Structural origin of the gradual spin transition in a mononuclear iron(II) complex

Variable temperature single crystal X-ray diffraction and SQUID magnetometry experiments have revealed a gradual spin transition in [Fe^II(L)](ClO₄)₂ (where L=1,4,7-tris(2-aminophenyl)-1,4,7-triazacyclononane), centred around room temperature. The gradual nature of the spin transition has been attributed to the lack of significant intermolecular interactions between iron centres and the propensity of the counter ions to accommodate the internal strain in the crystal caused by spin crossover.     

1H spin–lattice relaxation in water solution of 209Bi counterparts of Gd3+contrast agents

ABSTRACT

¹H spin–lattice relaxation studies of water solutions of Bismuth-ethylenediamine-tetraacetic acid (Bi-EDTA), Bismuth-ethylenediamine-tetrakis(methylenephosphonic) acid (Bi-EDTP), Bismuth-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (Bi-DOTA), Bismuth-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetrakis(methylenephosphonic acid) (Bi-DOTP) and Bismuth-1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid (Bi-DO3A) have been performed in order to compare Quadrupole Relaxation Enhancement (QRE) effects with Paramagnetic Relaxation Enhancement (PRE) from the perspective of exploiting the first one as a novel contrast mechanism for Magnetic Resonance Imaging (MRI). The selected compounds can be considered as ²⁰⁹Bi counterparts of Gd³⁺ complexes. The relaxation experiments have been performed in a broad frequency range of 5?kHz–30?MHz. The relaxation contribution associated with QRE has been extracted from the data and compared with PRE. Similarities and differences between the two effects have been discussed.     

Theory of magnetic shape memory effect and pseudoelastic deformation in Ni-Mn-Ga alloys

The effect of the magnetic field on the deformation behavior of magnetic alloys of the Heusler type under different loading conditions is discussed in terms of the theory of diffuse martensitic transitions. The effects of magnetic shape memory, pseudoelastic deformation, and generation of reactive stresses in response to the magnetic field are considered. The theoretical relationships are compared with the experimental data available in the literature.

     

Photoneutrino energy loss rates under magnetic field

The neutrino energy loss rate is calculated due to the photoneutrino process in a hot plasma, under magnetic field.

The calculations done for low densities and relatively low temperatures may be used for astrophysical estimations in neutron stars.

     

Inequalities for the lowest magnetic Neumann eigenvalue

We study the ground-state energy of the Neumann magnetic Laplacian on planar domains. For a constant magnetic field, we consider the question whether the disc maximizes this eigenvalue for fixed area. More generally, we discuss old and new bounds obtained on this problem.

     

Analysis of the interrelation between the processes of polarization and microstructuring in a magnetic fluid layer

The results of examining and analyzing the interrelation between the processes of polarization and structuring in magnetic fluid microlayers under the influence of the temperature, external electric and magnetic fields, and variations in the structure and amount of stabilizer have been presented.

     

Selected States Magnetic Resonance Spectroscopy (SSMRS): Detection of Paramagnetic Element Dynamics

Abstract

Magnetic resonance (MR) response obtained in a strongly heterogeneous magnetic field with a linear gradient is analysed. It is shown that the employment of strong magnetic field gradients enables the MR spectroscopy to be accomplished in a system with selectively populated energy states (SSMRS). The method can be applied for measuring such physical quantities as the spin diffusion coefficient, D, spin-lattice, T_1m and spin-spin, T_2m, relaxation times and mobility, p_m, of paramagnetic elements in individual Zeeman energy states.

     

Some principles in choosing parameters of magnetic resonance tomographs

The problem of amplifying the signal that ensures the visualization of internal organs in the magnetic resonance tomograph due to the optimal selection of some of its parameters has been considered. The operating principle of the tomograph has been analyzed. The relation between the angle of the magnetic moment precession in hydrogen nuclei in an organism, the frequency of the ac magnetic field exciting this precession, and the constant magnetic field used has been determined using quantum-mechanical concepts. This relation makes it possible to determine the optimal parameters for tomograph operation.

     

Monte Carlo simulations of magnetic particle suspensions with a simple assessment method for the particle overlap between magnetic spheroids

We have developed a simple assessment method for the overlap between spheroidal particles, which neither requires the complex manipulation of vectors and matrices that is indispensable in the ordinary methods, nor is based on a model potential. Moreover, we have developed an evaluation method for the interaction energy arising from the overlap of the steric layer coating spheroidal particles. This is based on a sphere-connected particle model, but some modifications are introduced in order to express an appropriate repulsive interaction energy at the deepest overlapping position. We have investigated the phase change in a magnetic spheroidal particle suspension for a two-dimensional system by means of Monte Carlo simulations. In the case of no external magnetic field, if the magnetic particle-particle interaction is sufficiently strong to favour cluster formation, long raft-like clusters tend to be formed in a dilute situation. With decreasing values of area fraction, a chain-like structure in a dense situation transforms into a raft-like structure within a narrow range of the particle area fraction. Similarly, the raft-like clusters are preferred in a weak applied magnetic field, but an increase in the field strength induces a phase change from a raft-like into a chain-like structure.

Highlights of the present paper:

1. A simple assessment method has been proposed for the overlap between two spheroidal particles.

2. The particle overlap assessment is free from a complex mathematical manipulation regarding vectors and matrices.

3. A modified sphere-connected model has been proposed in order to more accurately evaluate a repulsive interaction due to the overlap of the steric layers coating spheroidal particles.

4. 2D Monte Carlo simulations have been performed to elucidate the phenomenon of a phase change by magnetic spheroidal particles on a material plane surface.

5. A phase change between a raft-like and a chain-like aggregate structure is able to be controlled by the area fraction of particles and an external magnetic field.

     

NMR studies of the sedimentation of ferromagnetic nanoparticles in a magnetic fluid

A method has been proposed for studying the sedimentation of ferromagnetic nanoparticles in a magnetic fluid from the rate of decrease in its magnetization measured by the NMR technique. The dependence of the rate of variations in the magnetization of the magnetic fluid over time on the concentration of the stabilizer is investigated. A colloidal aqueous solution of magnetite nanoparticles with varying concentrations of the sodium oleate stabilizer was used as the magnetic fluid. It has been found that the ratio of the mass concentrations of the stabilizer and magnetite in a stable magnetic fluid must satisfy the condition C ≥ 0.7, which corresponds to the formation of a double layer of stabilizer on the surface of magnetite nanoparticles.

     

L'Étude de la Structure Électronique des Métaux des Terres Rares

In rare earth metals, one can neglect interactions between 4f shells centred on neighbouring sites. The conduction band is occupied by three sd electrons (eventually two in europium and ytterbium). These sd electrons are coupled to the f electrons through an interaction of the form where s _e is the spin of a conduction electron and Sf _i the spin of the ith f electron of a given ion. It is therefore possible to consider two groups of properties:

1. The ones, related to the nature of the conduction electrons, change very little through the series: this is the case of the crystalline structure, of the atomic volume.

2. The others, such as the magnetic properties, are related to the internal shells and vary with the filling of the 4f shell. Experiment shows a correlation between those two groups of properties. De Gennes formalism, essentially valid in the hypothesis of tightly bound 4f electrons, gives a satisfactory picture of the properties of the metals in the second half of the series, but it does not give as good a picture for the first rare earth metals, especially for cerium. In the cerium free atom, the 4f, 5d, 6s states have comparable energies and one might think that, in the trivalent metal, the 4f states are broadened in energy by resonances with the extended sd states, but still do not overlap from one atom to the other. They would then occupy virtual bound states analogous to the virtual bound states described by Blandin and Friedel for the transition impurities in noble metals.

An identical situation seems to occur in ytterbium under pressure: one observes a huge increase of the electrical resistivity which goes back to low values at very high pressures. This might also be the case of the actinide metals, especially of Plutonium, in which the 5f states begin to stabilize. So we have to consider two cases:

1. The 4f electrons occupy bound states.

2. The 4f electrons occupy virtual bound states.

In the first part (§ 2), we use de Gennes formalism for 4f bound states. The energy related to magnetic interactions is computed making the assumption of a spherical Fermi surface. A correlation between the crystalline structure and the magnetic properties shows up. In the second half of the series, one can neglect the crystalline field effects and the total energy is the sum of the magnetic term and of the elastic term due to the contribution of the conduction electrons. For every state of magnetic order, the crystalline structure is well defined, corresponding to the minimum of the total energy, and conversely. It is possible to explain in this manner:

1. The b.c.c. structure of europium, which is unusual for a divalent transition metal.

2. The variation of the c/a ratio of the h.c.p. structure both through the series and with temperature.

3. The anomalies in the thermal expansion coefficient observed below the magnetic order-disorder transitions.

4. The helix pitch of the magnetic configurations of this type.

The anomalies of the thermoelectric power observed at the transition points are related to the different dependences of the spin correlations above and below the transition temperatures. The agreement between theory and experiment is satisfactory. Some discrepancy can be attributed to the rather crude approximation of a spherical Fermi surface.

In the second part (§ 3), we deal with a situation where the 4f electrons occupy virtual bound states. These levels are very narrow, about 10^?2 ev wide, and separated in energy by the correlations between electrons. Using Blandin's formalism we calculate the electrical and magnetic properties associated with such a situation. Calculations lead to very strong magnetic coupling; the indirect interaction between magnetic ions is antiferromagnetic for first nearest neighbours, whereas in the case of 4f bound states it is ferromagnetic. Finally, it is possible to explain the properties of cerium and ytterbium.

1. In Cerium, the two first levels overlap at the Fermi level, in such a way that the f electron be almost entirely distributed in the first level.

2. In ytterbium, under pressure, the fourteenth level comes across and above the Fermi level. The maximum resistivity is obtained for a half filling of this level.

In the third part (§ 4), we attempt to apply this model of virtual bound states to plutonium, although in this metal, the 5f shells have a larger spatial extension than the 4f orbitals in rare earths. Anomalies in several physical properties of plutonium seem to indicate a magnetic transition at about 65° K, but no anomaly shows up in the magnetic susceptibility. Using a virtual bound state model associated with a very small polarization of the 5f states, it is possible to explain all the physical properties of plutonium. This model leads to a very small magnetic moment, that cannot be detected by experiment.     

Magnetic behaviour of cobalt,iron and manganese dissolved in palladium

This paper is meant to be a report on the experimental work on dilute Pd-based alloys with Co, Fe and Mn. These alloys exhibit the phenomenon of giant moments. The importance of measurements on paramagnetic alloys is emphasized. From these measurements the conclusion can be drawn that Co and Fe dissolved in Pd does not behave like a normal paramagnet, i.e. according to a Brillouin function. This result makes it possible to explain the existing discrepancy in the interpretations of magnetic measurements on one hand and of specific-heat experiments on the other.

The main conclusions of this paper are:

The giant moment should be accounted for by ‘normal’ values of the magnetic quantum number (3/2 for Co, 2 for Fe and 5/2 for Mn) and a large value of geff.

Paramagnetic alloys of Mn in Pd behave according to Brillouin functions, but alloys of Co or Fe in Pd do not. Hence, a number of interpretations of magnetic measurements should be considered as incorrect.

The localized model for ferromagnetism can well account for the magnetic ordering of dilute Pd-based alloys (certainly if c < 1 at.%). A straightforward generalization of the Weiss molecular-field model may be applied.

The transition temperature of Pd-Mn alloys is not proportional to the concentration, but after scaling the behaviour is similar to what has been found for Pd-Co and Pd-Fe alloys. The concentration dependence can be explained from a calculation of the strength of the interaction between two impurity atoms as a function of the distance.

Comparison between alloys with equal concentrations shows that the magnetic ordering in Pd-Mn is not at all exceptional, but analogous to that in Pd-Co and in Pd-Fe. It should be mentioned, however, that Pd-Mn at c > 3 at.% is a so-called spin glass.

Addition of Ag or Rh to Pd alloys with Co, Fe and Mn has important influences on their properties. Unfortunately these effects are not completely understood.     

Influence of Magnetic Field on Qutrit Teleportation under Intrinsic Decoherence

We study qutrit teleportation through a qutrit xyz chain, in the presence of intrinsic decoherence and a non-homogeneous magnetic field. We study the effects of intrinsic phase change, magnetic field and entanglement of the initial state of the channel. It is observed that while the intrinsic phase change and the non-homogeneity of the magnetic field have adverse effects on the teleportation fidelity, the entanglement of the initial state of the channeled enhances the latter. Moreover, the intrinsic decoherence may remove the ripples from the time curve that is delivered by the Schrödinger channel.

     

High-Pressure Mössbauer Spectroscopy of Perovskite Iron Oxides

Using a diamond anvil cell and a low-temperature cryostat with a superconducting solenoid, high-pressure ⁵⁷Fe Mössbauer spectroscopy with a radioactive source at 4.5 K under external magnetic fields up to 7 T has been developed. Pressure-temperature magnetic phase-diagrams for perovskite iron oxides, SrFeO₃, CaFeO₃ and Sr₂LaFe₃O₉ are presented up to about 70 GPa. High-pressure Mössbauer measurements under external magnetic fields proved that the electronic ground states of these oxides switch to uniform-charge and ferromagnetic (and most probably metallic) states under extremely high pressure.

     

Helimagnetism of CuxZn1−xCr2Se4 (for 0.0 ≤ × ≤ 0.1)

Abstract

The magnetization, the susceptibility and the magnetic anisotropy field of Cu _x Zn_1?xCr₂Se₄ compounds have been studied at low temperatures (down to 2.9 K) in: high magnetic stationary fields (up to 14 T), high pulsed magnetic fields (up to 25 T), medium magnetic stationary fields (up to 0.6 T). The magnetic structure of these spinels was studied by neutron powder diffraction.

The magnetic properties of Cu_xZn_1?xCr₂Se₄ are explained in terms of the molecular field approximation assuming the existence of 90° exchange interactions, ferromagnetic for Cr-Se-Cr between the nearest Cr ions and antiferromagnetic for Cr-Se-Se-Cr between the second-nearest Cr ions. The exchange parameters and integrals for the whole series under consideration are calculated. Taking into account the three magnetic phase transitions observed in these spinels (Juszczyk, Krok, Okońska-Koz?owska, Broda, Warczewski, Byszewski, 1981) and the neutron diffraction studies a modification of the simple spin spiral forced by a strong magnetic field is described.     

On Quantum Systems of Particles with Singular Magnetic Interaction

Abstract

For systems of particles with singular magnetic interation for special choice of a selfadjoint extension of the Hamiltoniam equilibrium reduced density matrices are calculated in the thermodynamic limit for simplest pair magnetic potentials.     

Reentrant phases in compensated ferrocholesterics

Influence of magnetic field on orientation and magnetic properties of a compensated ferrocholesteric, a suspension of needle-like ferromagnetic particles in a cholesteric liquid crystal, was studied theoretically. A phase transition from a ferrocholesteric to a ferronematic state in a magnetic field oriented normally to the axis of the helical structure was considered. The dependences of the transition field to the ferronematic phase on the material parameters of the suspension and of the helical structure pitch and magnetization on the field strength were investigated. A possibility of existence of a reentrant ferrocholesteric phase was shown.

     

Quasiclassical Solutions of the Schrödinger Equations as a Consequense of the Nonlinear Problem

Abstract

Quasiclassic method of solving of the Schrödinger equation with quadratic Hamiltonian is used to derive solutions of Klein-Fock equation for the particle in the constant magnetic field and the jumping magnetic field.     

Influence of ageing on the phase composition,structure, and magnetic properties of nanopowders of oxide ferrimagnets

This paper reports on the observation of the aging of cubic oxide ferrimagnet nanopowders prepared by mechanochemical synthesis, which manifests itself in a variation of the phase composition, structural parameters, and main magnetic properties.

     

Heat capacity and the magnetocaloric effect in Pr0.6Sr0.4Mn1–x Fe x O3 manganite

The heat capacity and the magnetocaloric effect of Pr_0.6Sr_0.4Mn_1–xFe_xO₃(x = 0 and 0.1) manganite have been studied in the temperature range 80–350 K and magnetic fields to 18 kOe. The magnetocaloric effect is estimated using two independent methods: the method of magnetic field modulation (direct method) and from the data on the heat capacity in magnetic field and without magnetic field (indirect method). The substitution of Fe atoms for Mn atoms (x = 0.1) shifts T _C by 167 K to lower temperatures; in this case, the magnetocaloric effect (MCE) is changed insignificantly in magnetic field 18 kOe with ΔS _M = 2.05 and 2.31 J/kg K for x = 0 and 0.10, respectively.