197 research outputs found
Spin canting and ferromagnetism in a AuFe alloy: Mössbauer and magnetic measurements
We have made Mössbauer polarization and susceptibility measurements above technical saturation on Au-19 at.% Fe. Just below the magnetic ordering temperature the system behaves as a ferromagnet, but random local canting sets in at lower temperatures. A well-defined canting transition temperature cannot be established from the data, but canting begins above the temperature at which the low-field susceptibility shows a sharp falloff
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
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
Size dependence of the photoinduced magnetism and long-range ordering in Prussian blue analog nanoparticles of rubidium cobalt hexacyanoferrate
Nanoparticles of rubidium cobalt hexacyanoferrate
(RbCo[Fe(CN)]HO) were synthesized using different
concentrations of the polyvinylpyrrolidone (PVP) to produce four different
batches of particles with characteristic diameters ranging from 3 to 13 nm.
Upon illumination with white light at 5 K, the magnetization of these particles
increases. The long-range ferrimagnetic ordering temperatures and the coercive
fields evolve with nanoparticle size. At 2 K, particles with diameters less
than approximately 10 nm provide a Curie-like magnetic signal.Comment: 10 pages, 6 figures in text, expanded text and dat
Dynamical Ising-like model for the two-step spin-crossover systems
In order to reproduce the two-step relaxation observed experimentally in spin-crossover systems, we investigate analytically the static and the dynamic properties of a two-sublattice Ising-like Hamiltonian. The formalism is based on a stochastic master equation approach. It is solved in the mean-field approximation, and yields two coupled differential equations that correspond to the HS fractions of the sublattices A and [email protected] ; [email protected]
Magnetization steps in a diluted Heisenberg antiferromagnetic chain: Theory and experiments on TMMC:Cd
A theory for the equilibrium low-temperature magnetization M of a diluted
Heisenberg antiferromagnetic chain is presented. The magnetization curve, M
versus B, is calculated using the exact contributions of finite chains with 1
to 5 spins, and the "rise and ramp approximation" for longer chains. Some
non-equilibrium effects that occur in a rapidly changing B, are also
considered. Specific non-equilibrium models based on earlier treatments of the
phonon bottleneck, and of spin flips associated with cross relaxation and with
level crossings, are discussed. Magnetization data on powders of TMMC diluted
with cadmium [i.e., (CH_3)_4NMn_xCd_(1-x)Cl_3, with 0.16<=x<=0.50 were measured
at 0.55 K in 18 T superconducting magnets. The field B_1 at the first MST from
pairs is used to determine the NN exchange constant, J, which changes from -5.9
K to -6.5 K as x increases from 0.16 to 0.50. The magnetization curves obtained
in the superconducting magnets are compared with simulations based on the
equilibrium theory. Data for the differential susceptibility, dM/dB, were taken
in pulsed magnetic fields (7.4 ms duration) up to 50 T, with the powder samples
in a 1.5 K liquid-helium bath. Non-equilibrium effects, which became more
severe as x decreased, were observed. The non-equilibrium effects are
tentatively interpreted using the "Inadequate Heat Flow Scenario," or to
cross-relaxation, and crossings of energy levels, including those of excited
states.Comment: 16 pages, 14 figure
Equivalence between non-bilinear spin- Ising model and Wajnflasz model
We propose the mapping of polynomial of degree 2S constructed as a linear
combination of powers of spin- (for simplicity, we called as spin-
polynomial) onto spin-crossover state. The spin- polynomial in general can
be projected onto non-symmetric degenerated spin up (high-spin) and spin down
(low-spin) momenta. The total number of mapping for each general spin- is
given by . As an application of this mapping, we consider a
general non-bilinear spin- Ising model which can be transformed onto
spin-crossover described by Wajnflasz model. Using a further transformation we
obtain the partition function of the effective spin-1/2 Ising model, making a
suitable mapping the non-symmetric contribution leads us to a spin-1/2 Ising
model with a fixed external magnetic field, which in general cannot be solved
exactly. However, for a particular case of non-bilinear spin- Ising model
could become equivalent to an exactly solvable Ising model. The transformed
Ising model exhibits a residual entropy, then it should be understood also as a
frustrated spin model, due to competing parameters coupling of the non-bilinear
spin- Ising model
Towards controlling the solid state valence tautomeric interconversion character by solvation
Crystals of [Co<i>(diox)</i><sub>2</sub>(4-NO<sub>2</sub>-py)<sub>2</sub>] (<b>1</b>) and [Co<i>(diox)</i><sub>2</sub>(4-CN-py)<sub>2</sub>] (<b>2</b>) where <i>diox</i> are the <i>o</i>-dioxolene 3,5-di-<i>t</i>-butylsemiquinonate (SQ<sup>•–</sup>) and/or
3,5-di-<i>t</i>-butylcatecholate (Cat<sup>2–</sup>) ions, 4-NO<sub>2</sub>-py is 4-nitro-pyridine, 4-CN-py is 4-cyano-pyridine,
are among the few known crystals presenting both thermally induced
and photoinduced <i>ls</i>-[M<sup>+3</sup>(SQ<sup>•–</sup>)(Cat<sup>2–</sup>)] ↔ <i>hs</i>-[M<sup>2+</sup>(SQ<sup>•–</sup>)<sub>2</sub>] valence tautomeric
interconversion (VTI). In <b>2</b>, the thermal-induced VTI
is cooperative, characterizing an abrupt conversion, and in <b>1</b> it is noncooperative. In this work, crystals of [Co<i>(diox)</i><sub>2</sub>(4-NO<sub>2</sub>-py)<sub>2</sub>]·benzene (<b>1BZ</b>), [Co<i>(diox)</i><sub>2</sub>(4-NO<sub>2</sub>-py)<sub>2</sub>]·toluene
(<b>1TL</b>), [Co<i>(diox)</i><sub>2</sub>(4-CN-py)<sub>2</sub>]·benzene (<b>2BZ</b>), and [Co<i>(diox)</i><sub>2</sub>(4-CN-py)<sub>2</sub>]·toluene (<b>2TL</b>) have been prepared and analyzed by single crystal X-ray
diffraction in order to investigate how solvation modulates thermally
induced VTI. Crystallographic data were also successfully used together
with the two-state equilibrium equation to estimate Δ<i>H</i>° and Δ<i>S</i>° VTI thermodynamic
parameters. The solvate crystals, like the nonsolvated ones, present
essentially reversible thermally induced VTI. The <b>1TL</b> crystal presents the same monoclinic symmetry and the same intermolecular
hydrogen-bonded network of <b>1</b>, and both present a noncooperative
thermal-induced VTI. The <b>1BZ</b> crystal has triclinic symmetry
and presents a cooperative VTI with a thermal hysteresis of ∼30
K. In contrast to <b>2</b>, thermally induced VTI in <b>2BZ</b> and <b>2TL</b> is noncooperative despite the fact that <b>2</b>, <b>2BZ</b>, and <b>2TL</b> crystals exhibit
the same monoclinic symmetry and the same intermolecular hydrogen-bonded
network. In <b>2BZ</b> and <b>2TL</b> benzene and toluene
molecules as well as the <i>t</i>-butyl groups of the <i>o</i>-dioxolene molecules convert gradually from being dynamically
disordered at about 300 K to a static disorder state below 150 K.
The layer separation distance of interacting [Co<i>(diox)</i><sub>2</sub>(4-X-py)<sub>2</sub>], X = CN and NO<sub>2</sub>, molecules in all solvate crystals is ∼15 Å, whereas
in <b>2</b>, which presents cooperative VTI, it is ∼12
Å. An order–disorder component might account for the stabilization
of the metastable <i>hs</i>-Co<sup>2+</sup> state in <b>2BZ</b> and in <b>2TL</b>, but no disorder was found in
the <b>1TL</b> crystals. Therefore, the lack of cooperativity
in the thermally induced VTI in these crystals seems to be due to
the large distance between the layers of interacting molecules. Cooperativity
in the VTI of <b>1BZ</b> crystal is likely to be related with
the unique molecular bond scheme network that connects neighboring
active [Co<i>(diox)</i><sub>2</sub>(4-NO<sub>2</sub>-py)<sub>2</sub>] molecules through the <i>o</i>-dioxolene
oxygen atoms bonded directly to the Co ion
Spectral sensitive phonon wipeout due to a fluctuating spin state in a Fe2+ coordination polymer
Raman scattering in the spin-crossover system [Fe(pmd)(H2O){Au(CN)2}2]*H2O
reveals a complex three-phase spin-state transition in contrast to earlier
observations in magnetization measurements. We observe different spin state
phases as function of temperature and electromagnetic radiation in the visible
spectral range. There exists a fluctuating spin state phase with an unexpected
wipeout of the low frequency phonon scattering intensity. Furthermore we
observe one phase with reduced symmetry that is attributed to a cooperative
Jahn-Teller effect. Pronounced electron-phonon interaction manifests itself as
a strong Fano-resonance of phonons related to {FeN6} and {FeN4O2} coordination
octahedra. Density functional theory supports this interpretation.Comment: 9 pages, 9 figures, 3 table
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