Magnetocaloric effect in La0.65−xEuxSr0.35MnO3

The magnetocaloric properties for the Eu-doped La_0.65?xEu_xSr_0.35MnO₃ samples with x = 0.05, 0.15, 0.20, and 0.30 upon 0.05T magnetic field have been investigated. It is found that the Eu doping in this system decreases the magnetocaloric properties lightly. Moreover, the results of Eu doping clearly indicate that the magnetocaloric effect in this system is tunable, which is beneficial for manipulating magnetocaloric refrigeration that occurs in various temperature ranges. This makes the La_0.65?xEu_xSr_0.35MnO₃ samples potential candidates for practical applications. A complete characterization of the magnetic properties of this material aids to the understanding required for the technological exploitation of such materials, and it suggests La_0.65?xEu_xSr_0.35MnO₃ perovskite as the promising magnetic refrigerant.     

Study of the magnetic phase separation in the Eu0.65Sr0.35Mn1−x Fe x O3 ceramics by EPR and Mössbauer spectroscopy

Physics of the Solid State - The structural and magnetic properties of the Eu0.65Sr0.35Mn1?x Fe x O3 (x = 0.2–0.4) ceramics have been investigated using EPR, Mössbauer...     

Neutron diffraction evidence for an antiferromagnetic ordering in the CMR manganites Pr0.7Ca0.3−xSrxMnO3

A powder neutron diffraction study on the giant magnetoresistive oxides Pr_0.70Ca_{0.3−xSrxMnO3} has been performed versus temperature for x = 0.1 and 0.05. The first one, Pr_0.7Ca_0.3Sr_0.1MnO₃, exhibits the smaller R₀/R_H ratio (100 at 100 K) and evidences a transition from paramagnetic to ferromagnetic state as the temperature decreases (T_c = 170 K). The second sample, Pr_0.70Ca_0.25Sr_0.05MnO₃, that exhibits an extremely high R₀/R_H ratio (2.5 × 10⁵ at 88 K), shows a transition from the paramagnetic state to an antiferromagnetic state and finally to a canted ferromagnetic state. The presence of an intermediate AF state explains the M(T) curves and the exceptional high magnetoresistive effect. The determination of the nuclear structure of these oxides confirms their ‘O₃’ oxygen stoichiometry and evidences a contraction of the lattice parameters at the ferromagnetic transition. The evolution of the MnO distances shows a decrease of the Jahn-Teller distortion at the magnetic transition.     

Calculation of the phase diagram of the TiZr alloy and investigation of the tendency toward separation of the ω phase

The electronic, structural, and thermodynamic properties of the TiZr equiatomic alloy have been calculated in terms of the electron density functional theory and Debye-Grüneisen model. The calculated lattice parameters a and c/a agree well with experimental data for the α, ω, and β phases. It has been shown that the ω phase is stable at atmospheric pressure and low temperatures, and it remains energetically more favorable up to T = 600 K. In the temperature range 600 K < T < 900 K, the α phase becomes stable, and above 900 K, the β phase of the TiZr alloy is stable. The phase diagram constructed in this study agrees qualitatively with the available experimental data. A tendency toward separation of the TiZr equiatomic alloy in the ω phase has been analyzed. It has been demonstrated that, in the ground state, the TiZr equiatomic alloy in the ω phase exhibits a tendency toward ordering rather than toward phase separation.     

The low temperature magnetic phase diagram of EuxSr1−xTiO3

Specific heat and magnetization measurements demonstrate that the antiferromagnetic (AFM) phase transition at T _N  = 5.7 K of EuTiO₃ is rapidly suppressed with Sr doping in Eu _x Sr_1?x TiO₃. Close to x = 0.25, T _N  = 0 K and AFM order vanishes. Above this critical concentration a finite transition temperature to an AFM phase is observed. The exchange couplings are derived as a function of x and the corresponding low temperature phase diagram is presented.     

Generic phase diagram of Nd2−xCexCuO4

We investigated the generic phase diagram of the electron doped superconductor, Nd_2?xCe_xCuO₄, using films prepared by metal organic decomposition. After careful oxygen reduction treatment to remove interstitial O_ap atoms, we found that the T_c increases monotonically from 24 K to 29 K with decreasing x from 0.15 to 0.00, demonstrating a quite different phase diagram from the previous bulk one. Based on the discussion on the tremendous influence of O_ap “impurities” on superconductivity in T′ cuprates, we conclude that our result represents the generic phase diagram for oxygen-stoichiometric Nd_2?xCe_xCuO₄.     

Phase diagram of perovskite manganites Sm1 − x Sr x MnO3

Physics of the Solid State - The magnetic and structural phase diagram of perovskite manganites Sm1 ? x Sr x MnO3 (0.16 ≤ x ≤ 0.67) are constructed based on systematic studies by...     

Reexamining the phase diagram of the Gay—Berne fluid

This work reexamines and updates earlier investigations on the phase behaviour of the Gay-Berne liquid crystal model, concentrating on the effect of varying temperature. Constant volume and constant pressure Monte Carlo simulations are combined for systems consisting of N = 500 molecules along different isotherms over the reduced temperature range 0.60 ≤ T ≤ 1.25. As in previous simulation studies of the model, the study identifies nematic and smectic B phases on compressing the isotropic fluid, the particular phase sequence depending on temperature. The nematic phase is found to be stable with respect to the isotropic phase for reduced temperatures T ≥ 0.75. In the temperature range 0.75 ≤ T ≤ 1.25, the phase boundaries of the isotropic-nematic transition are obtained by computing the Helmholtz free energy of both phases from thermodynamic integration. From the simulation data, the relative volume change at the isotropic-nematic transition is about 2%, and this value appears to be rather insensitive to changes in temperature. On compressing the nematic phase, the Gay-Berne fluid undergoes a strong first-order transition to the smectic B phase. This transition is studied by using constant pressure simulation, and the coexistence properties are estimated from the limits of mechanical stability of the nematic phase. Larger relative volume changes are found at the transition than those suggested by previous studies, with typical values increasing up to 10.5% as the temperature is decreased. The results are consistent with the existence of strong coupling between nematic and smectic order parameters. For temperatures T ≤ 0.70 the nematic phase is no longer stable, and the phase sequence isotropic-smectic B is observed. Therefore, the Gay-Berne model exhibits an isotropic-nematic-smectic B triple point. Extrapolating the present simulation data, this triple point is located approximately at reduced temperature T_INB ? 0.70 and reduced pressure P_INB ? 1.825.     

Isotopic effect in the solid–liquid phase diagram of quantum clusters

Applying the Fourier path integral formalism to the isothermal-isobaric ensemble, the solid–liquid transition for 13-atom pure Lennard–Jones clusters was characterized. The masses of the clusters were taken as the masses of hydrogen, deuterium and tritium, hence isotopic effects of quantum clusters were considered. The parallel tempering Monte Carlo algorithm was used to solve all multidimensional integral in the FPI method. The volume of the system was defined with respect to the centroids of the quantum particles and a variable constraining potential was used to restrict undesirable thermodynamic events. The maximum value of the constant pressure heat capacity at a given temperature was used to identify the melting temperature. Pressure versus temperature phase diagrams were constructed for these systems with and without the inclusion of quantum effects. A significant difference in the melting temperature was encountered for the different isotopes due to quantum contribution.     

Effect of local atomic phase separation in the x-ray absorption near edge structure spectroscopy of FeSexTe1−x

Ab initio X-ray absorption near edge structure (XANES) calculations for FeSe_xTe_1−x, using a structural model that combines FeSe and FeTe phases at the nanoscale, are compared with Fe K-edge XANES measurements in the “pre-edge” region. The important aspects of this model are (i) magnetic order in the FeTe phase; (ii) Se and Te atoms placed randomly in both FeSe and FeTe crystallographic positions and; (iii) the two distinct distances for Fe–Se and Fe–Te of the bulk phases. The calculated spectra reproduce the observed increase of spectral weight in the experiments on FeSe_xTe_1−x with Se concentration. This is consistent with an inhomogeneous local electronic structure of FeSe_xTe_1−x. Additionally, we have calculated projected electronic density of d-states for the Fe atom, revealing increased spectral weight in the “pre-edge” region of the XANES spectra, which correlates with the increase in Se concentration. The decrease of calculated Fe d-density of states for the Fermi level, N(ε_F), for high Te content is consistent with the suppression of superconductivity in the title system.     

Random-bond and random-anisotropy effects in the phase diagram of the Blume–Capel model

We report on Monte Carlo studies of the influence of quenched randomness on the phase diagram of the three-dimensional (3D) Blume–Capel model. The randomness is supposed to act either on the exchange coupling constants (bond randomness) or on the anisotropy distribution. With increasing disorder, first-order phase transitions are shown to change into second-order phase transitions. The trajectory of the tricritical point in the phase space as a function of disorder is presented. We have also calculated critical exponents at some points in the second-order phase region which show a change of universality class in agreement with the Harris criterion.     

Phase diagram of the system of solid solutions Pb1−X(Li½La½)X(Zr1-YTiY)O3 in the vicinity of the FE-AFE phase stability boundary. II. phase transitions and induced states

Results of the study of phase transitions in the system of solid solutions Pb_1-X(Li_½La_½)_X(Zr_1-YTi_Y)O₃ (PLLZT) at x = 0, 1 under the effect of an electric field are presented. It is shown that there are three regions of Ti-concentration on the “;composition-temperature”; phase diagram. The region of Ti-content from ～ 12 to ～ 19% is the most interesting. In this region the succession of phase transitions depends on the prehistory of the specimens: in the absence of an external electric field only the AFE-PE transition takes place, but after the action of the field, the succession of phase transitions FE-AFE-PE arises. Full “;temperature-field”; phase diagrams are constructed.     

Study of the glassy magnetic behaviour and charge-ordering phase transitions in La0.75Ca0.25FeO3−δ perovskite

Abstract

In this work, La_0.75Ca_0.25FeO_3?δ perovskite sample was prepared by the coprecipitation method. The nanoparticle was found to crystallize in the orthorhombic (Pbnm) phase as confirmed by X-ray diffraction (XRD) and transmission electron microscopic (TEM). The oxygen non-stoichiometry (δ) and magnetic states of iron ions (three magnetic sextets and non-magnetic doublet) were investigated by Mössbauer spectroscopy at room temperature (RT). The shape of the magnetic hysteresis loop of the sample reveals the existence of a weak ferromagnetism at RT. The magnetization vs. temperature curves, measured in the 9 to 200 K range, showed that the sample exhibits two magnetic-phase transition temperatures at 29 K (T_g) and 120 K (T_CO). The magnetization isotherms, M (H), around these magnetic-phase transition temperatures for the sample are analyzed.     

X-ray diffraction studies of specific features in the atomic structure of Fe-Si alloys in the α area of the phase diagram

The atomic structure of Fe-Si alloys with a silicon concentration of 5–8 at % (α-area of the phase diagram) was studied using X-ray diffraction. The effect of quenching after annealing at a disordering temperature of 850°C on the structural state of the alloys was elucidated. It is shown that the quenched samples are characterized by a short-range ordering; namely, there is a local B2-type order at a concentration of 5–6 at % Si and, in addition, DO₃-phase clusters are formed at 8 at % Si. The atomic structure of B2 clusters and their nearest surroundings is established.     

Superconductivity in the new Nb5Si3−xBx phase

In the Nb–Si system the αNb₅Si₃ phase is not a superconductor. Considering the high solubility of this phase for boron, in this work it has been evaluated the effect of boron doping in αNb₅Si₃ on the electrical and magnetic properties of the produced materials. It has been shown that boron doping promoted superconductivity for all studied compositions. The Nb₅Si_2.4B_0.6 sample presented the maximum superconducting critical temperature (7.8 K) with upper critical magnetic field close to μ_oH～4.2 T. Thus, this work shows for the first time superconductivity in a phase with Cr₅B₃-prototype structure.