The Paramagnetic Meissner Effect (PME) in Metallic Superconductors

Abstract

The experimental data in the literature concerning the Paramagnetic Meissner Effect (PME) or also called Wohlleben effect are reviewed with the emphasis on the PME exhibited by metallic, s-wave superconductors. The PME was observed in field-cool cooling (FC-C) and field-cool warming (FC-W) m(T)-measurements on Al, Nb, Pb, Ta, in compounds such as, e.g., NbSe2, In-Sn, ZrB12, and others, and also in MgB2, the metallic superconductor with the highest transition temperature. Furthermore, samples with different shapes such as crystals, polycrystals, thin films, bi- and multilayers, nanocomposites, nanowires, mesoscopic objects, and porous materials exhibited the PME. The characteristic features of the PME, found mainly in Nb disks, such as the characteristic temperatures T1 and Tp and the apparative details of the various magnetic measurement techniques applied to observe the PME, are discussed. We also show that PME can be observed with the magnetic field applied parallel and perpendicular to the sample surface, that PME can be removed by abrading the sample surface, and that PME can be introduced or enhanced by irradiation processes. The PME can be observed as well in magnetization loops (MHLs, m(H)) in a narrow temperature window Tp<Tc, which enables the construction of a phase diagram for a superconducting sample exhibiting the PME. We found that the Nb disks still exhibit the PME after more than 20 years, and we present the efforts of magnetic imaging techniques (scanning SQUID microscopy, magneto-optics, diamond nitrogen-vacancy (NV)-center magnetometry, and low-energy muon spin spectroscopy, (LE-μSR)). Various attempts to explain PME behavior are discussed in detail. In particular, magnetic measurements of mesoscopic Al disks brought out important details employing the models of a giant vortex state and flux compression. Thus, we consider these approaches and demagnetization effects as the base to understand the formation of the paramagnetic signals in most of the materials investigated. New developments and novel directions for further experimental and theoretical analysis are also outlined.

Complex AC Magnetic Susceptibility as a Tool for Exploring Nonlinear Magnetic Phenomena and Pinning Properties in Superconductors

“…However, for a superconductor with a particular granular morphology [52] and/or internal-void structure, the field penetration process can be much more sophisticated. A prominent example is the establishment of a paramagnetic Meissner (Wohlleben) effect [60]. It is based on the magnetic reentrance in the superconductor transition and formed due to interactions between the trapped remanence field profile in the sample, bulk superconductivity effects and especially the complex weak-link networktypes of Josephson junctions (π − π) and the symmetry of the superconducting order parameter (s-wave, d-wave).…”

Deposition of Superconducting Nb3Sn Coatings Using Multiple Magnetron Sputtering Techniques

“…This result also explains the large volume concentration of oxygen measured by energy dispersion spectroscopy. For the formation of the target Nb3Sn phase, selected samples were subjected to hightemperature annealing in a vacuum furnace (at a temperature of 800 °C) since as-deposited Nb3Sn coatings do not demonstrate high critical temperatures [15]. Before and after the temperature exposure, the microstructure and phase composition of the samples were controlled using XRD.…”

“…In this regard, the use of Nb 3 Sn coatings, which have almost double the magnetic superheating field of 400 mT and a higher superconducting critical temperature of 18.3 K, seems very promising [11][12][13][14][15][16]. To date, several different methods have been proposed and developed for Nb 3 Sn thin film deposition.…”