(a) Field lines from a current in a ring positioned on axis but slightly below the pickup loop, with the shielding tab shown in gray. (b) Field lines from the dipole moment of a mesoscopic sample located at the center of a pickup loop. The flux captured by the pickup loop increases as the loop diameter decreases. The net flux also increases if the sample is moved to a position directly under the wire forming the pickup loop. (c) Simplified diagram of the device, showing the general relationship between the pickup loops, the field coils, and the modulation coils. The center tap of the field coils allows compensation of lithographic imperfections between the two pickup loops. The shading represents the low inductance planar coaxial shield on the susceptometer arms.
(a) Photomicrograph of the full device, prior to polishing the tip. Material below the white dashed line is removed during polishing. Pads for wirebonding are at the top. The distance between pickup loops is . The area enclosed by the box near the center of the SQUID is enlarged in (b), and the area enclosed by the box near the tip is enlarged in (c). (b) Close-up view of the core area of the SQUID, including junctions, shunt resistors, and modulation coils. (c) Close-up view of the sensor area, after polishing. Note shielding layers above both the pickup loop leads and the field coil leads.
Circuit diagram for the device operation. Local compensated field coils apply magnetic field to the two ends of the SQUID susceptometer (S) with coupling . Modulation coils with allow for additional feedback circuitry to keep the device at an optimal working flux bias, while linearizing the response to an applied field. The susceptometer is voltage biased through and the SQUID current is coupled to a series array SQUID preamplifier (A) with input mutual inductance . The flux bias of S is set through an offset voltage in the feedback circuitry, and the flux bias of A is set through the mutual inductance .
Operating characteristics of the SQUID susceptometer. Main graph: Current-voltage characteristics of the SQUID at various flux bias points Inset: SQUID current, , as a function of the modulation coil current which couples flux through the modulation coils rather than the pickup loop. The voltage bias at this operating point is .
(a) Noise spectrum observed in a functional scanning setup at . The low frequency (-like) noise is believed to be associated with the magnetic field of spins, as discussed in the text. The rms white noise floor is approximately . (b) White noise floor (points) as a function of temperature. The shaded areas represent the quantum and thermal noise limits for the optimal performance of a resistively shunted device with an inductance of . The dashed line represents a fit to the Johnson noise temperature dependence that includes the effect of weak electron-phonon interactions limiting the minimum electron temperature to .
(a) Optical micrograph of a sample with aluminum rings and a gold meander wire. (b) Lock-in measurement of flux resulting from a current carried through the meander wire. This is a different grid section than in (a). (c) Response of several superconducting rings to magnetic field applied by the field coils (susceptibility scan). (d) Higher resolution image of a single ring showing the sensor’s imaging kernel in susceptibility mode. (e) Image of a single superconducting vortex in niobium, demonstrating the magnetometry imaging kernel. (f) Susceptibility scan of the grid lines (with zero grid current) at . The response is consistent with a concentration of spins.
(a) Lock-in measurement of the SQUID response to current applied by the field coil. The cross marks are centered on the susceptibility response of a gold ring with a sample structured similar to Figs. 6(a) and 6(f). The white circles and plus marks indicate positions sampled for weighted background subtraction. (b) Set of curves taken at all the points indicated in (a). The data are indistinguishable at this scale. (c) SQUID response after the points off of the ring (white circles) are subtracted from the points centered over the ring (white plus marks), weighted by the SQUID’s response kernel. (d) Subtracting a fitted linear response, presumably from spins in the ring, leaves a nonlinear response which we attribute to nonequilibrium effects in the metal.
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