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

TL;DR

This study reports the high-pressure flux growth of LnFeAsO single crystals, investigates their structural and superconducting properties, and compares their critical fields and current densities across different doping methods and rare-earth elements.

Contribution

It introduces a high-pressure flux method for growing high-quality LnFeAsO crystals and provides detailed superconducting property measurements, including critical fields and anisotropy, for the first time in some compositions.

Findings

Crystals grown up to 1 mm with good structural quality.

Superconductivity achieved via F and Co doping.

Critical current densities up to 10^9 A/m^2 at 2 K.

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 for the first time. 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 of = 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.