Milos Jirsa

Extension of the time window for investigation of relaxation effects in high-Tc superconductors

Abstract Measurements of the dependence of the magnetic hysteresis loop amplitude in a superconductor on the magnetic field sweep rate d H e /d t are shown to provide essentially the same information as magnetic relaxation experiments (flux-creep) at fixed external field H e . For practical reasons, however, magnetic hysteresis loop measurements are better suited to investigate fast flux-line movements than conventional flux-creep measurements. From the general flux movement differential equation it is shown that each magnetic hysteresis loop recorded at a given d H e /d t can be labelled by an effective time t M 0 so that this experimental information can be used to extend the time window of flux-creep measurements to short times. This method is applied to the experimental data obtained on a YBa 2 Cu 3 O 7 single crystal. In this way the time window is extended to less than 10 -2 s which is valuable for a more precise check of existing theoretical models for the activation energy of flux motion. It is found that the experimental data are well described by using an activation energy U ( j )=( U c / μ )[( j c / j ) μ -1] with μ=0.74 at 16 K and 1.8 T.

A comparative study of irreversible magnetisation and pinning force density in (RE) Ba2Cu3O7−δ and some other high-Tc compounds in view of a novel scaling scheme

Abstract We examined an extended set of experimental data on scaling of the field dependencies of the critical current density j s ( B ) and/or the related pinning force density F ( B ) as found in literature for various high- T c single crystals, mainly (RE)Ba 2 Cu 3 O 7−δ (RErare earth), exhibiting the fishtail effect. The j s ( B ) and/or F ( B ) curves, normalised with respect to the corresponding (fishtail) maximum, are shown to obey the functional dependencies j sc ( b ) = b m exp[( m / n )(1 − b n )] and ƒ(b f ) = b f m+1 exp {[(m + 1)/m](1 − b f n )} where b and b f , are the applied field normalised with respect to position of the corresponding (fishtail) maximum, and j sc and ƒ are the normalised values of the critical current density, and pinning force density, respectively. A possible link between the presented phenomenological scaling scheme and the theory of collective pinning is discussed. Most of the analyzed data could be well fitted with m set equal to 1 and n left as the only free parameter. n varied from 0.5 to 3.95 but most values fell into the interval (1,2). We point out that the value n = 3 2 in the middle of this interval is predicted by the collective pinning theory for the small bundle pinning regime.

Dynamic contribution to the fishtail effect in a twin-free DyBa2Cu3O7−δ single crystal

Abstract Superconducting current densities j s and dynamic relaxation rates Q  d ln j s /d In(d B e /d t ), where d B e /d t is the sweep rate of the external magnetic field B e , were measured as a function of temperature (5 K T 2 Cu 3 O 7−δ single crystal by means of a high-sensitivity capacitance torque magnetometer. Above 15 K, we observe a “fishtail” effect, i.e. a pronounced minimum in the j s ( B e ) curve at fields around B e = 1 T. The relaxation rate Q shows an anomalous increase at low fields which is correlated to the minimum in the j s ( B e ) curve. Both the j s versus B e and Q versus B e data are used as input parameters into the generalized inversion scheme developed by Schnack et al. [Phys. Rev. B 48 (1993) 13178] to calculate the true critical current density j c which is by definition independent of relaxation effects. Interestingly, the j c ( B e , T ) curves derived in this way do not show a minimum. This points clearly to a dynamic contribution to the fishtail effect. The true critical current density j c ( B e , T ) decreases weakly with increasing B e over the entire measured temperature and field range, as expected for single-vortex pinning. This indicates that the observed fishtail effect is not caused by a crossover from single-vortex pinning to pinning of flux bundles. The temperature dependence of j c is in good agreement with the predictions of a model based on single-vortex pinning caused by spatial fluctuations in the charge-carrier mean free path.

Magnetic properties of ceramics

Magnetic properties of a series of Mg-doped granular samples were studied by means of vibrating-sample and SQUID magnetometry. The equilibrium moment measured after the sample demagnetization gave evidence that the surface barrier effects can be neglected up to 60 K. Slope of the high-field dependence was used for determination of the temperature dependence of the penetration depth and its extrapolation to T = 0, . The latter quantity was found to be significantly affected by the expression used for the data fit. The irreversible moment deduced from the magnetic hysteresis loops showed a pronounced fishtail effect. In the temperature range 50-60 K, the curve was found to scale in a similar manner as in (RE)-123 single crystals. The conventional relaxation rate dramatically changes at the fields around the remanent state. The field range of this anomaly coincides with the central peak width, and we attribute this effect to redistribution of the magnetic flux in the sample. A simple non-monotonic dependence was observed of the fishtail maximum position on the nominal Mg content. A similar but mirror dependence was found of .

Relaxation in superconductors with large normal defects

Magnetic flux creep in a superconductor with large normal spherical defects is studied. The known coordinate dependence of the pinning energy allowed us to deduce the current dependence of the effective pinning barrier and the associated relaxation rate. A nonlinear differential equation governing the flux creep was derived. A macroscopic characteristic length determining the scale of the spatial flux changes was found. Magnetic flux density profiles during the penetration and relaxation processes were calculated for various mean defect size, R.

Flux pinning by twin planes in Nd-123 and Y-123 single crystals

Abstract The pinning by twin planes and an isotropic pinning disorder was investigated on single crystals of NdBa 2 Cu 3 O 7− δ and YBa 2 Cu 3 O 7− δ studying the angular dependence of total magnetic moment measured quasistatically by means of SQUID and dynamically by means of a vibrating sample magnetometer. The field dependence of the normalized relaxation rate of the vortex system pinned by twin planes, and by a random isotropic pinning disorder was determined. The qualitative difference of the two field profiles is clearly reflected by the shapes of the corresponding magnetic hysteresis loops. Material aspects of the pinning and the effect of the measurement method are discussed.

Formation of the low-field peak in magnetization loops of high-Tc superconductors

Abstract The positions of the central (low-field) peak in the magnetization hysteresis loops (MHLs) are analyzed in various high- T c superconducting samples comprising several RBa 2 Cu 3 O 7− δ (RBCO; R=rare earths) single crystals of different thicknesses, a laser-ablated YBa 2 Cu 3 O 7− δ (YBCO) thin film, Ag-sheathed (Pb,Bi) 2 Sr 2 Ba 2 Cu 3 O 10+ δ (Bi-2223) mono- and multifilamentary tapes, and a model sample designed to reproduce a layer of grains [M.R. Koblischka et al., Appl. Phys. Lett. 70 (1997) 514]. The single crystals and the thin film show the peak at zero-field or at negative applied fields on the descending field branch according to the critical state models, the Bi-2223 tapes are found to exhibit the peak anomalously in positive applied fields. In order to better understand the magnetization processes leading to the formation of the central peak in the MHLs, the local field distributions in applied fields close to zero were studied using magneto-optic (MO) flux visualization on the same samples. These flux patterns show how the vortices are rearranged when sweeping through zero-field. A large demagnetizing effect (“perpendicular geometry”) facilitates the penetration of vortices of opposite polarity, especially along structural defects, thus, forcing the central peak towards zero or even to very small positive fields. To explain the anomalous behaviour found in Bi-2223 tapes, effects of granularity have to be considered additionally. Further, we discuss the interaction of the central peak with other “peak effects” observed in MHLs.

Relaxation and scaling of magnetization around the fishtail minimum in single crystal with columnar tracks

Conventional and dynamic relaxation processes and scaling properties of the magnetic hysteresis loops (MHL) were studied on a Pb-ion irradiated single crystal by means of the torque magnetometry. The columnar pinning structure produced by irradiation enhanced considerably the induced critical current density and caused the high-field part of the MHL to be field-independent up to . The substantial change of the pinning structure shifted the appearance of the fishtail effect to higher temperatures and fields. The relaxation experiments confirm a correlation between the relaxation rate and the MHL shape and point to quite different relaxation regimes at high and low fields. Correspondingly, the scaling of the MHLs in both field ranges is different. This behaviour is modelled by two separable contributions to the MHL, one being active mainly at high fields and the other at low fields (central peak). Possible origins of these two contributions are discussed.

Relaxation and pinning in spark-plasma sintered MgB2 superconductor

The model of thermally activated relaxation developed and successfully tested on high-T c superconductors (Jirsa et al 2004 Phys. Rev. B 70 0245251) was applied to magnetic data of a bulk spark-plasma sintered MgB2 sample to elucidate its magnetic relaxation behavior. MgB2 and the related borides form a superconductor class lying between classical and high-T c superconductors. In accord with this classification, the relaxation phenomena were found to be about ten times weaker than in cuprates. Vortex pinning analyzed in terms of the field dependence of the pinning force density indicates a combined pinning by normal point-like defects and by grain surfaces. An additional mode of pinning at rather high magnetic fields (of still unknown origin) was observed.

Microstructure and Superconducting Properties of YBCO Bulk Superconductors With RE Substitutions

The influence of RE (RE = Yb, Sm) substitutions on the microstructure and superconducting properties of Y-Ba-Cu-O (YBCO) bulk superconductors has been studied for different RE concentrations and their combinations. Precursor powders YBa₂Cu₃O_7-δ (Y-123), Y₂O₃ and CeO₂ were enriched with different amounts of Yb₂O₃ and (or) Sm₂O₃ powders with the aim to increase critical current density, i.e., Jc, in self-field as well as in higher magnetic fields, by introducing additional pinning centers. YBCO bulk superconductors with RE₂O₃ addition were prepared by the optimized top-seeded melt-growth process in the form of single grains. Microstructure analysis revealed that RE₂O₃ addition leads to a higher amount of smaller Y₂BaCuO₅ (Y-211) particles, which is related to high critical current density (J_c ~ 7 × 10⁴ A/cm²) of the samples with RE₂O₃ addition in low magnetic fields. In this paper, the effect of RE₂O₃ addition on Y-211 particle size, critical temperature T_c, and critical current density J_c is reported.