BaT2As2 Single Crystals (T = Fe, Co, Ni) and Superconductivity upon Co-doping

The crystal structure and physical properties of BaFe2As2, BaCo2As2, and BaNi2As2 single crystals are surveyed. BaFe2As2 gives a magnetic and structural transition at TN = 132(1) K, BaCo2As2 is a paramagnetic metal, while BaNi2As2 has a structural phase transition at T0 = 131 K, followed by superconductivity below Tc = 0.69 K. The bulk superconductivity in Co-doped BaFe2As2 below Tc = 22 K is demonstrated by resistivity, magnetic susceptibility, and specific heat data. In contrast to the cuprates, the Fe-based system appears to tolerate considerable disorder in the transition metal layers. First principles calculations for BaFe1.84Co0.16As2 indicate the inter-band scattering due to Co is weak.Comment: Accepted to Physica

Iron substitution in NdCoAsO: crystal structure and magnetic phase diagram

The effects of replacing small amounts of Co with Fe in NdCoAsO are reported. Polycrystalline materials with compositions NdCo1-xFexAsO (x = 0.05, 0.10, 0.15, and 0.20) are studied and the results compared to previous reports for NdCoAsO. Rietveld analysis of powder x-ray diffraction data shows that as Fe replaces Co on the transition metal (T) site, the T-As distance increases, and the As tetrahedra surrounding the T-site become more regular. Electrical resistivity and magnetization measurements indicate that the three magnetic phase transitions in NdCoAsO are suppressed as Co is replaced by Fe, and these transitions are not observed above 1.8 K for x = 0.20. Based on these results, the magnetic phase diagram for the Co-rich side of the NdCoAsO-NdFeAsO system is constructed.Comment: Accepted for publication in Physical Review B, revised text and figures, 5 pages, 5 figure

Strain-activated structural anisotropy in BaFe2As2

High-resolution single crystal neutron diffraction measurements are presented probing the magnetostructural response to uniaxial pressure in the iron pnictide parent system BaFe2As2. Scattering data reveal a strain-activated, anisotropic broadening of nuclear Bragg reflections, which increases upon cooling below the resolvable onset of global orthorhombicity. This anisotropy in lattice coherence continues to diverge until a lower temperature scale---the first-order onset of antiferromagnetism---is reached. Our data suggest that antiferromagnetism and strong magnetoelastic coupling drive the strain-activated lattice response in this material and that the development of anisotropic lattice coherence under strain is the physical origin for the anomalous nematic anisotropy in this compound.Comment: 5 pages, 4 figure

Properties of RRe2Al10 (R=Y, Gd–Lu) crystals

Large single crystals of rare-earth rhenium aluminide RRe2Al10, with R=Y, and Gd–Lu were grown out of an Al-rich solution. Single crystal x-ray diffraction data confirmed the orthorhombic Cmcm structure for all members: R=Gd-Dy with TbRe2Al10-structure type (formula unit per cell Z=8); R=Y, and Ho–Lu with LuRe2Al10-structure type (Z=12). There is no evidence of a localized 3d electron moment in R=Y and Lu; R=Yb is nonmagnetic down to 1.8 K, but develops an enhanced electronic specific heat of ∼95 mJ mol−1 K−2. Ordering temperatures range from ferromagnetic order in R=Gd with Tc=7.2(1) K, antiferromagnetic order in R=Tbat TN=5.0(3) K, to R=Dy, Ho, and Er giving magnetic ordering temperatures of Tmag=1.7(1), ≤0.4, and 1.1(2) K, respectively. All compounds have effective moments close in value to that of free R3+ at high temperatures