Re-entrant Hysteresis Loop Tracer

Abstract

A simple method has been developed for displaying and measuring the important features of re-entrant hysteresis loops of ferromagnetic wires. A conventional ac loop tracer is modified by the addition of a pulse generator and an extra coil coaxial with the specimen. A current pulse in this coil is used to nucleate a reverse domain in the wire at the appropriate time so that domain walls are available to be moved by the field coil. The loop traced on the oscilloscope then shows the wall motion coercivity. When no current pulse is used, the conventional rectangular loop which shows the nucleation field is displayed. If the pulses are generated at half the applied field frequency, the inner and outer loops are displayed alternately and appear to be superimposed. This loop tracer has been used to measure the normal re-entrancy of permalloy wires, the stress-induced re-entrancy of ribbons with positive magnetostriction constants, and an unusual type of re-entrancy in nickel wires. With appropriate modifications this technique can be applied to other specimen configurations and loop tracers.