High-pressure flux growth, structural, and superconducting properties ofLnFeAsO (Ln=Pr, Nd, Sm) single crystals

Single crystals of the $Ln$FeAsO ($Ln$1111, $Ln$ $=$ 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 (PrFeAsO${}_{1\ensuremath{-}x}$F${}_{x}$ and NdFeAsO${}_{1\ensuremath{-}x}$F${}_{x}$) or when Fe is substituted by Co (SmFe${}_{1\ensuremath{-}x}$Co${}_{x}$AsO). From magnetization measurements, we estimate the temperature dependence and anisotropy of the upper critical field and the critical current density of underdoped PrFeAsO${}_{0.7}$F${}_{0.3}$ crystal with ${T}_{c}$ \ensuremath{\approx} 25 K. Single crystals of SmFe${}_{1\ensuremath{-}x}$Co${}_{x}$AsO with maximal ${T}_{c}$ up to 16.3 K for $x$ \ensuremath{\approx} 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 SmFeAsO${}_{1\ensuremath{-}x}$F${}_{x}$ with a much higher ${T}_{c}$ \ensuremath{\approx} 50 K. The magnetically measured critical current densities are as high as 10${}^{9}$ A/m${}^{2}$ 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 \ensuremath{\sim}\ensuremath{-}8.7 T/K ($H$||$ab$ plane) and \ensuremath{\sim}\ensuremath{-}1.7 T/K ($H$||$c$ axis). This anisotropy (\ensuremath{\sim}5) is similar to that in other $Ln$1111 crystals with various higher ${T}_{c}$'s.