Propagation of Large Barkhausen Discontinuities, III. Effect of a Circular Field with Torsion

Abstract

The propagation of large Barkhausen discontinuities along a nickel-iron (15 Ni 85 Fe) wire which is under tension alone is practically unaffected by the presence of a large circular field caused by a current through the wire, but such a circular field creates large effects if the wire is twisted. By treating the critical field as a vector having the main field as one component and the circular field as the other, critical field characteristics were obtained from which a simple relation was found between the strain applied to the wire and the critical field. This analysis shows that the component of the field perpendicular to that principal strain axis along which the extension is a maximum has no effect on the reversal of magnetization, thus establishing this principal strain axis as a direction of preferred orientation of the magnetic domains in the strained wire. Certain deviations of the critical field characteristics from the ideal simple form can be accounted for by taking account of (1) the change of strain with depth in a twisted wire, (2) the change in circular field with depth, and (3) the incomplete suppression by the applied strain of heterogeneous internal strains near the axis of a wire which is under torsion alone. Turning from directional properties to an analysis of the magnitude of the critical field, dependence of this on strain magnitude alone was expected. It appears that another unknown factor is of vital importance here. Velocity (longitudinal) vs. field plots for torsion alone with a succession of values of circular field show in one case the usual increasing slope with decreasing propagation range, in another very little correlation between these factors. Hysteresis curves taken with constant circular field show in some cases negative coercive force and negative remanence. A "hysteresis" curve of longitudinal magnetization as a function of circular field, taken in zero longitudinal field, was a closed symmetrical loop showing a large discontinuity in each branch. This behavior is readily accounted for.