Oxide electronics

Abstract:

We have used electron diffraction and neutron powder diffraction to elucidate the structural properties of superconducting NaxCoO 2•yD2O over a wide compositional range. Our measurements show that superconducting samples exhibit a number of supercells ranging from to , but the predominant modulation, observed also in the neutron data, is a double hexagonal cell with dimensions 2a × 2a × c. Rietveld analysis reveals that D2O is inserted between CoO 2 sheets to form a layered network of NaO6 triangular prisms. Our model removes the need to invoke a 5 K superconducting point compound and suggests that a solid solution of Na is possible within a relatively constant amount of water y. © IOP Publishing Ltd.

Abstract:

High-resolution x-ray and neutron powder-diffraction measurements were performed on polycrystalline VOMoO4. Below ≃ 40K the system orders in a simple Néel antiferromagnetic state (propagation vector k =0), indicating a dominant role of the nearest-neighbor interactions. The order is three dimensional but the reduced saturated magnetic moment m of 0.41 (1) μ V4+ at 2K indicates strongly two-dimensional character and enhanced quantum fluctuations. On cooling, there is no evidence of a reduction of the crystal symmetry. However, neutron diffraction indicates an anomalous evolution of the lattice parameters, which can be related to the onset of magnetic correlations. © 2005 The American Physical Society.

Abstract:

Transition-metal spinels (general formula AB 2X 4) have been, for many years, the subject of intense experimental and theoretical activity. Structurally, the most interesting feature of these systems is the fact that the B cation occupies the nodes of a pyrochlore lattice, which is known to be geometrically frustrated. Therefore, one can explore how the natural tendency of the transition metals to order in the charge, magnetic and orbital sectors is affected by geometrical frustration. Recently, orbital ordering has become a topical subject in a variety of both non-frustrated systems, such as manganites and other perovskites, and in the spinels. In this paper, I review the recent experimental activity on the subject of orbital ordering in transition-metal spinels and relate this to models of orbital ordering that are being developed by theoreticians. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.

Abstract:

We have studied the crystal and magnetic structures of the magnetoelectric materials RMn2O5 (R = Tb, Ho, Dy) using neutron diffraction as a function of temperature. All three materials display incommensurate antiferromagnetic ordering below 40 K, becoming commensurate on further cooling. For R = Tb, Ho, a commensurate-incommensurate transition takes place at low temperatures. The commensurate magnetic structures have been solved and are discussed in terms of competing exchange interactions. The spin configuration within the ab plane is essentially the same for each system, and the radius of R determines the sign of the magnetic exchange between adjacent planes. The inherent magnetic frustration in these materials is lifted by a small lattice distortion, primarily involving shifts of the Mn3+ cations and giving rise to a canted antiferroelectric phase.