Millimeter-sized single crystals of LaFeAsO, LaFeAsO 1-x F x , and LaFe 1-x Co x AsO were grown in NaAs flux at ambient pressure. The detailed growth procedure and crystal characterizations are reported. The as-grown crystals have typical dimensions of 3 × 4 × 0.05-0.3 mm 3 with the crystallographic c-axis perpendicular to the plane of the plate-like single crystals. Some crystals manifest linear dimensions as large as 4-5 mm. X-ray and neutron single crystal scattering confirmed that LaFeAsO crystals exhibit a structural phase transition at T s ~ 154 K and a magnetic phase transition at T SDW ~ 140 K. The transition temperatures agree with those determined by anisotropic magnetization, in-plane electrical resistivity and specific heat measurements and are consistent with previous reports on polycrystalline samples. Co and F were successfully introduced into the lattice leading to superconducting LaFe 1-x Co x AsO and LaFeAsO 1-x F x single crystals, respectively. This growth protocol has been successfully employed to grow single crystals of NdFeAsO. Thus it is expected to be broadly applicable to grow other

Large high-quality single crystals of hole-doped iron-based superconductor (Ba1-xKx)Fe2As2 were grown over a broad composition range 0.22 <= x <= 1 by inverted temperature gradient method. We found that high soaking temperature, fast cooling rate, and an adjusted temperature window of the growth are necessary to obtain single crystals of heavily K-doped crystals (0.65 <= x <= 0.92) with narrow compositional distributions as revealed by sharp superconducting transitions in magnetization measurements and close to 100% superconducting volume fraction. The crystals were extensively characterized by x-ray and compositional analysis, revealing monotonic evolution of the c-axis crystal lattice parameter with K substitution. Quantitative measurements of the temperature-dependent in-plane resistivity rho(T) found doping-independent, constant within error bars, resistivity at room temperature, rho(300 K), in sharp contrast with the significant doping dependence in electron and isovalent substituted BaFe2As2 based compositions. The shape of the temperature-dependent resistivity, rho(T), shows systematic dopingevolution, being close to T-2 in overdoped and revealing significant contribution of the T-linear component at optimum doping. The slope of the upper critical field, d H-c2/dT, scales linearly with T-c for both H parallel to c, H-c2,(c), and H parallel to ab, H-c2,H-ab. The anisotropy of the upper critical field. equivalent to Hc(2,ab)/H-c2,H-c determined near zero-field T-c increases from similar to 2 to 4-5 with increasing K doping level from optimal x similar to 0.4 to strongly overdoped x = 1. Large high-quality single crystals of hole-doped iron-based superconductor (Ba 1−x K x )Fe 2 As 2 were grown over a broad composition range 0.22 x 1 by inverted temperature gradient method. We found that high soaking temperature, fast cooling rate, and an adjusted temperature window of the growth are necessary to obtain single crystals of heavily K-doped crystals (0.65 x 0.92) with narrow compositional distributions as revealed by sharp superconducting transitions in magnetization measurements and close to 100% superconducting volume fraction. The crystals were extensively characterized by x-ray and compositional analysis, revealing monotonic evolution of the c-axis crystal lattice parameter with K substitution. Quantitative measurements of the temperature-dependent in-plane resistivity ρ(T ) found doping-independent, constant within error bars, resistivity at room temperature, ρ(300 K), in sharp contrast with the significant doping dependence in electron and isovalent substituted BaFe 2 As 2 based compositions. The shape of the temperature-dependent resistivity, ρ(T ), shows systematic doping-evolution, being close to T 2 in overdoped and revealing significant contribution of the T -linear component at optimum doping. The slope of the upper critical field, dH c2 /dT , scales linearly with T c for both H c, H c2,c , and H ab, H c2,ab . The anisotropy of the upper critical field γ ≡ H c2,ab /H c2,c determined near z...

The structural and magnetic phase transitions have been studied on NdFeAsO single crystals by neutron and x-ray diffraction complemented by resistivity and specific heat measurements. Two low-temperature phase transitions have been observed in addition to the tetragonal-to-orthorhombic transition at TS ∼ 142 K and the onset of antiferromagnetic (AFM) Fe order below TN ∼ 137 K. The Fe moments order AFM in the well-known stripe-like structure in the (ab) plane, but change from AFM to ferromagnetic (FM) arrangement along the c direction below T * ∼ 15 K accompanied by the onset of Nd AFM order below T Nd ∼ 6 K with this same AFM configuration. The iron magnetic order-order transition in NdFeAsO accentuates the Nd-Fe interaction and the delicate balance of c-axis exchange couplings that results in AFM in LaFeAsO and FM in CeFeAsO and PrFeAsO.