Unexpected Effect of Field Angle in Magnetoresistance Measurements of High-Purity Nb
Loren F. Goodrich, Theodore C. Stauffer, Jolene Splett, Dominic F. Vecchia
We report on unexpected field-angle dependence of the magnetoresistance measurements of commercial, high-purity Nb discovered during our study of residual resistivity ratio (RRR) measurements. The (RRR) value indicates the purity and the low-temperature thermal conductivity of the Nb and is used as a specification for superconducting radio-frequency cavities. The (RRR) is typically defined as the ratio of the electrical resistivities measured at 273 K and 4.2 K. One way to obtain the normal-state resistivity at 4.2 K is to measure the resistivity versus magnetic field at 4.2 K and extrapolate to zero field. The field-angle dependence was measured when a specimen was rotated while the field was transverse to the specimen current. The resistance changed by about 10 % as the angle varied at 8 T and the local maxima and minima of the resistance were separated by about 30'. This was observed on three polycrystalline Nb bar specimens with nominal dimensions of 64 mm × 6.7 mm × 3.3 mm and voltage taps separated by 30 mm. A similar field-angle dependence was obtained on a polycrystalline Cu wire specimen. This phenomenon has implications for Nb (RRR) measurements and interlaboratory comparisons.
, Stauffer, T.
, Splett, J.
and Vecchia, D.
Unexpected Effect of Field Angle in Magnetoresistance Measurements of High-Purity Nb, IEEE Transactions on Applied Superconductivity, [online], https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=31807
(Accessed November 26, 2022)