Measurements of Initial Relative Permeability of Ferromagnetic Material in Low Magnetic Fields Using a YBCO Superconductor at 77.4K

A measuring system for determining the initial relative permeability (μsi) of ferromagnetic materials at the boiling point of liquid nitrogen (77.4K) is developed by using a YBCO superconductor as the standard sample. The μsi is measured in low magnetic fields (over 10-6T to 10-4T) below the lower critical field (μ0Hc1, 5.0×10-4T) of YBCO superconductor. The values of μsi for the typical ferromagnetic materials such as the ferrite, 78-permalloy, soft iron, and electromagnetic stainless steel were 10.70, 10.77, 10.75, and 10.85, at a frequency of 60Hz, respectively. The μsi of a ferrite was constant over the frequency range from 10Hz to 20kHz at both temperatures of 77.4K and 300K. The μsi's of typical ferromagnetic materials without a ferrite were constant for frequency values up to 100Hz and then decreased as the frequency ranged from 100Hz to 20kHz. It was shown that the frequency dependence of μsi at 77.4K was more remarkable than that at 300K. The μsi's of typical ferromagnetic materials were also found to be independent of the magnetic field at both 77.4K and 300K over the range from 10-6T to 10-4T. The accuracy of μsi due to thermal expansions of the rod samples and the differential coil was about 0.1%.