An automatic frequency-sweeping SQUID susceptometer

A new, SQUID-based, measuring system has been developed for the investigation of the dynamic properties of magnetic materials. Its main advantages, compared to conventional mutual inductance systems, are its high sensitivity and its nearly frequency independent response, down to extremely low frequencies. With the SQUID system it is now possible to measure directly and rapidly the frequency dependence of the dynamic susceptibility of weakly magnetic materials in the range from 0.002 Hz to 5 kHz at a fixed value of a constant background field of up to 5 T. The frequency-swept operation allows experiments which are beyond the capabilities of conventional systems, for example the investigation of thermally isolated samples or the study of the dynamics at magnetic phase transitions in cases where the magnetic properties change very rapidly in a small field interval.