Direct-Current Measurements on Tape-Wound Cores

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Abstract
This paper reports hysteresis-loop measurements made on tape-wound cores of commercial-grade grain-oriented 50Ni-50Fe material at very low domain wall velocities. The experimental equipment includes a low-output-impedance core driver, a low-drift electronic integrator, and a 30 in.× 30 in. X-Y recorder. The duration of a complete flux reversal can be varied between 5 seconds and 30 minutes. The rate-of-change of flux during a reversal can be held constant within 1%, so that variations in the ease of domain wall motion (which appear as Barkhausen discontinuities in conventional measurements) are constrained to appear as variations of magnetomotive force. The coarse structure of these variations is repeatable on successive measurements; the fine structure, for the most part is not. In addition to showing these variations of mmf, the experimental arrangement displays minor loops in their correct locations with respect to the major loop. These measurements indicate that, under dc conditions, flux changes always begin at the inner circumference of the core, as predicted by a previously proposed model of flux change in polycrystalline tapes. Many other aspects of these minor-loop measurements can be explained by this model, which is currently being further developed. On the basis of the model, the recorded variations in mmf can easily be used to find the variations in the energy required to move domain walls in various portions of the core, under conditions of constant rate-of-change of flux. (Under such conditions, the product of wall velocity and wall area is a constant.) The repeatability of the coarse variations indicates their dependence on local metallurgical variations within the core. The nonrepeatability of the fine variations probably indicates a randomness in the configuration of the domain walls as they pass through the same region on successive measurements.

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