A measuring system for determining the initial relative permeability (μ
si) of ferromagnetic materials at the boiling point of liquid nitrogen (77.4K) is developed by using a YBCO superconductor as the standard sample. The μ
si is measured in low magnetic fields (over 10
-6T to 10
-4T) below the lower critical field (μ
0Hc1, 5.0×10
-4T) of YBCO superconductor. The values of μ
si 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 μ
si of a ferrite was constant over the frequency range from 10Hz to 20kHz at both temperatures of 77.4K and 300K. The μ
si'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 μ
si at 77.4K was more remarkable than that at 300K. The μ
si'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 μ
si due to thermal expansions of the rod samples and the differential coil was about 0.1%.
抄録全体を表示