Superconductivity in Co-doped LaFeAsO

Here we report the synthesis and basic characterization of LaFe1-xCoxAsO for several values of x. The parent phase LaFeAsO orders antiferromagnetically (TN ~ 145 K). Replacing Fe with Co is expected to both electron dope the system and introduce disorder in the FeAs layer. For x = 0.05 antiferromagnetic order is destroyed and superconductivity is observed at Tconset = 11.2 K. For x = 0.11 superconductivity is observed at Tc(onset) = 14.3 K, and for x = 0.15 Tc = 6.0 K. Superconductivity is not observed for x = 0.2 and 0.5, but for x = 1, the material appears to be ferromagnetic (Tc ~ 56 K) as judged by magnetization measurements. We conclude that Co is an effective dopant to induce superconductivity. Somewhat surprisingly, the system appears to tolerate considerable disorder in the FeAs planes.Comment: 19 pages, 9 figure

Magnetic properties of the geometrically frustrated S=1/2 antiferromagnets, La2LiMoO6 and Ba2YMoO6, with the B-site ordered double perovskite structure: Evidence for a Collective Spin Singlet Ground State

Two B-site ordered double perovskites, La2LiMoO6 and Ba2YMoO6, based on the S = 1/2 ion, Mo5+, have been investigated in the context of geometric magnetic frustration. Powder neutron diffraction, heat capacity, susceptibility, muon spin relaxation(_SR), and 89Y NMR- including MAS NMR- data have been collected. La2LiMoO6 deviates strongly from simple Curie-Weiss paramagnetic behavior below 150K and zero-field cooled/ field cooled (ZFC/FC)irreversibility occurs below 20K with a weak, broad susceptibility maximum near 5K in the ZFC data. A Curie-Weiss fit shows a reduced mu_eff=1.42\mu_B, (spin only = 1.73 muB) and a Weiss temperature, \theta_c, which depends strongly on the temperature range of the fit. Powder neutron diffraction, heat capacity and 7Li NMR show no evidence for long range magnetic order to 2K. On the other hand oscillations develop below 20K in muSR indicating at least short range magnetic correlations. Susceptibility data for Ba2YMoO6 also deviate strongly from the C-W law below 150K with a similarly reduced mu_eff = 1.72\mu_B and \theta_c = - 219(1)K. Heat capacity, neutron powder diffraction and muSR data show no evidence for long range order to 2K but a very broad maximum appears in the heat capacity. The 89Y NMR paramagnetic Knight shift shows a remarkable local spin susceptibility behavior below about 70K with two components from roughly equal sample volumes, one indicating a singlet state and the other a strongly fluctuating paramagnetic state. Further evidence for a singlet state comes from the behavior of the relaxation rate, 1/T1. These results are discussed and compared with those from other isostructural S = 1/2 materials and those based on S = 3/2 and S = 1.Comment: Accepted for publication in Phys. Rev.

Phase Stability, Structural Evolution and Magnetic Properties of Sc(1-x)LuxVO3 (0.0 x 1.0).

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF.NRC publication: Ye

Neutron diffraction determination of the cell dimensions and thermal expansion of the fluoroperovskite KMgF3 from 293 to 3.6 K

The cell dimensions of the fluoroperovskite KMgF3 synthesized by solid state methods have been determined by powder neutron diffraction and Rietveld refinement over the temperature range 293\u20133.6 K using Pt metal as an internal standard for calibration of the neutron wavelength. These data demonstrate conclusively that cubic $$Pm\overline{3} m$$ KMgF3 does not undergo any phase transitions to structures of lower symmetry with decreasing temperature. Cell dimensions range from 3.9924(2) at 293 K to 3.9800(2) at 3.6 K, and are essentially constant within experimental error from 50 to 3.6 K. The thermal expansion data are described using a fourth order polynomial function.NRC publication: Ye

The effect of hole-doping on the magnetic properties of Nd1-xTiO3

The magnetic properties of the hole(x)-doped Mott-Hubbard, antiferromagnetic insulator, NdTiO3, have been investigated in Nd1-xTiO3. For the materials in 0.010(6) < x < 0.112(4), correlation between the structural and magnetic properties is discussed with respect to the Ti - O - Ti angles and the Ti - O bond distances. For 0.010(6) < x < 0.071(10), long-range magnetic order is observed through low temperature neutron diffraction and heat capacity. Neel ordering temperatures of 88.2 K and 61.9 K are determined, respectively, for x = 0.019(6) and x = 0.064(10). From high resolution neutron diffraction, the Pnma magnetic structure is assigned, unambiguously, as GxCy on Ti3+ and Cy2 on Nd3+. The Ti3+ ordered moment decreases gradually from 0.45(8) uB for x = 0.019(6) to 0.31(7) uB for x = 0.071(10), but vanishes abruptly for higher x. Antiferromagnetic short-range order exists for 0.074(9) < x < 0.098(10) as determined by neutron diffraction and the field-dependence of the susceptibility. This is the first observation of short-range order for any hole-doped titanate system. For 29(4)% hole-doping in Nd1-xTiO3, x > 0.098(10), all magnetic ordering is destroyed.NRC publication: Ye

Use of neutron diffraction for development of metal hydrides : case of BCC alloys

NRC publication: Ye