Formation of Metallophthalocyanine Trimers in Liquid Solutions

For solutions of the complexes of phthalocyanine (PhC) with magnesium (Mg-PhC) and of tetrachloromethyl-PhC with zinc (Zn-PhCCh) in mixtures of glycerin with dimethyl formamide (DMF) and for Zn-PhCCh in toluene and DMF it has been found that the electron absorption spectra substantially depend on the substance concentration. It is shown that these changes are attributable to the association of the molecules of pigments. The number of molecules in the associates and the equilibrium constants for this association are determined. The formation of the trimers of Mg-PhC in binary solutions and the dimers of Zn-PhCCh in toluene, DNF, and binary solutions has been established. It has been found that on association the Q absorption bands of the pigments shift to shorter wavelengths as compared to the bands of the monomers. For the Mg-PhC trimers, the shift is = 1410 cm^–1, while for the Zn-PhCCh dimers it is 990 cm^–1 in toluene, 930 cm^–1 in DMF, and 925 cm^–1 in the mixture of DMF with glycerin. It is concluded that the structure of the trimers and dimers is of the type of a stack in which molecules are arranged in parallel one above the other at a distance of 3.5 to 4.0 Å between the planes of the molecules. In association, the fluorescence intensity of the pigments decreases substantially (the decrease corresponds to the decrease in the concentration of the monomer molecules of phthalocyanines), with the fluorescence spectra remaining unchanged. Consequently, the associates of the pigments do not display pronounced fluorescence.     

Domain structure in centroantisymmetric antiferromagnets

The domain structure of an antiferromagnet whose magnetic-symmetry group contains a center of antisymmetry is studied theoretically. The magnetoelectric effect and weak ferromagnetism are shown to coexist in a domain wall. It is established that when the inhomogeneous magnetic moment interacts with a sufficiently strong magnetic field H ‖ C ₃, a multidomain state with an odd number of 180° domain walls becomes energetically favorable. The critical field for the transition from a single-domain state to a multidomain state is found. It is shown that domain reversal occurs when the magnetic field H is reversed.     

Anomalies in the Magnetic Susceptibility in the Second-Order Phase Transitions beyond the Curie Point

Taking into account the inexhaustible interest in studying the peculiarities of physical properties in the neighborhood of phase transitions and the growth of experimental investigations of cobalt fluoride, we have studied the peculiarities of magnetic susceptibility in the vicinity of the critical field H_C at which cobalt fluoride performs the second-order phase transition from the antiferromagnetic phase to the angular phase. It is discovered that in the magnetic field H ‖ C₄, the magnetic susceptibility becomes infinite at H → H_C. It is shown that as the magnetic field direction deviates from the C₄ axis, the magnetic susceptibility in the critical field H_C proves to be finite. It is also shown that the change in the magnetic susceptibility with the change in the magnetic field considerably decreases at extremely insignificant deviations of the field H from the C₄ axis. Since the calculations are performed in terms of the Landau theory of phase transitions, we pay attention to the similarity and difference between the obtained results and those in the vicinity of the Curie point obtained by using the Landau theory of phase transitions.     

Magnetic circular dichroism and optical absorption in TmAl<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript>

The polarized spectra of absorption and magnetic circular dichroism in a TmAl₃(BO₃)₄ single crystal are studied in the region of ³ H ₆ → ³ F ₄, ³ H ₆ → ³ F ₃, and ³ H ₆ → ³ F ₂ electronic transitions in the Tm³⁺ ion. The structure of the spectra is interpreted qualitatively. It is shown that the magnetic circular dichroism of the ³ H ₆ → ³ F ₄ transition is determined by the contribution from the splitting of the ground state, whereas the magnetic circular dichroism of the ³ H ₆ → ³ F ₃ transition is governed by the contribution from the splitting of an excited state in a trigonal crystal field.     

A Mathematical Representation of Quantizing the Motion of Relativistic Electrons in a Magnetic Field

It is shown that the quadratic component of the kinetic energy of continuous longitudinal motion of relativistic electrons in the external magnetic field is varied continuously between 0 and 2(2m _e c ²_B H) within each Landau energy level, undergoing an abrupt change at the boundaries of the levels. This results in the fact that in the quantum limit of a superstrong magnetic field where all electrons are at the zero Landau level, the maximum quadratic component of the kinetic energy of free longitudinal electron motion along the direction of the magnetic field is twice as high as the maximum quadratic component of the kinetic energy of its bound transverse motion.     

Magnetic catalysis of stability of quark matter in the Nambu-Jona-Lasinio model

The effect of an external magnetic field H on the stability of quark matter is studied on the basis of the Nambu-Jona-Lasinio model. It is shown that, at H = 0, droplets of quark matter are stable only in the case where the coupling constant G is greater than some value G_bag. If H ≠ 0, stable multiquark formations may exist even for G ≤ G_bag (magnetic catalysis of stability). For G > G_bag, a magnetic field facilitates the formation of stable quark matter.     

FMR and EPR in Ni@C nanocomposites: Size and concentration effects

One-domain Ni@C nanoparticles encapsulated in carbon coating have been investigated depending on the size and concentration of Ni in carbon. The nanoparticles of nickel were prepared with the average diameters changing in a broad range of 4–45 nm, and the concentration of Ni in C varies in 2–12 wt%. To prepare the Ni@C nanocomposites the solid solutions of nickel phthalocyanine–metal-free phthalocyanine (NiPc) _x (H₂Pc)_1–x , 0 ≤ x ≤ 1 were synthesized and the solidphase pyrolysis of these compounds was performed. In the case of ultradispersive Ni nanoparticles (the interval of quantum dots is 1–10 nm), a considerable shift of the resonance field and broadening of resonance absorption field were revealed in the spectra of FMR at room temperature. The data were interpreted taking into account the essential contribution of the surface magnetic anisotropy, the magnetic field of which far exceeds the magnetic field of volume anisotropy.     

Relaxation of Paramagnetic Centers during Modulation of a Stationary Magnetic Field

It has been found that the phase shift _min in the synchronous detection block that ensures the minimum amplitude of the EPR lines of ruby, diphenylpicrylhydrazyl, and MnSO₄·5H₂O, depends on the amplitude of the modulationH _m of a stationary magnetic field. The dependence of _min on H _m is explained by the inertial nature of the recovery of the stationary states of paramagnetic centers on a change in the magnetic field strength.     

自旋为1/2的XY模型亚铁磁棱型链的物性和有序-无序竞争

利用不变本征算符法, 计算低温下自旋为1/2的XY模型一维亚铁磁棱型链系统的元激发谱, 讨论在此系统中不同的特殊情形下的元激发能量, 从而给出体系的三个临界磁场强度的解析解H_C1, H_C2, H_peak. 分析不同外磁场下 体系的磁化强度随温度的变化规律, 发现三个临界磁场强度的解析解H_C1, H_C2, H_peak是正确的, 并从三个元激发对磁化强度的贡献进行了说明. 低温下磁化强度随外磁场的变化呈现1/3磁化平台. 体系的磁化率随温度或者外磁场的变化都出现了双峰现象. 这说明双峰源于二聚体分子内电子自旋平行排列的铁磁交换作 用能和二聚体与单基体分子间电子自旋反平行排列的反铁磁交换作用能, 热无序能, 外磁场强度相关的自旋磁矩势能之间的竞争.     

Nonlinear waves in bismuth

The propagation of short radio waves in a bismuth crystal in a constant magnetic field H aligned parallel to the bisecting axis oriented normally to the surface of the crystal plate is investigated theoretically. In this geometry, spatial inhomogeneity of the wave field has a weak effect on electrons and a strong effect on holes. It is demonstrated that, in a certain range of magnetic field strengths H, the bismuth crystal is characterized by two modes, namely, a helicon and a doppleron, whose damping is governed by cyclotron absorption of holes. For small amplitudes of the wave field in a linear regime, the damping lengths of both modes are relatively short due to cyclotron absorption. In a nonlinear regime, the magnetic field of the radio wave captures holes responsible for cyclotron absorption. As a result, the absorption is suppressed and the damping lengths of the helicon and the doppleron increase drastically. The excitation of these modes in the bismuth plate results in the fact that the dependence of the impedance of the plate on the magnetic field strength H exhibits resonance behavior and the transmittance of the plate increases by more than two orders of magnitude. It is shown that this effect should manifest itself at frequencies of the order of a few megahertzes in relatively weak magnetic fields (of the order of a few tens of oersteds).     

Giant quantum oscillations of the skin layer in semimetals

We study theoretically the effect of electron energy quantization in a magnetic field on the penetration of radio waves into a semimetal in a geometry in which a constant magnetic field H is directed along the trigonal axis of a crystal. In this geometry, strong magnetic Landau damping in semimetals prevents wave propagation and is responsible for the skin effect. It is shown that quantization of the transverse energy of electrons considerably influences the effectiveness of collisionless absorption of the wave. As a result, the magnetic Landau damping and the skin depth experience giant oscillations upon a change in magnetic field H.     

Spectral Manifestations of Anion-Cation Interactions of Water-Soluble Porphyrins

The influence of anion-cation interactions on the structure and electronic absorption spectra of cationic 5,10,15,20-tetrakis-(4-N-methylpyridyl)porphyrin and anionic 5,10,15,20-tetrakis-(4-sulfonatophenyl)porphyrin has been investigated by absorption spectroscopy and molecular modeling. It is shown that the shifts of bands in the electronic absorption spectra appearing when passing from aqueous solutions to solutions in organic solvents are due to the interaction of ionized peripheral substituents of the porphyrin macrocycle with counterions. The bathochromic shift of the Q _x transition is due to the influence of counterions on the energy characteristics of the molecule itself, primarily, as a result of Coulomb interactions changing the relative position of the orbitals and the distribution of electron density on them. The hypsochromic shift of the Q _y transition is a consequence of the increase in the dihedral angle between the plane of the porphyrin macrocycle and the aromatic rings with ionized groups due to the decrease in the -electron interaction between them under the action of counterions.     

Semiempirical calculation of the absolute solvation shift of electronic and vibrational spectra of molecules in the gas-solution phase transition

Calculations of the main components of the absolute solvation shift of Δυ _∑ ^{a, f, R} optical absorption, emission, and Raman spectra of molecules in the gas-solution phase transition for solutions of several diatomic (HCl, DCl, H₂, D₂) and polyatomic (anthracene, 3-and 4-aminophthalimide) substances in some polar and nonpolar solvents are presented. The calculations are made by a new method of the semiempirical theory, which was proposed by the author earlier and makes such calculations possible for the first time. It is shown that the given theory adequately describes the photophysical effect under study and provides a correct prediction of the values of Δυ _∑ ^{a, f, R} , which differ for different systems by more than three orders of magnitude (from several inverse centimeters for vibrational spectra of H₂ and D₂ to many thousands of inverse centimeters for electronic fluorescence spectra of solutions of aminosubstituted derivatives of phthalimide).     

Polarisation effects in Mössbauer absorption by powder samples in external magnetic fields

The polarisation dependence of the absorption cross section must generally be taken into account in calculating Mössbauer absorption spectra of powder samples if they are partly polarised by an external magnetic field. The dependence of the intensity of the absorption lines on the aboorber thickness with an external field ofH _a=50 kG is shown in the case of the paramagnetic specimen Fe(ClO₄)₂·6H₂O. The calculation procedure is described and the results are compared with the usual thickness corrections neglecting polarisation effects.     

Mössbauer absorption by soft ferromagnets in radio-frequency magnetic field

The simultaneous influence of periodical magnetic field reversals at the nucleus between the values ±h ₀ and of magnetostrictive vibrations on the shape of the Mössbauer absorption spectrum is analyzed. The effect of a constant external magnetic field is taken into account by assuming unequal durations of the states ?h ₀ and +h ₀. It is shown that such asymmetric reversals of the magnetic field lead to splitting of the absorption lines into Zeeman patterns corresponding to the time-averaged magnetic field h ₀ R, where R is the asymmetry parameter of the reversals. The calculations agree well with experiment.     

Theory of Raman scattering and infrared absorption by interacting phonon-magnon states in FeF2

It is shown theoretically that, in the presence of a strong magnetic field, effects due to the interaction betweenB _1g ,E _g,TA phonons and magnons should be important in the Raman and infrared absorption spectrum of FeF₂. As a result the two magnon line should split. A microscopic Hamiltonian for the interactions has been derived. The calculated values of theE _g phonon-one magnon interaction, 9.9 cm^–1, and of theB _1g phonon-two magnon coupling, 6.3 cm^–1, are comparable to those in FeCl₂2H₂O metamagnet, in which C phonon-magnon hybrydization has been observed [1]. It is shown that hybridization between two magnon andB _1g orTA phonon can occur in FeF₂ if the magnon energy approaches the phonon energy due to increasing magnetic field or increasing temperature.On the leave from Technical University, Pozna, Institute of Physics, Pozna 61–138 Piotrowo 3, Poland     

Properties of phase transitions in easy-axis tetragonal antiferromagnets

Properties of phase transitions in a magnetic field H parallel to the easy axis have been investigated, and it has been shown, in particular, that the nature of the transition of the magnetic subsystem from the antiferromagnetic phase to the angular phase depends on the “sign” of the Dzyaloshinskii interaction. The conditions for orientation of the antiferromagnetism vector l in the basis plane in fields larger than the threshold field have been determined. It is shown that the transition from the angular phase to the state where the resulting magnetic moment m is parallel to the easy axis takes place in the field corresponding to a spin-flip transition. From an analysis of the configuration of the magnetic subsystem for arbitrary orientation of the external magnetic field, it follows that the values of the critical angle (ψ _cr ) for which a first-order phase transition takes place satisfy the condition in the case in which the anisotropy constant f in the basis plane is of the order of the first anisotropy constant b. Usually so that the tricritical point in the phase diagram H _y ,H _z satisfies the condition H _y ∼H _z . Fiz. Tverd. Tela (St. Petersburg) 41, 2044–2046 (November 1999)     

Anomalous absorption of ultrasound in gadolinium in a magnetic field

The absorption coefficient α _k for longitudinal ultrasonic (15 MHz) waves propagating transverse to the direction of a magnetic field H is measured in single crystal gadolinium. It is found that in fields H⩽600 Oe, the peak in α _k is shifted toward lower temperatures, while the absolute magnitude of the absorption rises with increasing H. It is shown on the basis of dynamic scaling that the anomalous behavior of α _k in fields H⩽600 Oe can be explained by introducing a magnetic field analog of the Landau-Khalatnikov relaxation mechanism. Fiz. Tverd. Tela (St. Petersburg) 39, 339–340 (February 1997)     

Thermodynamic functions of a two-sublattice ferrite in a magnetic field

This paper considers the effect of the magnetic field on the specific heat, entropy, and temperature behavior of the magnetization in a two-sublattice ferrite in the collinear phase. It is shown that the specific heat and entropy vary nonmonotonically with the magnetic field, while at low temperatures the magnetization increases with the temperature when the magnetic field takes values in the range H₁/2 < H h₁, where H₁ is the value of the magnetic field for which deviation from the collinear structure begins [1, 2].Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 33–36, January, 1977.In conclusion, the author wishes to express his indebtedness to G. M. Nedlin and to thank him for his valuable appraisal of the work.     

Basic Mechanisms of Thermal Degradation of the Compositions of Synthetic Restoration Polymers With Metal Oxides

Using the method of diffuse reflection spectroscopy, we studied thermal degradation of the compositions of white pigments with a number of synthetic polymers intended for restoration of paint layers of paintings. It has been established that, starting with 340 K, the heating of polymer compositions with TiO₂, regardless of the polymer type, induces absorption in the pigment at 2.88 and 2.50 eV, which can be referred to color centers (CC) fixed to oxygen vacancies. In the spectra of the compositions of fluorine-containing polymers with MgO and Al₂O₃, absorption bands in the blue region, which are induced when T > 315–320 K, can be ascribed to the hole color centers in oxides. The measurement of temperature dependences for the rate of increase in the absorption bands in programmed heating permitted determination of the activation energies of thermal degradation E _td. The dependence of E _td on the polymer type is attributed to the fact that reactions on the surface of pigments are limited by the activation of bonds in adsorbed macromolecules.