Superconducting susceptometer for high-accuracy routine operation

We present a new version of a SQUID susceptometer with the following performance. The overall accuracy is δχ≃3×10−9 (SI units). Within this figure is allowed the reproducibility in interchanging samples, the thermal expansion of the samples up to 10%, and the constancy of the calibration over a time scale of months. The sample can be interchanged in minutes and the overall time taken by the actual measurement procedure, to give results within the accuracy quoted above, is of the order of 5 min for one sample at one temperature. The temperature regulation is precise within 0.2 K between 40 and 300; the accuracy in the sample temperature has been tested to be within 0.5 K in the room temperature range. This performance is of great relevance for studies of biological molecules in solution, when different samples in different solution conditions must be compared with accuracies better than parts in a thousand of the magnetic susceptibility of the solution.