Doping Dependence of the Second Magnetization Peak, Critical Current Density, and Pinning Mechanism in BaFe_2–xNi_xAs₂ Pnictide Superconductors

Abstract

A series of high quality BaFe 2−x Ni x As 2 pnictide superconductors were studied using magnetic relaxation and isothermal magnetic measurements in order to study the second magnetization peak (SMP) and critical current behaviour in Ni-doped 122 family. The temperature dependence of the magnetic relaxation rate suggests a pinning crossover, whereas, it's magnetic field dependence hints a vortex-lattice structural phase-transition. The activation energy (U ) estimated using the magnetic relaxation data was analyzed in detail for slightly-underdoped, slightly-overdoped and an overdoped samples, using Maley's method and collective creep theory. Our results confirm that the SMP in these samples is due to the collective (elastic) to plastic creep crossover as has been observed for the other members of 122-family. In addition, we also investigated the doping dependence of the critical current density (J c ) and the vortex-pinning behaviour in these compounds. The observed J c is higher than the threshold limit (10 5 A/cm 2 ) considered for the technological potential and even greater than 1 MA/cm 2 for slightly underdoped Ni-content, x = 0.092 sample. The pinning characteristics were analyzed in terms of the models developed by Dew-Hughes and Griessen et al, which suggest the dominant role of δl-type pinning.

The Interplay of Pressure and Doping Effects on BaFe2As2 Superconductors

“…The study by Pervakov et al [134] even found the data to scale convincingly over a large temperature range in their Ba(Fe 1−x Ni x ) 2 As 2 samples (T = 1.4-7.9 K for x = 0.07, T = 7-17 K for x = 0.05) demonstrating that these results are only minorly affected by the elastic-plastic crossover. Values of h max obtained in reference [137] are a bit lower, ranging from 0.26-0.3 over a similar doping range to our samples.…”

“…, the fishtail effect has been quite concretely experimentally linked to a crossover in pinning behaviour via magnetic relaxation rate measurements [137].…”

Vortex dynamics and second magnetization peak in the iron-pnictide superconductor Ca$_{0.82}$La$_{0.18}$Fe$_{0.96}$Ni$_{0.04}$As$_2$

“…A positive value of exponent ν supports the collective pinning for H < H p [42,45], whereas a negative ν exponent for fields above H p supports plastic pinning. Although, the expected values of ν are 1 and -0.5 for collective and plastic pinning respectively, exponent ν smaller than 1, associated to collective pinning, have also been observed in other systems [6,17,18]. The inset of Fig.…”

“…Similar results have been observed in other iron pnictide superconductors as reported in Refs. [18,60,61,62]. Moreover, according to Dew-Hughes model [57], h max < 0.5 indicates a δl pinning, while h max > 0.5 suggests δT c pinning, therefore, h max = 0.27 in present case is suggestive of δl pinning.…”

“…For this reason the SMP has been studied in many different systems, usually by means of the vortex dynamics. Since the discovery of iron-pnictide superconductors [14] the SMP has been observed in many of these systems and explained in terms of a pinning-crossover as observed for Ba(Fe-Co) 2 As 2 , (Ba-K)Fe 2 As 2 , Ba(Fe-Ni) 2 As 2 , Ca(Fe-Co)As and (Ca-La)(Fe-Co)As 2 [15,16,17,18,19,20,21,22], in terms of a phase transition in the vortex lattice as observed for Ba(Fe-Co) 2 As 2 ,LiFeAs, and BaFe 2 (As-P) 2 [23,24,25,26], and also in terms of an order-disorder transition in some compounds [27,28,29]. It is to note that all the systems mentioned above share a similar layered crystal structure which consists of a spacer layer in-between of FeAs superconducting layers.…”