Propagation of Large Barkhausen Discontinuities. II

Large Barkhausen discontinuities have previously been observed by Forrer and by Preisach in nickel wires and hard-drawn wires of the nickel-iron series respectively under stress. A prediction that these discontinuities occur in form of a propagation along the wire, starting at a nucleus, has now been substantiated. In the experiments an additional local field was used to start the propagation at a definite point on the wire, which was in a uniform magnetic field, and the velocity was determined by measuring the short time interval elapsing between the passage through two search coils around the wire. With a fixed value of tension on the wire, the velocity $v$ was found to vary approximately linearly with the applied uniform field $H$, so that $v=A(H\ensuremath{-}{H}_{0})$. $A$ is the slope of the velocity-field characteristic and ${H}_{0}$ is called the critical field. Measured velocities range from 500 to 40,000 cm ${\mathrm{sec}}^{\ensuremath{-}1}$. ${H}_{0}$ varies with composition, amount of cold working, and with the stress applied to the wire. Increasing tension reduces the critical field over the greater part of the Ni-Fe alloy composition range. The behavior of ${H}_{0}$ with increasing and decreasing tension shows the presence of elastic hysteresis. $A$ is nearly constant for changes in tension, in diameter of wire, for composition of wire, and is the same for a strip. Its value is approximately 25,000 cm ${\mathrm{sec}}^{\ensuremath{-}1}$ ${\mathrm{gauss}}^{\ensuremath{-}1}$.The existence of eddy currents limits the speed with which magnetism can penetrate the wire. A rough calculation of this time gives values in the neighborhood of ${10}^{\ensuremath{-}2}$ sec. Thus a discontinuity travelling at ${10}^{4}$ cm per sec. occupies a length of some 100 cm on the wire. This was substantiated both by measurements of the peak voltage induced in a search coil and by oscillograms taken of the induced voltage. The observed passage times agree well with the theoretical penetration times.The constancy of the $v\ensuremath{-}H$ slope for wires of different diameters is believed toindicate that the velocity depends upon surface phenomena rather than volume phenomena. The velocity would thus be determined by conditions existing near the front edge of the discontinuity where the penetration is still slight. The critical field is believed to represent a threshold value of magnetic field which must be exceeded at all points of the wire before reversal of magnetism can occur. The excess of the impressed field over the critical is nullified during propagation by the fields arising from the eddy currents. A possible picture of the discontinuity is one in which the reversal occurs within a minute distance of an approximately conical surface in the wire, the edge of the base of the cone forming the front of the wave.The explanation advanced by Preisach for the asymmetric hysteresis loops found if one limit of the magnetization cycle was reduced has been extended. In this case magnetic inhomogeneities act as nuclei. Mechanical distortion introduces inhomogeneities of another type which also lead to the very easy formation of a nucleus. The phenomena found with torsion are more complicated than for tension. In some cases the slope of the $v\ensuremath{-}H$ lines shows appreciable variation with direction of twist. The results of tests with various compositions of the nickel-iron series are described using both tension and torsion, but no newrelations to other properties of these alloys can be given so far. Identifying ${H}_{0}$ with coercive field, R. Becker's theory has been compared with our results. It appears that in most cases increased elastic tension and increased cold working stresses shift ${H}_{0}$ in opposite directions.