Per G. Bjornsson

Gradiometric micro-SQUID susceptometer for scanning measurements of mesoscopic samples

We have fabricated and characterized micro-SQUID susceptometers for use in low-temperature scanning probe microscopy systems. The design features the following: a 4.6 mum diameter pickup loop; an integrated field coil to apply a local field to the sample; an additional counterwound pickup-loop/field-coil pair to cancel the background signal from the applied field in the absence of the sample; modulation coils to allow setting the SQUID at its optimum bias point (independent of the applied field), and shielding and symmetry that minimizes coupling of magnetic fields into the leads and body of the SQUID. We use a SQUID series array preamplifier to obtain a system bandwidth of 1 MHz. The flux noise at 125 mK is approximately 0.25 mu Phi 0/ sqrt Hz above 10 kHz, with a value of 2.5 mu Phi 0/ sqrt Hz at 10 Hz. The nominal sensitivity to electron spins located at the center of the pickup loop is approximately 200 muB/ sqrt Hz above 10 kHz, in the white-noise frequency region.

Scanning superconducting quantum interference device susceptometry

We report a scanning superconducting quantum interference device (SQUID) microsusceptometer with a spatial resolution of 8 μm, tested by measuring the susceptibility of individual 3 μm diam tin disks. Images of the disks agree well with numerical modeling based on the known geometry of the SQUID microsusceptometers. The low-field spin sensitivity between 1.5 and 6 K is 1×105 μB/Hz while scanning.

A scanning SQUID microscope in a dilution refrigerator

Abstract We have designed and built a scanning SQUID microscope in a dilution refrigerator, capable of magnetic imaging at temperatures down to 20 mK . As sensors we use susceptometer SQUIDs with two pickup loops and on-chip field coils to allow measurement of both the magnetic susceptibility of the sample and the magnetic field at the sample surface on a mesoscopic length scale. The instrument is useful for studying superconductivity and magnetic effects in novel materials and electronic coherence effects (such as persistent currents) in mesoscopic systems.