Magnetic clusters in Cu1 ? x In1 ? x Fe2x Se2 solid solutions

We have synthesized Cu_{1 ? x} In_{1 ? x} Fe_2x Se₂ solid solutions (CuInSe₂-FeSe join). The ??-FeSe end-member of the solid-solution system is an antiferromagnet and a superconductor with a superconducting transition temperature of 8 K. The magnetic properties of the solid solutions were studied at temperatures from 5 to 300 K in magnetic fields H = 3.98 and 3184 kA/m. By fitting experimental ??(H) magnetization data with Langevin and Curie equations, we determined the magnetic moment, size, and concentration of the clusters and the concentration of noninteracting Fe²⁺ ions. The magnetic moment of the Fe²⁺ ions in the clusters is ??0.8??_B at 2x = 0.06 and 0.08, with a tendency to decrease with increasing iron content. This points to antiferromagnetic exchange interaction, which becomes stronger with increasing cluster size. The number of clusters in the materials increases with iron content up to a critical level 2x _c < 2x = 0.06. In the range 2x = 0.06 to 0.10, the cluster size increases until a limiting composition determined by the boundary of the solid-solution range is reached.     

Photoinduced Superconductivity in La2?x Sr x CuO4

Dependences of the superconducting transition temperature (T _c) and chemical potential shift (????) on the hole concentration (n _h ) for La_2?x Sr _x CuO₄ cuprate is obtained taking into account of canonical two-band BCS model containing Fermi surfaces of p and d holes. The shift of chemical potential (??) leads to the curve T _c(n _h ) with a maximum. The dependences of T _c(n _h ) for our system compared with available experimental results. Downward shift (????) of the electron chemical potential (??) with the hole concentration (n _h ) have been found. Self-consistent equations for superconducting order parameters ( $\bar{\Delta}_{p}$ and $\bar{\Delta}_{d}$ ) for both p and d holes are derived using Green??s function and equation of motion method. The temperature dependences of superconducting gaps and specific heat based on this model are also calculated. The enhancement of T _c due to doping is observed.     

The Nodal SDW Gap and the Superconducting Gap in?BaFe2?x Co x As2

The spin density wave (SDW) transition in BaFe₂As₂ and the superconducting transition in BaFe_1.84Co_0.16As₂ were investigated by Raman scattering. The symmetries of the nodal SDW gap at 400?cm^?1 and the superconducting gap at 75?cm^?1 are both?B _2g. The superconducting coherent peak energy is smaller than the gap energy of the hole pocket, indicating that the peak is the resonant peak in the S _?? superconductor. The superconducting symmetry is given by B _2g in the orbital combination and A _1g (S _??) in the momentum space. The exchange interaction energies are estimated from the two-magnon peak.     

Magnetic Properties of Mg x Mn1?x Fe2O4 Nanoferrites

We report the magnetic properties of Mg _x Mn_1?x Fe₂O₄ (0??x??1.0) nanosize compounds with particle sizes between 8?nm and 15?nm. The evolutions of the properties as a function of composition have been investigated by X-ray diffraction, M?ssbauer, and SQUID magnetometry. Pure cubic spinel could be obtained under a low reaction temperature of 200°C in all the samples. Impurity phases have been observed in compounds annealed at above 900°C. Magnetic relaxation is observed in samples with particles of about 8?nm. The spectra with particle sizes larger than 10?nm could be fitted with at least two sextets attributed to Fe³⁺ ions on tetrahedral (A) and octahedral?(B) sites. The magnetization measurement indicates superparamagnetic behavior in nanosized compounds.     

Magnetic Transport Properties in GdBa2Cu3?x Ru x O7?δ Superconducting Phase

Bulk superconducting samples of type GdBa₂Cu_3?x Ru _x O_7?δ phase, Gd-123, with?x ranging from 0.0 to 0.15 were prepared by the conventional solid-state reaction technique. X-ray powder diffraction (XRD) and the electrical resistivity measurements were performed in order to investigate the effect of Ru⁴⁺ ions substitution on Gd-123 phase. Enhancement of the phase formation and the superconducting transition temperature T _c for GdBa₂Cu_3?x Ru _x O_7?δ phase up to x=0.05 was observed. The effect of magnetic field up to 4.4?kG on the electrical resistivity behavior of the prepared samples was studied to investigate the flux motion of this phase. The derived flux pinning energy?U, based on the thermally activated flux creep TAFC model, decreased with increasing the magnetic field?B. The flux pinning energy followed the exponent behavior as U(B)～B ^?β . The superconducting transition width ΔT increased as the magnetic field increased, showing the scaling relation as ΔT～B ⁿ . Using Ambegaokar and Halperin AH theory, the magnetic field and temperature dependence of U was found to be U(B,T)～ΔTB ^?η , η=β+n. The critical current density J _c (0) enhanced up to x=0.05, beyond which it decreased with further increase in Ru-content.     

Structural and Magnetic Studies of Ca2?x Sm x MnO Compounds (x=0–0.4)

The Ca_2?x Sm _x MnO₄ (x=0?C0.4) compounds were synthesized by a solid?Csolid method and characterized by XRD at room temperature. The XRD profiles were indexed with a tetragonal structure (I/4mmm space group) for x??0.3 and orthorhombic one (Pnma space group) for x=0.4. The magnetic measurements were studied as a function of temperature (T=2?C300?K) and applied field (?? ₀ H=0?C10?T). When the temperature decreases, each compound has shown first a ferromagnetic?Cparamagnetic (FM?CPM) transition and then an antiferromagnetic?Cferromagnetic (AFM?CFM) one. The transition temperatures are found to be Sm-doping dependent. For all compounds, a spin-glass phenomenon is evidenced by FC/ZFC magnetization curves.     

XRD, Magnetic and M?ssbauer Spectral Studies of?Ag x Ni1?x Fe2O4 Ferrite Nanoparticles

Nanophase Ag _x Ni_1?x Fe₂O₄ (x=0,0.2) ferrites were prepared by glycothermal method. The NiFe₂O₄ (x=0) nanosized sample was also produced by high-energy ball milling for comparison of properties. Structural investigations of the samples were carried out by X-ray diffraction. The experiment reveals that pure Ag-Ni ferrite materials with grain sizes of about 8 nm can be obtained after annealing at relatively low temperature of about 500?°C. The nanosized compounds produced by glycothermal reaction indicate superparamagnetic behavior. A higher value of coercive field (910 Oe) is observed in x=0 milled sample with similar particle size. The zero field cooled (ZFC) and field cooled (FC) magnetization measurements reveals spin glass like behavior of the nanosized compounds.     

Electrical properties of n-Cd1 ? x Co x GeAs2 (x = 0.05–0.15) at high pressures

The resistivity (??) and Hall coefficient (R _H) of n-Cd_{1 ? x} Co _x GeAs₂ with x = 0.05?C0.15 were measured as functions of temperature (at atmospheric pressure) and pressure (up to p = 7 GPa). The temperature-dependent ?? data were used to determine the ionization energy of a cobalt-related impurity center. In the ??(p) and R _H(p) curves, we identified structural semiconductor-semiconductor phase transitions under both increasing and decreasing pressure. Using a mixed-phase structure-effective medium model, we assessed the dynamics of the variation of phase composition with pressure.     

Thermodynamic properties of Ca1 − x Er x F2 + x and Ca1 − x Yb x F2 + x heterovalent solid solutions

The heat capacity of single crystals of the Ca_{1 ? x} Er _x F_{2 + x} (x = 0.05, 0.10) and Ca_0.95Yb_0.05F_2.05 fluorite solid solutions was determined by adiabatic calorimetry in the temperature range 55–300 K. The results were used to obtain temperature dependences of the Debye characteristic temperature, entropy, and enthalpy for the solid solutions.     

Study of Magnetic Properties in Spinels Co x Zn1?x Cr2O4 systems

The exchange interactions J _AA(x) and J _AB(x) are calculated by using a probability law for the Co _x Zn_1?x Cr₂O₄. The magnetic properties of a diluted ferromagnetic spinels Co _x Zn_1?x Cr₂O₄ system are investigated by using the high-temperature series expansions combined with the Padé approximants. The magnetic phase diagram, i.e., T _N versus?x, and the critical exponent associated with the magnetic susceptibility (??) are deduced.     

Gilbert Damping Constant of Quaternary Co2MnAl1?x Sn x Heusler Alloy Thin Films

Polycrystalline quaternary Co₂MnAl_1?x Sn _x films with x=0.25, 0.5, 0.75,?1 were prepared at room temperature using magnetron sputtering technique on SiO₂ substrates and post-annealed at various temperatures. We investigated the crystal structures, magnetic properties, and magnetic damping constants (??) of the prepared films. Out-of-plane angular dependences of the resonance field and the linewidth of the ferromagnetic resonance spectra were measured and analyzed using the Landau?CLifshitz?CGilbert equation to determine the magnetic properties and damping constant. Co₂MnAl_0.75Sn_0.25 and Co₂MnSn films had A2 ordered crystal structure while Co₂MnAl_0.25Sn_0.75 and Co₂MnAl_0.5Sn_0.5 films had A2 ordering up to 400?°C and 300?°C annealing temperature, respectively, and they had B2 ordering for the remaining temperatures. Also the crystal structure deteriorated at 600?°C for all of the film systems. The saturation magnetization, M _S , of films increased with annealing temperature till 400?°C except Co₂MnAl_0.5Sn_0.5 in which M _S increased till 500?°C, which is consistent with the structural analysis. The effective magnetization was obtained from the FMR spectra and it was found that it decreased with increasing Sn-concentration and reached a minimum value at Co₂MnAl_0.25Sn_0.75 composition. Lastly, Co₂MnAl_1?x Sn _x films annealed at 500?°C showed the best crystal ordering. The lowest ?? value was 0.008 and obtained from Co₂MnAl_0.5Sn_0.5 films annealed at 500?°C.     

The dielectic response of K x Al x Ti8−x O16 and K x Mg x/2 Ti8−x/2 O16

Materials of the hollandite structure with the general formulae K_x Al_x Ti_8–x O₁₆ and K_x Mg_x/2 Ti_8–x/2 O₁₆ have been synthesized in the composition range 1.6x2.0 and their dielectric properties have been measured in the temperature range 77 to 800 K and the frequency range 10^–3 to 10⁶ Hz. The observed response shows a whole range of features characteristic for both charge carrier and dipolar polarization processes and these are seen as being associated with the one-dimensional transport in channels in the hollandite structure. At low temperatures the dominant response is the universal dielectric relation in which the loss follows the law x() ^n–1, with the exponent n<1 and equal specifically to approximately 0.7. This is followed at 120 to 180 K by a distinct loss peak superimposed on the above law, and finally at higher temperatures by a region of strong dispersion which is associated with strongly interacting many-body processes between charged carriers restricted by defects to move in limited regions of the channels.     

Superconducting and Normal-State Properties of (Y1?x Bi x )Ba2Cu3O7?δ (x=0.00–0.30)

The effect of Bi in (Y_1−x Bi _x )Ba₂Cu₃O_7−δ for x=0.00–0.30 has been investigated by powder X-ray diffraction method, dc electrical resistance measurements and scanning electron microscopy. The resistance versus temperature curves showed metallic behavior for samples with x≤0.20 and semiconductor-like behavior for x=0.25 and 0.30. The resistance versus temperature curves showed onset transition temperature (T _{c onset}) between 89 and 93 K and a broadening of the superconducting-transition width with increasing Bi content for all samples. T _{c onset} decreased linearly indicating a pair-breaking mechanism in T _c suppression for x=0 to 0.3. The non-systematic broadening of the transition width may be due to the inhomogeneities in the samples as Bi content was increased. No substitution of Bi for Y was observed.     

Structure and Charge-Transfer Mechanism in Y1?x Ca x Ba2Cu3O7?δ Through Direct Doping

Here we study the effect of Ca doping on the charge-transfer mechanism of polycrystalline YBa₂Cu₃O_7??? compound. The samples of composition Y_1?x Ca _x Ba₂Cu₃O_7??? (x=0.00,0.05,0.10,0.20 and 0.30) are synthesized through standard solid-state reaction route. Carrier doping is controlled by annealing of samples in oxygen and subsequently in reducing atmosphere. Samples are investigated using resistivity, dc magnetization (M?CT) and magnetization with field (M?CH) measurements. With increase of Ca the transition temperature (T _c) decreases in oxygenated samples, whereas the same increases in reduced samples. Further reduction of samples at higher temperatures (>600?°C) though results in non-superconducting nature up to Ca concentration of x=0.20, the x=0.30 sample is superconducting below 30?K. This provides a remarkably simple and effective way to study the relationship between structure, superconductivity, and associated electronic properties. Variation in Cu₁?CO₄, Cu₂?CO₄ and Cu₂?CO₂ bond lengths with oxygen content, is seen through the structural refinement of XRD pattern. The effective coordination of Cu₂ atom with oxygen changes with the change in these bond lengths and hence the holes in the CuO₂ planes. The charge-transfer mechanism from CuO _x chains to CuO₂ planes and thus effective hole doping is discussed in the context of the results observed.     

Magnetic Properties of Ru-rich Ru2?x Fe x CrZ (Z=Si, Ge)

The properties of Heusler compounds Ru_2?x Fe _x CrGe are investigated and compared with those of Ru_1.9Fe_0.1CrSi. Ru₂CrGe is confirmed to exhibit an antiferromagnetic transition with Ne??l temperature 13?K by magnetic susceptibility and specific heat measurements. When Fe is substituted for Ru, a peak in the magnetic susceptibility is observed, and in the lower temperature region irreversibility between temperature dependences of magnetization for zero-field-cooling and field-cooling conditions is found. Nevertheless, in specific heat of Ru_1.9Fe_0.1CrGe, there is no anomaly to indicate phase transition. The specific heat is almost identical to that for Ru_1.9Fe_0.1CrSi. The above results demonstrate that the magnetic states in the low temperature region of Fe-substituted samples are spin-glass-like states. Slight substitution of Fe for Ru destroys the long-range-order and lead to peculiar spin-glass-like states.     

Appearance and Disappearance of Superconductivity with?Fe?Site?Co?Substitution in SmFe1?x Co x AsO (x=0.0 to 1.0)

We report synthesis, structural details, and magnetization of SmFe_1?x Co _x AsO with?x ranging from 0.0 to 1.0 at a close interval of 0.10. It is found that Co substitutes fully at the Fe site in SmFeAsO in an iso-structural lattice with a slightly compressed cell. The parent compound exhibited known spin density wave (SDW) character below 150?K. Successive doping of Co at the Fe site suppressed the SDW transition for x=0.05, and later induced superconductivity for x=0.10, 0.15, and 0.20, respectively, at 14, 15.5, and 9?K. The appearance of bulk superconductivity is established by wide open isothermal magnetization M(H) loops. For higher content of Co, i.e. x??0.30, superconductivity is not observed. Clearly, the Co substitution at the Fe site in SmFe_1?x Co _x AsO diminishes the Fe SDW character (x=0.05), introduces bulk superconductivity for?x from 0.10 to 0.20, and finally becomes a non-superconductor. The SmCoAsO also exhibits a secondary AFM-like transition below or around 50?K. The reported AFM ordering of Sm spins is seen from heat capacity C _p(T) at 4.5?K and the same remains invariant with Co doping in SmFe_1?x Co _x AsO. Further, the FM ordering of Co-spins (seen in magnetization measurements) is not evident in C _p(T) studies, suggesting weak correlations between ordered Co-spins in these systems.     

Metamagnetic Phase Transitions in La1−x Nd x Mn2Si2

Experimental measurements for the magnetization at various temperatures are analyzed using a mean field model for the purely ferromagnetic spin configuration near the metamagnetic phase transitions in La_1?x Nd _x Mn₂Si₂ (x=0.3) at a constant magnetic field (50 mT). By fitting the temperature dependence of the magnetization from the free energy in the mean field model to the experimental data for this compound, the coefficients in the free energy expansion are determined. Our analysis of the magnetization describes a first-order character of the metamagnetic transition in La_1?x Nd _x Mn₂Si₂ (x=0) on the basis of the mean field model studied here.     

Electrical and microstructural properties of Bi2−x Sb x Te and Bi2−x Sb x Te2 foils obtained by the ultrarapid quenching process

This paper is concerned with the feasibility and reproducibility of the ultrarapid quenching process used to fabricate Bi_2–x Sb _x Te and Bi_2–x Sb _x Te₂ alloys for thermoelectric applications. Microstructural properties of the materials, obtained in the shape of foils, were studied concerning the phase analysis, cell parameters, texture, and microstructure observations. The Bi_2–x Sb _x Te alloys were found to have the (2 0 3) texture. The (2 0 4) texture, with an additional (1 1 0) component for x values greater than 0.4, was predominant for Bi_2–x Sb _x Te₂ foils. The electrical properties of these materials were then characterized by measuring the Seebeck coefficient, Hall coefficient, and electrical resistivity. It was found that Bi_2–x Sb _x Te foils changed from n- to p-type for an x-value of about 1.2. A maximum Seebeck coefficient, ||, of 36×10^–6 V K^–1 was measured for Bi₂Te. In the case of Bi_2–x Sb _x Te₂ foils, the change from n- to p-type was observed for an x value of about 1. A maximum Seebeck coefficient, ||, of 32×10^–6 V K^–1, was measured for Bi_1.4Sb_0.6Te₂. Measurements of the temperature-dependent electrical resistivity, Hall and Seebeck coefficients of the foils were carried out and the analysis revealed a semi-metallic behavior.     

Direct Observation of k∥ and k⊥ Stripe Excitations in La2?x Sr x CuO4

The superconducting transition temperature in the cuprate is pointed to be enhanced by the self-organized spin-charge separated stripes. The magnetic excitations are very different in k?? and k???stripes. Therefore if one detects the different excitations, it is a clear evidence of the stripe. Neutron scattering, however, always observes the mixed excitations. We first succeeded to detect the k???stripe excitations of the separated dispersion induced by the super-lattice structure of the stripe in distinction from the k???stripe excitations by two-magnon Raman scattering. The stripe structure is ubiquitous in the whole carrier density range at low temperatures.