Frustration and Grace Under Pressure: Three Tales about Novel Oxides

CIC nanoGUNE Seminars

Speaker

John Mitchell, Argonne Natinal Lab, Argonne, USA

When

2010/09/15
14:00

Place

nanoGUNE seminar room, Tolosa Hiribidea 76, Donostia - San Sebastian

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Complex oxides provide an exceptionally powerful vehicle for understanding a wide range of electronic and magnetic phenomena in condensed matter science. Here I discuss some recent results in which control of material synthesis or crystal growth at ambient or high pressure yields new insights in magnetic frustration, oxide conductors, and multiferroicity. • The field of geometric frustration of magnetism has been widely studied using pyrochlore systems, calling for new materials and topologies. The RBaCo4O7 (R=Y, Tb-Lu, Ca) system is one such new topology, formed by an alternative stacking of the alternating Kagome and triangular layers found in pyrochlores. We have explored the chemistry, structure and magnetic properties of the Y member, demonstrating the extreme sensitivity of magnetic frustration to oxygen content. Magnetic diffuse scattering on single crystals reveal a microsopic, qualitative picture of the spin organization in this frustrated magnet. • Negative thermal expansion material orthorhombic Sc2W3O12, a white, insulating powder, transforms into a new phase, a black, metallic compound, under high P, T conditions. The crystal structure of the recovered phase Sc2/3WO4 was determined from synchrotron x-ray powder diffraction data. Sc2/3WO4, is a paramagnetic metal with a dense, monoclinic structure containing 1-D chains of edge-sharing WO6 octahedra. Sc2/3WO4 is unusual among conducting oxides, as there is little evidence of thermal activation or lattice involvement. Rather, Sc2/3WO4 bears resemblance to doped amorphous semiconductors. • Recent density functional theory calculations have predicted that the family of compounds MTiO3 (M = Mn, Fe, Ni) are promising candidates where a polar lattice distortion can induce weak ferromagnetism. We have prepared the corresponding Fe member at high pressure. It shows a sharp antiferromagnetic (AF) transition at as well clear evidence of ferroelectricity. Importantly, we have shown the existence of weak ferromagnetism, validating the ‘materials by design’ approach in this multiferroic. > > > >