Köszegi, J.; Kugeler, O.; Abou-Ras, D.; Knobloch, J.; Schäfer, R.: A magneto-optical study on magnetic flux expulsion and pinning in high-purity niobium. Journal of Applied Physics 122 (2017), p. 173901/1-9
10.1063/1.4996113
Open Access Version (externer Anbieter)
Abstract:
The magnetic flux trapped inside high-purity niobium samples after field cooling was investigated by indirect magneto-optical imaging using an indicator film. The detected magnetic field was compared with the field present during the phase transition, and the distribution of trapped flux was investigated. The measurements were performed on an untreated sample cut from an ingot and a sample that was heated at 1400 C for 4 h. For untreated niobium, the trapped flux was found to be homogeneously distributed and almost all applied field during phase transition was trapped. In contrast, the heat treated niobium showed significantly reduced trapping. Neither did the grain boundaries play a major role as pinning centers nor did the crystal orientation influence the amount of trapped flux significantly, which is contrary to recent assumptions. However, niobium hydrides, which formed during the cool-down-stage to cryogenic temperatures, were found to enhance trapping considerably.