460 research outputs found
Bulk and surface switching in Mn-Fe-based Prussian Blue Analogues
Many Prussian Blue Analogues are known to show a thermally induced phase
transition close to room temperature and a reversible, photo-induced phase
transition at low temperatures. This work reports on magnetic measurements,
X-ray photoemission and Raman spectroscopy on a particular class of these
molecular heterobimetallic systems, specifically on
Rb0.81Mn[Fe(CN)6]0.95_1.24H2O, Rb0.97Mn[Fe(CN)6]0.98_1.03H2O and
Rb0.70Cu0.22Mn0.78[Fe(CN)6]0.86_2.05H2O, to investigate these transition
phenomena both in the bulk of the material and at the sample surface. Results
indicate a high degree of charge transfer in the bulk, while a substantially
reduced conversion is found at the sample surface, even in case of a near
perfect (Rb:Mn:Fe=1:1:1) stoichiometry. Thus, the intrinsic incompleteness of
the charge transfer transition in these materials is found to be primarily due
to surface reconstruction. Substitution of a large fraction of charge transfer
active Mn ions by charge transfer inactive Cu ions leads to a proportional
conversion reduction with respect to the maximum conversion that is still
stoichiometrically possible and shows the charge transfer capability of metal
centers to be quite robust upon inclusion of a neighboring impurity.
Additionally, a 532 nm photo-induced metastable state, reminiscent of the high
temperature Fe(III)Mn(II) ground state, is found at temperatures 50-100 K. The
efficiency of photo-excitation to the metastable state is found to be maximized
around 90 K. The photo-induced state is observed to relax to the low
temperature Fe(II)Mn(III) ground state at a temperature of approximately 123 K.Comment: 12 pages, 8 figure
Novel mechanism of photoinduced reversible phase transitions in molecule-based magnets
A novel microscopic mechanism of bi-directional structural changes is
proposed for the photo-induced magnetic phase transition in Co-Fe Prussian blue
analogues on the basis of ab initio quantum chemical cluster calculations. It
is shown that the local potential energies of various spin states of Co are
sensitive to the number of nearest neighbor Fe vacancies. As a result, the
forward and backward structural changes are most readily initiated by
excitation of different local regions by different photons. This mechanism
suggests an effective strategy to realize photoinduced reversible phase
transitions in a general system consisting of two local components.Comment: 4 pages, LaTex, 3 figures, to appear in Phys. Rev. Let
Structure of Metastable States in Phase Transitions with High-Spin Low-Spin Degree of Freedom
Difference of degeneracy of the low-spin (LS) and high-spin (HS) states
causes interesting entropy effects on spin-crossover phase transitions and
charge transfer phase transitions in materials composed of the spin-crossover
atoms. Mechanisms of the spin-crossover (SC) phase transitions have been
studied by using Wajnflasz model, where the degeneracy of the spin states (HS
or LS) is taken into account and cooperative natures of the spin-crossover
phase transitions have been well described. Recently, a charge transfer (CT)
phase transition due to electron hopping between LS and HS sites has been
studied by using a generalized Wajnflasz model. In the both systems of SC and
CT, the systems have a high temperature structure (HT) and a low temperature
structure (LT), and the change between them can be a smooth crossover or a
discontinuous first order phase transition depending on the parameters of the
systems. Although apparently the standard SC system and the CT system are very
different, it is shown that both models are equivalent under a certain
transformation of variables. In both systems, the structure of metastable state
at low temperatures is a matter of interest. We study temperature dependence of
fraction of HT systematically in a unified model, and find several structures
of equilibrium and metastable states of the model as functions of system
parameters. In particular, we find a reentrant type metastable branch of HT in
a low temperature region, which would play an important role to study the
photo-irradiated processes of related materials.Comment: 19 pages, 11 figure
Microscopic Model for Photoinduced Magnetism in the Molecular Complex Perchlorate
A theoretical model for understanding photomagnetism in the heptanuclear
complex perchlorate is developed. It is a
many-body model involving the active orbitals on the transition metal ions. The
model is exactly solved using a valence bond approach. The ground state
solution of the model is highly degenerate and is spanned by five S=0 states,
nine S=1 states, five S=2 states and one S=3 state. The orbital occupancies in
all these states correspond to six ions and one diamagnetic
ion. The optically excited charge-transfer (CT) state in each spin sector occur
at nearly the same excitation energy of 2.993 eV for the physically reasonable
parameter values. The degeneracy of the CT states is largest in the S=3 sector
and so is the transition dipole moment from the ground state to these excited
states. Thus laser irradiation with light of this energy results in most
intense absorption in the S=3 sector. The life-time of the S=3 excited states
is also expected to be the largest as the number of states below that energy is
very sparse in this spin sector when compared to other spin sectors. These twin
features of our model explain the observed photomagnetism in the
complex.Comment: 8 pages, 6 figures and 1 tabl
Electronic structure study by means of X-ray spectroscopy and theoretical calculations of the "ferric star" single molecule magnet
The electronic structure of the single molecule magnet system
M[Fe(L)2]3*4CHCl3 (M=Fe,Cr; L=CH3N(CH2CH2O)2) has been studied using X-ray
photoelectron spectroscopy, X-ray absorption spectroscopy, soft X-ray emission
spectroscopy, and density functional calculations. There is good agreement
between theoretical calculations and experimental data. The valence band mainly
consists of three bands between 2 eV and 30 eV. Both theory and experiments
show that the top of the valence band is dominated by the hybridization between
Fe 3d and O 2p bands. From the shape of the Fe 2p spectra it is argued that Fe
in the molecule is most likely in the 2+ charge state. Its neighboring atoms
(O,N) exhibit a magnetic polarisation yielding effective spin S=5/2 per iron
atom, giving a high spin state molecule with a total S=5 effective spin for the
case of M = Fe.Comment: Fig.2 replaced as it will appear in J. Chem. Phy
Magnetic Behavior of a Mixed Ising Ferrimagnetic Model in an Oscillating Magnetic Field
The magnetic behavior of a mixed Ising ferrimagnetic system on a square
lattice, in which the two interpenetrating square sublattices have spins +- 1/2
and spins +-1,0, in the presence of an oscillating magnetic field has been
studied with Monte Carlo techniques. The model includes nearest and
next-nearest neighbor interactions, a crystal field and the oscillating
external field. By studying the hysteretic response of this model to an
oscillating field we found that it qualitatively reproduces the increasing of
the coercive field at the compensation temperature observed in real
ferrimagnets, a crucial feature for magneto-optical applications. This behavior
is basically independent of the frequency of the field and the size of the
system. The magnetic response of the system is related to a dynamical
transition from a paramagnetic to a ferromagnetic phase and to the different
temperature dependence of the relaxation times of both sublattices.Comment: 10 figures. To be published in Phys.Rev
Аномальная динамика намагниченности вблизи температуры спин-переориентационного перехода в нанопроволоках ε-In₀,₂₄Fe₁,₇₆O₃
Обнаружено, что увеличение частоты переменного магнитного поля приводит в нанопроволоках ε- In₀,₂₄Fe₁,₇₆O₃ к аномальному сдвигу максимума температурной зависимости динамической магнитной восприимчивости в противоположную сторону по сравнению с предсказаниями для термоактивированных процессов. Установлено, что наблюдаемый необычный эффект обусловлен перераспределением вкладов в динамическую магнитную восприимчивость высокотемпературной и низкотемпературной фаз, каждая из которых описывается в рамках модели кластерного стекла. В спектрах электронного спинового резонанса идентифицированы вклады, отвечающие этим фазам.Виявлено, що збільшення частоти змінного магнітного поля призводить в нанодротах ε- In₀,₂₄Fe₁,₇₆O₃ до аномального зсуву максимуму температурної залежності динамічної магнітної сприйнятливості в протилежну сторону в порівнянні з пророкуванням для термоактивованих процесів. Встановлено, що спостережуваний незвичайний ефект обумовлено перерозподілом вкладів в динамічну магнітну сприйнятливість високотемпературної та низькотемпературної фаз, кожна з яких описується у рамках моделі кластерного скла. У спектрах електронного спінового резонансу ідентифіковано вклади, що відповідають цим фазам.An anomalous direction of the shift of the temperature maximum of magnetic susceptibility with increasing magnetic field frequency was observed in ε-In₀,₂₄Fe₁,₇₆O₃ nanowires arrays in spite of the prediction of the theory of thermoactivated processes. The unusual effect can be explained by the redistribution of the contributions from low- and high-temperature phases to the temperature dependence of magnetic susceptibility. The magnetic state of each of the phases is described by the model of cluster magnetic glass. Separated electron spin resonance responses of these phases were distinguishe
A Kinetic Model for Photoswitching of magnetism in the High Spin Molecule [Mo(IV)(CN)2(CN-Cu(II)(tren))6](ClO4)8
The heptanuclear complex [Mo(IV)(CN)2(CN-CuL)6]8+ exhibits photomagnetism. An
earlier microscopic model showed that the transition dipole moments for
excitation in different spin manifolds are similar in magnitude. In this paper,
we attribute photomagnetism to the long lived S=3 charge transfer excited state
for which there appears to be sufficient experimental evidence. We model the
photomagnetism by employing a kinetic model which includes internal conversions
and intersystem crossings. The key feature of the model is assumption of the
existence of two kinds of S=3 states: one which has no direct pathway for
internal conversion and the other characterized by slow kinetics for internal
conversion to the low-energy states. The trapped S=3 state can decay via a
thermally activated barrier to the other S=3 state. The experimental
temperature dependence of magnetization plot is fitted using rate constants
with Arrhenius dependence. The two different experimental cMT vs. T curves
obtained with different irradiation times are fitted with our model. Our
studies show that the photomagnetism in these systems is governed by kinetics
and not due to differences in oscillator strengths for excitation of the
different spin states.Comment: 17 pages including 5 figures. Submitted to Phys. Rev.
A Unified Theoretical Description of the Thermodynamical Properties of Spin Crossover with Magnetic Interactions
After the discovery of the phenomena of light-induced excited spin state
trapping (LIESST), the functional properties of metal complexes have been
studied intensively. Among them, cooperative phenomena involving low spin-high
spin (spin-crossover) transition and magnetic ordering have attracted
interests, and it has become necessary to formulate a unified description of
both phenomena. In this work, we propose a model in which they can be treated
simultaneously by extending the Wajnflasz-Pick model including a magnetic
interaction. We found that this new model is equivalent to
Blume-Emery-Griffiths (BEG) Hamiltonian with degenerate levels. This model
provides a unified description of the thermodynamic properties associated with
various types of systems, such as spin-crossover (SC) solids and Prussian blue
analogues (PBA). Here, the high spin fraction and the magnetization are the
order parameters describing the cooperative phenomena of the model. We present
several typical temperature dependences of the order parameters and we
determine the phase diagram of the system using the mean-field theory and Monte
Carlo simulations. We found that the magnetic interaction drives the SC
transition leading to re-entrant magnetic and first-order SC transitions.Comment: 30pages, 11figure
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