  We demonstrate the integration of a mesoscopic ferromagnetic needle with a
cavity optomechanical torsional resonator, and its use for quantitative
determination of the needle's magnetic properties, as well as amplification and
cooling of the resonator motion. With this system we measure torques as small
as 32 zNm, corresponding to sensing an external magnetic field of 0.12 A/m (150
nT). Furthermore, we are able to extract the magnetization (1710 kA/m) of the
magnetic sample, not known a priori, demonstrating this system's potential for
studies of nanomagnetism. Finally, we show that we can magnetically drive the
torsional resonator into regenerative oscillations, and dampen its mechanical
mode temperature from room temperature to 11.6 K, without sacrificing torque
sensitivity.
