562 research outputs found
Elastic domains in antiferromagnets
We consider periodic domain structures which appear due to the magnetoelastic
interaction if the antiferromagnetic crystal is attached to an elastic
substrate. The peculiar behavior of such structures in an external magnetic
field is discussed. In particular, we find the magnetic field dependence of the
equilibrium period and the concentrations of different domains
Magnetism and local distortions near carbon impurity in -iron
Local perturbations of crystal and magnetic structure of -iron near
carbon interstitial impurity is investigated by {\it ab initio} electronic
structure calculations. It is shown that the carbon impurity creates locally a
region of ferromagnetic ordering with substantial tetragonal distortions.
Exchange integrals and solution enthalpy are calculated, the latter being in a
very good agreement with experimental data. Effect of the local distortions on
the carbon-carbon interactions in -iron is discussed.Comment: 4 pages 3 figures. Final version, accepted to Phys.Rev. Let
Influence of Strain on the Kinetics of Phase Transitions in Solids
We consider a sharp interface kinetic model of phase transitions accompanied
by elastic strain, together with its phase-field realization. Quantitative
results for the steady-state growth of a new phase in a strip geometry are
obtained and different pattern formation processes in this system are
investigated
A cascade of magnetic field induced spin transitions in LaCoO3
We present magnetization and magnetostriction studies of the insulating
perovskite LaCoO3 in magnetic fields approaching 100 T. In marked contrast with
expectations from single-ion models, the data reveal two distinct first-order
spin transitions and well-defined magnetization plateaux. The magnetization at
the higher plateau is only about half the saturation value expected for spin-1
Co3+ ions. These findings strongly suggest collective behavior induced by
strong interactions between different electronic -- and therefore spin --
configurations of Co3+ ions. We propose a model of these interactions that
predicts crystalline spin textures and a cascade of four magnetic phase
transitions at high fields, of which the first two account for the experimental
data.Comment: 5 pages + supplementary materials, 5 figure
Simulations of cubic-tetragonal ferroelastics
We study domain patterns in cubic-tetragonal ferroelastics by solving
numerically equations of motion derived from a Landau model of the phase
transition, including dissipative stresses. Our system sizes, of up to 256^3
points, are large enough to reveal many structures observed experimentally.
Most patterns found at late stages in the relaxation are multiply banded; all
three tetragonal variants appear, but inequivalently. Two of the variants form
broad primary bands; the third intrudes into the others to form narrow
secondary bands with the hosts. On colliding with walls between the primary
variants, the third either terminates or forms a chevron. The multipy banded
patterns, with the two domain sizes, the chevrons and the terminations, are
seen in the microscopy of zirconia and other cubic-tetragonal ferroelastics. We
examine also transient structures obtained much earlier in the relaxation;
these show the above features and others also observed in experiment.Comment: 7 pages, 6 colour figures not embedded in text. Major revisions in
conten
Coarse Grained Density Functional Theories for Metallic Alloys: Generalized Coherent Potential Approximations and Charge Excess Functional Theory
The class of the Generalized Coherent Potential Approximations (GCPA) to the
Density Functional Theory (DFT) is introduced within the Multiple Scattering
Theory formalism for dealing with, ordered or disordered, metallic alloys. All
GCPA theories are based on a common ansatz for the kinetic part of the
Hohenberg-Kohn functional and each theory of the class is specified by an
external model concerning the potential reconstruction. The GCPA density
functional consists of marginally coupled local contributions, does not depend
on the details of the charge density and can be exactly rewritten as a function
of the appropriate charge multipole moments associated with each lattice site.
A general procedure based on the integration of the 'qV' laws is described that
allows for the explicit construction the same function. The coarse grained
nature of the GCPA density functional implies great computational advantages
and is connected with the O(N) scalability of GCPA algorithms. Moreover, it is
shown that a convenient truncated series expansion of the GCPA functional leads
to the Charge Excess Functional (CEF) theory [E. Bruno, L. Zingales and Y.
Wang, Phys. Rev. Lett. {\bf 91}, 166401 (2003)] which here is offered in a
generalized version that includes multipolar interactions. CEF and the GCPA
numerical results are compared with status of art LAPW full-potential density
functional calculations for 62, bcc- and fcc-based, ordered CuZn alloys, in all
the range of concentrations. These extensive tests show that the discrepancies
between GCPA and CEF are always within the numerical accuracy of the
calculations, both for the site charges and the total energies. Furthermore,
GCPA and CEF very carefully reproduce the LAPW site charges and the total
energy trends.Comment: 19 pages, 11 figure
Study of Phase Stability in NiPt Systems
We have studied the problem of phase stability in NiPt alloy system. We have
used the augmented space recursion based on the TB-LMTO as the method for
studying the electronic structure of the alloys. In particular, we have used
the relativistic generalization of our earlier technique. We note that, in
order to predict the proper ground state structures and energetics, in addition
to relativistic effects, we have to take into account charge transfer effects
with precision.Comment: 22 pages, 7 figures. Accepted for publication in JPC
Tweed in Martensites: A Potential New Spin Glass
We've been studying the ``tweed'' precursors above the martensitic transition
in shape--memory alloys. These characteristic cross--hatched modulations occur
for hundreds of degrees above the first--order shape--changing transition. Our
two--dimensional model for this transition, in the limit of infinite elastic
anisotropy, can be mapped onto a spin--glass Hamiltonian in a random field. We
suggest that the tweed precursors are a direct analogy of the spin--glass
phase. The tweed is intermediate between the high--temperature cubic phase and
the low--temperature martensitic phase in the same way as the spin--glass phase
can be intermediate between ferromagnet and antiferromagnet.Comment: 18 pages and four figures (included
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