(from "Fundamentals of Magnetism and Magnetic Measurements" by Mike McElfresh of Purdue University)

As a sample is moved through the superconducting coils, the sample induces an electric current in the detection coils. The detection coils, the connecting wires and the SQUID input coils form a closed superconducting loop, so any change produced is detected and is proportional to the change in magnetic flux. The superconducting SQUID functions as a highly linear current-to-voltage convertor, so the variations in SQUID voltage output are proportional to the magnetic moment of the sample.

Since the SQUID is extremely sensitive to magnetic fields, care is taken to shield it from fluctuation in the ambient magnetic field of the lab and the large magnetic fields produced by its superconducting magnets. Thus a superconducting shield is used to provide a volume of relatively low magnetic fields for the SQUID to function in. While the SQUID does not need to have a low magnetic field, it does need a STABLE magnetic field. Thus the shield provides a stable field for the SQUID to work in. To realize why a stable field is needed, remember that the SQUID can detect a change of .001 of flux quantum, where flux quantum is 2.07 x 10^-7 G-cm². The magnetic flux in 1 cm² of the earth's magnetic field is about 2 million flux quanta.

There are many more details to the MPMS system. We recommend reading "Fundamental of Magnetism and Magnetic Measurements: Featuring Quantum Design's Magnetic Property Measurement System" by Mike McElfresh of Purdue University for an easy to understand overview of this piece of equipment. Much of the material on these pages has been obtained from the above site.

Information provided by: http://www.binghamton.edu