Magnetic structure of Ba7Co6BrO17
Résumé
At the basis of the search for original electronic and magnetic behaviour, a number of studies focus on the prospecting for new complex structural types within “hot” topics such as cobaltites (strong competition between magnetic ground states, attractive thermoelectricity, superconductivity ...). In this context, we recently synthesized by solid state reaction the new Ba7Co6BrO17 compound (P63/mmc: a=5.6611(1) Å and c=33.5672(8) Å) [1]. This cobaltite is built from a close-packing of [BaO3] and [BaOBr] layers with a 14H stacking sequence (c'chhhcc')2, which creates Co4O15 tetramers of face-sharing octahedra connected to their extremities to isolated tetrahedra by corner-sharing. This material is strongly related to the 12H-BaCoO2.6 [2] and 6H-Ba6Co6ClO16 [3] cobaltites, with the existence of common blocks. Measurement of the magnetic susceptibility Χ against T has been performed under applied magnetic field of 1 T. The experiment reveals the existence of two transitions at 60 K and 30 K. The curve was fitted to the Curie-Weiss law in the paramagnetic domain (above 60 K), leading to the values peff=3.12 ΜB/Co and Θ=55K This paramagnetic effective moment can be explained by assuming Co3+ S=1 and Co4+ S=3/2 i.e. cobalt atoms at the intermediate spin-state (considering the spin-only approximation). The positive Θ value indicates important ferromagnetic exchanges among the structures. In order to understand the magnetic properties of this cobaltite, we collected neutron powder diffraction data on the G41 diffractometer (LLB Saclay, France). The pattern below 60 K revealed the onset of new reflections, this fact being ascribed to the occurrence of long-range magnetic ordering and confirming the magnetic transition previously reported. All those magnetic reflections were indexed in a commensurate lattice related to the crystallographic one by a propagation vector k=[000]. Among all the possible magnetic models, only those for which the exchange interactions between two tetrahedral cobalt orders antiferromagnetically went to convergence with acceptable reliability factors. The magnetic structure can be described as the antiferromagnetic coupling of ferromagnetic blocks (Co4O15 + 2 CoO4 units). This kind of magnetic structure has been previously reported for 6H-Ba6Co6ClO16 [4].