Magnification of the frequency dispersion of the permittivity in ferroelectrics with defects

Experimental data about the behavior of the dynamic susceptibility and the damping factor of a number of perovskite ferroelectrics as a function of the defect concentration are discussed. The effect of an extraordinarily strong frequency dependence of the location of the permittivity temperature peak is explained on the basis of the Landau phenomenological theory of phase transitions.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 7–12, March, 1978.     

Reconstruction of dielectric permittivity profile of a plate with strong frequency dispersion

We demonstrate the possibility and propose the algorithm of unique reconstruction of the dielectric permittivity profile of a plate with a one-dimensional inhomogeneity (possibly, a plate with absorption), provided that the reflection and transmission coefficients of the plate for s-polarized plane waves are known in a certain range of incidence angles.     

Kubo formula for frequency dispersion of dielectric permittivity and static conductivity of the Coulomb system

It is proved that the Kubo formula for the conductivity σ(ω) is valid at real frequencies ω. On this basis, an exact relation is derived for the static conductivity σst of the Coulomb system. It is shown that the static conductivity is determined by the time correlation function in the limit t→∞. It is proved that the permittivity ε(ω) satisfies the Kramers-Kronig relations which take into account a singularity associated with static conductivity.     

The permittivity of a monatomic gas with spatial dispersion

The longitudinal, ε^l(ω, k), and transverse, ε^tr(ω, k), permittivities of a monatomic gas were calculated. The frequency ranges in which the permittivity ε(ω) and permeability μ(ω) of a gas without spatial dispersion have a physical meaning were determined. The limiting magnetic susceptibility χ(ω) at ω=0 and the static magnetic susceptibility were found. The question of whether an electromagnetic wave with antiparallel group and phase velocities can propagate through a monatomic gas is discussed.     

Dielectric dispersion in semiconducting ferromagnetic glass ceramic

Two regions of frequency dispersion of capacitance and conductivity have been discovered in the semiconducting glass ceramics BaTiO₃ and KNbO₃ which have a positive temperature resistance coefficient (PTRC). The low-frequency region (10²–10³ Hz) of dispersion is associated with the existence of barrier layers at the electrodes and the high-frequency region (beginning in the region 10⁵–10⁶ Hz), with the existence of barrier layers at grain boundaries. The frequencies f_m at which maxima of tan corresponding to both dispersion regions diminish in the temperature range of PTRC whereas in the higher region of PTRC they again increase with a rise in temperature. The anomalous temperature dependence of f_m is explained on the basis of the assumption that charge carriers in the barrier layers act as relaxation oscillators and the variations in the barrier heights are described by the Heywang model.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 76–81, September, 1978.     

High frequency magnetic properties of ferromagnetic thin films and magnetization dynamics of coherent precession

We focus on the ferromagnetic thin films and review progress in understanding the magnetization dynamic of coherent precession, its application in seeking better high frequency magnetic properties for magnetic materials at GHz frequency,as well as new approaches to these materials' characterization. High frequency magnetic properties of magnetic materials determined by the magnetization dynamics of coherent precession are described by the Landau–Lifshitz–Gilbert equation.However, the complexity of the equation results in a lack of analytically universal information between the high frequency magnetic properties and the magnetization dynamics of coherent precession. Consequently, searching for magnetic materials with higher permeability at higher working frequency is still done case by case.     

On the problem of the permittivity dispersion of barium titanate

The complex permittivity of multidomain single crystals of BaTiO₃ was measured in the decimetre and centimetre wave bands and it was proved that in this frequency region there is dispersion of the permittivity. Apparatus for measuring the permittivity of substances with a high is described and the influence of the inhomogeneity of the field in the sample on the characteristics of the cavity resonator is calculated. Present theories of the high-frequency properties of BaTiO₃ are discussed from the point of view of the experimental results.

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BaTiO₃ , . . BaTiO₃.

     

Classical theory of spin-wave dispersion for ferromagnetic metals

The properties of the four spin-wave dispersion branches for a planar ferromagnetic metal with TEM microwave excitation at normal incidence have been examined to derive physical insight into (a) their dependence on material parameters and orientation, (b) the interaction between branches, and (c) the polarization character associated with the related ecxitations. The Larmor electromagnetic and spin-wave branches are found to exhibit a peculiar crossover effect at a critical damping value ( 0·014 for permalloy). The polarizations vary from circular for the static field normal to the surface to elliptical for an in-plane static field. For the in-plane case, the major polarization axes are parallel and perpendicular to the surface for the Larmor and antilarmor branches, respectively. The well known Ament-Rado and Vittoria formulations have also been modified to include the effect of exchange in the damping term of the Landau-Liftshitz equation. The secular determinant is simplified considerably and the form of the dispersion equations is changed significantly. The only large effect on the branches themselves is to increase the attenuating component of the propagation parameterk for large ¦k¦.     

Frequency dispersive complex permittivity and permeability of ferromagnetic metallic granular composite at microwave frequencies

We experimentally studied the frequency dependent complex permittivity ε and permeability μ of composite composed of carbonyl iron powder (CIP) and epoxy resin in the frequency range 1-18 GHz. We found that the intrinsic ε and μ of CIP extracted from the measured ε and μ of composites follow the classical Maxwell equations and the Landau-Lifshitz-Gilbert (LLG) equation, respectively. The dependences of ε and μ of composites on the volume fraction of CIP (vf_CIP) were investigated using the two-exponent phenomenological percolation equation (TEPPE). We found that the TEPPE can fit the experimental results very well. Comparing the results of percolation parameters derived by experimental data at different frequencies, we show that the TEPPE is frequency independent for the composites at microwave frequencies. The results also show that the ε and μ spectrums of composites with definite vf_CIP can be correctly calculated by combining the TEPPE with the theoretical models of intrinsic ε and μ.     

Surface polaritons in composite media with time dispersion of permittivity and permeability

The propagation of electromagnetic waves in a composite medium based on an array of conducting wires in a ferromagnetic nonconducting matrix is discussed. It is demonstrated that, in certain ranges of frequencies and wavelengths, the composite under investigation can possess properties inherent in a “left-handed” medium. The regions of the existence of bulk and surface localized electromagnetic waves are explored. Consideration is given to the dispersion of surface electromagnetic waves in thick layers of the composite.     

The frequency dependence of demagnetizing factor of a Fe-based amorphous ribbon

The frequency dependence of total and internal demagnetizing factors has been investigated. A minimum of the demagnetizing factor at optimal frequency of magnetization reversal is revealed. The influence of the demagnetizing factor on the frequency dependence of magnetic properties of amorphous ribbon is studied.     

Thin Co films with tunable ferromagnetic resonance frequency

The tailored production of thin Co films of 50 nm thick with ferromagnetic resonance frequency in a range from 2.9 to 7.3 GHz using the DC magnetron sputtering is reported. The ferromagnetic resonance frequency, coercivity, effective magnetic field and nanocrystalline structure parameters are shown to be governed by the Co deposition rate. For this investigation, FMR, VSM and TEM techniques were used.     

Ferrite films with enhanced stability of ferromagnetic resonance frequency

The problem of providing the frequency stability of magnetic vibrations of a ferrite film upon simultaneous changes in two magnetic parameters has been stated. The method of stabilizing the frequency of uniform ferromagnetic resonance at small deviations of the effective magnetization and the field of crystallographic magnetic anisotropy of the ferrite from initial values has been proposed. The equation that relates the frequency variation with variations in the magnetic parameters within a linear approximation has been derived. The possibility of the coefficients at the variations in the magnetic parameters to turn to zero has been investigated. It has been shown that this possibility can be implemented via the proper choice of the crystallographic film orientation and the strength and direction of the magnetizing field. In this case, the frequency shift has been determined by quadratic form over the variations in the magnetic parameters. An example of calculating the quadratic form has been given. The optimized film is compared with a film, the frequency shift of which is determined by a linear dependence on the variations in its magnetic parameters.     

Normal group-velocity dispersion Kerr frequency comb

We show via numerical simulation that Kerr frequency combs can be generated in a nonlinear resonator characterized with normal group-velocity dispersion. We find the spectral shape of the comb and temporal envelope of the corresponding optical pulses formed in the resonator.     

Spin-wave dispersion softening in the ferromagnetic Kondo lattice model for manganites

Spin dynamics is calculated in the ferromagnetic (FM) state of the generalized Kondo lattice model taking into account strong on-site correlations between e_g electrons and antiferromagnetic (AFM) exchange among t_2g spins. Our study suggests that competing FM double-exchange and AFM super-exchange interaction lead to a rather nontrivial spin-wave spectrum. While spin excitations have a conventional Dq² spectrum in the long-wavelength limit, there is a strong deviation from the spin-wave spectrum of the isotropic Heisenberg model close to the zone boundary. The relevance of our results to the experimental data are discussed.     

Progress of novel diluted ferromagnetic semiconductors with decoupled spin and charge doping: Counterparts of Fe-based superconductors

Diluted ferromagnetic semiconductors(DMSs) that combine the properties of semiconductors with ferromagnetism have potential application in spin-sensitive electronic(spintronic) devices. The search for DMS materials exploded after the observation of ferromagnetic ordering in Ⅲ-Ⅴ(Ga,Mn)As films. Recently, a series of DMS compounds isostructural to iron-based superconductors have been reported. Among them, the highest Curie temperature T_Co f 230 K has been achieved in(Ba,K)(Zn,Mn)_2As_2. However, most DMSs, including(Ga,Mn)As, are p-type, i.e., the carriers that mediate the ferromagnetism are holes. For practical applications, DMSs with n-type carriers are also advantageous. Very recently,a new DMS Ba(Zn,Co)_2As_2 with n-type carriers has been synthesized. Here we summarize the recent progress on this research stream. We will show that the homogeneous ferromagnetism in these bulk form DMSs has been confirmed by microscopic techniques, i.e., nuclear magnetic resonance(NMR) and muon spin rotation(μSR).     

Low frequency dielectric permittivity of quasi-one-dimensional conductor (TMTTF)2Br

Conductivity and permittivity of the organic transfer salt (TMTTF)₂Br have been measured at low frequencies (10²-10⁷ Hz) between room temperature down to 4 K. The real part of the permittivity, , is shown to grow below the temperature at which the conductivity is maximum due to charge localization of Mott-Hubbard type. reaches a maximum of 10⁵-10⁶ at 35 K-50 K depending on the samples. Decreasing temperature below , sharply decreases down to helium temperature through the antiferromagnetic phase transition at T _N = 15 K. We explain the magnitude, the temperature and frequency dependence of as resulting from short range charge density wave states in the temperature range where charge localization occurs. This interpretation is supported by recent X-ray scattering measurements. Received: 10 October 1997 / Revised: 28 February 1998 / Accepted: 3 March 1998     

Forbidden frequency gaps in magnonic spectra of ferromagnetic layered composites

Spin wave spectra in layered composite materials are computed by means of a transfer matrix method with evidence for existence of numerous frequency gaps. Essential properties of these spectra are successfully explained within the framework of approximate models of independent films or an effective medium, according to the strength (strong and weak, respectively) of the magnetization and exchange contrasts; this includes an evidence of the existence of resonance states, for which both materials are simultaneously excited.     

Spatial dispersion in metamaterials with negative dielectric permittivity and its effect on surface waves

The effect of spatial dispersion on the electromagnetic properties of a metamaterial consisting of a three-dimensional mesh of crossing metallic wires is reported. The effective dielectric permittivity tensor epsilon(ij)(omega, k) of the wire mesh is calculated in the limit of small wavenumbers. The procedure for extracting the spatial dispersion from the omega versus k dependence for electromagnetic waves propagating in the bulk of the metamaterial is developed. These propagating modes are identified as similar to the longitudinal (plasmon) and transverse (photon) waves in a plasma. Spatial dispersion is found to have the most dramatic effect on the surface waves that exist at the wire mesh-vacuum interface.