Scanning SQUID microscopy of SFS $\pi$-Josephson junction arrays

We use a Scanning SQUID Microscope to image the magnetic flux distribution in arrays of SFS (superconductor-ferromagnet-superconductor) Josephson junctions. The junctions are fabricated with barrier thickness such that they undergo a transition to a $\pi $-junction state at a temperature T$_{\pi } \quad \approx $ 2-4 K. In arrays with cells that have an odd number of $\pi $-junctions, we observe spontaneously generated magnetic flux in zero applied magnetic field. We image both fully-frustrated arrays and arrays with non-uniform frustration created by varying the number of $\pi $-junctions in the cells. By monitoring the onset of spontaneous flux as a function of temperature near T$_{\pi }$,$^{ }$we estimate the uniformity of the junction critical currents.