2,186 research outputs found
Driving rate effects in avalanche-mediated, first-order phase transitions
We have studied the driving rate and temperature dependence of the power-law
exponents that characterize the avalanche distribution in first-order phase
transitions. Measurements of acoustic emission in structural transitions in
Cu-Zn-Al and Cu-Al-Ni are presented. We show how the observed behaviour emerges
within a general framework of competing time scales of avalanche relaxation,
driving rate, and thermal fluctuations. We have confirmed our findings by
numerical simulations of a prototype model.Comment: 4 pages, 3 figure
Pressure-induced amorphization, crystal-crystal transformations and the memory glass effect in interacting particles in two dimensions
We study a model of interacting particles in two dimensions to address the
relation between crystal-crystal transformations and pressure-induced
amorphization. On increasing pressure at very low temperature, our model
undergoes a martensitic crystal-crystal transformation. The characteristics of
the resulting polycrystalline structure depend on defect density, compression
rate, and nucleation and growth barriers. We find two different limiting cases.
In one of them the martensite crystals, once nucleated, grow easily
perpendicularly to the invariant interface, and the final structure contains
large crystals of the different martensite variants. Upon decompression almost
every atom returns to its original position, and the original crystal is fully
recovered. In the second limiting case, after nucleation the growth of
martensite crystals is inhibited by energetic barriers. The final morphology in
this case is that of a polycrystal with a very small crystal size. This may be
taken to be amorphous if we have only access (as experimentally may be the
case) to the angularly averaged structure factor. However, this `X-ray
amorphous' material is anisotropic, and this shows up upon decompression, when
it recovers the original crystalline structure with an orientation correlated
with the one it had prior to compression. The memory effect of this X-ray
amorphous material is a natural consequence of the memory effect associated to
the underlying martensitic transformation. We suggest that this kind of
mechanism is present in many of the experimental observations of the memory
glass effect, in which a crystal with the original orientation is recovered
from an apparently amorphous sample when pressure is released.Comment: 13 pages, 13 figures, to be published in Phys. Rev.
Field induced long-range-ordering in an S=1 quasi-one-dimensional Heisenberg antiferromagnet
We have measured the heat capacity and magnetization of the spin one
one-dimensional Heisenberg antiferromagnet NDMAP and constructed a magnetic
field versus temperature phase diagram. We found a field induced long-range
magnetic ordering. We have been successful in explaining the phase diagram
theoretically.Comment: 6 pages, 18 figure
Spin-Reorientation Transition of Field-Induced Magnetic Ordering Phases in the Anisotropic Haldane System
A possible spin-reorientation transition in field-induced magnetic ordering
phases of the S=1 Haldane system with large easy-plane anisotropy is proposed,
using an effective Lagrangian formalism as well as the density matrix
renormalization group method. Such a spin-reorientation transition is predicted
in the case where the applied magnetic field is inclined from the easy axis of
the anisotropy. We point out that this transition has a close connection with a
variation of the order parameter even at zero temperature, although it is
different from a quantum analog of the so-called spin-flop transition proposed
for the system having a strong easy axis anisotropy. In connection with a novel
phase observed recently in the Haldane system at high fields, we discuss
possible implications for the field-induced magnetic ordering.Comment: 14 pages, 7 figure
Phase diagram and hidden order for generalized spin ladders
We investigate the phase diagram of antiferromagnetic spin ladders with
additional exchange interactions on diagonal bonds by variational and numerical
methods. These generalized spin ladders interpolate smoothly between the
chain with competing nn and nnn interactions, the chain with
alternating exchange and the antiferromagnetic chain. The Majumdar-Ghosh
ground states are formulated as matrix product states and are shown to exhibit
the same type of hidden order as the af chain. Generalized matrix product
states are used for a variational calculation of the ground state energy and
the spin and string correlation functions. Numerical (Lanczos) calculations of
the energies of the ground state and of the low-lying excited states are
performed, and compare reasonably with the variational approach. Our results
support the hypothesis that the dimer and Majumdar-Ghosh points are in the same
phase as the af chain.Comment: 23 pages, REVTEX, 7 figure
Realization of the mean-field universality class in spin-crossover materials
In spin-crossover materials, the volume of a molecule changes depending on
whether it is in the high-spin (HS) or low-spin (LS) state. This change causes
distortion of the lattice. Elastic interactions among these distortions play an
important role for the cooperative properties of spin-transition phenomena. We
find that the critical behavior caused by this elastic interaction belongs to
the mean-field universality class, in which the critical exponents for the
spontaneous magnetization and the susceptibility are and , respectively. Furthermore, the spin-spin correlation function is a
constant at long distances, and it does not show an exponential decay in
contrast to short-range models. The value of the correlation function at long
distances shows different size-dependences: , , and
constant for temperatures above, at, and below the critical temperature,
respectively. The model does not exhibit clusters, even near the critical
point. We also found that cluster growth is suppressed in the present model and
that there is no critical opalescence in the coexistence region. During the
relaxation process from a metastable state at the end of a hysteresis loop,
nucleation phenomena are not observed, and spatially uniform configurations are
maintained during the change of the fraction of HS and LS. These
characteristics of the mean-field model are expected to be found not only in
spin-crossover materials, but also generally in systems where elastic
distortion mediates the interaction among local states.Comment: 13 pages, 16 figure
Simulations of Spinodal Nucleation in Systems with Elastic Interactions
Systems with long-range interactions quenched into a metastable state near
the pseudospinodal exhibit nucleation that is qualitatively different than the
classical nucleation observed near the coexistence curve. We have observed
nucleation droplets in our Langevin simulations of a two-dimensional model of
martensitic transformations and have determined that the structure of the
nucleating droplet differs from the stable martensite structure. Our results,
together with experimental measurements of the phonon dispersion curve, allow
us to predict the nature of the droplet. These results have implications for
nucleation in many solid-solid transitions and the structure of the final
state
Origin of the reduced exchange bias in epitaxial FeNi(111)/CoO(111) bilayer
We have employed Soft and Hard X-ray Resonant Magnetic Scattering and
Polarised Neutron Diffraction to study the magnetic interface and the bulk
antiferromagnetic domain state of the archetypal epitaxial
NiFe(111)/CoO(111) exchange biased bilayer. The combination of
these scattering tools provides unprecedented detailed insights into the still
incomplete understanding of some key manifestations of the exchange bias
effect. We show that the several orders of magnitude difference between the
expected and measured value of exchange bias field is caused by an almost
anisotropic in-plane orientation of antiferromagnetic domains. Irreversible
changes of their configuration lead to a training effect. This is directly seen
as a change in the magnetic half order Bragg peaks after magnetization
reversal. A 30 nm size of antiferromagnetic domains is extracted from the width
the (1/2 1/2 1/2) antiferromagnetic magnetic peak measured both by neutron and
x-ray scattering. A reduced blocking temperature as compared to the measured
antiferromagnetic ordering temperature clearly corresponds to the blocking of
antiferromagnetic domains. Moreover, an excellent correlation between the size
of the antiferromagnetic domains, exchange bias field and frozen-in spin ratio
is found, providing a comprehensive understanding of the origin of exchange
bias in epitaxial systems.Comment: 8 pages, 5 figures, submitte
On the athermal character of structural phase transitions
The significance of thermal fluctuations on nucleation in structural
first-order phase transitions has been examined. The prototype case of
martensitic transitions has been experimentally investigated by means of
acoustic emission techniques. We propose a model based on the mean
first-passage time to account for the experimental observations. Our study
provides a unified framework to establish the conditions for isothermal and
athermal transitions to be observed.Comment: 5 pages, 4 figures, accepted in Phys. Rev. Let
Microstructural characterization of AISI 431 martensitic stainless steel laser-deposited coatings
High cooling rates during laser cladding of stainless steels may alter the microstructure and phase constitution of the claddings and consequently change their functional properties. In this research, solidification structures and solid state phase transformation products in single and multi layer AISI 431 martensitic stainless steel coatings deposited by laser cladding at different processing speeds are investigated by optical microscopy, Scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), orientation imaging microscopy (OIM), ternary phase diagram, Schaeffler and TTT diagrams. The results of this study show how partitionless solidification and higher solidification rates alter the microstructure and phase constitution of martensitic stainless steel laser deposited coatings. In addition, it is shown that while different cladding speeds have no effect on austenite–martensite orientation relationship in the coatings, increasing the cladding speed has resulted in a reduction of hardness in deposited coatings which is in contrast to the common idea about obtaining higher hardness values at higher cladding speeds.
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