194 research outputs found
High-pressure/high-temperature synthesis of transition metal oxide perovskites
Perovskite and related Ruddlesden-Popper type transition metal oxides synthesised at high pressures and temperatures during the last decade are reviewed. More than 60 such new materials have been reported since 1995. Important developments have included perovskites with complex cation orderings on A and B sites, multiferroic bismuth-based perovskites, and new manganites showing colossal magnetoresistance (CMR) and charge ordering properties
Very weak electron-phonon coupling and strong strain coupling in manganites
The coupling of the manganite stripe phase to the lattice and to strain has
been investigated via transmission electron microscopy studies of
polycrystalline and thin film manganites. In polycrystalline \PCMOfiftwo a
lockin to in a sample with has been observed for the first
time. Such a lockin has been predicted as a key part of the Landau CDW theory
of the stripe phase. Thus it is possible to constrain the size of the
electron-phonon coupling in the CDW Landau theory to between 0.04% and 0.05% of
the electron-electron coupling term. In the thin film samples, films of the
same thickness grown on two different substrates exhibited different
wavevectors. The different strains present in the films on the two substrates
can be related to the wavevector observed via Landau theory. It is demonstrated
that the the elastic term which favours an incommensurate modulation has a
similar size to the coupling between the strain and the wavevector, meaning
that the coupling of strain to the superlattice is unexpectedly strong.Comment: 6 pages, 7 figure
Materials chemistry under high pressures - Some recent aspects
Among the thermodynamic parameters governing the preparation of novel materials, temperature (T) and pressure (p) play an important role. In Materials Chemistry, the synthesis of materials needs energy in order to enhance the diffusion of atoms to the equilibrium positions required by the specific structure and to induce the formation of chemical bonds..
Metamagnetism and soliton excitations in the modulated ferromagnetic Ising chain CoV2O6
We report a combination of physical property and neutron scattering
measurements for polycrystalline samples of the one-dimensional spin chain
compound CoV2O6. Heat capacity measurements show that an effective S = 1/2
state is found at low temperatures and that magnetic fluctuations persist up to
6.Tn. Above Tn = 6.3 K, measurements of the magnetic susceptibility as a
function of T and H show that the nearest neighbour exchange is ferromagnetic.
In the ordered state, we have discovered a crossover from a metamagnet with
strong fluctuations between 5 K and Tn to a state with a 1/3 magnetisation
plateau at 2 < T < 5 K. We use neutron powder diffraction measurements to show
that the AFM state has incommensurate long range order and inelastic time of
flight neutron scattering to examine the magnetic fluctuations as a function of
temperature. Above Tn, we find two broad bands between 3.5 and 5 meV and
thermally activated low energy features which correspond to transitions within
these bands. These features show that the excitations are deconfined solitons
rather than the static spin reversals predicted for a uniform FM Ising spin
chain. Below Tn, we find a ladder of states due to the confining effect of the
internal field. A region of weak confinement below Tn, but above 5 K, is
identified which may correspond to a crossover between 2D and 3D magnetic
ordering.Comment: Expanded version, includes results from arXiv:0804.2966 and neutron
powder diffraction. To appear in PR
Coexisting commensurate and incommensurate magnetic orders in the double double perovskite CaMnCoWO<sub>6</sub>
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