Measurement of Nanoscale Magnetization Using a Torsional Optomechanical Cavity

Torque magnetometry is a sensitive and efficient tool for measuring the magnetic anisotropy of magnetic materials. Measurements rely on the detection of the rotational motion of a mechanical resonator. Torque is imparted by the magnetization of an attached magnetic sample under applied fields.  However, this method suffers from decreased sensitivity at the nanoscale compared to other magnetometry techniques even in cryogenic and vacuum conditions.

We introduced optomechanical sensing and demonstrated torque magnetometry sensitive enough to be operational in room conditions.  We fabricated a nanobeam photonic crystal cavity and made it mechanically compliant by introducing a split in the middle of the nanomechanical resonator. The optomechanical interaction was carefully tailored to optimize sensitive detection of the torsional mode of the device.

The chip-scale sensor was used to probe the net magnetization and RF susceptibility of a single micron-size thin film of permalloy. Torque measurements show magnetic events such as Barkhausen transitions in the magnetic hysteresis due to the interaction of spins with intrinsic defects on the film. This new method bridges optomechanics with nanomagnetism and offers a new tool to sensitively explore mesoscale condensed matter systems with potential application in magnetic memories.