Large magneto resistance in a ferromagnetic cobaltite:: The 12H Ba0.9CoO2.6

The magnetic and transport properties of a hexagonal cobaltite related to the perovskite structure have been studied. By combining transmission electron microscopy, X-ray powder diffraction and iodometric titration, it is found that Ba0.9Co0.6 crystallizes in the 12H structure [P6(3)/mmc, a = 5.6612 (1) angstrom and c = 28.4627(8) angstrom]. Interestingly, this compound is a ferromagnet with a Curie temperature T-C = 50 K and a saturation magnetization M-S(2.5K) = 1.2 mu(B)/Co. This value is smaller than expected from the effective paramagnetic moment, mu(eff) = 3.7 mu(B)/Co, corresponding to an average spin S similar to(3)/(2) per Co, from which one would expect M-S(exp.) = 3 mu B/Co. This suggests 2 s either a canted structure or a strong local magnetic anisotropy related to the crystal field of the CoOn polyhedra. A clear transition in the electrical resistivity is found at T-C consistent with a spin scattering reduction as the sample becomes ferromagnetic. The spincharge coupling is evidenced by the large negative magnetoresistance effect optimum near T-C = 50 K, with [100 X (rho(H=7 T) - rho(H=0 T))/ rho(H=0)gamma] = -60%. (c) 2006 Elsevier Inc. All rights reserved.