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.     

Spin disorder in Fe-doped manganites

Ln_0.7M_0.3MnO₃ compounds are well-known ferromagnets mediated by a double exchange mechanism. As Mn atoms are substituted by Fe in the ratio Mn_1−xFe_x the magnetic structure dramatically changes, because the ferromagnetic double exchange chain is broken. At low Fe concentrations all compounds are magnetically ordered. For intermediate values ferro (FM), antiferro (AF) and paramagnetic (PM) phases co-exist in a large temperature range, and at x ≈ 0.2 spin or cluster-glass behavior is found. Magnetization, Mössbauer, polarized and low angle neutron scattering as well as muon spin relaxation experiments have been performed on 0 ⩽ x ⩽ 0.30 compounds showing the transit from long range ferromagnetism to spin glass. Co-existence of FM and AF clusters of different size has been found for all doped compounds.     

Radiation effects in ZrF4 based glasses: I. Electron spin resonance

Binary ZrF₄BaF₂, HfF₄BaF₂, and multicomponent ZrF₄ based glasses have been studied by electron spin resonance spectroscopy after liquid-nitrogen-temperature X-irradiations and room-temperature γ-ray irradiations. Polycrystalline BaF₂, ZrF₄ and HfF₄ were investigated in the same manner for comparison purposes. Isochronal anneal experiments have distinguished several resonances in the glasses which are characterized as due to F₂^? molecular ions, F⁰ interstitial atoms and Zr³⁺ and Hf³⁺ ions. Two more signals are tentatively assigned as due to a hole trapped on several fluorine ions and a hole trapped on an oxygen impurity. In lead-containing glasses the formation of Zr³⁺ is strongly inhibited, presumably due to the competitive formation of Pb⁺ ions. However, the radiation production of Pb³⁺ ions was independently observed, thus suggesting that the Pb²⁺ ions may serve as traps for charge carriers of either sign. No defects were detected which could be explicitly associated with other cationic constituents, such as La³⁺, Al³⁺, or Li⁺.     

Phase separation in La1−x Sr x CoO3 solid solutions with a perovskite structure

The properties of La_1−x Sr _x CoO₃ (x = 0, 0.15, 0.20, 0.30) solid solutions are investigated using neutron diffraction, positive-muon spin relaxation (precession) measurements, and extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES) spectroscopy. The results obtained are interpreted within the model of phase separation into ferromagnetic regions enriched with Sr²⁺ ions and nonmagnetic regions similar in composition to the LaCoO₃ compound.

     

Electrical and magnetotransport properties of La0.7Sr0.3MnO3/TiO2 composites

The composite samples with nominal compositions of (1‐x) La_0.7Sr_0.3MnO₃ + x TiO₂ (x = 0, 0.05, 0.10, 0.15 and 0.20) were synthesized via solid state reaction process. The X‐ray diffraction and scanning electronic microscopy observations reveal no reaction between La_0.7Sr_0.3MnO₃ (LSMO) and TiO₂ phases. Temperature dependence resistivity measurements show that TiO₂ phase shifts the metal‐insulator transition temperature (T_p) towards lower temperature and increases the resistivity. Moreover, the magnetization of the composite samples decreases with TiO₂ content. An enhancement in magnetoresistance is observed in the composite samples with x = 0.05 and x = 0.10 at low magnetic fields, which is encouraging for potential application of magnetoresistive materials at low field. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

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.     

Growth Kinetics of High-Tc and Low-Tc Phases in Bi2-xPbxCa2Sr2Cu3Oy Superconducting Compounds

Superconducting samples of the composition Bi_2-xPb_xCa₂Sr₂Cu₃O_y (x = 0.0, 0.1, 0.2, 0.3, 0.4, and 0.5) are prepared by co-decomposition of metal nitrates. DTA, TGA, density, and porosity studies have been performed on these samples. Characterisation techniques like XRD, SEM, EDXA, d.c. resistance, and a.c. magnetic susceptibility have been employed to study the growth of High-T_c (HTP) and Low-T_c (LTP) phases upon lead substitution for bismuth. Results have revealed the growth of HTP with increase in Pb conc. up to x = 0.3. Further increase in Pb appears to results in the deterioration of HTP resulting in the formation of LTP and Ca₂PbO₄ impurity phase. Transport critical current density (J_c) measurements performed on the samples indicate that Pb doping followed by densification remarkably improves the current carrying capacity of the materials.     

Effect of alkaline earth oxides on ceramization of LaF3 in aluminosilicate glass: F MAS-NMR study

Local environment of fluorine atoms in the lanthanum oxyfluoride aluminosilicate glasses and glass-ceramics modified by alkaline earth oxides, i.e. MgO, CaO and BaO, was studied by solid state ¹⁹F MAS-NMR spectroscopy. The effect of the type and concentration of the modifier on the formation of crystalline LaF₃ phase was determined, as a function of heat treatment conditions. In all series of glasses studied, the F-Me(n), but no Na-F species were observed. The presence of F-La,Ba(n) units, in which fluorine is coordinated both by lanthanum and barium, was detected in the Ba-glass. Supplementary XRD analysis of this series confirmed that an increase of barium contents leads to the formation of Ba_0.3La_0.7O_0.7F_1.3, instead of pure LaF₃ only.     

Growth and characterization of large La1‐xPbxMnO3+δ (x=0.32–0.35) crystals

Large crystals of La_0.63Pb_0.37Mn O_3+δ with small La(Pb)‐ deficiency of about 0.005‐0.01 at.% were grown by high temperature solution growth method. The structure of the grown crystals was determined as rhombohedral with R‐3 space group by single‐crystal X‐ray diffractometry. The surface morphology of the crystals and the exact chemical composition was examined by scanning electron microscopy and energy dispersive X‐ray analysis methods, respectively. The IR‐transmission spectrum reveals the presence of Mn³⁺O₆‐ and Mn⁴⁺O₆‐ octahedra in the lattice of La_0.63Pb_0.37Mn O_3+δ crystals. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Enhancement of ferroelectricity in the compositionally graded (Pb,Sr)TiO3 thin films derived by a sol–gel process

The compositionally graded Pb_1−xSr_xTiO₃ (PST) films with a fine compositional gradient from Pb_0.6Sr_0.4TiO₃ to Pb_0.3Sr_0.7TiO₃ were fabricated on LNO-buffered Pt/Ti/SiO₂/Si substrates by a sol–gel deposition method. The graded films crystallized into a pure perovskite structure and exhibited highly (1 0 0) preferred orientation after post-deposition annealing. Dielectric and ferroelectric properties were investigated as a function of temperature, frequency and direct current bias field. Dielectric constant peaks, common to a ferroelectric transition, were not observed in the temperature range of 25–250 °C within which the dielectric constant showed weak temperature dependence. This compositionally graded thin film can result in a dielectric constant more than double and a remanent polarization at least two and a half times larger than conventional PST thin films.     

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.     

Neutron diffraction and X-ray diffraction investigations of langasite crystals

Crystals of langasite La₃Ga₄(GaSi)O₁₄ grown by the Czochralski method are studied using neutron diffraction for the first time. It is established that the compositions of the upper and lower parts of an orange crystal grown from the La₃Ga_5.14Si_0.86O₁₄ seed in an (Ar + O₂) atmosphere (the 〈 0001 〉 growth direction) can be written as(La_2.85(2)□_0.15)(Ga_0.95(2)□_0.05) Ga₃(Ga_1.15 Si_0.85(5))(O_13.72□_0.28(7)) and(La_2.89(1)□_0.11)·(Ga_0.98(1)□_0.02) Ga₃(Ga_1.06Si_0.94(4))(O_13.81□_0.19(9)), respectively. The La content in the upper and lower parts of this crystal is lower and higher than the Ga content, respectively, and the Ga content exceeds the Si content in the (GaSi) position. By contrast, in a colorless crystal of the composition(La _2.97(4)□_0.03) Ga(1)(Ga_2.94(9)□_0.06)(Ga_0.7(1) Si_1.3)(O_13.9□_0.1(1)), which is grown from the La₃ Ga₅SiO₁₄ seed in an argon atmosphere (the 〈 01\(\bar 1\)1 ?growth direction), the Ga content in the (GaSi) position is lower than the Si content. A relation between the Ga: Si ratio and the (Ga,Si)-3O interatomic distances is found.     

Magnetic properties of amorphous metals

Neutron scattering has provided unique information about the nature of magnetism in amorphous alloys. This paper reviews some of the results obtained principally on two ribbon-form metallic glass systems - (Fe_xNi_1?x)₇₅P₁₆B₅Al₃ and Fe_xB_1?x. The former exhibits three states depending on composition: pure ferromagnetism, re-entrant spin glass, and ordinary spin glass. The neutron results provide evidence for the coexistence of spin glass and ferromagnetic correlations in the intermediate regime. The alloys of Fe and B show Invar phenomena which result in anomalously small values of the effective spin-wave stiffness calculated from low-temperature magnetization and Mössbauer data. The long-wavelength spin waves as measured by the neutrons do not reflect the rapid demagnetization found in the bulk results, implying the presence of excitation processes other than long-wave-length spin waves.     

Electrical Properties of Praseodymium-doped GaAs and Al0.3Ga0.7As Epilayers

Praseodymium-doped GaAs and Al_0.3Ga_0.7As epilayers grown on Semi-Insulating (SI) GaAs substrates by Liquid Phase Epitaxy (LPE) were first studied in this present work. Measurement techniques, such as microscopic observation, X-ray diffraction, Secondary Ion Mass Spectroscopy (SIMS), and Hall measurement were employed. Layers doped with Pr resulted in a mirror-like surface, except several high Pr-doped layers having droplet surfaces. Hall measurements reveal that the grown layers contained p-type layers, carrier concentrations from 6.3 × 10¹⁵ to 1.2 × 10¹⁶ cm⁻³, and from 6.3 × 10¹⁵ to 3.5 × 10¹⁶ cm⁻³ for Pr-doped GaAs and Al_0.3Ga_0.7As epilayers, respectively. Although p-type conduction exists, in the light of electrical features, doping of Pr into the GaAs and Al_0.3Ga_0.7As growth melts, is still considered to exhibit gettering properties rather than to become a new acceptor itself. Additional photoluminescence examinations were taken. Their results also indicate that Pr-doped layers produce no new emission lines and support the electrical observations.     

Microscopic Studies of 2H-NbSe2 Probed by Positive Muon

Abstract

The microscopic state of the positively charged light particle in the transition metal dichalcogenide 2H-NbSe₂ was studied using the muon spin relaxation method (μ⁺SR) and muon level crossing resonance method (μ-LCR). Muons are expected to stay at interlayer position and behaves as a hydrogen like intercalant. We discuss the relation between conduction electron properties and the muon's behavior.     

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.     

Dynamical magnetic correlations in the YbB12 kondo insulator: Neutron investigations with a polarization analysis

The results of investigations into the spin dynamics in the YbB₁₂ Kondo insulator with the use of inelastic neutron scattering, including experiments with a polarization analysis, are discussed. It is shown that, at low temperatures, the dynamic magnetic response in the structure of the YbB₁₂ Kondo insulator is characterized by three dispersive excitations with a nontrivial q-dependence of the intensity. An increase in the temperature results in a crossover to the single-site spin fluctuation regime accompanied by suppression of collective excitations.     

Electrophysical characteristics and structural parameters of metamorphic HEMT nanoheterostructures In0.7Al0.3As/In0.7Ga0.3As/In0.7Al0.3As containing superlattices with different numbers of periods in the metamorphic buffer

The results of studying the electrophysical characteristics and structural parameters of metamorphic In_0.7Al_0.3As/In_0.7Ga_0.3As/In_0.7Al_0.3As HEMT nanoheterostructures epitaxially grown on GaAs (100) substrates have been presented. A linear metamorphic buffer with inserted unbalanced superlattices characterized by different numbers of periods is used. Transmission electron microscopy has shown that an increase in the number of superlattice periods from 5 to 30 promotes the improvement of the crystal structure. In this case, the electrophysical parameters of metamorphic HEMT nanoheterostructures are also significantly improved.     

On the structure of the molten mixture Ag(I0.3Br0.7)

We present neutron diffraction measurements of the total structure factor of the molten salt mixture Ag(I_0.3Br_0.7) at 650 °C, measured by using the high intensity total scattering time-of-flight spectrometer, HIT-II, at the spallation neutron source of the Neutron Science Laboratory in the High Energy Accelerator Research Organization of Japan. We have compared the experimental data with molecular dynamics simulations using Vashishta–Rahman type potentials. Within the known constraints of pairwise rigid ion potentials, the results of the simulations are in fair agreement with experiment. We also present results using a linear combination of the experimental and molecular dynamics simulations of the structure factors of the AgI and AgBr melts. The latter are in somewhat better agreement with experiment.     

Specific features of the structure and weight loss of aliovalent-substituted oxides based on lanthanum gallate (La,Sr)(Ga,Fe,Mg)O3 − y

The structure of the (La_0.9Sr_0.1)(Ga_0.56Fe_0.24Mg_0.20)O_{2.85 + y} oxide is investigated using neutron powder diffraction at temperatures of 10, 295, and 1000 K. The results of the structure refinement are in good agreement with the dilatometric and thermogravimetric data and confirm the inference that the valence state of iron cations is higher than 3+ and that the lattice undergoes an additional expansion due to the oxygen loss at temperatures above 870 K.