Suppression of the non-linear susceptibilities of ferromagnetic PdFe and PdMn

A detailed experimental investigation of the linear and non-linear AC susceptibilities of two ferromagnetic samples, namely PdMn with 0.75 at.% Mn and PdFe with 0.17 at.% Fe, has been carried out as a function of temperature, frequency and amplitude of the AC driving field. In order to explain this behaviour, we have also developed a very simple theory for ferromagnetics, based upon the mean-field model. For these samples the expected divergence of the higher-order susceptibility Xnl at Tc does not occur; instead the measured response tends to zero. According to this theory, the correction term for demagnetization effects seems to outweigh the critical divergence in Xnl at T to Tc. The extreme sensitivity of Xnl to demagnetization correction is not generally recognized in the recent literature. To obtain the transition temperatures and the critical exponent gamma of the samples, the well known Kouvel-Fisher method is used. We have found Tc=1.95+or-0.02 K for PdFe and Tc=2.65+or-0.02 K for PdMn. For PdMn and PdFe, the values of gamma are calculated to be 1.41+or-0.05 and 1.55+or-0.05, respectively

SUPPRESSION OF THE NONLINEAR SUSCEPTIBILITIES OF FERROMAGNETIC PDFE AND PDMN

WOS: A1994PK92600021A detailed experimental investigation of the linear and non-linear Ac susceptibilities of two ferromagnetic samples, namely PdMn with 0.75 at.% Mn and PdFe with 0.17 at.% Fe, has been carried out as a function of temperature, frequency and amplitude of the Ac driving field. In order to explain this behaviour, we have also developed a very simple theory for ferromagnetics, based upon the mean-field model. For these samples the expected divergence of the higher-order susceptibility (X) over tilde(nl) at T-c does not occur; instead the measured response tends to zero. According to this theory, the correction term for demagnetization effects seems to outweigh the critical divergence in (X) over tilde(nl) at T --> T-c. The extreme sensitivity of (X) over tilde(nl) to demagnetization correction is not generally recognized in the recent literature. To obtain the transition temperatures and the critical exponent gamma of the samples, the well known Kouvel-Fisher method is used. We have found T-c = 1.95 +/- 0.02 K for PdFe and T-c = 2.65 +/- 0.02 K for PdMn. For PdMn and PdFe, the values of gamma are calculated to be 1.41 +/- 0.05 and 1.55 +/- 0.05, respectively

A rectangular solar pond model incorporating empirical functions for air and soil temperatures

A novel theoretical model, capable of giving the temporal temperature variation at any point inside or outside a non-insulated rectangular solar pond at any time, is presented. Incorporating the finite difference approach, the model makes use of one- and two-dimensional heat balances written on discrete regions in the brine and in the soil adjacent to the pond. These simultaneous equations are solved for the local temperatures, using a computer program. Values of hourly averaged air temperature and daily averaged soil temperature for the site were used as input parameters, and empirical functions for the time-dependence of these variables were incorporated into the theoretical model. It was found necessary to use this level of detail of the meteorological data for reliable predictions on the solar ponds. The model results are compared with measured results on an actual solar pond built in Cukurova, Turkey. The modelled and experimental temperature profiles are found to be in a very good agreement. The results indicate that the thickness of the salt gradient region of a solar pond should not be less than 1.3 m. Heat losses form the pond side-walls was found not to effect the performance of solar ponds when the surface area is greater than 100 m2.A novel theoretical model, capable of giving the temporal temperature variation at any point inside or outside a non-insulated rectangular solar pond at any time, is presented. Incorporating the finite difference approach, the model makes use of one- and two-dimensional heat balances written on discrete regions in the brine and in the soil adjacent to the pond. These simultaneous equations are solved for the local temperatures, using a computer program. Values of hourly averaged air temperature and daily averaged soil temperature for the site were used as input parameters, and empirical functions for the time-dependence of these variables were incorporated into the theoretical model. It was found necessary to use this level of detail of the meteorological data for reliable predictions on the solar ponds. The model results are compared with measured results on an actual solar pond built in Cukurova, Turkey. The modelled and experimental temperature profiles are found to be in a very good agreement. The results indicate that the thickness of the salt gradient region of a solar pond should not be less than 1.3 m. Heat losses form the pond side-walls was found not to effect the performance of solar ponds when the surface area is greater than 100 m2

Anisotropy of induced birefringence in ferroelectrics with pseudoilmenite structure

An analysis of the nature of anisotropy of the optical properties of LiTaO3 is presented on the basis (of the features) of its intrinsic crystallographical properties. Electrooptically induced refractive-index change in the regular and reverse-poled LiTaO3 is calculated. Specifically we determine how the index change depends on electric field magnitude and direction in y-propagating extraordinary modes. To accomplish this, changes in index-ellipsoid shape and orientation are determined by use of a numerical eigenvalue procedure for diagonalizing the impermeability tensor; then the refractive index is calculated by vector reference-frame transformation and small perturbation approximation. The general formula is inferred from calculations for specific field directions. Electrooptic coefficients for the reverse-poled LiTaO3 are obtained tensor reference-frame transformation of those of LiTaO3

The magnetic and electrical properties of rare-earth Sm3+ substituted Bi1.7Pb0.3Sr2Ca2-xSmxCu3O12 system

WOS: 000229100800006In the present work, the magnetic properties of high-T Bi1.7Pb0.3Sr2Ca2-xSmxCu3O12 system (x = 0.03) have been investigated, for different temperatures below T,. Substitution of Sm3+ for Ca2+ is found to change the superconducting properties of the system, despite the weak paramagnetic nature of the Sm3+ ion. The dc magnetization measurements performed on the compound reveal that the superconducting regions decrease with increasing temperature and magnetic field. The results are explained on the basis of a possible reduction of hole concentration with trivalent rare-eaxth ion substitution, and also on the basis of the magnetic nature of the substituted Sm3+ ions

Effects of annealing time on the magnetic properties of a Bi1.7Pb0.29Gd0.01Sr2Ca3Cu4O12+y superconductor prepared by the melt-quenching method

WOS: 000228353900007Bi-Pb-Gd-Sr-Ca-Cu-O bulk samples with nominal composition Bi1.7Pb0.29Gd0.01Sr2Ca3Cu4O12+y (2234) were prepared by the melt-quenching method. The effects of different annealing times on the structure have been investigated by electrical resistance, scanning electron micrographs, and magnetization measurements. The magnetization measurements have been carried out as a function of the magnetic field up to 5 kOe for field temperatures below the zero resistance temperatures of the annealed samples. It has been found that the high-T-c superconducting phase, (2223), can be formed by annealing at 840 degrees C for 120 h. The decrease in magnitude of the initial susceptibility dM/dH and the fast decrease of hysteresis loops, related to the superconducting volume with increasing temperature, seem to imply an existence of flux pinning centers in our samples

The effect of Gd concentration on the physical and magnetic properties of Bi1.7Pb0.3-xGdxSr2Ca3Cu4O12+y superconductors

WOS: 000230366300008Bi-Pb-Gd-Sr-Ca-Cu-O bulk samples with nominal composition Bi1.7Pb 0.3-xGdxSr2Ca3Cu4O12+y (x=0.01, 0.05, 0.075, 0.10) were prepared by the melt-quenching method. The effects of different Gd doping on the structure have been investigated by electrical resistance, scanning electron micrographs, XRD, magnetization and magnetic hysteresis loop measurements. The magnetization measurements have been carried out as a function of magnetic field for fields up to 5 kOe at temperatures well below the zero resistance temperatures of the annealed samples. It has been found that the high-T-c superconducting phase, (2 2 2 3), is formed in the sample A with concentration x = 0.01, annealed at 840 degrees C for 120 h. However, with increasing Gd3+ doping for Pb2+ the (2 2 2 3) phase gradually transforms into the (2 2 1 2) phase. The magnitudes of magnetization and initial susceptibility, | M | and | dM/dH|, and the hysteresis loop areas decrease with increasing Gd concentration x and/or temperature T. The fast decreases in | M|, | dM/dH |, and the hysteresis loop areas related to the superconducting volume, with increasing x and/or T seem to imply an existence of flux pinning centres in our samples. In order to support this implication the critical current densities J(c), of the samples, have been estimated at two fixed temperatures, 9 and 30 K. Our data have indicated that J(c) decreases with increasing temperature and/or Gd concentration, as expected

Field dependence of magnetization and dM/dH for Sm- and Gd-doped Bi1.7Pb0.3Sr2Ca2-xRExCu3O10+y compounds

WOS: 000224453100049We have performed magnetization measurements as a function of magnetic fields up to 4 kOe on Bi1.7Pb0.3Sr2Ca2-xRExCu3O10+y (RE: Gd-0.01, SM0.03), at different fixed temperatures T Sm3+ and Gd3+ substitution decreases the hole carrier concentration and hence the volume fraction of the superconducting phase

A simple chaotic neuron model: Stochastic behavior of neural networks

We have briefly reviewed the occurrence of the post-synaptic potentials between neurons, the relationship between EEG and neuron dynamics, as well as methods of signal analysis. We propose a simple stochastic model representing electrical activity, of neuronal systems. The model is constructed using the Monte Carlo simulation technique. The results yielded EEG-like signals with their phase portraits in three-dimensional space. The Lyapunov exponent was positive, indicating chaotic behavior. The correlation of the EEG-like signals was .92, smaller than those reported by others. It was concluded that this neuron model may provide valuable clues about the dynamic behavior of neural systems

Critical current densities in Bi1.7Pb0.3-xGdxSr2Ca3Cu4O12+y (x=0.01, 0.1) superconductors prepared by melt-quenching method and annealed in different time intervals

WOS: 000226905700006We report critical current density (J(c)) behaviors of the Bi1.7Pb0.3-xGdxSr2Ca3Cu4-O12+y (x = 0.01, 0.1) superconductors prepared by melt-quenching method and annealed in different time intervals at 840 C. The effects of annealing times are found to be very important. For example, for the sample B2, the highest value of J(c) is 3.7 x 10(4) A/cm(2) at 9 K. This value is almost 1.5 and 2 times greater than that of the samples B3 and B1, respectively. With increasing annealing time, the 2223 phase increases, thus resulting in a decrease in the insulating phase. For lower annealing times, there are a large number of insulating phases. On the whole, our results indicate that there is an optimum annealing time t to form the 2223 phase, in a time interval of 120 less than or equal to t less than or equal to 192 h. It is clearly seen that. J(c) decreases smoothly with increasing temperature for all the samples studied. At lower temperatures, especially below 30 K, an extremely rapid decrease of J(c) with increasing temperature has been observed. The experimental values also show that between the temperature interval from 9-45 K, J(c) decreases by one order of magnitude. We have also observed that when the Gd-Pb substitution increases, the effective defect density increases, leading to small critical current densities