Magnetic measurements of AC loss in short multifilamentary tapes

The vibrating sample magnetometer (VSM) in conjunction with an iron-cored electromagnet is suitable for magnetic AC loss measurement within a field-sweep-rate range of typically zero (point-by-point) to 4 T/min (or 0 similar to 10 mHz). In that it is capable of measuring eddy current loss over that very low range of frequencies, the VSM can be regarded as spanning the gap between the "pick-up-coil regime" and the DC limit represented by the SQUID- and extraction-type magnetometers. Presented here is an example of the use of the VSM in the measurement of low frequency AC loss. Samples for measurement are two sets of multifilamentary CMF) NbTi-base strands, with different filament numbers, that have been flattened to several aspect ratios. It is shown that for the densely filamented strands the eddy-current-measured effective transverse matrix resistivity, p(perpendicular to), agrees well after shape-factor correction, with the standard effective-medium model. Not so for the coarsely filamented tapes which yield anomalously high apparent p(perpendicular to)s. The discrepancy is explained in terms of a reduced "eddy current activation volume" in the latter class of tape. The study demonstrates that when interpreting the results of the AC loss measurement of non-round MF strands, consideration must be given both to external-shape-governed demagnetization and to internal strand architecture.