Dynamic compensation temperature in the kinetic spin-1 Ising model in an oscillating external magnetic field on alternate layers of a hexagonal lattice

The dynamic behavior of a two-sublattice spin-1 Ising model with a crystal-field interaction (D) in the presence of a time-varying magnetic field on a hexagonal lattice is studied by using the Glauber-type stochastic dynamics. The lattice is formed by alternate layers of spins σ=1 and S=1. For this spin arrangement, any spin at one lattice site has two nearest-neighbor spins on the same sublattice, and four on the other sublattice. The intersublattice interaction is antiferromagnetic. We employ the Glauber transition rates to construct the mean-field dynamical equations. Firstly, we study time variations of the average magnetizations in order to find the phases in the system, and the temperature dependence of the average magnetizations in a period, which is also called the dynamic magnetizations, to obtain the dynamic phase transition (DPT) points as well as to characterize the nature (continuous and discontinuous) of transitions. Then, the behavior of the total dynamic magnetization as a function of the temperature is investigated to find the types of the compensation behavior. Dynamic phase diagrams are calculated for both DPT points and dynamic compensation effect. Phase diagrams contain the paramagnetic (p) and antiferromagnetic (af) phases, the p+af and nm+p mixed phases, nm is the non-magnetic phase, and the compensation temperature or the L-type behavior that strongly depend on the interaction parameters. For D<2.835 and H₀>3.8275, H₀ is the magnetic field amplitude, the compensation effect does not appear in the system.     

Performance of phase compensated coherent free space optical communications through non-Kolmogorov turbulence

The bit-error-rate (BER) performance of coherent free-space optical (FSO) links employing phase compensation techniques is investigated in weak non-Kolmogorov turbulence. Assuming that the amplitude fading and phase fluctuation follow lognormal model and Gaussian distribution respectively and using the expression of non-Kolmogorov turbulence in terms of Zernike polynomials, the signal-to-noise ratio (SNR) at the coherent receiver is analyzed and as a special case, a new closed-form expression using chi-square distribution is obtained. Thus, the influence of different compensation modes and normalized receiver diameter on BER performance is evaluated and an optimum normalized receiver diameter is suggested to achieve the minimum BER. Moreover, the impact of outer scale L₀ and the exponent value α in non-Kolmogorov spectrum is studied with the optimum diameter, which reveals that the BER has an obvious decrease with larger values of L₀ and α.     

Domain structure of a thin single-crystal plate of terbium iron garnet near the magnetic compensation point

The domain structure of a thin single-crystal plate of the iron garnet Tb₃Fe₅O₁₂ has been investigated using the magneto-optical method in the temperature range near the magnetic compensation point of this ferrimagnet T _c = 248.6 K. It has been shown that, when the temperature of the sample approaches the magnetic compensation point, the domain width significantly increases, but remains finite at T = T _c . The magnetic H-T phase diagram, which determines the boundary between the multidomain and domain-free (uniformly magnetized) states of the sample, has been constructed using the data on visual observations of the transformation of the domain structure with variations in the temperature and external magnetic field. The results obtained have been interpreted in terms of the thermodynamic theory of stability of different magnetic phases of a two-sublattice cubic ferrimagnet near T _c .     

Compensation temperature of 3d mixed ferro-ferrimagnetic ternary alloy

In this study, we have considered the three dimensional mixed ferro-ferrimagnetic ternary alloy model of the type AB_pC_1-p where the A and X (X=B or C) ions are alternately connected and have different Ising spins S^A=3/2, S^B=1 and S^C=5/2, respectively. We have investigated the dependence of the critical and compensation temperatures of the model on concentration and interaction parameters by using MC simulation method. We have shown that the behavior of the critical temperature and the existence of compensation points strongly depend on interaction and concentration parameters. In particular, we have found that the critical temperature of the model is independent on concentration of different types of spins at a special interaction value and the model has one or two compensation temperature points in a certain range of values of the concentration of the different spins.     

Paramagnetic–ferrimagnetic transition in strong coupling paramagnetic systems: effect of cubic anisotropy interaction

The purpose of the present work is a quantitative investigation of the biquadratic exchange interaction effects on the paramagnetic–ferrimagnetic transition arising from two strongly coupled paramagnetic (1-spin) sublattices, of respective moments m and M. The free energy describing the physics of the system is of Landau type. In addition to the quadratic and quartic terms, in both m and M, this free energy involves two mixing interaction terms. The first is a lowest order coupling −CmM, where C<0 stands for the coupling constant measuring the interaction between the two sublattices. While the second, which is relevant for 1-spin systems and which traduces the dipole–dipole (or biquadratic) interaction, is of type wm²M², where w>0 is the new coupling constant. These two interactions enter in competition, and then, they induce drastic changes of the magnetic behavior of the material. The main change is that, the presence of this high order coupling tends to destroy the ferrimagnetic order of the system. We first show that the introduction of this biquadratic interaction does not affect the values of critical exponents. Also, we find that the compensation temperature (when it exists) and the compensation magnetic field are shifted to their lowest values, in comparison with the w=0 case. The Arrott-phase-diagram shape is also investigated quantitatively. We show the existence of three regimes depending on the values of w. When the latter is small, we find that the region of competition between the coupling C and the applied magnetic field H becomes more narrow under the effect of w (by competition, we mean the passage from the antiparallel state to the parallel one). While for higher values of w, this competition disappears completely, and then, the system loses its ferrimagnetic character. Kinetics of the phase transition is also examined, when the temperature is lowered from an initial value T_i to a final one T_f very close to the critical temperature T_c. As in the w=0 case, we find that kinetics is controlled by two kinds of relaxation times τ₁ and τ₂. The former is the relevant time, and is associated to long-wavelength fluctuations driving the system to undergo a phase transition. The second is a short time, which controls local dynamics. Near T_c, we show that, in particular, the longest relaxation time τ₁ becomes less important in comparison with that relative to the w=0 case. Finally, we note that the existence of two relaxation times is consistent with the predictions of a recent experiment, which was concerned with the 1/2-spin compounds Li_xNi_2−xO₂, where the composition x is close to 1.     

Diffuse phase transition of Fe doped lead ytterbium tantalate ceramics

The effect of different concentration of Fe on the phase transition behavior of Lead ytterbium tantalate is investigated by dielectric and differential scanning calrimetry measurements. The samples are prepared through solid state reaction method and it has been found that the sintering temperature significantly lowered when the proportion of Pb(Fe_1/2Ta_1/2)O₃ increased. It has been observed that the doping in small amounts (0≤x≤0.2) of Fe could meliorate the dielectric and ferroelectric properties. The diffuseness in the mode of phase transition increases and the phase transition temperature decreases as a function of Fe content. It is revealed that the dielectric data and heat capacity data follow a similar trend in the variation of the mode of phase transition and phase transition temperatures. The phase transition temperature values obtained from the heat capacity measurement well agreed with the values obtained from dielectric measurement.     

New high negative dispersion photonic crystal fiber

A new high negative dispersion photonic crystal fiber is proposed. It has double-core structure. The inner core has a circle germanium-doped region. The outer core is formed by removing the 3rd ring air-holes around the core. There are two ring air-holes between the two cores, Diameter of the 1st ring air holes is bigger than that of the 2nd ring air-holes, this can make mode coupling between inner mode and outer mode and showed that the high negative PCF is the result of this structure characteristics. There are honeycomb photonic lattice in the PCF's cladding. The influence of the structure parameters deviated from the design those on the chromatic dispersion are evaluated. When the structure parameters Λ=1.50 μm, d_core=2.10 μm, d₁=0.90 μm, d₂=0.44 μm and d₃=1.04 μm, the dispersion coefficient D is −1320 ps/(nm·km) at 1550 nm. This is a new kind of chromatic dispersion compensation PCF.     

Corrosion and erosion monitoring in plates and pipes using constant group velocity Lamb wave inspection

Recent improvements in tomographic reconstruction techniques generated a renewed interest in short-range ultrasonic guided wave inspection for real-time monitoring of internal corrosion and erosion in pipes and other plate-like structures. Emerging evidence suggests that in most cases the fundamental asymmetric A₀ mode holds a distinct advantage over the earlier market leader fundamental symmetric S₀ mode. Most existing A₀ mode inspections operate at relatively low inspection frequencies where the mode is highly dispersive therefore very sensitive to variations in wall thickness. This paper examines the potential advantages of increasing the inspection frequency to the so-called constant group velocity (CGV) point where the group velocity remains essentially constant over a wide range of wall thickness variation, but the phase velocity is still dispersive enough to allow accurate wall thickness assessment from phase angle measurements. This paper shows that in the CGV region the crucial issue of temperature correction becomes especially simple, which is particularly beneficial when higher-order helical modes are also exploited for tomography. One disadvantage of working at such relatively high inspection frequency is that, as the slower A₀ mode becomes faster and less dispersive, the competing faster S₀ mode becomes slower and more dispersive. At higher inspection frequencies these modes cannot be separated any longer based on their vibration polarization only, which is mostly tangential for the S₀ mode while mostly normal for the A₀ at low frequencies, as the two modes become more similar as the frequency increases. Therefore, we propose a novel method for suppressing the unwanted S₀ mode based on the Poisson effect of the material by optimizing the angle of inclination of the equivalent transduction force of the Electromagnetic Acoustic Transducers (EMATs) used for generation and detection purposes.     

Performance comparison of pre-, post- and symmetrical-dispersion compensation techniques using DCF on 40 Gbps OTDM system for different fibre standards

Various factors like polarization mode dispersion, non-linear effects, Kerr effect, second and third order dispersion impose limit on the performance (transmission distance, pulse broadening) of Optical Time Division Multiplexed (OTDM) transmission system. In this paper, the performance comparison of 40 Gbps OTDM transmission system with pre-, post-, and symmetrical-dispersion compensation techniques for different fibre standards has been carried out. It has been observed that for pre compensation, ITU 655 [D = 3.78 ps/nm/km] came to be best suitable fibre with the dispersion compensating fibre length of 2.36 km for maximum reach of 3000 km. For post- and symmetrical-compensation, the behaviour of alcatel [D = 8 ps/nm/km] is almost similar with maximum transmission distance of 3200 and 3050 km respectively.     

Surface reconstruction and the Debye-Waller factor

A recently developed soft-mode theory of surface reconstruction¹ is used to calculate the surface Debye-Waller factor (SDWF) as a function of temperature near a supposed transition temperature (T₀) between two reconstruction patterns. The soft surface mode gives rise to a sharp decrease in the SDWF as T₀ is approached, suggesting that an examination the LEED Bragg intensities may help verify the soft mode theory.     

Crystallography,magnetic properties and magnetocaloric effect in Gd4(BixSb1−x)3 alloys

A study of magnetic and thermal properties has been carried out on the alloys from the Gd₄(Bi_xSb_1−x)₃ series with x=0, 0.25, 0.5, 0.75, and 1. All of the alloys are ferromagnetic below their respective Curie temperatures which vary from 266 K for x=0 to 332 K for x=1.0. The magnetocaloric effect calculated from the temperature and magnetic field dependencies of the magnetization and heat capacity is moderate when compared to that of other materials, which order in the same temperature range. Both the magnetic ordering and the magnetocaloric effect peak temperatures increase nearly linearly with the increasing Bi content. Experimental magnetocaloric effect data obtained from two different measurement techniques are in excellent agreement.     

Substitutions in manganese zinc ferrites

The effect of substitutions in manganese zinc ferrites Zn_xFe _y ²⁺ Mn_zFe₂O₄, (x+y+z=1) on the compensation pointT ₀ of the crystal anisotropy constants and on the shape of the permeability vs. temperature dependence has been related to the contenty of divalent iron ions. For small amountsy<0.15 a definite dependence betweeny and the reduced temperatureT ₀/T _c of the compensation point has been found, which is independent of the specific substitution. This enables the effect of any intralattice substitution on permeability, hysteresis, losses and 3-with exceptions—on the disaccommodation spectra to be estimated.     

Simulation of the magnetic and magnetocaloric properties of hydrides of the La(Fe0.88Si0.12)13 compound by applying a negative pressure

Based on the model calculation, it has been shown that the effect of hydration of the ferromagnetic compounds La(Fe_0.88Si_0.12)₁₃H _y is quantitatively adequately described by the introduction of a negative pressure. The calculations have been performed for compounds with hydrogen concentrations y = 0, 0.5, 1.0, and 1.5. It has been found that these concentrations correspond to the pressures P = 0, ?0.95, ?1.85, and ?2.80 GPa, respectively. The calculated Curie temperature, magnetization, characteristics of the magnetocaloric effect, and other properties are in satisfactory agreement with experiment.     

Carbon monoxide oxidation over well-defined Pt/ZrO2 model catalysts: Bridging the material gap

Four different Pt/ZrO₂/(C/)SiO₂ model catalysts were prepared by electron beam evaporation. The morphology of these samples was examined before and after the catalytic reaction by Rutherford back-scattering (RBS), transmission electron microscopy (TEM) and grazing-incidence small-angle scattering (GISAXS). The catalytic behavior of such model catalysts was compared to a conventional Pt/ZrO₂ catalyst in the CO oxidation reaction using different oxygen excess (λ = 1 and 2). The so-called material gap was observed: model catalysts were active at higher temperature (620-770 K) and resulted in higher activation energy values (E_a = 77-93 kJ mol⁻¹ at λ = 1 and 129-141 kJ mol⁻¹ at λ = 2) compared to the powdered Pt/ZrO₂ catalyst (370-470 K, E_a = 74-76 kJ mol⁻¹). This material gap is discussed in terms of diffusion limitations, reaction mechanism and apparent compensation effect. Diffusion processes seem to limit the reaction on planar samples in the reactor system that was shown to be appropriate for the evaluation of the catalytic activity of powder samples. Kinetic parameters obeyed the so-called apparent compensation effect, which is discussed in detail. Langmuir-Hinshelwood-type of reaction, between CO_ads and O_ads, was proposed as the rate-determining step in all cases. Pt particles deposited on planar structures can be used for modeling conventional powdered catalysts, even though some limitations must be taken into account.     

Guided waves for autonomous online identification of structural defects under ambient temperature variations

Ultrasonic guided waves that are excited by piezoelectric transducers can be used for the autonomous online identification of structural defects in thin structures. The proposed technique in this paper continuously analyzes a damage metric which is defined as the maximum residual amplitude of the differential signal. A special focus is on the decision making to discriminate the undamaged from the damaged state of the structure where the appropriate detection thresholds are derived statistically from the inverse cumulative distribution function of the damage metric during an initial training phase. An integrated trend analysis by means of the moving average mitigates the impact of statistical outliers and reduces the probability of erroneous identifications.Long-term measurements under ambient temperature variations have been conducted on an aluminum and a composite plate to study the properties of the proposed novelty detection framework. In this process the temperature effect was compensated by the well-known combination of optimal baseline selection (OBS) and baseline signal stretch (BSS). In case of the aluminum structure two artificial cracks with different sizes have been identified reliably. Consistent results were found on the composite specimen where an impact damage was identified for different excitation frequencies.     

Long-range correlations in heart rate variability during computer-mouse work under time pressure

The aim of this study was to investigate the influences of time pressure on long-range correlations in heart rate variability (HRV), the effects of relaxation on the cardiovascular regulation system and the advantages of detrended fluctuation analysis (DFA) over the conventional power spectral analysis in discriminating states of the cardiovascular systems under different levels of time pressure. Volunteer subjects (n=10, male/female=5/5) participated in a computer-mouse task consisting of five sessions, i.e. baseline session (BSS) which was free of time pressure, followed by sessions with 80% (SS80), 100% (SS100), 90% (SS90) and 150% (SS150) of the baseline time. Electrocardiogram (ECG) and task performance were recorded throughout the experiments. Two rest sessions before and after the computer-mouse work, i.e. RS1 and RS2, were also recorded as comparison. HRV series were subsequently analyzed by both conventional power spectral analysis and detrended fluctuation analysis (DFA). The long-term scaling exponent α₂ by DFA was significantly lower in SS80 than that in other sessions. It was also found that short-term release of time pressure had positive influences on the cardiovascular system, i.e. the α₂ in RS2 was significantly higher than that in SS80, SS100 and SS90. No significant differences were found between any two sessions by conventional power spectral analysis. Our results showed that DFA performed better in discriminating the states of cardiovascular autonomic modulation under time pressure than the conventional power spectral analysis.     

Dielectric investigations of solid solutions SrTiO3-KTaO3 and SrTiO3-KNbO3

The temperature dependences of the dielectric constant and dielectric hysteresis loops in ceramic samples of (1 ? x)SrTiO_3?x KNbO₃ and (1 ? x)SrTiO_3?x KTaO₃ (0 ≤ x ≤ 0.3) solid solutions prepared using different heat treatments have been investigated. Phase diagrams of the studied solid solutions have been constructed in the T-x coordinates. It has been shown that, after quenching of samples (spontaneous cooling at room temperature after long-term heating at the sintering temperature of the ceramic samples), the temperature of the induced phase transition increases because of the weakening of random electric fields associated with nonisovalent impurities due to their “frozen” nonequilibrium redistribution. For small concentrations x, strong dielectric relaxation is observed in the temperature range of 150–250 K. A model of relaxing centers, which is based on the local charge compensation of heterovalent impurities, has been proposed.     

Magnetic structure and collective Jahn-Teller distortions in nanostructured particles of CuFe2O4

The aim of the present work is to compare the structural, the composition and chemical state of the surface and magnetic properties of different nanosized CuFe₂O₄ powders exhibiting collective Jahn-Teller effect. The samples under examination consist of edged nanosized particles (needle like) with average length 1300 ± 20 nm and diameter 300 ± 20 nm obtained after high temperature synthesis, and superparamagnetic (at room temperature) spherical particles (d = 6 ± 2 nm), obtained by soft chemistry techniques. The surface composition of the particles was investigated by X-ray photoelectron spectroscopy (XPS). Mössbauer spectroscopy (MöS), including at high magnetic field up to 5 T and 4.2 K, was used for characterization of cation distribution in the samples. The data yielded by the XPS and MöS analyses for spherical nanosized particles led us to the assumption for the existence of a Jahn-Teller effect gradient—from the B-sublattice on the surface to a compensation of the tetragonal distortion in the two sublattices in the core. The analysis of the contribution of the anisotropy energy in edged and spherical nanoparticles shows that it must be considered as an effective value reflecting the influence of the individual factors depending on the particle shape and surface.     

Vacancy-induced spin-glass behavior of Prussian blue analogue Fe1.1Crx[Cr (CN)6]0.6−x·nH2O nanowires

Prussian blue analogue Fe^II_1.1Cr^II_x[Cr^III(CN)₆]_0.6−x·nH₂O nanowires were synthesized by electrodeposition. The magnetic properties investigation indicates that the nanowires exhibit cluster spin-glass behavior, which undergoes a magnetic transition to a frozen state below about 62 K. Spin disorder arising from reduced coordination and broken exchange bonds between spin centers due to the structural defects may be the reason that causes the spin-glass freezing behavior. The negative magnetization observed at temperature lower than the compensation temperature (T_comp∼43 K) at a field of 10 Oe may be due to the different temperature dependences of the ferromagnetic site Fe-Cr and antiferromagnetic site Cr-Cr.