Modeling Resonances in the Current-Voltage Characteristics of SQUID Susceptometers*

In Scanning SQUID Microscopy, SQUID susceptometers are tools for achieving extremely precise magnetic imaging and local susceptibility measurements with high spatial resolution. Further development of scanning SQUID microscopy demands a thorough understanding of the properties of SQUID susceptometers. One mysterious behavior is the presence of resonances in the current-voltage characteristics of SQUID susceptometers for scanning applications. The origins and the effects of the resonances on the performance of these SQUIDs is unknown. To illuminate the origin and impact of the resonances, we have developed a model that successfully reproduces the experimentally-determined current-voltage characteristics of SQUIDs for one resonance. We explore the limitations of our model by calculating the noise characteristics of SQUIDs of different designs and comparing the results with experimental noise measurements. Looking forward, we aim to extend the model to encompass all the resonances that appear in experimental data. We will then use the model to optimize the signal-to-noise characteristics of scanning SQUID susceptometers.