A SQUID-BASED AC SUSCEPTOMETER

We have developed a high-resolution ac susceptometer that uses a rf su perconducting quantum interference device to directly measure the flux coupled into a superconducting detection coil from a sample's changin g magnetic moment in an applied ac field. The system operates in a fre quency range from 0.01 to 1500 Hz and an applied ac field range of 0.1 -400 mu T with a sensitivity of about 5X10(-12) A m(2) for magnetic mo ment measurement, and at a reduced sensitivity down to 0.001 Hz. The i nstrument is based on an existing de magnetometer system and uses that system's temperature control and dc superconducting magnet to allow o peration over a temperature range from 2 to 400 K and in applied dc fi elds of +/-5.0 T. During a measurement all operations are controlled a utomatically by computer from a menu-driven software system, with user input required only on initiation of a measurement sequence. Both rea l and imaginary components of the ac susceptibility can be determined.