Critical current densities and flux creep rates in near optimally doped BaFe_2−xRu_xAs₂ (x≈0.7) single crystals

Highlights

• Vortex dynamics for a BaFe_2−xRu_xAs₂ (x≈0.7) single crystal is reported.

• J_c at low magnetic fields is well explained by low density of strong pinning centers.

• The collective pinning energy and the glassy μ exponent were estimated.

Abstract

We present an investigation of the critical current densities J_c and flux creep rates in a near optimally doped BaFe_2−xRu_xAs₂ (x≈0.7) single crystal by (measuring magnetization). The superconducting critical temperature is 18 K. The in-field dependences of the critical current density J_c are due to a mixed pinning scenario produced mainly by large precipitates and a less significant contribution of random disorder. Furthermore, a Maley analysis in the regime dominated by strong pinning centers (μ₀H=0.1 T) is well described through a glassy exponent μ=1.9 and a collective pinning energy (U₀) smaller than 100 K.

Introduction

Vortex dynamics in the so-called 122 iron-based superconductors has been the focus of many studies during the last years [1], [2], [3]. Superconductivity can be induced by substituting the different sites, e.g., for BaFe₂As₂ with K (Ba site) [4], Co (Fe site) [5], Ru (Fe site) [6] and P [7]. These compounds show superconducting transition temperatures (T_c) between conventional and cuprate superconductors, low anisotropy (γ≈2), and high upper critical fields H_c2 (small coherence lengths ξ). The vortex matter in 122 systems shows features in common with those observed in cuprates [1], [2], [3], [8]. Small pinning energies (U₀) and glassy relaxation with characteristic exponent μ similar to those predicted by the collective pinning theory have been reported [1], [2], [9]. The short ξ usually present in 122 systems is particularly susceptible to pinning due to the vortex interaction with small imperfections of the crystalline structure. The resulting pinning is the non-trivial sum over all the contributions of the different types of crystalline defects. Pristine 122 single crystals usually present precipitates [8], twin boundaries [10], chemical inhomogeneities and random disorder [11]. Noticeable enhancement in the critical current density (J_c) has been reported when including artificial pinning centers such as random point defects and amorphous tracks [3], [12], [13], [14], [15]. Combining random point defects and strong pinning centers produces a noticeable improvement of in-field J_c [16]. On the other hand, high J_c values, which are promising for construction of superconducting magnets, have been reported in isovalent BaFe₂ (As_0.66P_0.33)₂ thin films with nanoparticles [17].

The absolute J_c values and their in-field dependences are given by the interaction of the flux vortices and defects [18]. The resulting vortex dynamics depends on the superconducting material, and on the size, geometry and density of the pinning centers. The effectiveness of the pinning depends primarily on the ratio between ξ and the size of the defects, since it is the smallest length scale resolvable by the vortex core [18]. Pinning from inhomogeneities smaller than ξ is much less effective than that produced by large defects. The resulting vortex dynamics for random point disorder has been explained by the weak collective theory. This theory considers that each single vortex line is pinned by the collective action of many weak point-like pins. The pinning energy results from a competition between the pinning potential and the elastic deformation of the vortices. At low fields (single vortex regime SVR), the vortices weakly interact and a single vortex line is collectively trapped by various pins. When the magnetic field is raised, the vortex–vortex interaction increases and the vortices are trapped as bundles [18]. Experimentally, the resulting mechanisms for vortex pinning in iron-based superconductors are more complex as a consequence of mixed pinning landscapes. The temperature and in-field dependences of J_c are well described by superposed collective and strong pinning regimes [8], [19].

To understand the role of intrinsic superconducting parameters on the resulting vortex dynamics it is of great importance to compare the magnetic field–temperature (H–T) vortex phase diagrams of iron-based superconductors with modified doping levels [2]. However, the superconducting properties in 122 systems are very sensitive to doping [20], [21]. Inhomogeneities and chemical gradients are usually present in as-grown single crystals [21] and are expected to be higher in non-optimal doped systems [5], [20]. Among the different 122 systems, Ba(Fe_1−xRu_x)₂As₂ is an isovalent substituted one with a maximum T_c around 20 K [22], which presents a strong competition between superconductivity and antiferromagnetic order. This order is suppressed by increasing the Ru doping and thus, superconductivity occurs at x>0.4 [22]. The optimal doped Ba(Fe_1−xRu_x )₂As₂ (x≈0.7) has a lower upper critical field (H_c2) than other superconducting optimal doped 122 compounds [23], [24], [25], [26], [27]. Therefore, it is special for investigating the interaction of large ξ (0)≈4 nm with the mix pinning landscape usually present in as-grown single crystals [8], [9].

Here, we examine the resulting J_c and vortex dynamics in an as-grown near optimally doped BaFe_2−xRu_xAs₂ (x≈0.7) single crystal. We find that the resulting temperature dependence of J_c is well explained by the combination of random point defects and strong pinning centers. Furthermore, collective creep regime (usually manifested as a SPM) is strongly suppressed. This fact can be attributed to the large coherence length ξ (0)≈4 nm, which reduces the pinning produced by random disorder. For fields where pinning is mainly dominated by strong centers (μ₀H=0.1 T), the vortex dynamics is well described by collective pinning theory with a glassy exponent μ≈1.9 and a pinning energy (U₀) smaller than 100 K.