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At least 1,729 records · Page 96
All sourcesNASA NTRSDOE OSTI
DOE OSTI2017

Analysis of Magnetization as a Function of Temperature for CoMn{sub 1−x}Fe{sub x}Ge

Our experimental measurements for the magnetization M as a function of temperature are analyzed for the multilayer structure of CoMn{sub 1−x}Fe{sub x}Ge for x = 0.085 and x = 0.12 at constant magnetic fields of 3, 5, and 7 T. For this analysis, a power-law formula is used below the transition temperature T{sub C} for these compositions and the values of the critical exponent β for the magnetization M (order parameter) are determined. Our analysis indicates that a first order transition at lower concentrations (x = 0.085) changes toward a second order at higher concentrations (x = 0.12) at constant magnetic fields (3, 5, and 7 T) for the multilayer CoMn{sub 1−x}Fe{sub x}Ge, as observed experimentally.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Influence of Ag Substitution on Structural, Critical Behavior and Magnetocaloric Characteristics in La{sub 0.5}Ca{sub 0.5−x}Ag{sub x}MnO{sub 3} (0 ≤ x ≤ 0.1) Systems

Based on the modified Arrott plots and the critical isotherm analysis, the critical exponents of both La{sub 0.5}Ca{sub 0.5}MnO{sub 3} and La{sub 0.5}Ca{sub 0.4}Ag{sub 0.1}MnO{sub 3} systems have been comprehensively studied and investigated near the paramagnetic-ferromagnetic (PM-FM) phase transition. Critical exponents, β, γ, and δ, which are corresponding to the thermal variation of the spontaneous magnetization, initial susceptibility, and the isothermal magnetization, respectively, obeyed the Widom scaling law δ = 1 + γ/β. It indicates thus the validity the obtained results. We performed then another analysis of the critical behavior via the magnetocaloric effect presenting an accord with the analysis of magnetization measurements. Relative cooling power (RCP) follows the evolution of the magnetic field as RCP ∼ H{sup 1+1/δ}. The acquired critical exponents were in conformity with those gotten from the modified Arrott plots and Widom scaling law.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Critical Behavior of the Specific Heat of Pr{sub 0.6}Sr{sub 0.4}Mn{sub 1−x}Fe{sub x}O{sub 3} Manganites

The specific heat of Pr{sub 0.6}Sr{sub 0.4}Mn{sub 1−x}Fe{sub x}O{sub 3} (x = 0 and 0.1) manganites was measured in the temperature range 80–350 K. A nature of the ferromagnetic phase transition is studied via the analysis of a specific heat critical behavior near the Curie temperature with account of correction to the scaling. The critical indices calculated with the account of correction to the scaling α = −0.0981 and −0.1208 for x = 0 and 0.10, correspondingly, are close to those of the 3D Heisenberg model.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Unordinary Ferromagnetic Phase Transitions in Mn- and Fe-doped Half-Heusler CoTiSb

Unordinary ferromagnetic character and high Curie temperature around 900 K have been detected for CoTi {sub 1−x}M{sub x}Sb (M = Mn and Fe, x = 0.1) multiphases systems. The synthesis process was carried out at high temperature in evacuated quartz tubes. The x-ray powder diffraction analysis through Rietveld method was applied to refine the crystal structure. By manipulating the search match program, three nanocrystalline phases of CoTiSb (64.5%), CoSb (15.5%), and TiSb{sub 2} (20.5%) for Mn-doped system and CoTiSb (33%), Ti{sub 3}Sb (35.3%), and CoTi{sub 2} (30.7%) for Fe-doped system were recognized. The term “phase” was used to address the coexistence of different stable chemical compositions for the same half-Heusler alloy. The scanning electron microscope (SEM) and the high-resolution transmission electron microscope (HR-TEM) were applied to characterize the morphology, size, shape, crystallinity and lattice spacing. Good matching was detected between the obtained d values using electron diffraction data and those calculated from x-ray analysis. Well defined nanocrystalline structure with average interatomic distances equaling 0.229 nm, 0.286 nm together with many successive rings with sharp diffraction spots characterized the Fe-doped system. In contrast, partially distorted structured planes and smeared halo surrounded by weak rings were detected in the Mn-doped system. Thermo-magnetic measurements (M-T) were performed for both systems. In addition to Curie transition, many magnetic phase transitions were detected and confirmed through the ΔM/ ΔT vs. T relationship. The field-dependence magnetization (M-H) was measured at different temperatures. The M-H hysteresis loops showed the ferromagnetic character for both systems. Tentative magnetic phase diagrams are suggested as a guide for future research.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Effect of Sodium Doping on Magnetic and Magnetocaloric Properties of La{sub 0.65}Sr{sub 0.35}MnO{sub 3} Manganites

A series of Na-doped manganites La{sub 0.65}Sr{sub 0.35−x} Na{sub x}MnO{sub 3} (x = 0.20, 0.25, 0.30, and 0.35) were synthesized via sol-gel method. The structural, magnetic, and magnetocaloric properties of the samples were investigated by XRD, TEM, Raman spectroscopy, and magnetization measurement. Rietveld refinements of XRD patterns shown that samples were single phase with a space group R-3c. The variation of magnetization versus temperature in a magnetic applied field demonstrated a phase transition from ferromagnetic to paramagnetic. The value of T{sub C} decreased from 345 to 330 K when x increases from 0.20 to 0.35. Arrott plots shown that all samples exhibit a second-order magnetic phase transition. The maximum absolute values of the magnetic entropy change were found to be 0.91, 0.93, 1.12, and 1.10 J/kg for x = 0.2, 0.25, 0.3, and 0.35, respectively. Moreover, relative cooling power has been discussed.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Critical Behavior Near the Paramagnetic-Ferromagnetic Phase Transition in La{sub 1−x}Sr{sub x}CoO{sub 3} (x = 0.3 and x = 0.5) Cobaltites

In this work, we present the study of structural, magnetic and critical behavior of La{sub 1−x}Sr{sub x}CoO{sub 3} (x = 0.3 and 0.5) cobaltites. X-ray diffraction analysis using the Rietveld refinement shows that the sample La{sub 0.7}Sr{sub 0.3}CoO{sub 3} crystallizes in a rhombohedral distorted perovskite structure with R3̄c space group, while La{sub 0.5}Sr{sub 0.5}CoO{sub 3} exhibits both phases rhombohedric and cubic, with R3̄c and Pm3̄m space groups. Our samples exhibit thermo-magnetic irreversibility behavior in the magnetization data under a magnetic applied field of 0.05 T. The temperature dependence of magnetic susceptibility at higher temperatures for La{sub 0.7}Sr{sub 0.3}CoO{sub 3} sample reveals the presence of the Griffiths’ phase above the Curie temperature, while it follows the Curie–Weiss law perfectly for La{sub 0.5}Sr{sub 0.5}CoO{sub 3}. Different techniques including the modified Arrott plot, the Kouvel-Fisher method and the critical isotherm analysis have been used to analyze the paramagnetic-ferromagnetic phase transition. The values of the critical exponents associated to this transition were close to those expected in the mean-field model. The validity of the calculated critical exponents has been confirmed by the scaling equation of state. The determined critical exponents using various methods are found to follow the scaling equation with the magnetization data scaled into two different curves below and above the critical temperature.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Kinetic Phase Transition in the Semi-infinite Spin-1 Ising Model Under a Periodically Oscillating Magnetic Field

The kinetics of the semi-infinite Ising model in the presence of a time-dependent oscillating external field is studied within the framework of the mean-field approach. We use the Glauber-type stochastic dynamics to describe the evolution time of this system. We present a variety of phase in a semi-infinite cubic ferromagnet with spin-1 in two different, planes; phase diagrams contain (BF, S {sub 1}P, SP) phase, (BF, S {sub 1}F, SP) phase, completely ordered phase (BF, S {sub 1}F, SF) and completely disordered phase (BP, S {sub 1}P, SP), which strongly depend on interaction parameters. The nature first and second orders of the transitions is characterized by studying the thermal behaviors of the dynamic magnetizations. Furthermore, the system exhibits dynamical tricritical phenomenon and the reentrant behaviors. The magnetizations and phase space trajectories are given and discussed.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Characterization of Polycrystalline Fe{sub 2}B Compound with High Saturation Magnetization

The crystal structure, magnetic phase transition, high saturation magnetization, and chemical status of polycrystalline Fe{sub 2}B compound were investigated. The results show that a single phase with the CuAl{sub 2}-type tetragonal structure with space group of I4/mcm is clearly observed, and the Curie temperature is 1017 K. A maximum value of saturation magnetization (M) of 173 Am{sup 2}⋅kg{sup −1} was achieved when the saturated magnetic field was ∼5 kOe at 300 K. Moreover, XPS proved that the chemical status of Fe and B elements existed in the form of Fe{sub 2}B in this compound. The polycrystalline Fe{sub 2}B compound with large saturation magnetization is economical and non-toxic raw materials, which would be of considerable importance for exploring the potential applications in the magnetic functional material field.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Magnetic and Transport Properties of Mn{sub 2}Ni{sub 1.8}In{sub 0.2} Alloy

The structural, magnetic, and transport properties of the Mn{sub 2}Ni{sub 1.8}In{sub 0.2} alloy have been presented. It was observed to form in tetragonal structure. Multiple magnetic phase transitions have been detected in magnetization data over the temperature range from 2 to 350 K. From the resistivity, ac susceptibility, and heat capacity data, the superconducting transition are observed at T = 5.6 K. The alloy is considered to be one of the potential candidates from the Mn-rich alloys, which has shown both magnetic and superconducting properties, which can be useful from application point of view.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Mixed Spin (1, 5/2) Ising Ferromagnetic Blume-Capel Model Under Time-Dependent Sinusoidal Magnetic Field: an Effective-Field Theory Analysis

Non-equilibrium magnetic properties are studied in a two-dimensional mixed spin (1, 5/2) Blume-Capel Ising system under the influence sinusoidal magnetic fields using the effective-field theory based on a decoupling approximation. A dynamic equation of motion has been solved for a square lattice by utilizing a Glauber-type stochastic process. Dynamic phase diagrams are presented in the reduced magnetic field amplitude and reduced temperature plane (h,T) and reduced crystal-field and reduced temperature plane (Δ,T). We observe that the system exhibits dynamic tricritical behaviors. Phase diagrams contain the paramagnetic (p), ferrimagnetic (i), and coexistence or mixed-phase regions, namely the i,p, and i + p, which strongly depend on the crystal-field interaction parameter and oscillating magnetic field frequency. We have found that the system only exhibits a second-order transition line in the higher-frequency region. We have also compared our result with the mean-field prediction in order to indicate the effects of correlations and found that some of the dynamic first-order phase lines disappeared.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

The Mean-Field Theory for the Study of the Magnetocaloric Effect of Nd{sub 0.67}Ba{sub 0.33}Mn{sub 0.98}Fe{sub 0.02}O{sub 3} Manganite

In this paper, we exploit the mean-field theory and the Bean-Rodbell model to justify the magnetocaloric effect (MCE) on Nd{sub 0.67}Ba{sub 0.33}Mn{sub 0.98}Fe{sub 0.02}O{sub 3} manganite. The modeling of magnetic properties was investigated. The Bean-Rodbell equation of state leads the system to second-order ferromagnetic (FM)-paramagnetic (PM) phase transition. The expressions describing the magnetic entropy change under the influence of magnetic field have been derived. A large magnetic entropy has been observed in our sample with a peak centered on the Curie temperature. The theoretical calculations are compared with experimental behavior.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Phase Diagrams and Magnetic Properties of a Spin-3/2 Blume-Capel Nanoparticle

Within the framework of Monte Carlo simulation, the critical behavior and the magnetic properties of a spherical nanoparticle with the presence of a crystal field are studied. The particle is formed by a ferromagnetic core of spin-3/2 surrounded by a ferromagnetic shell of spin-3/2 with antiferromagnetic interface coupling located on a simple cubic lattice. The ground-state phase diagrams are obtained in the plane of the crystal field versus the shell coupling and the interface coupling versus the crystal field. The effects of the shell coupling and the crystal field on both the critical and compensation temperatures are investigated. The phase diagrams exhibit a rich variety of behaviors, which include the first- and second-order phase transitions, the multiphase critical point, the isolated critical point, and the compensation point.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Lattice Location Effect of Ni{sub 50}Mn{sub 36}Sn{sub 14} Heusler Alloy

The magnetic properties of Ni{sub 50}Mn{sub 36}Sn{sub 14} Heusler alloy with L{sub 21} structure (L{sub 21}-NiMnSn HA) are investigated by using the Kaneyoshi approach (KA) within the effective field theory. Mn atoms have two different magnetic properties, and Sn atoms also have two different magnetic properties, according to the lattice location. The L{sub 21}-NiMnSn HA and its components (Mn1, Mn2, Sn1, Sn2, and Ni) exhibit a second-order phase transition from the ferromagnetic phase to the paramagnetic phase at T{sub C} = 2.824. The magnetizations of the HA are obtained as Sn2 > Ni > Mn2 > L{sub 21}-NiMnSn HA > Sn1 > Mn1 at T < T{sub C}. We suggest that the magnetizations of the HA decrease from that of its core atom to its corner atoms. We refer to the different magnetization behavior of the Mn atoms and the Sn atoms of the HA as “lattice location effect (LLE)” because the magnetic difference of the Mn atoms and the Sn atoms of the HA results from their lattice location on the L{sub 21} structure. While the coercive fields are the same with each other, their remanence magnetizations are different. Our theoretical results of M(T) and M(H) of L{sub 21}-NiMnSn HA are in quantitatively good agreement with some experimental results of NiMnSn alloys. Furthermore, the magnetization critical exponent is obtained. Its value is in good agreement with some experimental results and the mean field theory. This case indicates that the model exhibits a universal critical behavior.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Monte Carlo Study of the Core-Shell Phase Transitions in Nanostructure Systems

We present in this work the effects of the external magnetic field, the exchange anisotropy, the coupling exchange interaction and the temperature on the magnetic properties of the nanostructure systems. More exactly, the effect of the core-shell spin Ising systems has been also examined. A number of favorable phenomena such as the wetting magnetic and the layering transitions have been investigated. The Monte Carlo simulations have been used to investigate the behavior of these nanostructure systems and to obtain the interesting results presented in the form of the phase diagrams.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Monte Carlo Study of Magnetic and Thermal Phase Transitions of a Diluted Magnetic Nanosystem

Monte Carlo (MC) simulation method with the Metropolis algorithm is used to study the magnetic and thermal phase transition properties of a spherical nanoparticle. The system consists of two concentric spheres of rays R{sub C} and R{sub S}, respectively (R{sub c} < R{sub s}). For r < R{sub c}, the spin is σ = ±3 /2 and ±1 /2, and for R{sub C} < r ≤ R{sub S}, the spin is S = ±7 /2, + 5/2, ±3 /2, and ±1 /2 with antiferromagnetic interface coupling. Between R{sub C} and R{sub S}, the sites are populated with the probability (p). We present a detailed discussion on the magnetic and thermal phase transition characteristics of the system under consideration. Our investigations show that this system can be used as a magnetic nanostructure possessing potential applications in magnetism.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Evolution of the Fe{sup 3+} Ion Local Environment During the Phase Transition ε-Fe{sub 2}O{sub 3} → α-Fe{sub 2}O{sub 3}

Evolution of the local environment of Fe{sup 3+} ions in deposited Fe{sub 2}O{sub 3}/SiO{sub 2} nanoparticles formed in samples with different iron contents was investigated in order to establish the conditions for obtaining the stable ε-Fe{sub 2}O{sub 3}/SiO{sub 2} samples without impurities of other iron oxide polymorphs. Microstructure of the samples with an iron content of up to 16% is studied by high-resolution transmission electron microscopy, X-ray diffraction analysis, and Mössbauer spectroscopy, and their magnetic properties are examined. At iron concentrations below 6%, calcinations of iron-containing precursor nanoparticles in a silica gel matrix lead to the formation of the ε-Fe{sub 2}O{sub 3} iron oxide polymorphic modification without foreign phase impurities, while at the iron concentration in the range of 6–12%, the hematite phase forms in the sample in the fraction of no more than 5%. It is concluded on the basis of the data obtained that the spatial stabilization of iron-containing particles is one of the main factors facilitating the formation of the ε-Fe{sub 2}O{sub 3} phase in a silica gel matrix without other iron oxide polymorphs. It is demonstrated that the increase in the iron content leads to the formation of larger particles in the sample and gradual changes of the Fe{sup 3+} ion local environment during the phase transition ε-Fe{sub 2}O{sub 3} → α-Fe{sub 2}O{sub 3}.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Model Proposed to Calculate the Exchange Interaction in Ni/Pt Multilayers

The behavior of spin waves is mainly influenced by all parameters that characterize the magnetic material such as exchange interactions, surface and volume anisotropy effects, dipole interactions, and phase transitions and imperfections. In this article, we develop a simple calculation model based on a ferromagnetic anisotropic system that has allowed us to obtain numerical estimates for the Pt exchange interaction and the interlayer coupling force at various Ni/Pt multilayers.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Interplay Between Superconductivity and Magnetism in Cu-Doped FeSe Under Pressure

Superconductivity in iron selenide is very sensitive to any dopants at levels of only a few percent. Here, we demonstrate how iron substitution by copper destroys superconductivity in FeSe, and argue why superconductivity reemerges under pressure. Detailed Mössbauer spectroscopy studies on {sup 57}Fe-enriched Cu{sub 0.04}Fe{sub 0.97}Se reveal that at low temperatures part of the iron sites is magnetically ordered, and static magnetic moments destroy the superconducting pairing. Application of pressure leads to a collapse of the static magnetism and restoration of superconductivity with the maximal T{sub c} value at pressure ∼ 8 GPa. The further pressure increase provokes a structural tetragonal-to-hexagonal phase transition and disappearance of superconductivity similarly to FeSe. The hyperfine parameters of the non-superconducting hexagonal phase of Cu{sub 0.04}Fe{sub 0.97}Se reveal a less covalent character of Fe–Se bonds and a noticeable higher value of the electric field gradient comparing to the tetragonal phase.

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