Large single crystal growth of BaFe1.87Co0.13As2 using a nucleation pole

Co-doped iron arsenic single crystal of BaFe1.87Co0.13As2 with dimension up to 20 x 10 x 2 mm3 were grown by a nucleation pole: an alumina stick served as nucleation center during growth. The high quality of crystalline was illustrated by the measurements of neutron rocking curve and X-ray diffraction pattern. A very sharp superconducting transition temperature Tc~25 K was revealed by both resistivity and susceptibility measurements. A nearly 100% shielding fraction and bulk nature of the superconductivity for the single crystal were confirmed using magnetic susceptibility data.Comment: 4 pages, 5 figure

Rare earth magnetism in CeFeAsO: A single crystal study

Single crystals of CeFeAsO, large enough to study the anisotropy of the magnetic properties, were grown by an optimized Sn-flux technique. The high quality of our single crystals is apparent from the highest residual resistivity ratio, RRR = 12, reported among undoped RFeAsO compounds (R=rare earth) as well as sharp anomalies in resistivity, specific heat, C(T), and thermal expansion at the different phase transitions. The magnetic susceptibility chi(T) presents a large easy-plane anisotropy consistent with the lowest crystal electric field doublet having a dominant Gamma_6 character. Curie-Weiss like susceptibilities for magnetic field parallel and perpendicular to the crystallographic c-axis do not reveal an influence of a staggered field on the Ce site induced by magnetic ordering of the Fe. Furthermore, the standard signatures for antiferromagnetic order of Ce at T_N = 3.7 K observed in chi(T) and C(T) are incompatible with a Zeeman splitting Delta = 10 K of the CEF ground state doublet at low temperature due to the Fe-magnetic order as previously proposed. Our results can be reconciled with the earlier observation by assuming a comparatively stronger effect of the Ce-Ce exchange leading to a reduction of this Zeeman splitting below 15 K.Comment: 15 pages, 6 figures, added section on magn. susceptibilit

Iron pnictides: Single crystal growth and effect of doping on structural, transport and magnetic properties

We demonstrate the preparation of large, free standing iron pnictide single crystals with a size up to 20 x 10 x 1 mm3 using solvents in zirconia crucibles under argon atmosphere. Transport and magnetic properties are investigated to study the effect of potassium doping on the structural and superconducting property of the compounds. The spin density wave (SDW) anomaly at Ts ~138 K in BaFe2As2 single crystals from self-flux shifts to Ts ~85 K due to Sn solvent growth. We show direct evidence for an incorporation of Sn on the Fe site. The electrical resistivity data show a sharp superconducting transition temperature Tc~38.5 K for the single crystal of Ba0.68K0.32Fe2As2. A nearly 100% shielding fraction and bulk nature of the superconductivity for the single crystal were confirmed by magnetic susceptibility data. A sharp transition Tc~25 K occurred for the single crystal of Sr0.85K0.15Fe2As2. There is direct evidence for a coexistence of the SDW and superconductivity in the low doping regime of Sr1-xKxFe2As2 single crystals. Structural implications of the doping effects as well as the coexistence of the two order parameters are discussed.Comment: 22 pages, 9 figure