Doping dependent Irreversible Magnetic Properties of Ba(Fe1-xCox)2As2 Single Crystals

We discuss the irreversible magnetic properties of self-flux grown Ba(Fe1-xCox)2As2 single crystals for a wide range of concentrations covering the whole phase diagram from the underdoped to the overdoped regime, x=0.038, 0.047, 0.058, 0.071, 0.074, 0.10, 0.106 and 0.118. Samples were characterized by a magneto-optical method and show excellent spatial uniformity of the superconducting state. The overall behavior closely follows classical Bean model of the critical state. The field-dependent magnetization exhibits second peak at a temperature and doping - dependent magnetic field, Hp. The evolution of this fishtail feature with doping is discussed. Magnetic relaxation is time-logarithmic and unusually fast. Similar to cuprates, there is an apparent crossover from collective elastic to plastic flux creep above Hp. At high fields, the field dependence of the relaxation rate becomes doping independent. We discuss our results in the framework of the weak collective pinning and show that vortex physics in iron-based pnictide crystals is much closer to high-Tc cuprates than to conventional s-wave (including MgB2) superconductors.Comment: for the special issue of Physica C on iron-based pnictide superconductor

Uniaxial-strain mechanical detwinning of CaFe2As2 and BaFe2As2 crystals: Optical and transport study

The parent compounds of iron-arsenide superconductors, AFe(2)As(2) (A = Ca, Sr, Ba), undergo a tetragonal to orthorhombic structural transition at a temperature T-TO in the range 135-205 K depending on the alkaline-earth element. Below T-TO the free standing crystals split into equally populated structural domains, which mask intrinsic, in-plane, anisotropic properties of the materials. Here we demonstrate a way of mechanically detwinning CaFe2As2 and BaFe2As2. The detwinning is nearly complete, as demonstrated by polarized light imaging and synchrotron x-ray measurements, and reversible, with twin pattern restored after strain release. Electrical resistivity measurements in the twinned and detwinned states show that resistivity, rho, decreases along the orthorhombic a(o) axis but increases along the orthorhombic b(o) axis in both compounds. Immediately below T-TO the ratio rho(bo)/rho(ao) = 1.2 and 1.5 for Ca and Ba compounds, respectively. Contrary to CaFe2As2, BaFe2As2 reveals an anisotropy in the nominally tetragonal phase, suggesting that either fluctuations play a larger role above T-TO in BaFe2As2 than in CaFe2As2 or that there is a higher temperature crossover or phase transition.This article is published as Tanatar, M. A., E. C. Blomberg, A. Kreyssig, M. G. Kim, N. Ni, A. Thaler, S. L. Bud’Ko et al. "Uniaxial-strain mechanical detwinning of CaFe 2 As 2 and BaFe 2 As 2 crystals: Optical and transport study." Physical Review B 81, no. 18 (2010): 184508. DOI: 10.1103/PhysRevB.81.184508. Copyright 2010 American Physical Society. Posted with permission