Nonlinear Cantilever AC Magnetometry with Feedback Linear Subtraction

In order to characterize quantum interactions in condensed matter systems, increasingly precise and efficient measurements of responses that transcend linear behavior become important. Here we present a novel experimental method to directly measure AC nonlinear magnetic susceptibility using a torque magnetometer. Torque magnetometry has emerged in recent decades as a very sensitive tool to measure the linear susceptibility of extremely small samples and is able to compete with SQUID based detection of the magnetization. Its use in extracting the nonlinear susceptibility is not yet well established. Here we report on a measurement method where we employ a micromachined silicon cantilever with integrated piezoresistors to extract the nonlinear part of the magnetic response of a sample. A calibrated current loop on the cantilever serves to provide an AC magnetic field that counterbalances the oscillation of the cantilever due to the linear response.of the sample. Application of this technique for studies of Kitaev quantum spin liquid candidates and Kagome superconductors will be presented.