444 research outputs found
Orbital correlations in the pseudo-cubic \emph{O} and rhombohedral -phases of LaMnO
The local and intermediate structure of stoichiometric LaMnO has been
studied in the pseudocubic and rhombohedral phases at high temperatures (300 to
1150 K). Neutron powder diffraction data were collected and a combined Rietveld
and high real space resolution atomic pair distribution function analysis
carried out. The nature of the Jahn-Teller (JT) transition around 750 K is
confirmed to be orbital order to disorder. In the high temperature orthorhombic
() and rhombohedral () phases the MnO octahedra are still fully
distorted locally. The data suggest the presence of local orbitally ordered
clusters of diameter \AA (four MnO octahedra) implying
strong nearest neighbor JT anti-ferrodistortive coupling.Comment: 4 pages, 4 figures, submitted to Phys. Rev. Le
Dielectric anomalies and spiral magnetic order in CoCr2O4
We have investigated the structural, magnetic, thermodynamic, and dielectric
properties of polycrystalline CoCrO, an insulating spinel exhibiting
both ferrimagnetic and spiral magnetic structures. Below = 94 K the
sample develops long-range ferrimagnetic order, and we attribute a sharp phase
transition at 25 K with the onset of long-range spiral magnetic
order. Neutron measurements confirm that while the structure remains cubic at
80 K and at 11 K; there is complex magnetic ordering by 11 K. Density
functional theory supports the view of a ferrimagnetic semiconductor with
magnetic interactions consistent with non-collinear ordering. Capacitance
measurements on CoCrO, show a sharp decrease in the dielectric constant
at , but also an anomaly showing thermal hysteresis falling between
approximately = 50 K and = 57 K. We tentatively attribute the
appearance of this higher temperature dielectric anomaly to the development of
\textit{short-range} spiral magnetic order, and discuss these results in the
context of utilizing dielectric spectroscopy to investigate non-collinear
short-range magnetic structures.Comment: & Figure
Local structure of In_(0.5)Ga_(0.5)As from joint high-resolution and differential pair distribution function analysis
High resolution total and indium differential atomic pair distribution
functions (PDFs) for In_(0.5)Ga_(0.5)As alloys have been obtained by high
energy and anomalous x-ray diffraction experiments, respectively. The first
peak in the total PDF is resolved as a doublet due to the presence of two
distinct bond lengths, In-As and Ga-As. The In differential PDF, which involves
only atomic pairs containing In, yields chemical specific information and helps
ease the structure data interpretation. Both PDFs have been fit with structure
models and the way in that the underlying cubic zinc-blende lattice of
In_(0.5)Ga_(0.5)As semiconductor alloy distorts locally to accommodate the
distinct In-As and Ga-As bond lengths present has been quantified.Comment: 9 pages, 7 figur
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