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.     

Growth,composition, ferroelectric and magnetic properties of new multiferroic Pb3.3Mn4.8Ni1.1Ti0.56O15.3 single crystals

Multiferroic single crystals in the novel system Pb‐Mn‐Ni‐Ti‐O have been grown by the high temperature solution growth method. At room temperature the crystals are indexed in the hexagonal space group P6₃cm. The dielectric and magnetic properties along with the temperature dependence of the c‐lattice parameter have been studied in the temperature range 2 K ‐ 500 К. The magnetic measurements reveal a paramagnetic to antiferromagnetic phase transition around 48 K. The dielectric permittivity exhibits a maximum at 430 K, indicating ferroelectric to paraelectric phase transition. The temperature dependent Raman and XRD measurements around 430 K reveal an anomaly and abrupt change of the lattice parameter along the z‐axis respectively, thus confirming the ferroelectric‐to‐paraelectric phase transition.     

Structural aspects of the giant oxygen isotope effect in perovskite manganese oxides

The structure of manganites R _1?x A _x MnO₃ (R = La, Sm, or a mixture of rare earth cations; A = Ca, Sr; x = 0.3, 0.5) has been studied by neutron diffraction to find out the reasons for the giant oxygen isotope effect—the transition from a low-temperature metallic state to an insulating state as a result of replacement of ¹⁶O with ¹⁸O. It is shown that this effect is observed in compositions in which the two phases P1 and P2 coexist at low temperatures. They have the same crystal symmetry (sp. gr. Pnma) but different types of Jahn-Teller distortions of oxygen octahedra and different magnetic structure. Phase P1 has a conductivity of the metallic type, weakly distorted MnO₆ octahedra, and a ferromagnetic structure. Phase P2 is insulating, with MnO₆ octahedra extended or compressed in the apical direction and the moments of Mn ions forming an antiferromagnetic structure. The relative volume of phases P1 and P2 in samples depends on the average radius of the A cation and changes occurring upon replacement of ¹⁶O with ¹⁸O. The percolation transition from the metallic to insulating state upon substitution of ¹⁶O with ¹⁸O is caused by the sharp decrease in the volume of the ferromagnetic metallic phase in favor of the insulating antiferromagnetic phase. An effect of the sample microstructure on the formation of the two-phase state is found.     

Neutron powder diffraction studies in CaMn1‐xCuxO3 (x = 0, 0.2)

Neutron powder diffraction patterns were recorded on CaMn_1‐xCu_xO₃ (x = 0 and 0.20) compounds at different temperatures down to 11K. All the patterns were analyzed by employing Rietveld refinement technique and using the Fullprof program. The observed crystallographic peaks could be refined by using Pbnm space group and no structural transition has been observed down to 11K. An additional peak at 2θ = 16.7° has been observed with decrease in temperature below T_N and its intensity was found to increase with decrease in temperature. It could be indexed to magnetic (101) plane. The magnetic ordering is found to be G‐type antiferromagnetic behaviour. The magnetic moment at 11K for the samples x = 0.0 and 0.20 are found to be 2.69 and 2.42μ_B. The doped Cu ions are found to be in Cu²⁺ state and take part antiferromagnetic interactions with Mn ions. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Magnetic structure of Er5Si3 at T ≥ 20 K

The symmetry analysis of magnetic structures of the Er₅Si₃ compound, possible at temperatures of 20 and 25 K, has been performed to interpret the experimental elastic neutron scattering data obtained at these temperatures. It was shown that the smallest dispersion factor R _m at these temperatures corresponds to the modulated magnetic structure in which the magnetic moments of Er atoms are directed along the a₃ axis of the unit cell and form an antiferromagnetic longitudinal spin wave characterized by the wave vector k = μb₃, where μ is approximately 0.264 and 0.274 at 20 and 25 K, respectively. The analysis of the temperature behavior of the magnetic reflection intensities demonstrated that the Er₅Si₃ compound is paramagnetic at 30 K.     

Topology and symmetry analysis of rare earth borocarbides structural family,analogy to hexaferrites and relation to properties

The topology and symmetry analysis was applied to a series of rare earth borocarbide compounds, which have been gaining increasing interest due to their magnetic and thermoelectric properties. Using principles of OD theory, the crystal structures were deconvoluted into L(1) (B₁₂ icosahedra and C‐B‐C chain) layers and L(2) (rare earth and B₆ octahedral) layers. The arrangement of B₁₂ icosahedra in the L(1) layer is equal to close packed spheres, however, symmetry of the B₁₂ block lowers symmetry of the resulting layer from P 6/mmm to P 3m1. Both layers, L(1) and L(2) possess symmetry P 3m1 and the conjugation of L(1) with L(2) layers occurs in accordance with the symmetry elements. No disorder may appear here because of equal symmetry of single layers and layer pairs and it is not a classical OD family. Only the increasing of the amount of one type of layers, namely L(1), provides the structural variations. Close analogy to the hexagonal ferrites family has been found. Topology and symmetry analysis reveals principles in the building up of the structural family, gives an insight into the particular order‐disorder formation mechanism/criteria of these homologous borocarbide compounds and as the result relation to the properties (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Crystallization temperature of amorphous Fe80B20 under pressure

The resistance of rapidly quenched Fe₈₀B₂₀ was recorded versus T at a rate of 7 K/min and various pressures up to 2.5 GPa. The onset of crystallization, T_x, was defined by extrapolation at the sharp change in resistance associated with crystal growth. The resulting slope of T_x is 15 K/GPa. Separate measurements at atmospheric pressure yielded T_x versus rate of temperature increase, R = dT/dt. crystallization times from the literature were recalculated to yield T_x, and the results are compared with our data. Nominally amorphous materials of the same composition show markedly different properties. It is also shown that the variation of T_x with R is mostly due to the change in viscosity.     

Magnetic and magnetoimpedance properties of Mn-doped FINEMET

In this study, the effect of substituting Mn for Fe on the magnetic and magnetoimpedance (MI) properties of amorphous and nanocrystalline Fe_73.5?xMn_xCu₁Nb₃Si_13.5B₉ (x = 1, 3, 5, 7) alloys has been investigated. A higher Mn content causes a decrease in the saturation magnetization and Curie temperature. For as-received Fe_73.5?xMn_xCu₁Nb₃Si_13.5B₉ (x = 0, 1, 3, 5, 7) ribbons, the measured values of the MI are 11%, 12%, 16%, 12% and 15% for x = 0, 1, 3, 5, 7, respectively. The magnitude of the MI ratio increases in annealed samples, indicating that they are magnetically softened by nanocrystallisation. The change in MI is found to be ～70% for the x = 3 sample annealed at 550 °C. It is also found that replacing a small amount of Fe by Mn improves the magnetic softness of Fe_73.5?xMn_xCu₁Nb₃Si_13.5B₉ alloys.     

Growth of big single crystals of a new magnetic superconducting double perovskite Ba2PrRu1–xCuxO6

Single crystals of Ba₂PrRu_1–xCu_xO₆ with x = 0 to 0.2, have been grown from high temperature solutions of a mixture of PbO‐PbF₂ in the temperature range 1100–1200 °C. Thin crystals with mostly a hexagonal and triangular plate like habit measuring up to 1–2 mm across and 0.1–0.2 mm thick were obtained. The size, quality and morphology of the crystals were improved by varying the solution volume as well as additives like B₂O₃. Large crystals measuring up to 3 mm across and 0.3 to 0.5 mm thick were obtained with 5–7 wt% solute concentration and 0.51 wt% of B₂O₃. The ZFC curves exhibit a spin glass like behavior with x = 0 and a superconducting transition at 8 to 11 K depending on x = 0.05 to 0.1. The transition was also influenced by the growth temperature and post growth annealing. Powder x‐ray diffraction, EDS and Raman spectroscopic measurements confirm the presence of Cu in the crystals. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Magnetic properties of Fe-Co-Nd-Y-B magnet prepared by suction casting

The magnetic properties of Fe_67−xCo₁₀Nd₃Y_xB₂₀ (x = 0, 2, 6, 10) alloys prepared by suction casting had been investigated. It was found that a small addition of Y (6%) was very effective in improving glass-forming ability (GFA) of Fe-Co-Nd-Y-B alloys. The as-cast Fe₆₁Co₁₀Nd₃Y₆B₂₀ alloy presented soft magnetic behavior. However, the alloy showed the hard magnetic behavior due to exchange coupling between soft and hard magnetic phases after annealing at 948 K for 30 min. The results provide a promising approach for the production of bulk magnets by the simple process of copper mold casting and subsequent heat treatment.     

Disorder in LixFePO4: From glasses to nanocrystallites

Li_xFePO₄ glasses have been prepared by fast-quenching method in the whole range of composition 0 ? x ? 1. The amorphous state of glassy materials is confirmed by X-ray diffraction. Information concerning the local environment of Li and Fe cations and the configuration of (PO₄)³⁻ oxo-anions is obtained by Fourier transform infrared (FTIR) spectroscopy. While the LiFePO₄ crystalline materials undergo a transition from the paramagnetic to the antiferromagnetic ordering at 52 K, no magnetic ordering is observed in the vitreous samples that realize random field systems, so that a spin glass-like freezing is observed at low temperature. The paramagnetic Curie temperature of Li_xFePO₄ is independent of x and shifted to θ = −60 K in the glassy state, due to a significant distortion of the FeO₆ octahedra that alters the superexchange path inside the atomic FeO₄ layers of the crystallized structure. On another hand, the PO₄ tetrahedra are not significantly distorted in the glassy phase. The results are compared with highly disordered, but nanocrystallized LiFePO₄ recently obtained at the early stage of synthesis by solid state reaction at 300 °C. In this latter case, the lack of long-range antiferromagnetic ordering is due to substitutional disorder among the cationic sublattice.     

Elasticity, thermal expansion and compressive behavior of Mg65Cu25Tb10 bulk metallic glass

The existence of special covalently bonded short-range ordering structures in a Mg₆₅Cu₂₅Tb₁₀ bulk metallic glass (BMG) is confirmed by thermal expansion and compression behavior. Under ambient conditions the linear thermal expansion coefficient obtained is almost constant in the glassy state with a value of 4.0 × 10⁻⁵ K⁻¹. By fitting the static equation of state at room temperature under ambient conditions we find the value for bulk modulus B of 48.7 GPa, which is in excellent agreement with the experimental study by pulse-echo techniques of 44.7 GPa. Unlike many bulk metallic glasses, such as Zr- and Pd-based, which bulk modulus is much larger than 100 GPa, the value B of Mg₆₅Cu₂₅Tb₁₀ BMG falls into the range of SiO₂ and fluorozirconate glass ZBLAN. Moreover, the elastic constant of the Mg₆₅Cu₂₅Tb₁₀ BMG is almost the same as those of ZBLAN. No evidence for the high-pressure phase transitions of the Mg₆₅Cu₂₅Tb₁₀ BMG has been found up to 31.19 GPa at room temperature.     

Study of the structure of PyHReO4 under high pressure

The structure of deuterated pyridinium perrhenate (d₅PyH)ReO₄ (C₅D₅NHReO₄) is studied by X-ray diffraction at room temperature and pressures up to 3.5 GPa and by neutron diffraction in the temperature range 10–293 K and at pressures up to 2.0 GPa. Under normal conditions, this compound belongs to the orthorhombic space group Cmc2₁ (ferroelectric phase II). At room temperature and pressures above P > 0.7 GPa, a transition to an orthorhombic phase (paraelectric phase II) is observed. This paraelectric phase is described by the space group Cmcm. At a pressure as high as P = 2.0 GPa, phase I remains stable at temperatures down to 10 K. This fact indicates that the high pressure suppresses the ferroelectric state in deuterated pyridinium perrhenate (d₅PyH)ReO₄.     

Electron spin resonance of Mn2+ IN As–Se and As–Te glasses

The electron spin resonance of Mn has been studied in As_xSe_100?x with 0 ? x ? 70 and As_xTe_100?x with 40 ? x ? 70. All samples, except those with x < 20 in As_xSe_100?x, exhibit six hyperfine lines centered at g = 4.3. A g = 2.0 line is observed in As–Se with largely scattered linewidth by samples, but not in As–Te unless oxygen contamination is included in the samples. The g = 4.3 line in As–Se is closely related to a formation of As₂Se₃-type layer structure and interpreted as being caused by Mn situated at the interlayer position and surrounded by four Se atoms in an arrangement of rhombic symmetry. In As–Te, a similar model by four Te atoms is valid in composition near As₂Te₃, but the surrounding Te is replaced by As as As content increases. The g = 2.0 line is concluded to come from phase-separated antiferromagnetic particles of Mn–O and MnSe. The linewidth is scattere by differences in the relative amounts of the two kinds of particles and in particle size.     

New family of LnBaMn2O6 − γ manganites (Ln = Nd, Sm, and Gd)

A new family of manganites, LnBaMn₂O_{6 ? γ}(Ln = Nd, Sm, and Gd), is obtained by the method of topotactic reactions. In these compounds, rare-earth and barium ions are ordered, which results in a dramatic increase in the temperature of transition to the paramagnetic state. Thus, a SmBaMn₂O₆ compound with a disordered arrangement of Sm and Ba ions is a spin glass with the freezing temperature of magnetic moments T _f = 40 K, whereas in the state with an ordered Sm and Ba arrangement, the Curie temperature T _C, is ～ 280 K. Below T _C, a maximum in resistivity and magnetoresistance is observed. The possible models of magnetic ordering are discussed.     

Neutron and X-ray diffraction study of superstructure and localized magnetic moments in Cu0.5Fe0.5Cr2S4 and Cu0.5In0.5Cr2S4 compounds

The superstructure parameters for the Cu_0.5Fe_0.5Cr₂S₄ and Cu_0.5In_0.5Cr₂S₄ compounds have been determined by neutron and X-ray diffraction. The localized magnetic moments in different sublattices measured for Cu_0.5Fe_0.5Cr₂S₄ are equal to 3.06 ± 0.17 μB for Fe³⁺ ions in the A-site and 2.76 ± 0.22 μB for Cr³⁺ ions in the B-site (Cu⁺ possess no magnetic moment), which are much less than the magnetic moments for the ions in the purely ionic state.     

In situ energy dispersive X-ray diffraction analysis of the high-pressure–high-temperature stability of amorphous Co–Fe–Zr–B alloys

The pressure–temperature stability of Co₅₆Fe₁₆Zr₈B₂₀ amorphous alloys was investigated by energy dispersive X-ray diffraction. The in situ experiments were performed at the MAX80/F2.1 high-pressure beamline at the HASYLAB synchrotron radiation facility (Hamburg, Germany). Crystallization experiments were performed under different applied pressures up to 2 GPa. The effect of static pressure on the crystallization temperature stems from the pressure effect on the nucleation barriers and modified kinetics of solid-state diffusion under high-pressure–high-temperature conditions.     

Thermomagnetic study of the amorphous cobalt-erbium borides Co80ErxB20−x (0 ? x ? 4)

Calorimetric, magnetic and X-ray diffraction measurements have been used to study the magnetic susceptibility and thermal stability of Co₈₀Er_xB_20−x with (0 ? x ? 4) amorphous ribbons. The compounds are found to crystallize in Co₂B and β-Co, after precipitation of the tetragonal Co₃B phase. The addition of erbium shifts up the crystallization temperature leading to the increase in the stability of the amorphous state. Magnetic susceptibility measurements show that the addition of erbium increases the Curie temperature and induces noncollinear magnetic behavior. This latter fact is explained on the basis of random local magnetic anisotropy related to the rare earth atoms in amorphous materials.     

Crystallization kinetics of Fe73.5−xMnxCu1Nb3Si13.5B9 (x = 0, 1, 3, 5, 7) amorphous alloys

In this study, we have investigated the effect of substituting Mn for Fe on the crystallization kinetics of amorphous Fe_73.5−xMn_xCu₁Nb₃Si_13.5B₉ (x = 1, 3, 5, 7) alloys. The samples were annealed at 550 °C and 600 °C for 1 h under an argon atmosphere. The X-ray diffraction analyses showed only a crystalline peak belonging to the α-Fe(Si) phase, with the grain size ranging from 12.2 nm for x = 0 to 16.7 nm for x = 7. The activation energies of the alloys were calculated using Kissinger, Ozawa and Augis-Bennett models based on differential thermal analysis data. The Avrami exponent n was calculated from the Johnson-Mehl-Avrami equation. The activation energy increased up to x = 3, then decreased with increasing Mn content. The values of the Avrami exponent showed that the crystallization is typical diffusion-controlled three-dimensional growth at a constant nucleation rate.     

Bioactive glasses in the system CaO-B2O3-P2O5: Preparation, structural study and in vitro evaluation

Glasses in the system x B₂O₃(1 − x) [y CaO P₂O₅], (x = 0, 0.1, 0.2, 0.3, y = 2, 2.6, 3, 4, 5) have been prepared by fast quenching of high temperature melts. The presence of B₂O₃ affected the glass forming ability, allowing the preparation of calcium phosphate glasses with y ? 2.6. The structure of glasses was analyzed by μ-Raman and infrared spectroscopy. The analysis indicated that the glass network is dominated by highly charged species from phosphate tetrahedra with 3 (pyro) or 4 (ortho) NBOs, while the boron atoms are incorporated mainly in 3 coordinated sites in the form of B∅₃ or B∅₂O⁻ units. A small fraction of units was also evident from the spectra analysis of glasses with high CaO content. All calcium borophosphate glasses exhibited bioactivity after soaking in SBF solution within a few days. This was observed by μ-Raman and SEM microscopy, while XRD patterns clearly revealed growth of hydroxyapatite phase. The presence of boron in the glass network has a catalytic effect at favoring bioactivity of the otherwise bioinert calcium phosphate glasses.