Very strong intrinsic flux pinning and vortex avalanches in(Ba,K)Fe2As2superconducting single crystals

Abstract

We report that the ͑Ba, K͒Fe 2 As 2 crystal with T c = 32 K shows a pinning potential, U 0 , as high as 10 4 K, with U 0 showing very little field dependence. The ͑Ba, K͒Fe 2 As 2 single crystals become isotropic at low temperatures and high magnetic fields, resulting in a very rigid vortex lattice, even in fields very close to H c2 . The isotropic rigid vortices observed in the two-dimensional ͑2D͒ ͑Ba, K͒Fe 2 As 2 distinguish this compound from 2D high-T c cuprate superconductors with 2D vortices. The vortex avalanches were also observed at low temperatures in the ͑Ba, K͒Fe 2 As 2 crystal. It is proposed that it is the K substitution that induces both almost isotropic superconductivity and the very strong intrinsic pinning in the ͑Ba, K͒Fe 2 As 2 crystal.A high critical current density, J c , upper critical field, B c2 , and irreversibility field, B irr , a high superconducting transition temperature, T c , strong magnetic-flux pinning, good grain connectivity, and isotropic superconductivity are the major physical requirements for superconducting materials used in practical applications operating at low and, in particular, high magnetic fields. The conventional low-T c superconductors, where H c2 is also small, can only carry large J c at very low temperatures. The cuprate high-T c superconductors suffer from poor grain connectivity and easy melting of the vortex lattice, leading to small J c in high magnetic fields at relatively high temperatures. For MgB 2 superconductor with T c of 39 K, B irr is far below H c2 , and J c drops quickly with both field and temperature, preventing its use above 20 K. The newly discovered Fe-based superconductors 1-7 show T c as high as 55 K and B c2 above 200 T, in combination with a small anisotropy for REFeAsO 1−x F x ͑RE-1111 phase, with RE a rare-earth element͒ 8 and an almost isotropic superconductivity for ͑Ba, K͒Fe 2 As 2 ͑122 phase͒. 9 These properties make the Fe-based superconductors extremely promising candidates for high magnetic field applications at relatively high temperatures. The current carrying ability of these superconductors at high fields and temperatures is largely determined by the flux-pinning strength and the behavior of the vortex matter. Therefore, the determination of their intrinsic vortex pinning strength is a central issue from both an applied and a fundamental perspective. Both 1111 and 122 phase compounds have typical two-dimensional ͑2D͒ crystal structures. In RE-1111 phase, where RE is a rare-earth element, the FeAs superconducting layers are separated by insulating LaO layers 10 while in Ba͑K͒-122 phase, the FeAs layer is sandwiched between conductive Ba layers. 5 It is expected that the 122 phase containing two FeAs layers would have small anisotropy and thus higher intrinsic pinning compared to the single layer 1111 phase. Co-doped BaFe 2 As 2 single crystal shows an anisotropy of 1-3 and upper criticalfield values of B c2 ͑B ʈ ab͒ = 20 T and B c2 ͑B ʈ c͒ = 10 T at 20 K, with dB c2 / dT Ϸ 5 T/ K. 11 For single crystals of the optimally do...

Magnetoresistance, critical current density, and magnetic flux pinning mechanism in nickel doped BaFe2As2 single crystals

“…Flux jumping has been also observed in the Ba 1-x K x Fe 2 As 2 single crystal. 20 However, flux jumping and pinning potential behavior have not been reported in the electron doped BaFe 2-x Ni x As 2 so far. In this paper we present the pinning potential behavior in optimal electron doped BaFe 1.9 Ni 0.1 As 2 single crystal.…”

“…15 The absence of the second magnetization peak effect in CaFe 2-x Co x As 2 indicated that the vortex dynamics in this compound is consistent with plastic creeping rather than the collective creep model. 18 (Ba,K)Fe 2 As 2 superconductor with nearly isotropic superconductivity, 19 very high intrinsic pinning, 20 H c2 as high as 200 T, and high pinning potential, as high as 10 4 K 20 is a promising candidate for high magnetic field application. Flux jumping has been also observed in the Ba 1-x K x Fe 2 As 2 single crystal.…”

SANS study of vortex lattice structural transition in optimally doped (Ba1−xKx)Fe2As2

“…Nevertheless, a SMP feature has been observed [29,30,31]. The SANS results on Ba 0.64 K 0.36 Fe 2 As 2 single crystals presented below reveal that vortex lattice disorder can be sufficiently weak to permit the observation of well-defined Bragg peaks corresponding to a long-range orientationally ordered triangular lattice.…”

“…The value of the upper critical field B c2 has been taken from Refs. [29], [39]. The measured intensity has its maximum value at low fields, and drops abruptly above 0.5 T, well below B c2 .…”

Thin Film Growth and Device Fabrication of Iron-Based Superconductors

“…100,102) They concluded that these observations originate from intrinsic pinning due to the correlated ab-planes, i.e., three dimensional pinning. 106) Both the aforementioned buffer layers are electrically conductive because STO buffer layers become conductive upon the high-temperature/high-vacuum deposition of a Ba122:Co film, and Fe is a metal. Hence, it is natural to consider that these buffer layers affect the normal-state resistivity and J c .…”