Detailed investigation of the superconducting transition of niobium disks exhibiting the paramagnetic Meissner effect

Abstract

The superconducting transition region in a Nb disk showing the paramagnetic Meissner effect (PME) has been investigated in detail. From the field-cooled magnetization behavior, two well-defined temperatures can be associated with the appearance of the PME: ${T}_{1}$ $(<{T}_{c})$ indicates the characteristic temperature where the paramagnetic moment first appears and a lower temperature ${T}_{p}$ $(<{T}_{1})$ defines the temperature where the positive moment no longer increases. During the subsequent warming, the paramagnetic moment begins to decrease at ${T}_{p}$ and then vanishes at ${T}_{1}$ with the magnitude of the magnetization change between these two temperatures being nearly the same as that during cooling. This indicates that the nature of the PME is reversible and not associated with flux motion. Furthermore, the appearance of this paramagnetic moment is even observable in fields as large as 0.2 T even though the magnetization does not remain positive to the lowest temperatures. Magnetic hysteresis loops in the temperature range between ${T}_{1}$ and ${T}_{p}$ also exhibit a distinct shape that is different from the archetypal shape of a bulk type-II superconductor. These behaviors are discussed in terms of the so-called ``giant vortex state.''