Gradiometric micro-SQUID susceptometer for scanning measurementsof 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 μ m 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 μ Φ 0 ∕ Hz above 10 kHz , with a value of 2.5 μ Φ 0 ∕ Hz at 10 Hz . The nominal sensitivity to electron spins located at the center of the pickup loop is approximately 200 μ B ∕ Hz above 10 kHz , in the white-noise frequency region.