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

TL;DR

This study provides a detailed analysis of the superconducting transition in niobium disks exhibiting the paramagnetic Meissner effect, revealing reversible PME behavior and characteristics of the giant vortex state.

Contribution

It offers new insights into the PME in niobium disks, identifying characteristic temperatures and the nature of the effect as reversible and not flux-motion related.

Findings

PME appears at T_1 and vanishes at T_p during both cooling and warming.

Magnetization behavior suggests the PME is reversible and not flux motion related.

Hysteresis loops indicate a giant vortex state in the superconductor.

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'.