Magnetic and electrical properties of bismuth cobaltite Bi24(CoBi)O40 with charge ordering

The compound Bi₂₄(CoBi)O₄₀ has been synthesized using the solid-phase reaction method. The temperature and field dependences of the magnetic moment in the temperature range 4 K < T < 300 K and the temperature dependences of the EPR line width and g-factor at temperatures 80 K < T < 300 K have been investigated. The electrical resistivity and thermoelectric power have been measured in the temperature range 100 K < T < 1000 K. The activation energy has been determined and the crossover of the thermoelectric power from the phonon mechanism to the electron mechanism with variations in the temperature has been observed. The thermal expansion coefficient of the samples has been measured in the temperature range 300 K < T < 1000 K and the qualitative agreement with the temperature behavior of the electrical resistivity has been achieved. The electrical and structural properties of the compound have been explained in the framework of the model of the electronic-structure transition with inclusion of the exchange and Coulomb interactions between electrons and the electron-phonon interaction.     

The resistance jump in the pure type-I superconductors tin,indium and lead

It is shown that for tin, indium and lead samples of sufficiently high purity, sufficiently large diameter (2–3mm), and sufficiently close to the critical temperature (ΔT<0.15K), there exists a common value close to $\text{2}\text{3}$(0.64?R(i_c)/R_n ? 0.69) of the resistance jump at the transition from the superconducting to the intermediate state.     

Transport properties of CeCu6 single crystals

The electrical resistivity ? and the thermoelectric power of CeCu₆ single crystals are strongly anisotropic. The inverse of the temperature of the Kondo resistivity maximum (T_max) roughly scales the linear temperature coefficient B of ? as well as the residual value (?₀ ÷ B ÷ 1/T_max). Along the [1 0 0] direction ? follows a T² Fermi-liquid law between 30 and 90 mK. The thermoelectric power is positive over the investigated temperature range (1–300 K) and shows two contributions.     

Electron interaction effects on the thermoelectric power of a quantum dot at T &gt; T<Subscript>K</Subscript>

In the absence of phonon thermal conductivity, we theoretically investigate the output power of an interacting quantum dot thermoelectric setup that is moderately coupled to two electronic reservoirs in the regime T ? T _K . In the noninteracting case, the output power is maximized when the energy level of the dot is around a critical value ε _c . We find that when the energy level of the dot is lower than ε _c , Coulomb interaction can enhance the maximum thermoelectric power that can be achieved by tuning the bias and a wider operating region is also observed. However, when the energy level of the dot is higher than ε _c , Coulomb interaction suppresses the maximum power. Finally when the dot level is around ε _c , Coulomb interaction has minimal effects on the maximum power.     

Anomalous thermoelectric power behaviour in PrSn3 and NdSn3

Results of the thermoelectric power (TEP) measurements done on monocrystalline samples of RESn₃ compounds (RE=La, Pr, Nd, and Gd) are presented for the temperature range of 5.5-300 K. It was found that the TEP is positive and weakly temperature dependent at temperatures T>100 K. For T<100 K pronounced anomalies have been observed for the PrSn₃ and the NdSn₃ compounds in the vicinity of 10 K.We argue that the Kondo and crystal field effects cause these anomalies. A shape of the TEP anomaly found for PrSn₃ resembles very much that observed in the electrical resistivity.     

Thermoelectric power of potassium doped lanthanum manganites at low temperatures

The temperature-dependent resistivity and thermoelectric power of monovalent (K) doped La_1−xK_xMnO₃ polycrystalline pellets (x=0.05, 0.10 and 0.15) between 50 and 300 K are reported. K substitution enhances the conductivity of this system. Curie temperature (T_C) also increases from 260 to 309 K with increasing K content. In the paramagnetic region (T>T_C), the electrical resistivity is well represented by adiabatic polaron hopping, while in the ferromagnetic region (T<T_C), the resistivity data show a nearly perfect fit for all the samples to an expression containing, the residual resistivity, spin-wave and two-magnon scattering and the term associated with small-polaron metallic conduction, which involves a relaxation time due to a soft optical phonon mode. Small polaron hopping mechanism is found to fit well to the thermoelectric power (S) data for T>T_C whereas at low temperatures (T<T_C) in ferromagnetic region (S_FM), S_FM is well explained with the spin-wave fluctuation and electron–magnon scattering. Both, resistivity and thermopower data over the entire temperature range (50–300 K) are also examined in light of a two-phase model based on an effective medium approximation.     

On the coexistence of spin and lattice polarons in the La0.67−xEuxCa0.33MnO3 CMR system

The electrical and magnetic transport properties of the La_0.67−xEu_xCa_0.33MnO₃ system exhibit lowering of insulator to metal and paramagnetic to ferromagnetic transition temperature (T_C) with the increase of Eu concentration in addition to possessing CMR property. The temperature variation of electrical resistivity and magnetic susceptibility for x=0.21 is found to have two distinct regions in the paramagnetic state for T>T_P; one with the localization of lattice polaron in the high-temperature region (T>1.5T_P) satisfying the dynamics of variable range hopping (VRH) model and the other being the combination of the spin and lattice polarons in the region T_P<T<1.5T_P. The resistivity variation with temperature and magnetic field, the cusp in the resistivity peak and CMR phenomenon are interpreted in terms of coexistence of spin and lattice small polarons in the intermediate region (T_P<T<1.5T_P). The spin polaron energy in the La_0.46Eu_0.21Ca_0.33MnO₃ system is estimated to be 106.73±0.90 meV and this energy decreases with the increase of external magnetic field. The MR ratio is maximal with a value of 99.99% around the transition temperature and this maximum persists till T→0 K, at the field of 8 T.     

Low temperature thermoelectric power of LaB6, PrB6 and NdB6

We have measured the thermoelectric power of LaB₆, PrB₆ and NdB₆ in the temperature range 2–20K. PrB₆ and NdB₆ order antiferromagnetically at T_N = 6.99K and 7.74K respectively, whereas LaB₆ is non-magnetic. The thermoelectric power data have a negative maximum near 5.5K in all three hexaborides. This negative peak is thought to be mainly due to phonon drag in LaB₆. In PrB₆ and NdB₆ there is an additional contribution due to magnon drag. It is expected that the high temperature thermoelectric power is mainly due to an electron diffusion term and a contribution due to spin disorder scattering. The thermoelectric power and the resistivity have been compared in the vicinity of T_N. A qualitative similarity in the temperature derivatives of these two quantities has been found for PrB₆ and NdB₆ near T_N.     

Thermoelectric power and thermal conductivity study of the Y3Ba5Cu8Ox system

In the present study, the effect of heat treatment conditions on the structural/microstructural, electrical and transport properties, such as thermoelectric power and thermal conductivity, of the Y-358 system was investigated. The Y-123 phase instead of Y-358 were obtained with heat treatments. XRD and SEM analyses showed that the best structural formation was obtained for the sample prepared at 900 °C for 24 h. All the samples exhibited the T_c value around 91 K. Positive S(T) value was obtained in all the samples. S(T) data was analyzed by “Two band model with linear T-term”. In thermal conductivity of the samples, small peak with broad maximum was found just below T_c, unlike that of the HT_c cuprates.     

Complex competition between ferromagnetism and antiferromagnetism in the CMR manganites Pr1−x Ca x MnO3

The systematic investigation of the magnetic susceptibility of the CMR manganites Pr_1?x Ca _x MnO₃ versus temperature has been performed for 0.25 ≤ x ≤ 0.50. Due to the similar size of calcium and praseodymium, these results show the important role of the mixed valence of manganese upon the complex magnetic behaviour of these compounds. They demonstrate that the appearance of antiferromagnetism coincides with charge ordering, T _N = T _CO varying with x, from 250K for x = 0.50 to 225K for x = 0.35. A strong competition between ferromagnetism and antiferromagnetism is observed for 0.35 ≤ x < temperature (T > 170 K) and going through canted antiferromagnetic or weak ferromagnetic states for intermediate temperatures (70 K < T < 170 K). For 0.25 ≤ x ≤ 0.30, a strong ferromagnetic state is observed for 95 K ≤ T ≤ 150 K, with a transition to a spin glass like state below 95–110 K.     

Superconducting Properties of Indium Nanostructured in Pores of Thin Films of SiO<Subscript>2</Subscript> Microspheres

Samples of a superconducting indium nanocomposite based on a thin-film porous dielectric matrix prepared by the Langmuir–Blodgett method are obtained for the first time, and their low-temperature electrophysical and magnetic properties are studied. Films with thickness b ≤ 5 μm were made from silicon dioxide spheres with diameter D = 200 and 250 nm; indium was introduced into the pores of the films from the melt at a pressure of P ≤ 5 kbar. Thus, a three-dimensional weakly ordered structure of indium nanogranules was created in the pores, forming a continuous current-conducting grid. Measurements of the temperature and magnetic field dependences of the resistance and magnetic moment of the samples showed an increase in the critical parameters of the superconductivity state of nanostructured indium (critical temperature T_c ≤ 3.62 K and critical magnetic field H_c at T = 0 K H_c(0) ≤ 1700 Oe) with respect to the massive material (T_c = 3.41 K, H_c(0) = 280 Oe). In the dependence of the resistance on temperature and the magnetic field, a step transition to the superconductivity state associated with the nanocomposite structure was observed. A pronounced hysteresis M(H) is observed in the dependence of the magnetic moment M of the nanocomposite on the magnetic field at T < T_c, caused by the multiply connected structure of the current-conducting indium grid. The results obtained are interpreted taking into account the dimensional dependence of the superconducting characteristics of the nanocomposite.     

Ferromagnetic semiconductor Ge1−x−ySnxMnyTe with phase transformation of ferroelectric type

It is expected that joint existence of ferromagnetic properties and ferroelectric structural phase transition in diluted magnetic semiconductors IV-VI leads to new possibilities of these materials. Temperature of ferroelectric transition for such crystals can be tuned by the change of Sn/Ge ratio. Magnetic susceptibility, Hall effect, resistivity and thermoelectric power of Ge_1−x−ySn_xMn_yTe single crystals grown by Bridgeman method (x=0.083-0.115; y=0.025-0.124) were investigated within 4.2-300 K. An existence of FM ordering at T_C∼50 K probably due to indirect exchange interaction between Mn ions via degenerated hole gas was revealed. A divergence of magnetic moment temperature dependences at T?T_C in field-cooled and zero-field-cooled regimes is obliged to magnetic clusters which are responsible for superparamagnetism at T>T_C≈T_f (freezing temperature) and become ferromagnetic at T_C arranging spin glass state at T<T_f≈T_C. Phase transition of ferroelectric type at T≈46 K was revealed. Anomalous Hall effect which allows to determine magnetic moment was observed.     

Electrical and thermal properties of Pr2/3(Ba1−xCsx)1/3MnO3 manganites

Electrical and thermal properties of Pr_2/3(Ba_1?x Cs _x )_1/3MnO₃ (0 ≤ x ≤ 0.25) manganite perovskites are reported here. Two insulator-metal (I-M) transitions (T _P1 &T _P2) are observed in the electrical resistivity (ρ) of the pristine Pr_2/3Ba_1/3MnO₃ (PBMO) sample, and they are systematically shifted to lower temperatures with increasing Cs substitution. An upturn in ρ is noticed below 50 K in these perovskites, presumably due to the combined effect of weak localization, electron-electron and electron-phonon scattering. It is found that the absolute value of room-temperature thermoelectric power (TEP) gradually decreases with increasing Cs content, implying the annihilation of the charge carriers with doping. An analysis of the electrical resistivity and thermoelectric power data indicates that the paramagnetic insulating state above T _P1 is governed by the small polaron hopping due to a non-adiabatic process. It is argued that the electron-magnon scattering processes are responsible for low temperature metallic behavior of TEP. A distinct specific heat peak below T _P1 is observed, attributed to the magnetic ordering, and its broadening with Cs-doping corresponds to the increase of magnetic inhomogeneity. Further, the temperature variation of thermal conductivity and the low temperature plateau in κ has been associated with the delocalization of Jahn-Teller polarons and transition from Umklapp scattering to a defect-limited scattering, respectively.     

Thermoelectric power in ferromagnetic Pb0.16Sn0.72Mn0.12Te semiconductor

Thermoelectric power was studied in semimagnetic (diluted magnetic) Pb_1−x−ySn_yMn_xTe (x=0.12,y=0.72) semiconductor in the temperature range T=10–100 K covering both ferromagnetic and paramagnetic range of temperatures of this material. In addition to standard diffusion contribution to the thermoelectric power (α_D∝T), we found in PbSnMnTe an additional “magnetic” contribution (α_FM). Employing the effect of carrier-concentration-controlled ferromagnetism in PbSnMnTe, we demonstrate that the temperature dependence of α_FM(T) shows a clear maximum at the ferromagnetic Curie temperature. This experimental finding is discussed in terms of Kasuya model for the thermoelectric properties of diluted magnetic metallic systems with sd-exchange interaction.     

Oxidizing effects of high temperature annealing in reducing atmosphere in Ca-doped YIG films

The existence of an oxidation mechanism is reported in Ca-doped YIG films, when high temperature annealing is carried out in N₂ atmosphere. The annealing were performed at successively increasing temperatures, and optical absorption, lattice parameter and thermoelectric power were measured at each step. Optical absorption was observed going down to a minimum (T = 450°C in one cycle of annealing, T = 400°C in a different cycle) and then rise again. The minimum in absorption corresponds to a maximum of the lattice parameter, while the thermoelectric power is always p-type. Analysis of the data leads to the conclusion that we are observing a reoxidation process, triggered both by temperature and oxygen vacancy concentration. This process is subject to exhaustion. Previous annealing experiments are analyzed in the light of these results.     

Surface properties of liquid In-Zn alloys

The measurements of surface tension and density of zinc, indium and liquid In-Zn alloys containing 0.9, 0.85, 0.75, 0.70, 0.60, 0.40, 0.25 and 0.10 mole fraction of In were carried out using the method of maximum pressure in gaseous bubbles (MBP) as well as dilatometric technique. The technique of sessile drop was additionally applied in the measurements of surface tension for pure indium and zinc. The measurements were performed at temperature range 474-1151 K. The isotherms of surface tension calculated based on Butler's equation at 700 and 1100 K corresponded well with the experimental values for zinc content lower than 0.6 mole fraction. The surface tension calculated for alloys of higher zinc concentrations (0.6 < X_Zn < 0.95) had a positive value of the surface tension temperature coefficient (dσ/dT), which did not coincide with the experimental results. The density as well as molar volume of liquid In-Zn alloys showed almost identical behaviour like the ideal solutions. The observed little deviations were contained within assessed experimental errors.     

AC-susceptibility of Ni/W(1 1 0) ultrathin magnetic films: determination of the Curie temperature and critical behavior

The temperature dependence of the magnetic susceptibility of 6–8 ML Ni/W(1 1 0) is measured in situ in UHV by means of an AC-susceptibility mutual inductance bridge. At sufficient small driving magnetic fields ⩽11 A/m a susceptibility maximum and an interval of constant susceptibility on the high temperature side of the peak are observed. The Curie temperature is identified as the low temperature limit of this region of constant susceptibility. The appearance of the maximum at lower temperatures is interpreted as a ferro-magnetic response at T<T_C. The critical exponent γ is extracted from a power law fit at T>T_C. For the smallest field of 3 A/m, the determined γ of 1.26 (7) is consistent with γ of the three-dimensional Ising model. For larger fields, the exponent depends on the field and presents an effective value.     

Effects of indium doping on the superconducting properties of YBa2Cu3Oy sintered compounds and thin films

We investigated the effects of indium doping on the superconducting properties of YBCO sintered samples and thin films. In₂O₃-doped YBCO and YBa₂Cu_3−xIn_xO_y sintered samples showed a gradual decrease in the critical temperature (T_c) with increasing indium content; however, a T_c value above 80 K was maintained even up to 30 vol.% addition and x = 0.4, respectively. Ba₃Cu₃In₄O₁₂ was detected by X-ray diffractometry and energy-dispersive X-ray spectroscopy as a reaction product for both sintered samples. The normalized J_c under a magnetic field of 0.1 T showed a maximum at x = 0.3. Indium-doped YBCO films prepared by pulsed laser deposition showed a similar dependence of T_c on indium content as the sintered samples.     

Stoichiometry,electrical properties,and electronic structure of Cr1.70Nb2.30Se10

Both powder and single-crystal X-ray investigations show that Cr_{1 + x}Nb_3?xSe₁₀ which belongs to the FeNb₃Se structure type, exists only in a narrow range of stoichiometry close to x=0.70. In contrast to the Fe analog, Cr_1.70Nb_2.30Se₁₀ undergoes no disticnt metal-insulator transition; the resistivity and the thermoelectric power remain of the same order as the temperature is lowered from 300 K to 4.2 K. The thermoelectric powers S of Fe_{1 + x}Nb_3?xSe₁₀ (0.25<x<0.40) and FeVNb₂Se₁₀, however, follow such a temperature T dependence as $\text{S∝}\text{1}\text{T}$ even at room temperature. On the basis of light-binding band calculations, these observations are interpreted in terms of the contribution of Cr or Fe d-orbitals to electron conduction.     

Very-low-frequency ac susceptibility of the spin-glass system PrPy (y ⪅ 1)

The frequency dependence of the ac susceptibility, χ_ac, is reported for off-stoichiometric PrP_y (y ? 1) for 0.003 < f < 10⁴ Hz and for temperatures 1.4 < T < 20 K. For this spin-glass system, T_f, the temperature at which χ_ac is a maximum, reaches a minimum and constant value for f ? 0.1 Hz. The complex initial susceptibility for 0.003 < f < 0.03 Hz is presented.