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At least 1,765 records · Page 98
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

The Random Change of Coordination Number in the Blume-Capel Model

The spin-1 Blume-Capel model is investigated under the random change of coordination numbers on the Bethe lattice (BL). Various coordination numbers are taken as couples and varied randomly on the shells of the BL. The randomization is realized in the standard-random approach and the formulation is obtained in terms of exact recursion relations. The phase diagrams are obtained by calculating the places of the second- and first-order phase transitions and tricritical points.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Modeling the Magnetocaloric Effect of La{sub 0.67}Pb{sub 0.33}MnO{sub 3} by the Mean-Field Theory

In this paper, a theoretical analysis linked to the molecular field theory and the Bean-Rodbell model has investigated the magnetocaloric effect (MCE) in a La{sub 0.67}Pb{sub 0.33}MnO{sub 3} sample. We have demonstrated how the mean-field model can adequately simulate the magnetic and magnetocaloric properties of this magnetic system, which may be a candidate material to magnetic refrigerants. The second-order ferromagnetic (FM)-paramagnetic (PM) phase transition has been confirmed. The expressions describing the entropy and its change under the influence of the magnetic field have been derived. A scaling method based on the mean-field theory leads to a direct calculation of the exchange parameter (λ), the saturation magnetization (M{sub 0}), the total angular momentum (J), and the gyromagnetic factor (g) of our sample. These constants were used to simulate M(H,T) isotherms, which are used again for the determination of the magnetic entropy change (−ΔS{sub M}) of La{sub 0.67}Pb{sub 0.33}MnO{sub 3}.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Observation of Magnetic Phase Transition and Magnetocaloric Effect in Ba{sub 1−x}Sr{sub x}MnO{sub 3−δ}

A series of perovskite materials (BaMnO{sub 3−δ} and Ba{sub 1−x}Sr{sub x}MnO{sub 3}) were synthesized by a solid-state reaction method. The content of oxygen vacancy can be controlled by verifying the sintering temperature and atmosphere. Two obvious divarications between zero-field-cooling curves and field-cooling curves were observed in a high-temperature region of 250–300 K and at a low temperature of 40 K, respectively. It is found that the different oxygen vacancy contents in BaMnO{sub 3−δ} cause the change of crystalline structure and variation of the divarication temperature at a high-temperature region. The factor that controls the negative magnetic susceptibility was discussed. The superexchange interaction between Mn ions can give rise to the coexistence of the ferromagnetic phase and antiferromagnetic phase in the system. We further performed the study of magnetocaloric effect in Sr-doped BaMnO{sub 3}. However, the values of magnetic entropy change and refrigeration coefficient are much smaller than those of rare-earth manganites due to the weak magnetocrystalline anisotropy in Ba{sub 1−x}Sr{sub x}MnO{sub 3}.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Heat Capacity and Mössbauer Study of Self-Flux Grown FeTe Single Crystal

We report mainly the heat capacity and Mössbauer study of self-flux grown FeTe single crystal, which is a ground state compound of the Fe chalcogenides superconducting series i.e. FeTe{sub 1−x}(Se/S){sub x} The as grown FeTe single crystal is large enough to the tune of a few centimetres and the same crystallizes in tetragonal structure having space group of P4/nmm. FeTe shows the structural/magnetic phase transition at 70 K in both magnetic and resistivity measurements. Heat capacity measurement also confirms the coupled structural/magnetic transition at the same temperature. The Debye model fitting of low temperature (below 70 K) heat capacity exhibited Debye temperature (𝜃{sub D}) to be 324 K. Mössbauer spectra are performed at 300 and 5 K. The 300-K spectra showed two paramagnetic doublets and the 5-K spectra exhibited hyperfine magnetic sextet with an average hyperfine field of 10.6 Tesla matching with the results of Yoshikazu Mizuguchi et al.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Insight into Phase Transition, Electronic, Magnetic, Mechanical, and Thermodynamic Properties of TbTe: a DFT Investigation

Theoretical investigation on TbTe for its structural, electronic, magnetic, and thermodynamic stuffs has been carried within density functional theory (DFT) as implemented in WIEN2K code. TbTe was found stable in ferromagnetic phase. The calculated ground-state parameters were found in a good agreement with the experimental data. The compound was found to have a structural stability in cubic B1 (NaCl-type structure) phase, but under the application of high pressure (at 27 GPa), it undergone to B2 (CsCl-type structure) phase of pressure. The second-order elastic constants and mechanical properties like Young’s modulus, Shear modulus, Poisson ratio, Cauchy pressure (C{sub 12}–C{sub 44}), and Pugh’s ratio (B/G) were calculated. The present calculations confirmed the ductile nature of TbTe. Further, the thermodynamic investigations have been carried using quasi-harmonic Debye approximation. We have calculated the pressure and temperature dependence of Debye temperature (𝜃{sub D}), bulk modulus (B), thermal expansion (α), heat capacities (C{sub V}), and entropy (S) in the temperature range of 0 to 1000 K and pressure range of 0 to 25 GPa.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Synthesis and Magnetic Properties of New Pyrochlore Fe{sub 2}Mn{sub 2}O{sub 7} Compound

The compound Fe{sub 2}Mn{sub 2}O{sub 7} was prepared using ceramic method Fe{sub 2}O{sub 3} and MnO{sub 2} as a precursor. It was crystallized in pyrochlore oxides type monoclinic structure in the P2/M space group. Concerning the magnetic study, they showed that the compound undergoes a ferromagnetic (FM) transition near 68 K. As for the Arrott’s plots, they revealed a second-order phase transition. The variation of the applied magnetic field revealed a magnetocaloric effect that is manifested by a maximum variation of the magnetic entropy of 1 J/kgK for a field change of 5 T, with a cooling power of 63J/kg.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Observation of Room-Temperature Range Magnetocaloric Effect in PrSr{sub 1−x}Pb{sub x}Mn{sub 2}O{sub 6} (0.4 ≤ x ≤ 0.6) Double-Perovskite Manganite System

The Pr(Sr{sub 1−x}Pb{sub x})Mn{sub 2}O{sub 6} (0.4 ≤ x ≤ 0.6) double-perovskite manganite samples have been produced by using a sol-gel method. The Rietveld refinement method has been used to analyse the structural properties, and the results show that all samples are crystallized in monoclinic structure with P21/n space group without any impurity phase. The temperature dependence of magnetization measurement shows that samples exhibit paramagnetic to ferromagnetic phase transition by decreasing temperature. The values of the T{sub C} are determined as 286, 280 and 278 K for x= 0.4, 0.5 and 0.6 in Pr(Sr{sub 1−x}Pb{sub x})Mn{sub 2}O{sub 6} samples, respectively. Isothermal applied magnetic field-dependent magnetization curves are used to determine both order of magnetic phase transition and magnetic entropy change (ΔS{sub M}) value. −ΔS{sub M} values are found as 3.68, 3.15 and 2.85 J kg{sup − 1} K{sup − 1} for PSPM-4, PSPM-5 and PSPM-6 samples, respectively. Additionally, magnetic phase transition of the samples is identified as second-order magnetic phase transition that indicates accompanying small thermal and magnetic hysteresis to the samples during the phase transition. Relative cooling power (RCP) values are determined as 282, 233 and 268 J kg{sup − 1} for PSPM-4, PSPM-5 and PSPM-6 samples, respectively.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Magnetic Field Induced Metal-Insulator Transition and Reentrant Metallic Behaviors in Kish Graphite

The metal-insulator transition on electronic transport properties and the extraordinarily large magnetoresistances (MR) at low temperature had been found in Kish graphite under low magnetic field. Along with the large ordinary magnetoresistances (MR), Kish graphite had clearly shown the reentrant metallic behavior at low temperature under 600 and 900 Oe applied magnetic fields. Besides a dominating ordinary MR (OMR) mechanism in the free-electron mode, the Coulomb interacting quasiparticles within graphite sheet play some role to extraordinarily large OMR.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Study of Magnetocaloric Effect on Strontium Ferrite SrFe{sub 12}O{sub 19} Ceramic

Magnetocaloric effect on SrFe{sub 12}O{sub 19} ceramic have been studied using Monte Carlo simulation. The thermal magnetization, dM/dT, magnetic entropy, and the specific heat of SrFe{sub 12}O{sub 19} ceramic are obtained for several magnetic fields. The temperatures dependence of the magnetic entropy and of the adiabatic temperature for a several magnetic field have been obtained. The field dependence of relative cooling power (RCP) of SrFe{sub 12}O{sub 19} ceramic has been determined for a several magnetic fields. The magnetic hysteresis cycle of SrFe{sub 12}O{sub 19} ceramic has been obtained for a several temperatures. The obtained values are close to the experimental values. The transition paramagnetic to ferromagnetic is found at the Curie temperature. The second phase transition is also obtained around the Curie temperature.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

The Magnetic Properties of the Spin-1 Ising Fullerene Cage with a Core-Shell Structure

Magnetic properties of Ising-type endohedral fullerene (IEF), which are formed by an atom encapsulated inside a diluted magnetic spherical cage, have been investigated theoretically as a function of its system parameters, and magnetism calculations by using effective field theory (EFT) were performed. Theoretical studies have been carried out to understand the effect on magnetic behaviors by the diluted magnetic surface, crystal field, and exchange couplings. The results display the first second-order phase transitions and tricritical point. The reentrant phenomena have been observed within a certain parameter range.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

The Structural, Electronic, and Magnetic Features of Co{sub 2}Mn{sub 1−x}Al{sub x}Ti (0 ≤ x ≤ 0.6) Heusler Alloys

The structural, electronic, and magnetic features of Co{sub 2}Mn{sub 1−x}Al{sub x}Ti (0 ≤ x ≤ 0.6) Heusler alloys (HAs) have been systematically investigated by experimental techniques (X-ray powder diffraction, scanning electron microscopy, and magnetization measurements) and ab initio theoretical calculations. Whole alloys have cubic structures at room temperature and exhibit ferromagnetic properties. The Curie temperature of Co{sub 2}Mn{sub 1−x}Al{sub x}Ti (x = 0) is approximately 790 K, which is compatible with other studies. By increasing Al content, the temperature dependence of magnetization trials has proved a linear diminishment on Curie temperature and saturation magnetization.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Structural Characterization and Magnetic Properties of SrCo{sub 2}Fe{sub 16}O{sub 27} Hexaferrite Synthesized by Citric Acid Sol-Gel Method

The W -type SrCo{sub 2}Fe{sub 16}O{sub 27} (SCFO) hexagonal ferrite samples were prepared by the sol-gel method using metal nitrate as raw material, citric acid as complexing agent, and water as solvent. The effects of different sintering temperatures and holding times on the phase composition and morphology properties were explored in detail by using the X-ray diffractometer and field emission scanning electron microscope. The X-ray diffraction, microstructure, and magnetic performance of most optimum sintering temperature and holding time of preparing single-phase SCFO hexagonal ferrite were investigated. The experimental results indicate that the best optimum sintering temperature and holding time of SCFO hexagonal ferrite are 1280 °C and 4 h, respectively. Field emission scanning electron microscopy results indicate that the size of the particles is about 5–10 μ m. The result of magnetic measurement shows that SCFO hexagonal ferrite exhibits the soft magnetic property and the Curie temperature (T{sub C}) is ∼ 850 K. Furthermore, by analyzing the Arrott plot, the phase transition near the T{sub C} is a secondary phase transition.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Comparative Study of Magnetocaloric Effect in La{sub 0.67}Ca{sub 0.20}Sr{sub 0.13}MnO{sub 3} Manganite Prepared by Sol-Gel Method and Spark Plasma Sintering

The La{sub 0.67}Ca{sub 0.20}Sr{sub 0.13}MnO{sub 3} nanoparticles and bulks were prepared by using sol-gel method (SGM) and spark plasma sintering (SPS), respectively. The X-ray diffraction analysis indicates each sample crystal in a single rhombohedral perovskite structure (space group R 3-bar c). The Curie temperature is slightly improved and the phase transition is sharpened after processing with SPS. The enhancement of magnetocaloric effect (MCE) is also observed in the samples prepared by SPS. For a field change of 3 T, the maximum isothermal entropy change is improved by 32 and 117% when different sintering temperatures and sintering pressures are applied. The magnetic phase transition and MCE can be well described by the Landau theory. The present studies suggest that the perovskite bulks prepared by SPS could be useful for the application of magnetic refrigeration.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2019

Microstructural, Magnetic, Magnetocaloric, and Electrical Properties of Ni{sub 0.4}Mg{sub 0.3}Cu{sub 0.3}Fe{sub 2}O{sub 4} Ferrite Prepared Using Sol–Gel Method

Detailed investigations of the microstructural, magnetic, magnetocaloric, and electrical properties of Ni{sub 0.4}Mg{sub 0.3}Cu{sub 0.3}Fe{sub 2}O{sub 4} ferrite synthesized by sol-gel method have been investigated. XRD pattern indicates that sample has cubic spinel structure with Fd 3-bar m symmetry. The cation distribution of the sample has been determined by Rietveld refinement. Temperature dependence of magnetization shows that sample exhibits a second-order PM to FM phase transition at the Curie temperature T{sub C} = 690 K. From the M(μ{sub 0}H,T) data, we found that the maximum entropy change |ΔS{sub M}{sup max}| reaches value of about 1.56 J kg{sup − 1} K{sup − 1} and relative cooling power (RCP) of 136 J kg{sup − 1} at μ{sub 0}H = 5 T. From electrical conductivity curves, the estimated value of the activation energy is equal to 0.349 eV. The Nequist diagram at different temperatures reveals that the grain boundary contribution is responsible to the conduction process for the studied sample.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Spontaneous Magnetization Estimation and Magnetocaloric Effect Study by Means of Theoretical Models in La{sub 0.67}Pb{sub 0.33}MnO{sub 3}

In this work, the dependence of the spontaneous magnetization on temperature of La{sub 0.67}Pb{sub 0.33}MnO{sub 3} manganite has been experimentally estimated from the Arrot plots and from the magnetic entropy change. The magnetocaloric effect (MCE) in this sample has been simulated by the Landau theory and the phenomenological model. Theoretical simulations correlated well with the experimental results. A secondorder ferromagnetic-paramagnetic (FM-PM) phase transition has been proved for La{sub 0.67}Pb{sub 0.33}MnO{sub 3} compound.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Radiative Transfer, Non-transparency, Stability Against Quench in Superconductors and Their Correlations

Radiative transfer calculations are presented for multi-filamentary BSCCO 2212 and 2223 and for thin film-coated YBaCuO 123 superconductors. A strictly radiative view allows conductor morphology of these superconductors to be interpreted as particulate or quasi-particulate objects. Problems then arising from non-regular particle shape, missing refractive indices, dependent scattering and superconductor material (diamagnetic) properties are solved in a multi-step, approximation approach. Both types of conductors are shown to be non-transparent to mid-IR radiation. Non-transparency may become critical for superconductor stability. The obtained extinction cross sections converge near the phase transition. A substantial difference of extinction properties between the superconducting and normal conducting state thus cannot be observed. As a first corollary from non-transparency, application of the additive approximation for the total thermal conductivity used in previous numerical calculations of the BSCCO and YBaCuO superconductors is confirmed. Second, within transit time intervals, the length of which depends on optical thickness, no uniform equilibrium conditions of electron pair density within the conductor cross section and near critical temperature are observed. Phase transition does not proceed uniformly, neither spatial nor temporal, and the order (succession on time scales) of local events (temperature variations, quench), in extreme cases, is completely dissolved. As another corollary, non-transparency even makes the existence of uniquely defined, physical time scales doubtful in a superconductor. The obtained results are expected to improve existing stability models and will be of practical value for future materials development.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Magnetocaloric Effect and Critical Behavior Investigations in La{sub 0.45}Pr{sub 0.2}Sr{sub 0.35}MnO{sub 3} Manganite Oxide

Structural, magnetic, and magnetocaloric effect (MCE) properties and the critical behavior of the La{sub 0.45}Pr{sub 0.2}Sr{sub 0.35}MnO{sub 3} sample prepared by the sol–gel method have been investigated. The X-ray diffraction characterization reveals that our sample crystallizes in the orthorhombic structure with Pnma space group. Magnetization versus temperature measurement shows that our compound exhibits a ferromagnetic–paramagnetic transition. The Curie temperature is found to be 348 K. Close to this temperature, the maximum values of the magnetic entropy change (ΔS{sub M}) and the relative cooling power (RCP) are found to be 2.84 J/kg/K and 230 J/K under µ{sub 0}H = 5 T, respectively. The Arrott plots revealed that this compound exhibits a second-order magnetic phase transition at T{sub C}. The critical behavior has been investigated by several techniques which are the modified Arrott plot method, the Kouvel–Fisher method, and the critical isotherm analysis. The obtained critical exponents β, γ, and δ are consistent with those predictions of the mean-field model. Their reliability was confirmed by Widom’s scaling relation, as well as the universal scaling hypothesis.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Nickle Concentration Effect on Low Magnetic Field Magnetocaloric Properties for Ni{sub 2+x}Mn{sub 1−x}Ge

Low magnetic field magnetocaloric properties of Ni{sub 2+x}Mn{sub 1−x}Ge near phase transition from a ferromagnetic to a paramagnetic state were investigated theoretically for level of doping x = 0.1 and 0.2. It is shown that the change of Ni content allows the magnetocaloric effect in Ni{sub 2+x}Mn{sub 1−x}Ge to be tunable, which is more practical for construction magnetocaloric refrigeration. Furthermore, the results show that the magnetocaloric properties of Ni{sub 2+x}Mn{sub 1−x}Ge samples are significantly larger and comparable with some magnetocaloric properties of many materials like La{sub 1−x}Cd{sub x}MnO{sub 3}, Gd{sub 1−x}Ca{sub x}BaCo{sub 2}O{sub 5.5}, Ge{sub 0.95}Mn{sub 0.05}, La{sub 1.25}Sr{sub 0.75}MnCoO{sub 6}, Ni{sub 58}Fe{sub 26}Ga{sub 28}, and MnAs films.

36 MATERIALS SCIENCE↗