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 (PrFeAsO 1-x F x and NdFeAsO 1-x F x ) or when Fe is substituted by Co (SmFe 1-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 ≈ 25 K. Single crystals of SmFe 1-x Co x AsO with maximal T c 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 SmFeAsO 1-x F x with a much higher T c of ≈ 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 ∼ -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 T c 's.

An extended study of the superconducting and normal-state properties of various as-grown and post-annealed RbxFe2−ySe2 single crystals is presented. Magnetization experiments evidence that annealing of RbxFe2−ySe2 at 413 K, well below the onset of phase separation Tp 489 K, neither changes the magnetic nor the superconducting properties of the crystals. In addition, annealing at 563 K, well above Tp, suppresses the superconducting transition temperature Tc and leads to an increase of the antiferromagnetic susceptibility accompanied by the creation of ferromagnetic impurity phases, which are developing with annealing time. However, annealing at T = 488 K Tp increases Tc up to 33.3 K, sharpens the superconducting transition, increases the lower critical field, and strengthens the screening efficiency of the applied magnetic field. Resistivity measurements of the as-grown and optimally annealed samples reveal an increase of the upper critical field along both crystallographic directions as well as its anisotropy. Muon spin rotation and scanning transmission electron microscopy experiments suggest the coexistence of two phases below Tp: a magnetic majority phase of Rb2Fe4Se5 and a non-magnetic minority phase of Rb0.5Fe2Se2. Both microscopic techniques indicate that annealing the specimens just at Tp does not affect the volume fraction of the two phases, although the magnetic field distribution in the samples changes substantially. This suggests that the microstructure of the sample, caused by mesoscopic phase separation, is modified by annealing just at Tp, leading to an improvement of the superconducting properties of RbxFe2−ySe2 and an enhancement of Tc.

Here, we present a combination of magnetization and magnetic torque experiments to investigate the magnetic orders in undoped EuFe2As2 and Co doped EuFe1.8Co0.2As2 single crystals. Although at low temperatures typical results for an antiferromagnetic (AFM) state in EuFe2As2 were found, our data strongly indicate the occurrence of a canted antiferromagnetic (C-AFM) order of the Eu 2+ moments between 17 K and 19 K, observed even in the lowest studied magnetic fields. However, unlike in the parent compound, no low-field and low-temperature AFM state of the Eu 2+ moments was observed in the doped EuFe1.8Co0.2As2. Only a C-AFM phase is present at low fields and low temperatures, with a reduced magnetic anisotropy as compared to the undoped system. We present and discuss for both, EuFe2As2 and EuFe1.8Co0.2As2, the experimentally deduced magnetic phase diagrams of the magnetic ordering of the Eu 2+ sublattice with respect to the temperature, the applied magnetic field, and its orientation to the crystallographic axes. It is likely that the magnetic coupling of the Eu and the Fe sublattice is strongly depending on Co doping, having detrimental influence on the magnetic phase diagrams as determined in this work. Their impact on the occurrence of superconductivity with higher Co doping is discussed.

We report crystal growth at high pressure, structure determination, and magnetic and transport studies of an oxypnictide superconductor Pr 4 Fe 2 As 2 Te 1−x O 4 . Its structure resembles the known 1111 phase except for the considerably larger c lattice constant and intercalated tellurium atoms and it crystallizes in a tetragonal lattice [a = 4.0165(2)Å, c = 29.8572(16)Å, and space group I 4/mmm (no. 139)]. The electrical resistivity ρ(T) and magnetization measurement show a transition at T c ≈ 25 K. The lower (H c1 ) and upper (H c2 ) critical fields are 2 mT and 6.5 T, respectively. The Ginzburg-Landau parameter of κ ≈ 80 places this compound in the family of strong type-II superconductors.The layered d-metal pnictide and pnictide oxides (oxypnictides) have been intensively studied in recent years (see the review article 1 and references therein). This family consists of anti-fluorite-type M 2 P 2 (M is a transition metal and P a pnictogen) layers alternating for some representatives with fluorite-type L 2 O 2 layers (where L is a lanthanide). There are more than ten known structural types of this kind. 2,3 Some of them reveal superconductivity with a critical temperature T c up to 58.1 K. 4 The "new generation of high-T c materials" 5 provides a broad research area because of the huge variability in chemical composition with a potential to form new structural types. 1,3,6 Some of these compounds, like SmFeAs(O,F), due to the optimal combination of high isotropic critical current densities and the relatively high T c , could be considered as materials which meet requirements for possible applications. 7 Here we report on another compound of the layered d-metal oxypnictide superconductors, Pr 4 Fe 2 As 2 Te 1−x O 4 , its crystal structure and basic physical properties such as T c , dc resistivity, lower and upper critical fields, and Ginzburg-Landau parameters. For its synthesis a mixture of chemicals PrAs, PrTe, and FeO in molar proportion 0.9:0.1:1 was thoroughly ground in a malachite mortar and pressed into a pellet inside a glovebox in argon gas. The pellet was annealed at 1050 • C for 7 days in an Al 2 O 3 crucible in a sealed quartz ampoule filled with Ar gas (≈0.2 atm). After that it was quenched in cold water. This precursor was used for crystal growth at high pressure (3 GPa) in NaCl/KCl flux employing a high-pressure cubic anvil system. The temperature was raised to 1500 • C in 1 h and then kept constant for 60 h, followed by cooling (45 • C/h) to 1050 • C and cooling in 1 h to room temperature. The flux was dissolved in water. The crystals of platelike shape were selected from the remaining mixture.Single-crystal x-ray diffraction was performed on a Bruker diffractometer equipped with a CCD detector. Data reduction and multiscan absorption correction were done using APEX2 (Ref. 8) and SAINT (Ref. 9) software. The crystal structure was determined by a direct method and refined on F 2 , employing the programs SHELXS-97 (Ref. 10) and . The lattice parameters were determined as a = 4.0165(2)Å and c = 29.8...

An overdoped YBa2Cu3O7 single crystal was studied by SQUID and torque magnetometry in order to investigate the temperature dependence of the anisotropy parameter close to the transition temperature Tc (0.87 Tc < T < Tc). Angle dependent torque measurements were performed and analyzed with the widely used Kogan model [Phys. Rev. B 38, 7049 (1988)] as well as with an extended model by Clem [Phys. Rev. Lett. 67, 2371 (1991)], taking into account the influence of the vortex cores on the magnetization. Both approaches yield similar results, with an out-of-plane anisotropy parameter around 6.5 which slightly increases with decreasing temperature, and a temperature independent in-plane anisotropy parameter γ ab = 1.12(5).