Magnetic and transport studies of σ-phase Fe50V50 alloys with different thermal history

The influence of the formation conditions of the sigma phase in an equiatomic FeV alloy on the magnetic and electric transport properties is studied. It is found that a sigma phase sample with higher resistivity (subject to a previous long annealing) has a much sharper magnetic transition than one formed after a shorter heat treatment and quenching, although both have very similar magnetic moments and ferromagnetic transition temperatures (T_c ～ 15 K from minimum dM/dT).     

Transport behavior of yttrium substituted polycrystalline La0.7Pb0.3MnO3

The transport properties in La_0.7?xY_xPb_0.3MnO₃ (0.0 ? x ? 0.2) is investigated. The substitution of La³⁺ ions by smaller nonmagnetic Y³⁺ leads to greater spin disorder and induces variations in the magnetotransport behavior. From resistivity versus temperature curves a metal–insulator transition phenomenon is observed at the transition temperature, T_P, decreases as the Y content increases. The resistivity is well fitted using the equation ρ(T) = ρ_nexp[(T¹/T)ⁿ] with n = 1/4 and n = 1/2 at high and intermediate temperatures, respectively. The characteristic temperature T¹ varies with Y content in a manner consistent with the localization model of variable range hopping. Below T_P, resistivity varies as a function of power law contributions, ρ = ρ₀ + ρ₂T² + ρ_5/2T^5/2, corresponding to the electron scattering processes in the ferromagnetic phase.     

Electronic properties of liquid noble metal–Si alloys

The electrical resistivity, ρ, of liquid noble metal–Si alloys (M_1−cSi_c, M = Cu, Ag, Au) increases rapidly with the addition of Si to liquid noble metals. The composition dependence of ρ exhibits a maximum at the composition c = 0.3, 0.4 and 0.4 for liquid Cu–Si, Ag–Si and Au–Si, respectively. A positive to negative transition of thermoelectric power, S, is observed at c = 0.3, 0.15 and 0.1 for liquid Cu–Si, Ag–Si and Au–Si, respectively. Their composition dependence of S reaches a minimum value in the composition range c = 0.6–0.8. The experimental results of ρ and S have been analyzed using the Faber–Ziman theory. The composition dependence of χ for liquid Cu–Si and Au–Si alloys exhibits a clear deviation from a straight line on the noble metal-rich side. As a result, liquid Cu–Si and Au–Si alloys have a minimum in the composition dependence of paramagnetic susceptibility.     

Preparation and properties of chalcogenide glasses in the GeS2–Sb2S3–CdS system

In searching for new kind of photoelectric material, chalcogenide glasses in the GeS₂–Sb₂S₃–CdS system have been studied and their glass-forming region was determined. The system has a relatively large glass-forming region that is mainly situated along the GeS₂–Sb₂S₃ binary side. Thermal, optical and mechanical properties of the glasses were reported and the effects of compositional change on their properties are discussed. These novel chalcogenide glasses have relatively high glass transition temperatures (T_g ranges from 566 to 583 K), good thermal stabilities (the maximum of deference between the onset crystallization temperature, T_c, and T_g is 105 K), broad transmission region (0.57–12 μm) and large densities (d ranges from 2.99 to 3.34 g cm^?3). These glasses would be expected to be used in the field of rare earth doped fiber amplifiers and nonlinear optical devices.     

Mössbauer study of multiphase iron oxide composites

A representative nanocomposite made of ferrimagnetic γ-Fe₂O₃ and antiferromagnetic α-Fe₂O₃ nanoparticles with estimated weight fractions of ～90 and ～10%, respectively, and grown in transparent silica xerogels has been characterized by Mössbauer spectroscopy with respect to the temperature. The changes of the hyperfine parameters of the different subspectral components have been compared with other macroscopic magnetic and structural measurements. This comparison has allowed us to identify two different superparamagnetic transitions, located at ～50 K and at ～250 K, as well as the phases involved in each transition.     

Pressure-induced suppression of ferromagnetic phase in LaCoO3 nanoparticles

Magnetic and structural properties of nanocrystalline LaCoO₃ with particle size ranging from 25 to 38 nm, prepared by the citrate method, were investigated. All nanoparticles exhibit ferromagnetism below T_C ≈ 85 K. It was found that the unit-cell volume increases monotonically with decreasing particle size and ferromagnetic (FM) moment increases simultaneously with lattice expansion, whereas T_C remains nearly unchanged. It appears that both magnetic and structural properties of LaCoO₃ nanoparticles are size-dependent due to the surface effect. On the other hand, an applied pressure suppresses strongly the FM phase leading to its disappearance at ～11 kbar. Remarkably, the T_C does not change visibly under pressure. Our data reveal that the ferromagnetism in LaCoO₃ nanoparticles, likely related to the intermediate-spin (IS) Co³⁺ state, is simply controlled by the unit-cell volume. Within this scenario, the FM coupled IS states appear/disappear with expanding/compressing the lattice and/or Co–O bonds.     

Interplay between spherical periodicity and angular correlation

We report on structural properties and the resistivity of amorphous (In₅₀Sb₅₀)_100−xAu_x (0 < x < 80). Immediately after deposition at T = 4 K the static structure was measured by electron diffraction and the resistivity by a four-probe technique. The structural data can be described as induced by a resonance effect (Hume–Rothery-, Peierls-like) between the electronic system and the forming static structure. If the electronic system is changed, the structure adjusts to the new situation. With increasing Au-content (shrinking the Fermi-sphere diameter), for example, a resonance-induced structural peak at 2k_F shifts to lower scattering values. By analyzing the static structure in even more detail, indications of angular correlations appear, quite similar as has been observed in amorphous precursor alloys of quasicrystals. After deposition the resistivity is quite large at the In₅₀Sb₅₀ – rich side. Annealing alloys with x > 0 gives a sharp decrease by roughly 10% around T = 160 K which is interpreted as a separation into two amorphous phases. One which is In₅₀Sb₅₀ – rich or may even consist of pure In₅₀Sb₅₀, and another one which is enriched by Au. Around T = 300 K there is a second resistivity drop, interpreted as the crystallization of a spherically-periodic ordered Au-rich metallic phase which itself can be described as a so-called amorphous Hume–Rothery phase, stabilized by the electronic system.     

Effects of hydration,annealing, and melting on heat transport properties of fused quartz and fused silica from laser-flash analysis

Thermal diffusivity (D) at high temperature (T) was measured from 15 samples of commercial SiO₂ glasses (types I, II, and III with varying hydroxyl contents) using laser-flash analysis (LFA) to isolate vibrational transport, in order to determine effects of impurities, annealing, and melting. As T increases, D_glass decreases, approaching a constant (~ 0.69 mm²s^? 1) above ~ 700 K. From ~ 1000 K to the glass transition, the slope of D is small but variable. Increases of D with T of up to 6% correlate with either low water and/or low fictive temperature and are attributed to removal of strain and defects during annealing. Upon crossing the glass transition, D substantially decreases to 0.46 mm²s^? 1 for the anhydrous melt. Hydration decreases D_glass, makes the glass transition occur over wider temperature intervals and at lower T, and promotes nucleation of cristobalite from supercooled melt. Due to the importance of thermal history, a spread in D of about 5% occurs for any given chemical type. Combining prior steady-state, cryogenic data with our average results on type I glass provides thermal conductivity (k_lat = ρC_PD) for type I: k_lat increases from ~ 0 K, becoming nearly constant above 1500 K, and drops by ~ 30% at T_g. We find that D^? 1(T) correlates with thermal expansivity times temperature from ~ 0 K to melting due to both properties arising from anharmonicity.     

Nano-particles of La0.9Ca0.1MnO3 manganite: Size-induced change of magnetic ground state and interplay between surface and core contributions to its magnetism

The structure, dc and ac magnetic properties of sonochemically prepared nano-particles of La_0.9Ca_0.1MnO₃ compound were studied as compared to those of the bulk sample of the same composition. It is shown that transition to the nanometer sized scale results in a change of the magnetic order from a mixed antiferro + ferromagnetic to a pure ferromagnetic one due to the improvement of crystallinity and suppression of chemical/magnetic disorder in nano-particles. The surface contribution to its magnetism leads to a super-paramagnetic-like ordering supposedly due to the interfacial exchange coupling via surface tunneling of carriers above the Curie point (T_C). Below T_C ∼ 90 K, a notable contribution of surface spin disorder reduces the spontaneous magnetization of the nano-sample as compared to the bulk-one.     

Thermal properties and glass formation in the SiO2–B2O3–Bi2O3–ZnO quaternary system

Lead-free glasses in the SiO₂–B₂O₃–Bi₂O₃–ZnO quaternary system were studied. The glass formation region, as determined by XRD patterns of bulk samples, was limited to glasses having more than 40 mol% of the glass-forming oxides SiO₂ and B₂O₃. Crystalline phases of Zn₂SiO₄ (willemite) were detected in compositions of 30SiO₂ · 10B₂O₃ · 20Bi₂O₃ · 40ZnO and 20SiO₂ · 10B₂O₃ · 25Bi₂O₃ · 45ZnO. Glass transition temperatures (T_g), dilatometric softening points (T_d) and linear coefficients of expansion in the temperatures range of 25–300 °C (α_25–300) were measured for subsystems along the B₂O₃ join of 10, 20 and 30 mol%. For these subsystems, T_g ranged from 411 to 522 °C, and T_d ranged from 453 to 563 °C, both decreasing with increasing Bi₂O₃ content. The measured α_25–300 ranged from 53 to 95 × 10⁻⁷ °C⁻¹, with values increasing with increasing Bi₂O₃ content. The ZnO content had the opposite effect to the Bi₂O₃ content. It appears that Bi³⁺ acts as a glass-modifier in this quaternary system.     

Structural,dielectric and optical properties of transparent glasses and glass-ceramics of SrBi2B2O7

Optically clear glasses of SrBi₂B₂O₇ (SBBO) were fabricated via the conventional melt-quenching technique. The amorphous nature of the as-quenched samples of this compound was confirmed by X-ray powder diffraction (XRD) studies. Its glassy nature was established by differential scanning calorimetry (DSC). However, the optical microscopy revealed the presence of isolated hexagonal shaped crystallites especially at the edges of the as-quenched glasses. The glass plates that were heat-treated around the onset of the glass transition temperature (670 K) for 12 h yielded transparent (～60% transmission) glass-ceramics of SrBi₂B₂O₇ (SBBO) with well defined microstructure. These were found to be textured associated with an orientation factor of about 0.77 (77%). The optical transmission studies carried out in the 100–1200 nm wavelength range confirmed both the as-quenched and heat-treated samples to be transparent from 400 to 1200 nm. The dielectric properties of the as-quenched as well as the heat-treated (670 K/12 h) samples were studied as a function of frequency (100 Hz–10 MHz) at various temperatures (303–873 K). The dielectric dispersion at higher temperatures in the as-quenched glass was rationalized using Jonscher’s dielectric dispersion relations. The prefactor A(T) and the exponent n(T) in the Jonscher’s expression were found to be maximum and minimum respectively around the crystallization temperature (T_cr) of the as-quenched SBBO glasses.     

Magnetic and structural characterization of silver-iron oxide nanoparticles obtained by the microemulsion technique

In the present paper we report the magnetic characterization of silver-iron oxide nanocomposite obtained by the chemical microemulsion method. TEM images and X-ray diffractograms show that the nanocomposite consists of Ag nanoparticles of ～ 7 nm surrounded by a quasiamorphous matrix. The ZFC–FC curves and Mössbauer spectra obtained at different temperatures show a typical evolution of a system composed of weakly interacting nanoparticles with a blocking temperature (T_b) of ～50 K. The analysis of the magnetic data reveals that the matrix is formed by γ-Fe₂O₃ phase with a structural range order of ～2 nm.     

FMR and DSC study of maghemite nanoparticles in PMMA polymer matrix

Nanocomposite samples of poly(methyl methacrylate) (PMMA) polymer with a γ-Fe₂O₃ (maghemite) filler have been synthesised and studied by ferromagnetic resonance (FMR) and differential scanning calorimetry (DSC) methods. Two types of samples have been investigated: containing 5 wt%, 10 wt% of a maghemite filler. DSC measurements have revealed an increase in the glass transition temperature T_g and a decrease in the heat capacity c_p with an increase in the concentration of nanoparticles. A FMR study in the 4–300 K temperature range has shown the presence of an asymmetric spectrum that has been analyzed in terms of two Gaussian-shaped components arising from the assumed magnetic anisotropy of the nanoparticles. The FMR investigation has exposed the temperature range of a superparamagnetic regime (60–290 K) and the blocking temperature of T_B ～ 60 K. In that range a shift in the resonance line δH_r and the linewidth ΔH is related by δH_r ～ (ΔH)ⁿ, where n = 2.79 indicates a fair amount of disorder in the maghemite system. An analysis of the FMR spectra in terms of two component lines has shown the importance of the dipole–dipole interaction for higher concentrations of maghemite and for T > 220 K.     

The iso-structural viscosity,configurational entropy and fragility of oxide liquids

This paper describes how the fragility of a liquid is linked to the ratio between the energy barrier (E_eq) for the equilibrium viscous behavior and that (E_iso) for the non-equilibrium iso-structural viscous behavior. Using the concept of iso-structural viscosity, two functions describing the variation of the configurational entropy (S_c) with temperature (T) are obtained from the Avramov-Milchev (AM) and the Vogel-Fulcher-Tammann (VFT) viscosity equations, respectively. The two S_c(T) functions exhibit different relations to the liquid fragility. The AM S_c(T) function is a power function with the exponent of F ? 1, where F is the AM fragility index. In this case, S_c vanishes at T = 0 K. For the VFT function, S_c vanishes as T is lowered to a finite temperature T₀, whereas it reaches the maximum value S_c,max at infinitively high T. S_c,max is proportional to the VFT fragility index. Thus, the VFT equation is not only a dynamical, but also a thermodynamic model. It is proved that for oxide liquids, the VFT equation describes viscosity data better than the AM equation, provided the pre-exponential factor η₀ is fixed to a generally accepted value, e.g., 10^?3.5 Pa s.     

Effect of laser irradiation on thermal and optical properties of selenium–tellurium alloy

The crystallization parameters such as glass transition temperature (T_g), onset crystallization temperature (T_c), peak crystallization temperature (T_p) and enthalpy released (ΔH_C) of the bulk Se–Te chalcogenide glass has been studied by using Differential Scanning Calorimeter (DSC), under non-isothermal condition at a heating rate of 20 K/min. The values of T_g, T_c, T_p and ΔH_C with and without laser irradiation for different exposure time have been studied. The optical absorption of pristine and laser irradiated thermally evaporated Se–Te films has been measured. The films shows indirect allowed interband transition that is influenced by the laser irradiation. The optical energy gap has been found to decrease from 1.61 to 1.38 eV with increasing irradiation time from 5 to 20 min. The results have been analyzed on the basis of laser irradiation-induced defects in the film.     

Synthesis and characterization of TM-doped CuO (TM = Fe,Ni)

Polycrystalline Cu_1?xTM_xO samples (x = 0 and 0.06; TM = Ni²⁺ and Fe³⁺) were grown using a co-precipitation method. The structural and magnetic properties were investigated by means of temperature dependent magnetic susceptibility and room temperature X-ray powder diffraction (XRPD). The XRPD analyses of the samples reveal the formation of single phase with structure isomorphous to the CuO. Interestingly, T-dependent magnetization shows the reduction of Néel temperature, T_N, from 213 K in the copper oxide to 70 K in the Fe-doped sample (x = 0.06). Because in the Ni-doped samples T_N seems to be unaffected, this decrease in T_N is believed to be due to the different electronic structure of the dopant. The ferromagnetic behavior observed at room temperature in all samples can be related to both the level of oxygen (excess or vacancy) of our samples and to the difference in the magnetic structure of the dopant.     

Effect of mixed transition-metal ions in glasses. Part III: The P2O5–V2O5–MnO system

Electrical and optical properties of phosphate glasses containing vanadium and manganese ions in the xP₂O₅–[(100 − x)(V₂O₅ + MnO)] (PVM) system have been investigated. This is the last article of a III-part series devoted to the electronic properties of phosphate glasses containing a mixture of transition ions. The first article was devoted to the electrical conductivity of glasses having the general composition: xP₂O₅–[(100 − x)(V₂O₅ + Fe₂O₃)] (PVF). Competitive transport of small polarons on V and Fe ion sites was found to contribute to a mixed transition-ion effect (MTE) in PVF glasses. Several features of MTE were found to be similar to the well known mixed alkali effect, observed in glasses containing two alkali ions. In the second article, optical absorption and electronic conduction of xP₂O₅–[(100 − x)(Fe₂O₃ + MnO)] (PFM) glasses were reported. In the absence of competitive transport between the two transition ions (since Mn ions were determined not to contribute to dc conduction), MTE was not observed. The most important feature of PFM glasses was a sharp increase in resistivity at a critical concentration of iron ions, similar to ‘metal–insulator transition’ (MIT). In the present article, we report a resistivity transition in PVM glasses which is similar to that exhibited by the glasses of the PFM series. While Fe ions contributed the carriers in the PFM glasses, V ions serve the same purpose in the PVM compositions. As the concentration of vanadium ions, n_V, is decreased in the composition range 0.82 > n_V > 0.40, resistivity (ρ) increases marginally. For glasses with 0.2 < n_V < 0.40, resistivity and the activation energy for dc conduction (W) increase sharply with decreasing n_V, marking the incidence of an MIT-type transition. As in the PFM glasses, the observation of MIT coincides with the transformation of small polarons to small bipolarons, which is confirmed by the shifting of the small polaron optical absorption band to higher energies with decreasing V concentration.     

Vapour diffusion growth and characterisation of fully deuterated triglycine sulphate (ND2CD2COOD)3D2SO4

A new vial-in-vial vapour diffusion method for growing single crystals of fully deuterated triglycine sulphate (TGS) has been developed. Single crystals of hydrogenous TGS were also grown for comparison purposes. The crystals have been characterised using x-ray diffraction and differential scanning calorimetry. The phase transition temperature was 334.0±0.5 K for fully deuterated TGS compared to 322.3±0.3 K for hydrogenous TGS. These values compare well with the expected T_C.     

Fast relaxation processes in glasses as revealed by depolarized light scattering

Applying tandem-Fabry-Perot interferometry together with a double monochromator, depolarized light scattering spectra were measured in order to investigate the fast relaxation processes and vibrations in molecular, ionic and polymeric glasses in the 1–5000 GHz range covering temperatures from the glass transition temperature T_g down to some 10 K. In addition to the boson peak, the spectra reveal quasi-elastic contributions that we attribute to (i) a nearly constant loss in the frequency range below ≅10 GHz and (ii) a power-law contribution with positive exponent α at higher frequencies. In the majority of glasses the latter may be attributed to thermally activated dynamics in asymmetric double well potentials as previously found for the light scattering spectra in silica. Following the Gilroy–Phillips model the exponent α shows a master curve as a function of T/V₀ for the various glasses, where V₀ specifies the width of the exponential distribution of barriers g(V), i.e., g(V) ∝ exp(−V/V₀). The parameter V₀ is found to be ∼T_g/2 in most cases. The relative strength of the dynamics in asymmetric double well potentials and the nearly constant loss contribution is different in the glasses studied.