2,121 research outputs found

    A single crystal high-temperature pyrochlore antiferromagnet

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    We report the magnetic characterization of the frustrated transition metal pyrochlore NaCaCo2_2F7_7. This material has high spin Co2+^{2+} in CoF6_6 octahedra in a pyrochlore lattice, and disordered non-magnetic Na and Ca on the large-atom sites in the structure. Large crystals grown by the floating zone method were studied. The magnetic susceptibility is isotropic, the Co moment is larger than the spin-only value, and in spite of the large Curie Weiss theta (-140 K), freezing of the spin system, as characterized by peaks in the ac and dc susceptibility and specific heat, does not occur until around 2.4 K. This yields a frustration index of f = θCW-\theta_{CW}/TfT_f \approx 56, an indication that the system is highly frustrated. The observed entropy loss at the freezing transition is low, indicating that magnetic entropy remains present in the system at 0.6 K. The compound may be the realization of a frustrated pyrochlore antiferromagnet with weak bond disorder. The high magnetic interaction strength, strong frustration, and the availability of large single crystals makes NaCaCo2_2F7_7 an interesting alternative to rare earth oxide pyrochlores for the study of geometric magnetic frustration in pyrochlore lattices.Comment: Submitted to PRL; 14 pages, 4 figures, 2 table

    The trimer-based spin liquid candidate Ba4NbIr3O12

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    Ba4NbIr3O12, a previously unreported material with a triangular planar geometry of Ir3O12 trimers, is described. Magnetic susceptibility measurements show no magnetic ordering down to 1.8 K despite the Curie-Weiss temperature of -13 K. The material has a very low effective magnetic moment of 0.80 {\mu}B/f.u. To look at the lower temperature behavior, the specific heat (Cp) was measured down to 0.35 K; it shows no indication of magnetic ordering and fitting a power law to Cp vs. T below 2 K yields the power {\alpha} = 3/4. Comparison to the previously unreported trimer compound made with the 4d element Rh in place of the 5d element Ir, Ba4NbRh3O12, is presented. The analysis suggests that Ba4NbIr3O12 is a candidate spin liquid material.Comment: 19 pages, 7 main figures, 2 SI figure

    Superconductivity in WO2.6F0.4 synthesized by reaction of WO3 with Teflon

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    WO3-xFx (x < 0.45) perovskite-like oxyfluorides were prepared by a chemically reducing fluorination route using the polymer polytetrafluoroethylene (Teflon). The symmetry of the crystal structures of WO3-xFx changes from monoclinic to tetragonal to cubic as the fluorine content increases. Fluorine doping changes insulating WO3 to a metallic conductor, and superconductivity (Tc = 0.4 K) was discovered in the samples with fluorine contents of 0.41 < x < 0.45. This easy fluorination method may be applicable to other systems and presents an opportunity for finding new oxyfluoride superconductors.Comment: 9 pages, 3 figures, 2 tables. To be published in Phys Rev

    The Reactivity of MgB2 with Common Substrate and Electronic Materials

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    The reactivity of MgB2 with powdered forms of common substrate and electronic materials is reported. Reaction temperatures between 600 C and 800 C, encompassing the range commonly employed in thin-film fabrication, were studied. The materials tested for reactivity were ZrO2, yttria stabilized zirconia (YSZ), MgO, Al2O3, SiO2, SrTiO3, TiN, TaN, AlN, Si, and SiC. At 600 C, MgB2 reacted only with SiO2 and Si. At 800 C, however, reactions were observed for MgB2 with Al2O3, SiO2, Si, SiC, and SrTiO3. The Tc of MgB2 decreased in the reactions with SiC and Al2O3.Comment: 5 figure