Advanced sensors for scanning SQUID microscopy

As part of a joint Stanford/IBM effort to build a scanning SQUID microscopy user facility at Stanford, we have designed and fabricated three types of scanning SQUID microscope sensors. The first is a SQUID susceptometer, with a symmetric, gradiometric design, pickup loops with 0.1 micrometer minimum feature size integrated into the SQUID body through coaxially shielded leads, integrated flux modulation coils, and counterwound one-turn field coils. The second is a SQUID sampler, in which a picosecond current pulse generated on chip is inductively coupled into a hysteric scanning SQUID sensor. The feedback flux to keep the average SQUID voltage at a constant value is proportional to the flux through the sensor pickup loop at a fixed time delay. JSPICE simulations indicate that time resolutions below 10 picosec can be obtained. The third type is a dispersive SQUID, in which the capacitance and Josephson inductance of a one-junction SQUID are chosen so it has an LC resonance in the GHz range. The Josephson inductance depends on the magnetic flux through the SQUID. The magnetic flux is sensed through phase shifts in the reflected microwave signals at resonance. Calculations indicate spin sensitivities better than 1 Bohr magneton per root Hz for a 0.3 micrometer pickup loop diameter, with bandwidths of about 100 MHz possible.