Measurements of the Domain Wall Area-Mobility Product during Slow Flux Reversals

摘要

A previous paper has shown that the sides of slow constant-voltage hysteresis loops on polycrystalline tape-wound cores are not smooth; instead the loop sides show irregular, imperfectly reproducible variations. These variations indicate changes in the ease of motion of the so-called transition region (the outward-moving region, formed of moving domain walls, in which the flux is changing). Such changes imply rearrangements of the domain walls within the transition region; and rearrangements of the walls imply changes in their area and mobility. Measurements have been made of a factor K, defined from ec = K(H-Ht), where ec is the induced voltage-per-turn, H is the field applied at the center of the transition region, and Ht is a threshold parameter. K is proportional to the average of the area-mobility product over the walls in the transition region. Using feedback of the induced voltage, K was measured by modulating the applied field H in such a way that the resulting modulation in ec was a small-amplitude square wave. The ratio of the modulation amplitudes in ec and H gave K. This technique, which is similar in principle to Becker''s, has shown that the area-mobility product varies directly with the average rate of flux change and inversely with the level of prior saturation. In the 50-50 Ni-Fe grain-oriented 2-mil tape core for which results are presented, with a prior saturating field of 10 times the coercive force Hc, as the average induced voltage was varied from 1.2 to 20 μv per turn, the mean value of K increased from 8 to 80 μv-per-turn per ampere-turn-per-meter. With the average induced vol-ntage at 1.2, as the prior saturating field was increased from the vicinity of the coercive force to 2Hc, K dropped from 14 to 9, then remained constant at 8 as HS, the prior saturating field, was increased to 100Hc. It is shown that these results are qualitatively consistent with the results from nonmodulated measurements. It is also shown that the number of active domain walls for K = 15 is of the order of 3, if each wall is assumed to be one wrap-of-tape long and if several other drastic assumptions are made.