Dispersive microwave magnetometry of single molecule magnet crystals

Dispersive microwave readout of SQUIDs is a promising technique for magnetometry of molecular magnets, as it offers good sensitivity, high bandwidth, and minimal back action. We have fabricated a sample consisting of a 1.5 GHz aluminum coplanar stripline resonator with a quality factor Q=1000, terminated with a Al-AlO$_{x}$-Al unshunted tunnel junction SQUID with loop area 9 $\mu $m$^{2}$. Flux signals coupled to the SQUID loop are detected as changes in the phase of the reflected microwave signal. We present data characterizing the effective flux noise of the detector as a function of microwave drive power and flux bias. By exploiting the enhanced sensitivity when the resonator is operated in the nonlinear regime we achieve a minimum effective flux noise of 0.5-1 $\mu \Phi _{0}$ / rt Hz. This effective flux noise in the nonhysteretic regime is presently limited by our system noise temperature of 10 K. Data showing low field T$_{1}$ dispersion measured on a small crystal of Cr$_{7}$Ni diluted in Cr$_{8}$ will be presented. We acknowledge support from AFOSR and the US DOE.