Magnetic hysteresis in the microwave surface resistance of Nb samples in the critical state

Abstract

a b s t r a c tThe magnetic-field-induced variations of the microwave surface resistance have been investigated in a heavily neutron-irradiated MgB 2 sample, in which the irradiation has caused the merging of the two gaps into a single value. The experimental results have been analyzed in the framework of the Coffey and Clem model. By fitting the experimental data, we have determined the field dependence of the depinning frequency, x 0 , at different values of the temperature. Although the pinning is not particularly effective, the value of x 0 obtained at low temperatures is considerably higher than that observed in conventional low-temperature superconductors.

Microwave surface resistance of pristine and neutron-irradiated MgB2 samples in magnetic field

“…Microwave losses induced by static magnetic fields have been investigated by several authors [27,28,29,30,31,32,42,48]. At low temperatures and for applied magnetic fields lower enough than the upper critical field, the main contribution arises from the fluxon motion; however, it has been pointed out that a noticeable contribution can arise from the presence of normal fluid, especially at temperatures near T c and for magnetic fields of the same order of H c2 (T ).…”

“…The majority of the models assume an uniform distribution of fluxons inside the sample; so, they disregard the effects of the critical state. Very recently, we have investigated the field-induced variations of the mw surface resistance in superconductors in the critical state [42,48], and have accounted for the magnetic hysteresis in the R s (H 0 ) curves.…”

“…1a), the effects of the nonuniform B distribution on R s are particularly enhanced because in the two surfaces of the sample normal to the external magnetic field the mw current and fields penetrate along the fluxon axis and, consequently, the mw losses involve the whole vortex lattice. However, in this case, one can easily take into account the non-uniform B distribution by calculating a proper averaged value of R s over the whole sample as follows [42,48] …”

Influences of Ferromagnetic Substrate on Microwave Surface Resistance of Type-II Superconductors

“…The governing equations which facilitate calculation of microwave surface resistance within the framework of the critical-state model proposed by Bonura et al [24,25] through the theory proposed by Coffey and Clem [26][27][28] are the same as those which can be found in other studies [24,25,29,30]. In the present work, a method based on the finite element method (FEM) is used, which is similar to the approach employed by Gömöry et al [31].…”

“…For calculating surface resistance using Coffey-Clem equations [26][27][28], some physical parameters such as the ratio λ 0 /δ 0 , the lower and upper critical fields, and the depinning frequency must be known. Moreover, in considering the critical-state effects by the model due to Bonura et al [24,25], it is also essential to know the B profile inside the sample. In this case, COMSOL Multiphysics software, which is a commercial implementation of FEM, can be employed to compute geometry-dependent field profile in the sample via its AC/DC module [32].…”