Structures of Metastable States in Phase Transitions with a High-Spin Low-Spin Degree of Freedom

Microscopic Cutoff Dependence of an Entropic Force in Interface Propagation of Stochastic Order Parameter Dynamics

The steady propagation of a (d-1)-dimensional planer interface in d-dimensional space is studied by analyzing mesoscopic nonconserved order parameter dynamics with two local minima under the influence of thermal noise. In this analysis, an entropic force generating interface propagation is formulated using a perturbation method. It is found that the entropic force singularly depends on an ultraviolet cutoff when d≥2. The theoretical calculation is confirmed by numerical simulations with d=2. The result means that an experimental measurement of the entropic force provides an estimation of the microscopic cutoff of the mesoscopic description.

Giant Magnetoelectric Coupling and Magnetic-Field-Induced Permanent Switching in a Spin Crossover Mn(III) Complex

We investigate giant magnetoelectric coupling at a Mn³⁺ spin crossover in [Mn^IIIL]BPh₄ (L = (3,5-diBr-sal)₂323) with a field-induced permanent switching of the structural, electric, and magnetic properties. An applied magnetic field induces a first-order phase transition from a high spin/low spin (HS-LS) ordered phase to a HS-only phase at 87.5 K that remains after the field is removed. We observe this unusual effect for DC magnetic fields as low as 8.7 T. The spin-state switching driven by the magnetic field in the bistable molecular material is accompanied by a change in electric polarization amplitude and direction due to a symmetry-breaking phase transition between polar space groups. The magnetoelectric coupling occurs due to a γη² coupling between the order parameter γ related to the spin-state bistability and the symmetry-breaking order parameter η responsible for the change of symmetry between polar structural phases. We also observe conductivity occurring during the spin crossover and evaluate the possibility that it results from conducting phase boundaries. We perform ab initio calculations to understand the origin of the electric polarization change as well as the conductivity during the spin crossover. Thus, we demonstrate a giant magnetoelectric effect with a field-induced electric polarization change that is 1/10 of the record for any material.

Stress-Induced Domain Wall Motion in a Ferroelastic Mn3+ Spin Crossover Complex

Domain wall motion is detected for the first time during the transition to a ferroelastic and spin state ordered phase of a spin crossover complex. Single-crystal X-ray diffraction and resonant ultrasound spectroscopy (RUS) revealed two distinct symmetry-breaking phase transitions in the mononuclear Mn3+ compound [Mn(3,5-diBr-sal2 (323))]BPh4 , 1. The first at 250 K, involves the space group change Cc→Pc and is thermodynamically continuous, while the second, Pc→P1 at 85 K, is discontinuous and related to spin crossover and spin state ordering. Stress-induced domain wall mobility was interpreted on the basis of a steep increase in acoustic loss immediately below the the Pc-P1 transition.

Microscopic theory of cooperative spin crossover: Interaction of molecular modes with phonons

In this article, we present a new microscopic theoretical approach to the description of spin crossover in molecular crystals. The spin crossover crystals under consideration are composed of molecular fragments formed by the spin-crossover metal ion and its nearest ligand surrounding and exhibiting well defined localized (molecular) vibrations. As distinguished from the previous models of this phenomenon, the developed approach takes into account the interaction of spin-crossover ions not only with the phonons but also a strong coupling of the electronic shells with molecular modes. This leads to an effective coupling of the local modes with phonons which is shown to be responsible for the cooperative spin transition accompanied by the structural reorganization. The transition is characterized by the two order parameters representing the mean values of the products of electronic diagonal matrices and the coordinates of the local modes for the high- and low-spin states of the spin crossover complex. Finally, we demonstrate that the approach provides a reasonable explanation of the observed spin transition in the [Fe(ptz)6](BF4)2 crystal. The theory well reproduces the observed abrupt low-spin → high-spin transition and the temperature dependence of the high-spin fraction in a wide temperature range as well as the pronounced hysteresis loop. At the same time within the limiting approximations adopted in the developed model, the evaluated high-spin fraction vs. T shows that the cooperative spin-lattice transition proves to be incomplete in the sense that the high-spin fraction does not reach its maximum value at high temperature.

Monte-Carlo simulations of spin-crossover phenomena based on a vibronic Ising-like model with realistic parameters

A new vibronic Ising-like model considering harmonic stretching-and-bending intermolecular interactions with realistic parameters describes thermal hysteresis in spin-crossover phenomena well.

Cooperative spin-crossover-like transitions in the inhomogeneous chain of exchange clusters

A statistical theory of temperature-induced spin-crossover-like transitions has been developed for spin chains with a few exchange clusters in a unit cell. Using static magnetic susceptibility measurements, a general expression for the effective magnetic moment μ(eff) of exchange clusters is obtained for an arbitrary structure of cluster terms. For heterospin Cu(hfac)2L(R) complexes ("breathing crystals") with a "head-to-head" chain motif, a two-point approximation for a partition function of the Jahn-Teller Cu(+2) paramagnetic center (spin 1/2) is suggested. Theoretical results are compared with available experimental data on thermal spin-crossover-like transitions in the "breathing crystal" compounds. It is shown that the model developed is able to successfully describe both of the cases of smooth and abrupt (cooperative) spin transitions for chains of three-spin exchange clusters. Spin-crossover-like transitions of a new type with step-wise changes in a positive exchange integral are predicted.

Photomagnetism in cyano-bridged bimetal assemblies

The study of photoinduced phase-transition materials has implications for the fields of inorganic chemistry, solid-state chemistry, and materials science. Cyano-bridged bimetal assemblies are promising photomagnetic materials. Because cyano-bridged bimetal assemblies possess various absorption bands in the visible light region, their electronic and spin states can be controlled by visible light irradiation. Moreover, the selection of magnetic metal ions and organic ligands provide a way of controlling spin-spin interactions through a cyano bridge. In this Account, we describe cyano-bridged bimetal assemblies developed in our laboratory. Cu(II)(2)[Mo(IV)(CN)(8)]·8H(2)O (CuMo), Rb(I)Mn(II)[Fe(III)(CN)(6)] (RbMnFe), and Co(II)(3)[W(V)(CN)(8)](2)·(pyrimidine)(4)·6H(2)O (CoW) induce photomagnetism via photoinduced metal-to-metal charge transfers (MM'CT), while Fe(II)(2)[Nb(IV)(CN)(8)]·(4-pyridinealdoxime)(8)·2H(2)O (FeNb) exhibits a photoinduced magnetization via a photoinduced spin crossover. Irradiation with 473 nm light causes the CuMo system to exhibit a spontaneous magnetization with a Curie temperature (T(C)) of 25 K, but irradiation with 532, 785, and 840 nm light reduces the magnetization. In this reversible photomagnetic process, excitation of the MM'CT from Mo(IV) to Cu(II) produces a ferromagnetic mixed-valence isomer of Cu(I)Cu(II)[Mo(V)(CN)(8)]·8H(2)O (CuMo'). CuMo' returns to CuMo upon irradiation in the reverse-M'MCT band. RbMnFe shows a charge transfer (CT)-induced phase transition from the Mn(II)-Fe(III) phase to the Mn(III)-Fe(II) phase. Irradiation with 532 nm light converts the Mn(III)-Fe(II) phase into the Mn(II)-Fe(III) phase, and we observe photodemagnetization. In contrast, irradiation of the Mn(II)-Fe(III) phase with 410 nm light causes the reverse phase transition. A CT-induced Jahn-Teller distortion is responsible for this visible light-induced reversible photomagnetic effect. In the CoW system, a CT-induced spin transition causes the thermal phase transition from the Co(II)-W(V) phase to the Co(III)-W(IV) phase. Irradiation of the Co(III)-W(IV) phase with 840 nm light causes ferromagnetism with a T(C) of 40 K and magnetic coercive field (H(c)) of 12,000 Oe, but excitation of the back M'MCT (Co(II) → W(V)) with 532 nm light leads to the reverse phase transition. These examples of the photomagnetic effect have occurred by exciting MM'CT bands. In the fields of inorganic chemistry and materials science, researchers have studied extensively the photoinduced phase transitions between low-spin (LS) and high-spin (HS) transition metal ions. Recently, we have observed the first example of photoinduced spin crossover ferromagnetism with a FeNb system (T(C) = 20 K and H(c) = 240 Oe), in which a strong superexchange interaction between photoproduced Fe(II)(HS) and neighboring paramagnetic Nb(IV) operates through a CN bridge. The optical switching magnets described in this Account may lead to novel optical recording technologies such as optomagnetic memories and optical computers.

Molecular spin crossover phenomenon: recent achievements and prospects

Recently we assisted a strong renewed interest in the fascinating field of molecular spin crossover complexes by (1) the emergence of nanosized spin crossover materials through direct synthesis of coordination nanoparticles and nanopatterned thin films as well as by (2) the use of novel sophisticated high spatial and temporal resolution experimental techniques and theoretical approaches for the study of spatiotemporal phenomena in cooperative spin crossover systems. Besides generating new fundamental knowledge on size-reduction effects and the dynamics of the spin crossover phenomenon, this research aims also at the development of practical applications such as sensor, display, information storage and nanophotonic devices. In this critical review, we discuss recent work in the field of molecule-based spin crossover materials with a special focus on these emerging issues, including chemical synthesis, physical properties and theoretical aspects as well (223 references).

Asymptotic forms and scaling properties of the relaxation time near threshold points in spinodal-type dynamical phase transitions

We study critical properties of the relaxation time at a threshold point in switching processes between bistable states under change in external fields. In particular, we investigate the relaxation processes near the spinodal point of the infinitely long-range interaction model (the Husimi-Temperley model) by analyzing the scaling properties of the corresponding Fokker-Planck equation. We also confirm the obtained scaling relations by direct numerical solution of the original master equation, and by kinetic Monte Carlo simulation of the stochastic decay process. In particular, we study the asymptotic forms of the divergence of the relaxation time near the spinodal point and re-examine its scaling properties. We further extend the analysis to transient critical phenomena such as a threshold behavior with diverging switching time under a general external driving perturbation. This models photoexcitation processes in spin-crossover materials. In the ongoing development of nanosize fabrication, such size-dependence of switching processes should be an important issue, and the properties obtained here will be applicable to a wide range of physical processes.

Phase transition in spin systems with various types of fluctuations

Various types ordering processes in systems with large fluctuation are overviewed. Generally, the so-called order-disorder phase transition takes place in competition between the interaction causing the system be ordered and the entropy causing a random disturbance. Nature of the phase transition strongly depends on the type of fluctuation which is determined by the structure of the order parameter of the system. As to the critical property of phase transitions, the concept "universality of the critical phenomena" is well established. However, we still find variety of features of ordering processes. In this article, we study effects of various mechanisms which bring large fluctuation in the system, e.g., continuous symmetry of the spin in low dimensions, contradictions among interactions (frustration), randomness of the lattice, quantum fluctuations, and a long range interaction in off-lattice systems.

Monte Carlo simulation of pressure-induced phase transitions in spin-crossover materials

Pressure-induced phase transitions of spin-crossover materials are studied in a microscopic model taking into account the elastic interaction among distortions of lattice due to the difference of the molecular sizes between the high-spin state and the low-spin state. We perform Monte Carlo simulations in the constant pressure ensemble and reproduce several important properties of the pressure effect in a unified way with a microscopic mechanism for the first time. The simulation newly reveals how the temperature dependence of the ordering process changes with the pressure.

Simple two-dimensional model for the elastic origin of cooperativity among spin states of spin-crossover complexes

We study the origin of the cooperative nature of spin crossover (SC) between low-spin and high-spin (HS) states from the viewpoint of elastic interactions among molecules. As the size of each molecule changes depending on its spin state, the elastic interaction among the lattice distortions provides the cooperative interaction of the spin states. We develop a simple model of SC with intra and intermolecular potentials which accounts for the elastic interaction including the effect of the inhomogeneity of the spin states and apply constant temperature molecular dynamics based on the Nosé-Hoover formalism. We demonstrate that, with increase of the strength of the intermolecular interactions, the temperature dependence of the HS component changes from a gradual crossover to a first-order transition.