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 [Mo(IV)(CN)2(CN−CuL)6]8+[Mo(IV)(CN)_2(CN-CuL)_6]^{8+} Perchlorate

A theoretical model for understanding photomagnetism in the heptanuclear complex $[Mo(IV)(CN)_2(CN-CuL)_6]^{8+}$ 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 $Cu(II)$ ions and one diamagnetic $Mo(IV)$ 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 $[Mo(IV)(CN)_2(CN-CuL)_6]^{8+}$ 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