The role of temperature in the magnetic irreversibility of type-I Pb superconductors

Abstract

Evidence of how temperature plays a role in the magnetic irreversibility in the intermediate state of a cylinder and various disks of pure type-I superconducting lead is presented. Isothermal measurements of first magnetization curves and hysteresis cycles are analyzed in a reduced representation that defines an equilibrium state for flux penetration in all the samples and reveals that flux expulsion depends on temperature in the disks but not in the cylinder. The magnetic field at which irreversibility sets in along the descending branch of the cycle and the remnant magnetization at zero field are found to decrease with temperature. The contributions to irreversibility of the geometrical barrier and the energy minima associated with stress defects that act as pinning centers on normal-superconductor interfaces are discussed in an effort to account for the results obtained.

Magnetic field dependence of the quantum tunneling of normal-superconductor interfaces in a type-I Pb superconductor

“…In this region, the two m 1st (h) curves largely superimpose whereas the m des (h) branches depend on the value of T . Actually, the m des (h) branches obtained as T increases progressively from 2.00 K to 6.00 K (not shown here) span the space bound by the two branches in the figure, while all the corresponding m 1st (h) curves scale onto a single one, in good agreement with previous results obtained in similar samples [15]. In a defect-free sample, in which only geometrical effects are the source for irreversibility, a large energy barrier separates the states that can be attained by submitting the sample to different magnetic history processes and provides strong stability to the system [8].…”

“…At a certain temperature, m 1st (h) should thus follow a series of equilibrium states corresponding to the flux penetration as h increases, while m des (h) and m FC (h) should actually coincide because both curves would correspond to a succession of equilibrium states that would be followed during the flux expulsion as the magnetic field decreases [7]. The lack of isothermal relaxation along the whole m(h) curve and the thermal independent behavior of m des (h) [15] in a sample of this kind are strong proofs of this fact. When other sources of irreversibility like stress defects are present, the ability of the system to trap magnetic flux during the expulsion is enhanced with respect to the defect-free case and is substantially influenced by temperature [15].…”

“…The properties of type-I superconductors have been studied, both theoretically and experimentally, for nearly one century [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15]. Historically, the presence of irreversible phenomena in the magnetic hysteresis cycle had been associated to physical or chemical defects such as dislocations or impurities [2].…”

“…However, if the sample also contains other irreversibility sources, metastability develops and a good amount of magnetic flux can be trapped at zero field [12,15]. The isothermal evolution of such remnant state has been reported to follow a logarithmic law with time in flat disks of Pb with stress defects [14].…”

“…A model for quantum tunneling of interfaces (QTI) based on the Caldeira-Leggett theory for quantum dissipation [16] was recently developed considering that the magnetization evolves with time due to the formation and decay of mesoscopic bumps in the NSI, and was found to fit rather well the experimental data [14]. The pinning barriers act as a source for metastability along the descending branch of the hysteresis cycle and are responsible for the temperature-dependent irreversibility exhibited by the samples in which both geometrical barrier and stress defects are present [15]. The aim of this paper is to investigate the effect of an external magnetic field on the dynamics of NSI in a type-I Pb superconductor.…”