance-enhanced response. Previous studies have successfully achieved
scalable and reproducible microcavity optomechanical magnetometry (MCOM) by
incorporating Terfenol-D thin films into high-quality ($Q$) factor whispering
gallery mode (WGM) microcavities. However, the sensitivity was limited to 585
pT/Hz$^{1/2}$, over 20 times inferior to those using Terfenol-D particles. In
this work, we propose and demonstrate a high-sensitivity and scalable MCOM
approach by sputtering a FeGaB thin film onto a high-$Q$ SiO$_2$ WGM microdisk.
Theoretical studies are conducted to explore the magnetic actuation constant
and noise-limited sensitivity by varying the parameters of the FeGaB film and
SiO$_2$ microdisk. Multiple magnetometers with different radii are fabricated
and characterized. By utilizing a microdisk with a radius of 355 $\mu$m and a
thickness of 1 $\mu$m, along with a FeGaB film with a radius of 330 $\mu$m and
a thickness of 1.3 $\mu$m, we have achieved a remarkable peak sensitivity of
1.68 pT/Hz$^{1/2}$ at 9.52 MHz. This represents a significant improvement of
over two orders of magnitude compared with previous studies employing sputtered
Terfenol-D film. Notably, the magnetometer operates without a bias magnetic
field, thanks to the remarkable soft magnetic properties of the FeGaB film.
Furthermore, as a proof-of-concept, we have demonstrated the real-time
measurement of a pulsed magnetic field simulating the corona current in a
high-voltage transmission line using our developed magnetometer. These
high-sensitivity magnetometers hold great potential for various applications,
such as magnetic induction tomography and corona current monitoring.
