Abstract
Single crystals of the LnFeAsO (Ln1111, Ln = Pr, Nd, and Sm) family with lateral dimensions up to 1 mm were grown from NaAs and KAs flux at high pressure. The crystals are of good structural quality and become superconducting when O is partially substituted by F (PrFeAsO1−xFx and NdFeAsO1−xFx) or when Fe is substituted by Co (SmFe1−xCoxAsO). From magnetization measurements, we estimate the temperature dependence and anisotropy of the upper critical field and the critical current density of underdoped PrFeAsO0.7F0.3 crystal with Tc ≈ 25 K. Single crystals of SmFe1−xCoxAsO with maximal Tc up to 16.3 K for x ≈ 0.08 were grown. From transport and magnetic measurements, we estimate the critical fields and their anisotropy and find these superconducting properties to be quite comparable to the ones in SmFeAsO1−xFx with a much higher Tc ≈ 50 K. The magnetically measured critical current densities are as high as 109 A/m2 at 2 K up to 7 T, with indication of the usual fishtail effect. The upper critical field estimated from resistivity measurements is anisotropic with slopes of ∼−8.7 T/K (H||ab plane) and ∼−1.7 T/K (H||c axis). This anisotropy (∼5) is similar to that in other Ln1111 crystals with various higher Tc's.
Zhigadlo, N