The superconducting quantum interference device (SQUID) is functionally a magnetic flux to voltage converter. One advantage of SQUID magnetometry is that it directly measures magnetic flux as opposed to torque (torque magnetometer) or current induced by a magnetic field (vibrating sample magnetometer). SQUID magnetometry is also very sensitive measuring moments as small as 80-100 nEMU over a dynamic range of ten orders of magnitude.
A simple loop coupled to a SQUID device would provide some challenges. One, it would detect the flux from the earth's magnetic field, as well as any flux from nearby magnetic systems. To use the SQUID device more effectively, one uses a gradiometer-- a configuration of coils that detects the gradient of the magnetic field. In the case of the Quantum Design MPMS a second derivative gradiometer is used. Such a gradiometer rejects all constant and linearly diverging magnetic flux. Since the sample can be assumed to be a dipole, the geometry of the second derivative gradiometer can be optimized for the detection of dipole moments. Such a gradiometer consists of two coils with the same sense very closely spaced and two coils of the opposite sense spaced symmetrically outside. All four of these coils are tied together. The flux penetrating all four coils is detected by the SQUID device.
In practice the sample is moved or extracted through the gradiometer and the SQUID output measured as a function of position. Based on this profile of voltage (and thus flux) versus position, given the assumption that the signal is from a point dipole, the magnetic moment can be determined. Double-click on the image to see the animated GIF (image borrowed from ICMMO). The gradiometer is interior to the superconducting magnet of the magnetometer. When fields are changed the gradiometer is decoupled from the SQUID and recoupled after field changes are complete. This decoupling is achieved by using a small heater to drive normal a superconducting transformer that couples the gradiometer to the SQUID device.
Proper use of the gradiometer requires not only centering the sample in the gradiometer coils, but mounting samples in such a way that the mounting material provides a continuous signal throughout the gradiometer coils and is thus not detected by the gradiometer.