1,012 research outputs found
Microscopic origin of Magnetic Ferroelectrics in Nonlinear Multiferroics
A simple but general microscopic mechanism to understand the interplay
between the electric and magnetic degrees of freedom is developed. Within this
mechanism, the magnetic structure generates an electric current which induce an
counterbalance electric current from the spin orbital coupling. When the
magnetic structure is described by a single order parameter, the electric
polarization is determined by the single spin orbital coupling parameter, and
the material is predicted to be a half insulator. This mechanism provides a
simple estimation of the value of ferroelectricity and sets a physical
limitation as well.Comment: 4 pages, 1 figur
Surface reconstruction and ferroelectricity in PbTiO thin films
Surface and ferroelectric properties of PbTiO thin films are investigated
using an interatomic potential approach with parameters computed from
first-principles calculations. We show that a model developed for the bulk
describes properly the surface properties of PbTiO. In particular, the
antiferrodistortive surface reconstruction, recently observed from X-ray
scattering, is correctly reproduced as a result of the change in the balance of
long-range Coulombic and short-range interactions at the surface. The effects
of the surface reconstruction on the ferroelectric properties of ultrathin
films are investigated. Under the imposed open-circuit electrical boundary
conditions, the model gives a critical thickness for ferroelectricity of 4 unit
cells. The surface layer, which forms the antiferrodistortive reconstruction,
participates in the ferroelectricity. A decrease in the tetragonality of the
films leads to the stabilization of a phase with non-vanishing in-plane
polarization. A peculiar effect of the surface reconstruction on the in-plane
polarization profile is found.Comment: 6 pages, 5 figure
Phase stability and structural temperature dependence in sodium niobate: A high resolution powder neutron diffraction study
We report investigation of structural phase transitions in technologically
important material sodium niobate as a function of temperature on heating over
300-1075 K. Our high resolution powder neutron diffraction data show variety of
structural phase transitions ranging from non-polar antiferrodistortive to
ferroelectric and antiferroelectric in nature. Discontinuous jump in lattice
parameters is found only at 633 K that indicates that the transition of
orthorhombic antiferroelectric P (space group Pbcm) to R (space group Pbnm)
phase is first order in nature, while other successive phase transitions are of
second order. New superlattice reflections appear at 680 K (R phase) and 770 K
(S phase) that could be indexed using an intermediate long-period modulated
orthorhombic structure whose lattice parameter along direction is 3 and 6
times that of the CaTiO3-like Pbnm structure respectively. The correlation of
superlattice reflections with the phonon instability is discussed. The critical
exponent ({\beta}) for the second order tetragonal to cubic phase transition at
950 K, corresponds to a value {\beta}, as obtained from the
temperature variation of order parameters (tilt angle and intensity of
superlattice reflections). It is argued that this exponent is due to a second
order phase transition close to a tricritical point. Based on our detailed
temperature dependent neutron diffraction studies, the phase diagram of sodium
niobate is presented that resolves existing ambiguities in the literature.Comment: 21 Pages, 8 Figure
An economic survey of New Zealand wheatgrowers : survey no. 2
This is survey number 2. No. 1 was know as "National wheatgrowers' survey"This Report is the second in an annual series of economic
surveys of New Zealand wheatgrowing farms. These surveys have been undertaken by the Agricultural Economics Research Unit
at Lincoln College on behalf of Wheat Growers Sub-Section of
Federated Farmers of New Zealand Inc.
Specific attention has been focused on the physical
characteristics of wheatgrowing farms, the area of wheat and
other crops sown, wheat yields, cultural practices and costs and
returns for the 1977/78 wheat crop. An attempt has also been
made to allocate plant and machinery overhead costs to the wheat
enterprise on both an historical and current cost basis.
The need for current and detailed information from the
Survey involved two visits to the farms in the sample; one in
the spring following drilling and the second in the autumn after
harvest
Engaging and empowering first-year students through curriculum design: perspectives from the literature
There is an increasing value being placed on engaging and empowering first-year students and first-year curriculum design is a key driver and opportunity to ensure early enculturation into successful learning at university. This paper summarises the literature on first-year curriculum design linked to student engagement and empowerment. We present conceptualisations of ‘curriculum’ and examples from first-year curriculum design. We also note the limited literature where students have been involved in designing first-year curricula. The results of the literature review suggest that key characteristics of engaging first-year curricula include active learning, timely feedback, relevance and challenge. The literature also points to the importance of identifying students' abilities on entry to university as well as being clear about desired graduate attributes and developmental goals. Acknowledging realities and constraints, we present a framework for the first-year curriculum design process based on the literature
Electrostatic model of atomic ordering in complex perovskite alloys
We present a simple ionic model which successfully reproduces the various
types of compositional long-range order observed in a large class of complex
insulating perovskite alloys. The model assumes that the driving mechanism
responsible for the ordering is simply the electrostatic interaction between
the different ionic species. A possible new explanation for the anomalous
long-range order observed in some Pb relaxor alloys, involving the proposed
existence of a small amount of Pb^4+ on the B sublattice, is suggested by an
analysis of the model.Comment: 4 pages, two-column style with 1 postscript figure embedded. Uses
REVTEX and epsf macros. Also available at
http://www.physics.rutgers.edu/~dhv/preprints/index.html#lb_orde
CVM studies on the atomic ordering in complex perovskite alloys
The atomic ordering in complex perovskite alloys is investigated by the
cluster variation method (CVM). For the 1/3\{111\}-type ordered structure, the
order-disorder phase transition is the first order, and the order parameter of
the 1:2 complex perovskite reaches its maximum near x=0.25. For the
1/2\{111\}-type ordered structure, the ordering transition is the second order.
Phase diagrams for both ordered structures are obtained. The order-disorder
line obeys the linear law.Comment: 10 pages, 6 figure
First-principles investigation of 180-degree domain walls in BaTiO_3
We present a first-principles study of 180-degree ferroelectric domain walls
in tetragonal barium titanate. The theory is based on an effective Hamiltonian
that has previously been determined from first-principles
ultrasoft-pseudopotential calculations. Statistical properties are investigated
using Monte Carlo simulations. We compute the domain-wall energy, free energy,
and thickness, analyze the behavior of the ferroelectric order parameter in the
interior of the domain wall, and study its spatial fluctuations. An abrupt
reversal of the polarization is found, unlike the gradual rotation typical of
the ferromagnetic case.Comment: Revtex (preprint style, 13 pages) + 3 postscript figures. A version
in two-column article style with embedded figures is available at
http://electron.rutgers.edu/~dhv/preprints/index.html#pad_wal
Superparaelectric phase in the ensemble of non-interacting ferroelectric nanoparticles
For the first time we predict the conditions of superparaelectric phase
appearance in the ensemble of non-interacting spherical ferroelectric
nanoparticles. The superparaelectricity in nanoparticle was defined by analogy
with superparamagnetism, obtained earlier in small nanoparticles made of
paramagnetic material. Calculations of correlation radius, energetic barriers
of polarization reorientation and polarization response to external electric
field, were performed within Landau-Ginzburg phenomenological approach for
perovskites Pb(Zr,Ti)O3, BiFeO3 and uniaxial ferroelectrics rochelle salt and
triglycine sulfate.Comment: 28 pages, 7 figures, 3 Appendices, to be submitted to Phys. Rev.
The temperature dependent bandstructure of a ferromagnetic semiconductor film
The electronic quasiparticle spectrum of a ferromagnetic film is investigated
within the framework of the s-f model. Starting from the exact solvable case of
a single electron in an otherwise empty conduction band being exchange coupled
to a ferromagnetically saturated localized spin system we extend the theory to
finite temperatures. Our approach is a moment-conserving decoupling procedure
for suitable defined Green functions. The theory for finite temperatures
evolves continuously from the exact limiting case. The restriction to zero
conduction band occupation may be regarded as a proper model description for
ferromagnetic semiconductors like EuO and EuS. Evaluating the theory for a
simple cubic film cut parallel to the (100) crystal plane, we find some marked
correlation effects which depend on the spin of the test electron, on the
exchange coupling, and on the temperature of the local-moment system.Comment: 11 pages, 9 figure
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