  Optomechanical magnetometers enable highly sensitive magnetic field sensing.
However, all such magnetometers to date have been optically excited and
read-out either via free space or a tapered optical fiber. This limits their
scalability and integrability, and ultimately their range of applications.
Here, we present an optomechanical magnetometer that is excited and read out
via a suspended optical waveguide fabricated on the same silicon chip as the
magnetometer. Moreover, we demonstrate that thermomechanical noise limited
sensitivity is possible using portable electronics and laser. The magnetometer
employs a silica microdisk resonator selectively sputtered with a
magnetostrictive film of galfenol (FeGa) which induces a resonant frequency
shift in response to an external magnetic field. Experimental results reveal
the retention of high quality-factor optical whispering gallery mode resonances
whilst also demonstrating high sensitivity and dynamic range in ambient
conditions. The use of off-the-shelf portable electronics without compromising
sensor performance demonstrates promise for applications.
