Self-oscillating micro-SQUIDs for application in a scanning SQUID microscope

The obtainable spatial resolution of a scanning SQUID microscope depends among others on the effective SQUID area. An effective area of only a few /spl mu/m/sup 2/ is desirable as it allows for a spatial resolution of a few /spl mu/m. SQUIDs with conventional tunnel junctions are not suitable for this task, since tunnel junctions usually have a relatively large area and are therefore sensitive to magnetic fields themselves. A SQUID with nanobridges as weak links provides very small junction areas of below 0.1 /spl mu/m/sup 2/. When making use of the thermal hysteresis in the I-V-characteristics of these bridges, a SQUID relaxation oscillator can be realised. In such a self-oscillating SQUID (SOS) the oscillation frequency depends periodically on the applied magnetic flux. By reading out the frequency change instead of a voltage change, a simple and compact SQUID readout results. As a first application of such micro-SQUIDs, a scanning SQUID microscope was built and the magnetic field of a nickel tip with 100 nm apex radius was measured.