Magnetic phase diagram and critical current of BaFe_2As_2 single crystals with hole- and electron-doping

摘要

We report on comparative study of magnetic phase diagram and critical current of the hole- and electron-doped BaFe_2As_2 single crystals with close values of superconducting critical temperature, T_c, (slightly underdoped Ba_(0.64)K_(0.36)Fe_2As_2 with T_c=25K and optimally doped BaFe_(1.9)Ni_(0.1)As_2 with T_c=20K) obtained from measurements of the temperature dependence of ac-susceptibility and isothermal irreversible magnetization loops, M(H), in magnetic fields parallel to the c-axis of the crystal. From ac-susceptibility measurements we get estimation of a slope of the upper critical field, H_(c2), in dependence on temperature, dH_(c2)/dT ≈ -4.2T/K for BaFe_(1.9)Ni_(0.1)As_2 single crystal and H_(c2)/dT ≈ -1.75T/K for Ba_(0.64)K_(0.36)Fe_2As_2 sample that in accordance with Werthamer, Helfand, and Hohenberg (WHH) model gives H_(c2)(0) = 0.69T_c((dH_(c2))dT) ≈ 56T for BaFe_(1.9)Ni_(0.1)As_2 sample and lower value of H_(c2)(0) ≈ 31T for Ba_(0.64)K_(0.36)Fe_2As_2 crystal. However, obtained from M(H) measurements temperature dependence of the irreversibility field, H_(irr)(T), for BaFe_(1.9)Ni_(0.1)As_2 crystal located below the one for Ba_(0.64)K_(0.36)Fe_2As_2 crystal. Furthermore, at T=4.2K and higher temperatures our results for critical current density, J_c, calculated from M(H) curves clearly show slower reduction of J_c with increasing field in even underdoped Ba_(0.64)K_(0.36)Fe_2As_2 sample compared to optimally doped BaFe_(1.9)Ni_(0.1)As_2 crystal demonstrating higher capacity of K-doped 122 compounds for production of superconducting cables and wires with high critical current in strong magnetic fields.