Slow Vortex Creep Induced by Strong Grain Boundary Pinning in Advanced Ba122 Superconducting Tapes*

Abstract

We report the temperature, magnetic field and time dependences of magnetization in advanced Ba122 superconducting tapes. The sample exhibits a peculiar vortex creep behavior. Below 10 K, the normalized magnetization relaxation rate S=dln(-M)/dln(t) shows a temperature insensitive plateau with a value comparable to that of the low temperature superconductors, which can be explained within the framework of the collective creep theory. It then enters into a second collective creep regime when the temperature increases. Interestingly, the relaxation rate below 20 K tends to saturate with the increasing field. However, it changes to a power law dependence on field at a higher temperature. A vortex phase diagram composed of the collective and the plastic creep regions is concluded. Benefit from the strong grain boundary pinning, the advanced Ba122 superconducting tape has promising potential to be applied not only in liquid helium but also in liquid hydrogen or at the temperature accessible with cryocoolers.PACS numbers: 74.25. Qt, 74.25.Sv, 84.71.Mn Understanding the behavior of vortex matter in a superconductor is vitally important for both basic physics and technological applications. Vortex creep caused by thermal fluctuation introduces measurable dissipation and a reduction of the maximum loss-less current. The scale of the thermal fluctuation is usually parameterized by the dimensionless Ginzburg number Gi∝γ 2 T c 2 λ 4 /ξ 2 , where γ is the anisotropy parameter, T c is the superconducting transition temperature, λ is the penetration depth and ξ is the coherence length. Due to high T c , small ξ and large γ, the thermal fluctuation in cuprates is significant, giving rise to the so-called giant vortex creep [1]. It is thus a major task to reduce the detrimental effect of vortex motion in cuprates by incorporating effective pinning centers. Iron-based superconductors (IBSC), on the contrary, have larger ξ, lower γ, and consequently smaller Gi than that of the cuprates [2]. In combination with their high upper critical field H c2 and moderate T c [3], IBSC are considered as potential candidates in largescale application of superconductivity.After ten years of research and design since the discovery of IBSC, the (Ba/Sr) 0.6 K 0.4 Fe 2 As 2 superconducting tapes fabricated by the powder in tube (PIT) method [4,5] are now the prevalent materials for applied research. Up to now, the critical current density J c (4.2 K, 10 T) of the (Ba/Sr) 0.6 K 0.4 Fe 2 As 2 tape has already surpassed the practical application level [6]. Recently, we optimized the hot pressing method and enhance the transport J c to 1.5×10 5 A/cm 2 at 4.2 K and 10 T [7]. It even retains a J c of 5.4×10 4 A/cm 2 at 20 K and 5 T. It was found that the grain size is only 0.5-1 µm, much smaller than that of the Sr 0.6 K 0.4 Fe 2 As 2 counterpart [8] whose grains are *

Current transport, magnetic and elemental properties of densified Ag-sheathed Ba1−xKxFe2As2 tapes

“…This is consequence of the higher critical current density of this conductor with respect to the flat rolled one, as shown in figure 1. In the case of the hot-pressed sample, we observed flux jump instabilities at low fields and for temperatures up to ∼20 K, similarly to what reported in [27]. Critical state models predict that the maximum of the M(H) hysteresis loop occurs at H = 0 [28].…”

“…At 15 K, the signal was too noisy to determine S with sufficient accuracy above 3 T. S is expected to increase when the vortex pinning becomes less effective. Normally, this occurs upon increasing the temperature or the field because of a change in the vortex dynamics regime or in the nature itself of the vortex matter, as in the case of lattice melting [26,27,30]. We observe a clear increase of S with the temperature when H is on the tape plane.…”

Flux pinning and the vortex phase diagram in optimized CaKFe4As4 single crystals fabricated by a one-step method

“…The critical fields between the vortex elastic lattice and vortex glass state or the vortex glass state and vortex liquid state is strongly affected by the anisotropic behaviors and correlated disorder of the superconductors. For example, in high-T c cuprate superconductors and (Li 1−x Fe x )OHFeSe with large anisotropic factor, vortex lattices possess twodimensional characteristic, which gives rise to the vortex glass field of H g is far from upper critical field H c2 [8][9][10].…”

Vortex creep activation energies and depinning currents in CaKFe4As4 and Ba0.6K0.4Fe2As2 revealed by AC susceptibility measurements

“…There are many ways, such as dynamical (static) magnetic relaxation, magneto-transport measurements, and ac susceptibility measurements, to study the vortex dynamics in iron-based superconductors. Based on the model of thermally activated flux creep within the collective vortex pinning, most of the experimental phenomena can be understood [30,31]. In contrast to other inductive measurements, ac susceptibility technology has the advantages of high sensitivity and acquiring data in a much shorter time window of 10 −1 -10 −4 s. It is well known that the temperature dependence of out of phase χ in susceptibility reveals the dissipation mechanism due to the vortex motion, and the maximum in the energy dissipation usually appears at a temperature T p slightly lower than the diamagnetic onset temperature in χ [32].…”