Specific effects of nanometer scale size on magnetic ordering in La1−xCaxMnO3 (x = 0.1, 0.3 and 0.6) manganites

To check the impact of nano-size originated effects on the magnetic ordering in doped manganites, X-band electron magnetic resonance measurements were performed on nanometer sized and bulk samples of La_1?xCa_xMnO₃ (x = 0.1, 0.3 and 0.6). The model fittings of EPR signal parameters and complementary magnetic measurements indicate that bulk La_0.9Ca_0.1MnO₃ shows inhomogeneous confinement of charge carriers, leading to its mixed magnetic ordering. The carriers mobility within impurity-like band and ferromagnetic (FM)-like ground state are observed on nanocrystals of the same composition. The bulk La_0.7Ca_0.3MnO₃ demonstrates homogeneous FM order and band-like carrier mobility. The surface magnetic disorder in its nano-counterpart leads to appearance of two magnetic phases – a core phase (bulk-like in properties) and a surface phase with notably reduced temperature of magnetic ordering; neither double-exchange interaction nor carriers mobility exist between these phases. The size reduction induced effects in La_0.4Ca_0.6MnO₃ are: an increase of surface FM-like component and a decrease of the charge-ordering temperature. These findings allow us to conclude that the nano-scale effect on magnetic ordering in La_1?xCa_xMnO₃ system weakens progressively upon stabilization of the low temperature magnetic ground state with Ca-doping.     

Studies on electrical conduction behavior of La1‐3xCaxBaxSrxMnO3 synthesized by chemical route

In the manganite La_1‐xM_xMnO₃ (M = Ca, Ba, Sr) the doping concentration introduces a mixed valency (Mn³⁺, Mn⁴⁺) which governs the magnetic and electrical properties of the compound. The perovskite oxides La_1‐3xCa_xBa_xSr_xMnO₃ (x = 0.00, 0.05, 0.10) were prepared by chemical method. Single‐phase formation is confirmed by XRD studies. The electrical behavior of compositions with x = 0.00, 0.05 and 0.10 in the system La_1‐3xCa_xBa_xSr_xMnO₃ was studied in the temperature range 300‐420 K. It is observed that conductivity decreases with increasing temperature as well as dopants concentration. Metallic behavior of these compositions decreases with increasing dopants concentration (x). The microstructures of these samples have been characterized using scanning electron microscopy (SEM). (© 2007 WILEY ‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Effect of Lithium Doping in the Bi-2212 Superconducting System

Samples with the nominal composition Bi₂Sr₂Ca_1–xLi_xCu₂O_y (x = 0.2,0.4, and 0.6) and Bi₂Sr₂CaCu_2−x L1_xO_z (x = 0.4, 0.6, and 1.0) were prepared by the solid state reaction method. The role of L1 at both Ca and Cu sites in the Bi-2212 composition were studied. From the X-ray diffraction data it was found that the L1-doped at the Ca site increases the c-axis and that doped at the Cu site decreases the c-axis. From the D.C. four-probe resistivity data it was found that Li-doped at the Ca site reduces the T_c and the L1-doped at the Cu site gives T_c(0) above the liquid nitrogen temperature. It was observed that the L1 doping reduces the melting point of Bi-2212 composition. The presence of Lithium was confirmed by inductive the coupled plasma method.     

Resistance and inductance studies of Bi2−xPbxCa2Sr2Cu3Oy superconducting compounds

Bi_2−xPb_xCa₂Sr₂Cu₃O_y compounds with (x = 0, 0.1, 0.2, 0.25, 0.3 and 0.4) were prepared by solid state reaction. The resistance and inductance results showed that transition temperature increases with increase in lead concentration. The highest value of T_c(0) observed is 109 K for Bi_1.70Pb_0.30Ca₂Sr₂ · Cu₃O_y. The change of inductance per unit volume at liquid nitrogen temperature as a function of composition showed maximum change in x = 0.30 Pb-doped sample. The results showed that the inductance change is maximum in samples which also showed highest T_c.     

High-pressure synthesis and magnetic properties of Bi<Subscript>0.5</Subscript>D<Subscript>0.5</Subscript>MnO<Subscript>3</Subscript> (<Emphasis Type="Italic">D</Emphasis> = Pb,Ba) solid solutions

Perovskites Bi_0.5D_0.5MnO₃(D = Pb, Ba) were prepared under high pressure (4 GPa) at 1200–1300°C. According to the X-ray diffraction data, crystalline Bi_0.5Pb_0.5MnO₃ has a tetragonal unit cell with the parameters a = 3.940 Å and c = 3.800 Å, whereas Bi_0.5Ba_0.5MnO₃ crystals are cubic with a = 3.940 Å. It is concluded from magnetic studies that lead-containing manganite is an antiferromagnet with T_N = 120 K, whereas Bi_0.5Ba_0.5MnO₃ is a spin glass with spin-freezing temperature T _f = 38 K. Both compounds are decomposed upon heating in air at temperatures above 500°C. With the use of synthesis in air, Bi_0.5Ca_{0.5 ? x}D _x MnO₃ solid solutions with x as high as 0.25 were obtained.     

Crystal growth and characterization of two-leg spin ladder compounds: Sr14Cu24O41 and Sr2Ca12Cu24O41

Single crystals of Sr_14−xCa_xCu₂₄O₄₁ (x=0 and 12) are grown by the travelling solvent floating zone technique using an image furnace. The grown crystals are characterized for their single crystallinity by the X-ray and Neutron Laue method. The magnetic susceptibility measurements in Sr₁₄Cu₂₄O₄₁ show considerable anisotropy along the main crystallographic axes. Low-temperature specific heat measurement and DC susceptibility measurement in Ca-doped crystal showed antiferromagnetic ordering at 2.8 K at ambient pressure. High-pressure AC susceptibility measurement on Ca-doped crystal showed a sharp superconducting transition at 2 K under 40 kbars. T_c onset reached a maximum value of 9.9 K at 54 kbars. The bulk superconductivity of the sample is confirmed by the high-pressure AC calorimetry with T_c max=9.4 K and T_N=5 K at 56 kbars.     

Growth of bulk zinc chalcogenide single crystals from the vapour phase and their use for laser screens of projection colour television

Results on ZnSe, ZnSe_xS_1−x and ZnS crystal growing from the vapour phase up to 7.5 cm³ in volume are described. Crystals were grown on sapphire, ZnS, ZnSe_xS_1−x and quartz glass substrates without a contact of the growing crystal with a growth ampoule and using the molten tin as a heating medium. Large high-purity crystals with a density of etch pits of 10³ cm⁻² were obtained They exhibited an effective exciton luminescence and rather high radiation efficiency (30 ± 10% for ZnSe at T = 77 K). This made it possible to use these crystals for fabricating laser screens for a cathode ray tube. The main laser parameters obtained on a ZnSe screen at T = 80 and 300 K using a 75 keV electron beam excitation are presented. The light power output reached 0.8 W at T = 80 K; this allowed to obtain a 10 cd · m⁻² TV image of 1.5 × 2 m² in area.     

Evidence of hopping of charge carriers in the clustered state of manganites

The electrical resistivity and magnetization measurements as a function of both temperature and magnetic field provide clear evidence for three temperature intervals in which the magnetic and transport properties are altered in the La_0.55Y_0.15Ca_0.3MnO₃ compound. The intermediated-T regime, called clustered state, involves the presence of both ferromagnetic and paramagnetic phases and is shown to be closely related to the colossal magnetoresistivity effect. An extra hopping conducting process is proposed to exist in this clustered state. We use both the Abeles’ model and microstructural parameters to estimate the hopping activation energy between ferromagnetic clusters. The size (density) temperature dependence and the shape of the ferromagnetic metallic clusters are revealed to play a major role in the clustered state of this manganite.     

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.     

Ferromagnetic insulating and reentrant spin glass behavior in Mg doped La0.75Sr0.25MnO3 manganites

In this study, the effect of Mg substitution on structural, magnetic and electrical properties of La_0.75Sr_0.25Mn_1?xMg_xO₃ and La_0.75Sr_0.25?xMg_xMnO₃ (nominal compositions) samples are investigated by XRD, Ac susceptibility and electrical resistivity measurements. It is found that Mg does not replace La in the perovskite lattice. Also the results show that by increasing Mg doping levels, the paramagnetic–ferromagnetic and metal–insulator transition temperatures decrease. The reason for decreasing transition temperatures with increasing Mg concentration is, that the long-range FM order has been destroyed by the Mg, which is randomly occupying Mn site. This leads to the suppression of double-exchange interaction in the Mn³⁺–O–Mn⁴⁺ networks. Also the reentrant spin glass (RSG) state accompanied by FM transition, exists in high doped samples. The RSG state could be understood on the basis of double exchange ferromagnetic interaction in Mn³⁺–O–Mn⁴⁺ and super-exchange antiferromagnetic interaction in the Mn⁴⁺–O–Mn⁴⁺ networks.     

Paramagnetic behavior of Zn1−xMnxO at room temperature

Mn‐doped ZnO were synthesized by solid state reaction and sol‐gel method respectively. It was found that samples synthesized by solid state reaction containing Mn₂O₃ and MnO₂ are a mixture of ferromagnetic and paramagnetic phases. Contrary, samples without second phases were found to be paramagnetic at room temperature. According to previous report, interface effects between Zn‐rich Mn₂O₃ and MnO₂ interfaces may be the origin of the ferromagnetic behavior observed in our samples prepared by solid reaction, so the alloy of Zn_1−xMn_xO may be paramagnetic at room temperature. Prepared by sol‐gel technique, the samples without second phases in the XRD patterns are also room‐temperature paramagnetic. Therefore we believe that the magnetism of Zn_1−xMn_xO is paramagnetic at room temperature. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Studies of local fields in the Pr1−xCaxMnO3 system using perturbed angular correlation spectroscopy

In this work a local probe study using perturbed angular correlation spectroscopy in Pr_1?xCa_xMnO₃ small bandwidth manganites is presented. The magnetic hyperfine field study across the Pr_1?xCa_xMnO₃ phase diagram is provided and the results are discussed in terms of mixed-phase ferromagnetic/antiferromagnetic states versus canted antiferromagnetism.     

Effective electro‐optic coefficient of (1–x)Pb(Zn1/3Nb2/3)O3–xPbTiO3 single crystals

Refractive indices and effective electro‐optic coefficient γ_c of (1–x)Pb(Zn_1/3Nb_2/3)O₃–xPbTiO₃ (PZN‐xPT, x = 0.05, 0.09 and 0.12) single crystals were measured at 532 nm wavelength. Orientation and temperature dependences of the electro‐optic coefficient were investigated. Large electro‐optic coefficient (γ_c = 470 pm/V) was observed in [001]‐poled PZN‐0.09PT crystal. More importantly, γ_c of tetragonal PZN‐0.12PT is almost unchanged in a temperature range −20 ∼ 80 °C. The γ_c of PZN‐xPT single crystals are much higher than that of widely used electro‐optic crystal LiNbO₃ (γ_c = 20 pm/V). These results show that PZN‐xPT single crystals are very promising materials for electro‐optic modulators in optical communications.     

Magnetic Susceptibility in 2D Superconductor NaxHfNCl System

Abstract

Superconducting critical temperature, T_c and the shielding volume fraction, SVF, of layered nitride superconductor Na_xHfNCl have been studied as a function of x, i.e. Na concentration. Although T_c decreases gradually with increasing x from 20.0 K at x=0.11 to 16.5 K at x=0.85, SVF has a sharp peak around x=1/6, where strong coupling between local ordering of Na atoms and Fermi surface instability can be expected. Structural disorder in the samples above x=0.5, observed by powder neutron diffraction, does not affect the superconductivity appreciably. Electronic specific heat coefficient, γ, is estimated to be about 7.7 mJ/mol/K² by its difference of magnetic susceptibility between HfNCl and Na_0.5HfNCl. The γ value is relatively small compared with the high T_c value, revealing double honeycomb lattice system as new potential higher T_c superconductor series by intercalation.     

Influence of simultaneous doping of Tl and Bi on microstructure and critical current density of HgBa2Ca2Cu3O8+δ

It has been found that simultaneous doping of Tl and Bi at Hg site of HgBa₂Ca₂Cu₃O_8+δ HTSC phase induces interesting microstructural variants in the form of long period polytypoid (LPP) like structure embodying native defect substructures. It has been observed that the as synthesized (HgTl_0.2−xBi_x)Ba₂Ca₂Cu₃O_8+δ (with x = 0.05, 0.10, 0.15) phases have superconducting onset transition temperature (T_c) of about ∼133 K. However, the intragrain critical current density (J_c) for the various materials with different compositions varies significantly. It varies from 6.2 × 10⁶ A/cm² to 2.9 × 10⁶ A/cm² at 5 K and 1 T for (HgTl_0.10Bi₁₀)Ba₂Ca₂Cu₃O_8+δ and (HgTl_0.15Bi_0.05)Ba₂Ca₂Cu₃O_8+δ HTSC phase respectively. A correlation between the intragrain J_c and the defect substructures has been found to be present. This correlation has been described and disscussed. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Structural investigation of anion-deficient manganites La<Subscript>0.7</Subscript>Sr<Subscript>0.3</Subscript>MnO<Subscript>3 − δ</Subscript>

The crystal structure of the anion-deficient manganites La_0.7Sr_0.3MnO_{3 ? δ} (δ = 0, 0.10, 0.15, 0.20) is investigated using high-resolution neutron diffraction. At room temperature, the crystal structure of the stoichiometric manganite La_0.7Sr_0.3MnO₃ and the anion-deficient manganite La_0.7Sr_0.3MnO_2.9 is satisfactorily described in rhombohedral space group R \(\bar 3\) c. The anion-deficient manganite La_0.7Sr_0.3MnO_2.85 is characterized by two perovskite phases with space groups R \(\bar 3\) c and 14/mcm. The crystal structure of the La_0.7Sr_0.3MnO_2.8 manganite corresponds to the structure of a perovskite phase with space group I4/mcm. It is established that the phase separation in the crystal structure of the La_0.7Sr_0.3MnO_{3 ? δ} manganites at a temperature of 293 K is associated with a nonuniform distribution of oxygen vacancies.     

Magnetic properties of TiCx/C nanocomposites

The magnetic properties of nanocrystalline titanium carbide dispersed in a carbon matrix (TiC_x/C) prepared by the non-hydrolytic sol–gel process have been studied by dc magnetization measurements. The superconducting phase of titanium carbide has been observed at low temperatures with the onset of the superconducting transition temperature T_c at about 3.5 K, superimposed on a ferromagnetic component. At T > T_c the magnetic response of TiC_x/C is determined by the interplay of the ferromagnetic contribution with the paramagnetic/diamagnetic signal of the metallic system and the contribution of exchange coupled paramagnetic ions. Moreover, significant differences are observed in the magnetic response for samples of the same preparation batch, indicative of the magnetic/electronic inhomogeneity of nanocrystalline titanium carbide which is important for its practical applications.     

Thermodynamic analysis and technology of preparation of high-Tc superconducting system YBa2Cu3O7−x

By modeling the equilibrium state of the phase formation reaction of High-T_c superconducting phase 1:2:3 (YBa₂Cu₃O_7−x) and using the results of P-T−x correlation investigation of some authors, the variation of Gibbs free energy (ΔGm) versus temperature has been calculated. On the basis of the variation of ΔGm, the technology procedure of preparation of phase 1:2:3 with desired x has been established to meet requirement of basic investigation and application of High-T_c superconductors YBa₂Cu₃O_7−x.     

Thermal Expansion and Superconductivity in Alkali Metal- and Silver-doped Bi?Sr?Ca?Cu?O System

Samples with the nominal composition Bi₂Sr₂CaCu₂O_y with na doped at the Ca site and K doped at the Sr site are prepared by solid state reaction method. From the X-ray diffraction data it is found that all the the samples have exhibited a single phase 2212. The D.C. electrical resistivity data show that for Na-doped samples the T_c (zero) varies from 80 K to 85 K and for K-doped samples it is from 79 K to 82 K. The loss of oxygen from these samples around 400°C was confirmed by high temperature dilatometry. The variation of the thermal expansion coefficient (“α”) with temperature for different alkali dopings are discussed. Also the samples with the nominal composition Bi₄Sr₃Ca₃Cu_4−xAg_xO_y (with x = 0, 0.1, 0.2, 0.3) were studied.     

Growth,structure, and superconducting properties of Bi2Sr2Ca2Cu3O10 and (Bi,Pb)2Sr2Ca2Cu3O10‐y crystals

Large and high‐quality single crystals of both Pb‐free and Pb‐doped high temperature superconducting compounds (Bi_1‐xPb_x)₂Sr₂Ca₂Cu₃O_10‐y (x = 0 and 0.3) were grown by means of a newly developed “Vapour‐Assisted Travelling Floating Zone” technique (VA‐TSFZ). This modified zone‐melting technique was realised in an image furnace and allowed for the first time to grow Pb‐doped crystals by compensating for the Pb losses occurring at high temperature. Crystals up to 3×2×0.1 mm³ were successfully grown. Post‐annealing under high pressure of O₂ (up to 10 MPa at T = 500°C) was undertaken to enhance T_c and improve the homogeneity of the crystals. Structural characterisation was performed by single‐crystal X‐ray diffraction (XRD) and the structure of the 3‐layer Bi‐based superconducting compound was refined for the first time. Structure refinement showed an incommensurate superlattice in the Pb‐free crystals. The space group is orthorhombic, A2aa, with cell parameters a = 27.105(4) Å, b = 5.4133(6) Å and c = 37.009(7) Å. Superconducting studies were carried out by A.C. and D.C. magnetic measurements. Very sharp superconducting transitions were obtained in both kinds of crystals (ΔT_c ≤ 1 K). In optimally doped Pb‐free crystals, critical temperatures up to 111 K were measured. Magnetic critical current densities of 2�10⁵ A/cm² were measured at T = 30 K and μ₀H = 0 T. A weak second peak in the magnetisation loops was observed in the temperature range 40‐50 K above which the vortex lattice becomes entangled. We have measured a portion of the irreversibility line (0.1‐5 Tesla) and fitted the expression for the melting of a vortex glass in a 2D fluctuation regime to the experimental data. Measurements of the lower critical field allowed to obtain the dependence of the penetration depth on temperature: the linear dependence of λ(T) for T < 30 K is consistent with d‐wave superconductivity in Bi‐2223. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)