Comparative study of upper critical fieldHc2and second magnetization peakHspin hole- and electron-doped

Abstract

A comparative study of the upper critical field H c2 and second magnetization peak H sp was performed using high-quality single crystals of hole-doped Ba 0.68 K 0.32 Fe 2 As 2 and electron-doped BaFe 1.85 Co 0.15 As 2 and BaFe 1.91 Ni 0.09 As 2 . The H c2 was extracted from both resistivity and magnetization measurements using varying magnetic fields on H ʈ c and H Ќ c orientations. The anisotropic ratio, ␥ = H c2Ќc / H c2 ʈc , was observed to decrease to ϳ2.5 for the hole-doped and ϳ3.0 for both electron-doped samples as the magnetic fields were increased up to 9 T. It demonstrates that the anisotropic properties only show slight change by doping aliovalent ions either in or out-of the basal plane of FeAs. For the hole-doped Ba 0.68 K 0.32 Fe 2 As 2 the H c2 and H sp shift toward the higher temperature and higher field regime in the temperature-normalized ͑T / T c ͒ vortex phase diagram, suggesting a stronger vortex pinning by the comparison with the electron-doped BaFe 1.85 Co 0.15 As 2 and BaFe 1.91 Ni 0.09 As 2 . In contrast to the As-deficiency or inhomogeneous doping distribution of K, Co, and Ni, the dense pinning centers in Ba 0.68 K 0.32 Fe 2 As 2 may be attributed to the disordered structural domains and suggested to be responsible for the intrinsic disorder and anisotropy of iron arsenides.

Flux pinning mechanism in BaFe1.9Ni0.1As2 single crystals: Evidence for fluctuation in mean free path induced pinning

“…24 For example, an under-doped Ba 1Àx K x Fe 2 As 2 system does not show the SMP. Also SMP has been reported for Ba(Fe 1Àx Ni x ) 2 As 2 , 19,25 LiFeAs, 26 FeTe 1Àx Se x (Ref. 27), and PrFeAsO 0.6 F 0.1 .…”

“…The obtained value of J c ¼ 0.14 Â 10 6 A/cm 2 at zero field and T ¼ 10 K is comparable with the reported value of J c ¼ 0.23 Â 10 6 for the optimally doped sample. 19 The inset of Figure 2 illustrates a normalized J c versus B plot at selected temperatures. The onset position, B onset SMP , and peak position, B peak SMP , of the SMP are marked for T ¼ 4 K. The position of SMP shifts toward lower field with increasing temperature, for example, B peak SMP decreases considerably from 6.8 T at 4 K to 0.7 T at 15 K, but B onset SMP drops slowly from 1.7 T at 4 K to 0.2 T at 15 K, respectively.…”

“…The SMP has also been observed in optimally doped Ba 1Àx K x Fe 2 As 2 . 19,23 However, it is worth to note that most form of in-homogeneity like T c variation, impurity phase, doping variation, etc., might prevent the occurrence of SMP. 24 For example, an under-doped Ba 1Àx K x Fe 2 As 2 system does not show the SMP.…”

“…The SMP observed only for the samples near optimally doping for NdFeAsO 0.85 , 18 Ba(Fe 1-x Co x ) 2 As 2 (Refs. [19][20][21][22] and weak and collective pinning are concluded for most studies. The SMP has also been observed in optimally doped Ba 1Àx K x Fe 2 As 2 .…”

“…28 The observed SMP for electron and hole doped Ba-122 crystals for H//c 19 was disappeared for H//ab, indicating an anisotropic effect of flux pinning for these compounds. 19 There are two main pinning mechanisms in type II superconductors: (I) dl pinning from spatial variation in the charge carrier mean free path, l, and (II) dT c pinning due to randomly distributed spatial variation in T c . It has been reported that strong pinning centres in PrFeAsO 0.9 and NdFeAsO 0.9 F 0.1 arise from oxygen deficiency and dopant atoms, which results in pinning by local variations in the mean-free path.…”