Bekir Özçelik

Field Dependence of Magnetization anddM/dHfor Sm- and Gd-Doped Bi1.7Pb0.3Sr2Ca2 xRExCu3O10+yCompounds

We have performed magnetization measurements as a function of magnetic fields up to 4 kOe on Bi1.7Pb0.3Sr2 Ca2−xRExCu3O10+y (RE: Gd0.01, Sm0.03), at different fixed temperatures T<Tc in the range 15≤T≤75 K. We have observed three important features. Firstly, the M–H curves of both the Sm- and Gd-doped samples are similar and each shows a minimum of M at a certain critical field Hc, above which there appears to be a monotonic decrease in magnitude. Secondly, the magnitude of the initial susceptibility dM/dH, related to the superconducting volume, decreases with increasing temperature. In other words, the decrease of |dM/dH| with T indicates that the superconducting regions reduce in size and the paramagnetic domains probably grow at the expense of superconductivity. The third feature is the fast decrease of hysteresis loops in both the Gd and Sm doped samples with increasing temperature, which implies existence of flux pinning centres. Thus the results show that the Ca2+→Sm3+ and Gd3+ substitution decreases the hole carrier concentration and hence the volume fraction of the superconducting phase.

THE MAGNETIC AND ELECTRICAL PROPERTIES OF RARE-EARTH Sm3+ SUBSTITUTED Bi1.7Pb0.3Sr2Ca2-xSmxCu3O12 SYSTEM

In the present work, the magnetic properties of high-TcBi1.7Pb0.3Sr2Ca2-xSmxCu3O12 system (x = 0.03) have been investigated, for different temperatures below Tc. 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-earth ion substitution, and also on the basis of the magnetic nature of the substituted Sm3+ ions.

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

We report critical current density (Jc) behaviors of the Bi1.7Pb0.3-xGdxSr2Ca3Cu4O12+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 Jc is 3.7×104A/cm2 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<t<192 h. It is clearly seen that Jc decreases smoothly with increasing temperature for all the samples studied. At lower temperatures, especially below 30 K, an extremely rapid decrease of Jc with increasing temperature has been observed. The experimental values also...

Effect of Yb substitution in Bi-2212 ceramics prepared by laser floating zone technique

In the present work, the influence of Yb substitution for Bi on the structural, electrical and magnetic properties of textured Bi-2212 ceramics has been investigated. X-ray diffraction studies indicated that the major peaks correspond to the Bi-2212 phase. The SEM images clearly show that there is an increase in the number of secondary phases with raising Yb content while all samples maintain similar grain morphology. Magnetic analysis of all samples has been done by magnetic hysteresis measurements, indicating that an enhancement in the M-H loops is obtained by a low Yb doping. The critical current density of samples was calculated from the M-H curves by using Bean’s critical state model, indicating that sample with 0.05 Yb substitution possesses the highest Jc values.

Observation of magnetocaloric effect in the LaMn1.9Fe0.1Si2 compound at low fields in the vicinity of phase transition around room temperature

Low field magnetic and magnetocaloric properties of the compound LaMn1.9Fe0.1Si2 have been investigated. Using the M-H data measured at various temperatures, we have derived the magnetic entropy change |ΔS| as a function of temperature and magnetic field, by using the Maxwell relation. The temperature dependence of the Landau coefficient has been derived by fitting the magnetization, using the Landau expansion of magnetic free energy and Arrott plots. The results show that LaMn1.9Fe0.1Si2 is ferromagnetically ordered around room temperature (285 K) and the nature of the phase transition at this temperature is found to be second order.

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.

The synthesis and magnetic structure of the iron selenide Ba0.8Fe2Se2

The BaxFe2-ySe2 (x=0.8) single crystal has been synthesized by using the one step technique at relatively lower temperatures. We have investigated structural properties of samples by using the XRD, SEM-EDX and magnetic properties. The SEM results clearly demonstrate that Ba ions are intercalated in the FeSe lattice. XRD result shows that the sample prepared has multi-phase structure. According to our M-H measurements perfect diamagnetism has been observed only in low field at 5 K. In addition, those curves indicate that the high field value and some impurities reveal ferromagnetic interactions. The superconducting transition was observed at around Tc≈10.9 K.

The physical and magnetic properties of FeSe-11 superconductor

In this study, polycrystalline of FeSe0.98 and FeSe1.2 samples were synthesized by a solid-state reaction method to investigate the microstructural and magnetic properties in these systems. Different heating cycles were applied and the best synthesis cycle has been established by optimization. Samples were characterized regarding structure, composition and superconducting properties. The microstructural investigations showed that structural formation of the samples was not strictly depending on the heating cycles. Transition temperature is obtained as 10 K from Zero Field Cooled magnetization measurement.

Thermal Conductivity and Thermoelectric Power of Yb-Substituted Bi-2212 Superconductor

In the present study, Bi2Sr2CaCu2-xYbxO8+y where x = 0.0, 0.05, 0.1 and 0.25, superconductor materials were textured using the Laser Floating Zone technique. After annealing the as-grown materials, they were characterized by thermal conductivity (κ-T), thermoelectric power (S-T), and electrical resistivity (R-T) measurements performed from room temperature down to 20 K. The thermoelectric power is positive in all cases and is found to increase with decreasing temperature, reaching the maximum value (peak) around the samples TC values and dropping rapidly to zero below TC. Electrical resistivity measurements show a slight decrease of TC when Yb substitution is raised. In spite of this effect, TC values are higher than 90 K in all cases. On the other hand, Yb substitution also decreases thermal conductivity, compared with the undoped samples.

Physical and Magnetic Properties of Sm0.2Gd0.8Ni4B Compound

Physical properties of the Sm0.2Gd0.8Ni4B compound have been investigated by means of the X-ray powder diffraction, DC and AC-susceptibility techniques. The compound studied crystallizes in CeCo4B type structure with P6/mmm space group. The unit-cell parameters a and c are determined as 5.01 and 6.95 Å, respectively, and the unit-cell volume V is calculated as 151.08 Å3. DC and AC magnetic measurements present the visible magnetic phase transition from paramagnetic to ferromagnetic, around definite transition temperature. The magnetic phase transition temperature of the compound is obtained from DC magnetization, AC-susceptibility and the well known Kouvel-Fisher method as 36.6, 35.7 and 35.2 K, respectively. The saturation magnetization (Ms) and the coercive fields (Hc) of the compound are found to be 3.7µB/f.u and 277 Oe, respectively, by using the hysteresis loops at 9.5 K. We have also investigated the non-linear AC-susceptibility of the compound, around its ferromagnetic transition temperature, as a function of temperature, frequency and amplitude of the AC-driving field. In order to explain the measured experimental results, we have used the theory developed for ferromagnetic, based upon the mean field model. The measurements exhibit both frequency and amplitude dependencies. Observed dependencies are compared with the existing theories of linear and nonlinear susceptibilities with reference to short- and long-range interactions. In Kouvel-Fisher method, one plots 1/χ1*(dχ-1/dT) against T, obtaining a straight line. The slope of this line gives the critical exponent γ, and it intersects the T axis at Tc. In order to obtain dχ-1/dT and the best straight line, we used a two-point numerical differentiation program and linear regression method, respectively. The critical exponent γ of the sample is calculated to be 2.78 ± 0.05. The value of the critical exponent β, which is characteristic of static phase transition to a ferromagnetic state, is estimated as 2.41±0.3 from the slope of the line obtained the plot of the absolute third-harmonic values versus the reduced temperature on a log–log scale.