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Schematic diagram of the SQUID susceptometer chip (inside dashed line). Input current is controlled with the internal oscillator of the lock-in amplifier and series resistance . couples flux to the first order gradiometer of the intermediate loop through mutual inductances and . These mutual inductances have the same geometry and their values differ only when a magnetic sample is present, in that case , where is the magnetic susceptibility of the sample and is a calibration factor. The flux coupled by the signal-SQUID is given by . A second stage 16-SQUIDs array is used to read-out . Magnicon XXF-1 electronics is used to operate the system in flux-locked loop (FLL) mode. Finally, the in-phase and out-of-phase components of are read-out with the lock-in amplifier.
(Left) Representation of the lower part of the dilution refrigerator insert. The sensor lies inside the mixing chamber immersed in the rich phase, with the PCB holder plugged into electrical feedthroughs connected to the cryocable in the IVC. (Right) photograph of SQUID-susceptometer chip mounted on PCB holder, it can be easily manipulated for changing sample. Chip size is .
Flux noise of the sensor measured at 4.2 K and 13 mK with a HP 3562A dynamic signal analyzer (limited to 100 kHz).
(Top) In-phase and out-of-phase ac susceptibility components of a single crystal of single-molecule magnets (SSM) at . Measurements within the whole bandwidth of the system enable us to observe the magnetic relaxation of the two molecular species present in this sample. The respective relaxation times have been determined by fits of a combination of two Cole–Cole laws to the experimental data (solid lines). The inset shows a larger single crystal of SSM attached with vacuum grease onto the intermediate loop (pick-up coil) of a SQUID susceptometer. (Bottom) Real and imaginary component of the susceptibility of the same crystal fitted to Cole–Cole functions (solid lines). Measurements at different temperatures down to 15 mK provide much information about the nature of the relaxation process taking place in the sample.
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