Measurement of Spin Susceptibility of Thin Films and Nano-Scale Structures

We report measurements of a spin-like paramagnetic susceptibility signal from high purity metallic and insulating thin films.~ The measurements were performed using a Superconducting Quantum Interference Device (SQUID) in a scanning microscope.~ By using the SQUID to scan areas of the sample both near and far from the metallic films, we found a paramagnetic susceptibility associated with both Au and AlOx films that was ten times larger than could be explained by the concentration of impurity spins expected for 6N gold.~ The 1/T temperature dependence and the paramagnetic sign indicate that the susceptibility signal is caused by localized spins that are at most weakly coupled to each other and to the conduction electrons.~ Moreover, the signal exhibits a measurable out of phase response which can be related to 1/f noise due to fluctuating spins [Koch, DiVincenzo, and Clarke, Phys. Rev. Lett. \textbf{98}, 267003 (2007)].~ These results demonstrate the utility of scanning SQUID based susceptibility measurements for characterizing spin related effects.~ Further applications of this technique may include probing 1/f noise origins in superconducting devices as well as imaging magnetic structures such as nanomagnets.