VARIABLE FREQUENCY 2-PHASE AC SUSCEPTOMETER FOR USE IN DC BIAS FIELDS OVER THE TEMPERATURE-RANGE OF 4.2-K TO 300-K

A system has been designed and constructed in order to detect differential ac susceptibility as a function of temperature between 4.2 and 300 K. A dc bias field in the range of 0-50 kOe is provided by a superconducting magnet. The available ac field range is 0-1 Oe. Using the reference source of a PAR lock-in amplifier, the measurements can be carried out in a frequency range of 0.1-10 kHz. A differential amplifier circuit is used to accurately detect the susceptibility signal. As this provides both in-phase and out-of-phase compensating signals, both the real and imaginary components of the susceptibility can be detected even when using a single-phase lock-in amplifier. The circuit diagram and principle of operation as well as the calibration procedure and data processing methods are described in detail.