A SQUID BASED AC SUSCEPTOMETER FOR THE INVESTIGATION OF LARGE SAMPLES

We developed a SQUID based susceptometer with a large available magnet ized volume for the investigation of large objects. The magnetizing fi eld is generated by a pair of Helmoltz coils. To achieve a high signal -to-noise ratio, the susceptometer is operated in a lock-in mode with an AC magnetizing field. A negative feedback control allows the reject ion of the applied field with a relative residual of 1 x 10(-7). The a pparatus was tested with substances of known magnetic susceptibility. The overall sensitivity, stated in terms of the magnetic moment, is be tter than 7 x 10(-11) A m(2) for small samples.