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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 901 records · Page 50
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Enhanced Magneto-optical and Photocatalytic Properties of Ferromagnetic Mg{sub 1−y}Ni{sub y}Fe{sub 2}O{sub 4} (0.0 ≤ y ≤ 1.0) Spinel Nano-ferrites

Spinel Mg{sub 1−y}Ni{sub y}Fe{sub 2}O{sub 4} (0.0 ≤ y ≤ 1.0) ferrite magnetic nanoparticles (MNPs) were synthesized by a sol-gel combustion method using urea as the reducing agent/fuel. Powder X-ray diffraction (XRD) analysis revealed the formation of cubic spinel Mg{sub 1−y}Ni{sub y}Fe{sub 2}O{sub 4} powders and confirmed pure crystalline phase and the average crystalline size of 21.25 to 17.04 nm. Functional group bonds between metal and oxygen (M–O) were confirmed by Fourier transform infrared (FT-IR) spectra. The microstructure of the powders was recorded by high-resolution scanning electron microscope (HR-SEM) and confirmed the particle-like surface morphology with smaller agglomeration, due to the magnetic interaction of the particles. Energy-dispersive X-ray (EDX) results showed the composition of the expected elements and confirmed the phase purity of the products. Vibrating sample magnetometer (VSM) technique recorded at room temperature was used to analyse the magnetic properties of the samples and the hysteresis loops showed the ferromagnetic behaviour. Moreover, the samples Mg{sub 1−y}Ni{sub y}Fe{sub 2}O{sub 4} NPs were tested for the photocatalytic degradation (PCD) of methylene blue (MB) dye and the sample y = 0.6 showed maximum degradation efficiency (96.83 %), due to the smaller particle size with higher surface area than other compositions. Furthermore, spinel Mg{sub 1−y}Ni{sub y}Fe{sub 2}O{sub 4} nano-photocatalysts can be reused several times without change of its catalytic activity.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

In-Plane Anisotropy Effect to the Spin-Wave Gap in Ultrathin Ferromagnetic Films at Finite Temperatures

In this paper, we investigate the calculated thickness- and temperature-dependent magnetization given in terms of the spin-wave gap alone for two-dimensional ultrathin ferromagnetic films with anisotropy and Zeeman energy sufficiently large to dominate over the dipolar interaction. The spin-wave gap was calculated for a magnetic field which is perpendicular to the plane. The calculated equations present a nonzero spin-wave gap at zero magnetic field which is strongly affected by anisotropies. The temperature-dependent magnetization strongly increases with decreasing thickness of the insulating spacer layer at the saturation field and at room temperature. We reported that the in-plane anisotropy strongly depends on the insulating spacer layer thickness and saturation field and overcomes the fourfold anisotropy in the spacer layers with thickness of below 0.8 nm. The results were also discussed in connection with experimental data given in Co{sub 0.9}Fe{sub 0.1}/MgO/C{sub 0.9}Fe{sub 0.1} films.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Propagation of Spin Waves Through an Interface Between Ferromagnetic and Antiferromagnetic Materials

Boundary conditions for order parameters at an interface between ferromagnetic (FM) and two-sublattice antiferromagnetic (AFM) materials are obtained in the continuous medium approximation similarly to the approach which allows one to take into account the finite thickness of the FM/FM interface, which is much less than spin wavelength. Three order parameters are considered inside an interface of finite thickness with the magnetization M of FM, magnetizations of both sublattices M{sub 1} and M{sub 2} of AFM. The uniform and non-uniform exchange between all order parameters are taken into account to the interface energy. Using these boundary conditions, the excitation of a surface evanescent spin wave is considered in AFM when the spin wave in FM falls onto this interface. The coefficients and the phases of transmission and reflection of spin wave through the FM/AFM interface are derived.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

A eutectic dual-phase design towards superior mechanical properties of heusler-type ferromagnetic shape memory alloys

Heusler-type ferromagnetic shape memory alloys possess attractive multifunctional properties, including magnetic field induced shape memory effect, magnetoresistance and magnetocaloric effect, owing to the unique concurrent magnetic and martensitic transformations. However, these intermetallics generally exhibit intrinsic high brittleness and low strength, which severely impede their workability for processing and applicability in real use. In this study, we demonstrate a new grain refining strategy by means of eutectic solidification to improve the mechanical properties in Ni-Mn-Sn-Fe alloys. In a fully eutectic microstructure, the average γ lamellae thickness was refined to ~170 nm and the composite showed a compressive strength of 1950 MPa, ductility of 19.5%, Young’s Modulus of 38 GPa, pseudoelasticity of 3.2% and high mechanical cyclic stability. The high mechanical performance is attributed to the effect of departmentalization of the brittle Heusler alloy by the densely distributed γ phase fine lamellae in resisting crack propagation. The eutectic Heusler composite exhibited a metamagnetic phase transformation, with a magnetic entropy change of 10.2 J/kg·K and a refrigeration capacity of 168 J/kg in a field change of 5 T.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Weyl type spin–orbit coupled ferromagnetic Bose–Einstein condensates under rotation

Highlights: Spinor Bose–Einstein condensates with Weyl type spin–orbit coupling and rotation are considered. Different topological patterns are obtained in the full parameter space. Effects of spin-independent and spin-dependent interactions on the ground state are also discussed. -- Abstract: We investigate the ground state of ferromagnetic Bose–Einstein condensates with Weyl type spin–orbit coupling and rotation. When the spin–orbit coupling is strong, the ground state of the system exhibits square lattice phase for the non-rotation case, while two half-skyrmion chains can occur, which is perpendicular to each other, for the rotation case. For the weak spin–orbit coupling case, half-skyrmion displays the symmetrical arrangement around Mermin–Ho vortex at the center of condensates as the rotation frequency is increased. Moreover, the effects of spin-independent and spin-dependent interactions on the ground state are also studied.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2017

A proposed method for electronic feedback compensation of damping in ferromagnetic resonance

Here, we propose an experimental technique for extending feedback compensation of dissipative radiation used in nuclear magnetic resonance (NMR) to encompass ferromagnetic resonance (FMR). This method uses a balanced microwave power detector whose output is phase shifted π/2, amplified, and fed back to drive precession. Using classical control theory, we predict an electronically controllable narrowing of field swept FMR line-widths. This technique is predicted to compensate other sources of spin dissipation in addition to radiative loss.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Metamagnetism and kinetic arrest in a long-range ferromagnetically ordered multicaloric double perovskite Y 2 CoMnO 6

A systematic magnetic study of the double perovskite oxide Y 2 CoMnO 6 (YCMO) has been performed. A monoclinic P21/n phase of YCMO was synthesized using a sol-gel method. Neutron diffraction (ND) measurements evidence the onset of long-range ferromagnetic (FM) ordering at TC ~ 76 K, which persists down to 5 K. The presence of 25% antisite disorder, estimated from the ND data, leads to the appearance of short-range antiferromagnetic (AFM) interactions. The existence of thermal hysteresis due to competing interactions between FM and AFM phases is observed in the M vs. T measurements. The pinning of magnetic domain walls at the Co/Mn antiphase boundaries results in a metamagnetic-like behavior. The field dependence of thermomagnetic irreversibility and the nature of virgin curves indicate the occurrence of a kinetic arrest phenomenon, which is further verified via cooling and heating of the system in an unequal fields (CHUF) protocol. In full agreement with the ND data, a detailed analysis of critical exponents near the paramagnetic (PM)-FM phase transition also establishes YCMO as a long-range interacting mean-field system. A close examination of the temperature- and field-dependent magnetic entropy change yields an in-depth understanding of coexisting magnetically ordered and disordered phases in YCMO, leading to a comprehensive magnetic phase diagram of this multifunctional double perovskite system.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Unconventional critical magnetic behavior in the Griffiths ferromagnet La₀.₄Ca₀.₆MnO₂.₈□₀.₂ oxide

The effects of oxygen vacancy on the critical magnetic behavior in La₀.₄Ca₀.₆MnO₂.₈□₀.₂ around the paramagnetic-ferromagnetic (PM-FM) phase transition were investigated through various techniques such as modified Arrott plot, Kouvel-Fisher method and critical isotherm analysis via dc magnetization measurements recorded around the Curie temperature T C . The obtained critical exponents values are β~0.8, γ~0.7 and δ~1.882 with T C ~164.5 K. Thus the scaling law γ+β=δβ is fulfilled. The critical exponents obey the single scaling-equation of state M(H,ε)|ε|{sup -β}=f{sub ±}(H|ε|{sup -(β+γ)}) where, f₊ for T>T C and f⁻ for T>T C . The found exponents are inconsistent with any known universality class. These results attributed to the existence of Griffiths Phase (Triki et al. (2012) [1]) seem to actually reflect the unconventional critical scaling of the magnetic susceptibility. - Graphical abstract: Modified Arrott plots: M{sup 1/β} vs. (μ₀H){sup 1/γ} with (a) mean-field model (β=0.5, γ=1), (b) 3D-Heisenberg model (β=0.365, γ=1.336), (c) 3D-Ising model (β=0.325, γ=1.24), (d) tricritical mean-field model (β=0.25, γ=1) and (e) (β=0.79, γ=0.71). Highlights: • Study of the critical behavior for La₀.₄Ca₀.₆MnO₂.₈□₀.₂ compound. • A typical second-order magnetic transition near T C . • Unconventional critical exponents were found.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Annealing effects on the structural and magnetic properties of off-stoichiometric Fe-Mn-Ga ferromagnetic shape memory alloys

Annealing plays an important role in modifying structures and properties of ferromagnetic shape memory alloys (FSMAs). The annealing effect on the structures and magnetic properties of off-stoichiometric Fe 45 Mn 26 Ga 29 FSMA has been investigated at different elevated temperatures. Rietveld refinements of neutron diffraction patterns display that the formation of the γ phase in Fe 45 Mn 26 Ga 29 annealed at 1073 K increases the martensitic transformation temperature and reduces the thermal hysteresis in comparison to the homogenized sample. The phase segregation of a Fe-rich cubic phase and a Ga-rich cubic phase occurs at the annealing temperature of 773 K. The atomic occupancies of the alloys are determined thanks to the neutron's capability of differentiating transition metals. The annealing effects at different temperatures introduce a different magnetic characteristic that is associated with distinctive structural changes in the crystal.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Dielectric, ferromagnetic and maganetoelectric properties of BaTiO{sub 3}–Ni{sub 0.7}Zn{sub 0.3}Fe{sub 2}O{sub 4} composite ceramics

Graphical abstract: - Highlights: • The lead-free ME ceramic composites BaTiO{sub 3}–Ni{sub 0.7}Zn{sub 0.3}Fe{sub 2}O{sub 4} were successfully synthesized. • The composites showed high dielectric constant and low dielectric loss. • The composite with 30% NiZn ferrite presented good soft magnetic properties and ME performance. - Abstract: Lead-free magnetoelectric composite ceramics (1 − x)BaTiO{sub 3}–xNi{sub 0.7}Zn{sub 0.3}Fe{sub 2}O{sub 4} (x = 0.15, 0.3, 0.45) were successfully prepared by conventional oxide ceramic process. The tetragonal perovskite BaTiO{sub 3} and cubic spinel Ni{sub 0.7}Zn{sub 0.3}Fe{sub 2}O{sub 4} were confirmed by X-ray diffraction. The dielectric behaviors of all composite samples show a normal response to the increasing measurement frequency and Ni{sub 0.7}Zn{sub 0.3}Fe{sub 2}O{sub 4} ferrite content. Well defined ferromagnetic hysteresis loops and obvious magnetoelectric coupling effect are observed in the composite ceramics. The influence of applied dc bias magnetic field and Ni{sub 0.7}Zn{sub 0.3}Fe{sub 2}O{sub 4} ferrite content on the magnetoelectric coupling responding voltage is investigated. The strongest peak magnetoelectric coupling voltage coefficient is 124 μV/cm Oe when x = 0.3, which corresponds to the maximum magnetoelectric coupling responding voltage of 200 μV.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Enhancement of microwave absorption bandwidth of polymer blend using ferromagnetic gadolinium silicide nanoparticles

Ferromagnetic gadolinium silicide (Gd 5 Si 4 ) nanoparticles significantly enhance the microwave absorption bandwidth of a polymer blend (PVB-PEDOT:PSS). These materials are essentially needed for various military and civilian applications such as X-band (8.2–12.4 GHz) and Ku-band (12.4–18 GHz) absorption. A single 1.2 mm thick layer of PVB-PEDOT:PSS-Gd 5 Si 4 (PPGS) nanocomposite film demonstrates the most promising bandwidth (8.2–18 GHz) with a minimum reflection loss of -14 dB. Mechanistically, dielectric loss ($\mathcal {tan}$δ e ≈2.4) and magnetic loss ($\mathcal {tan}$δ m ≈1.1) contributes more efficiently, and standard microwave simulation confirms the stored energy is predominant in PPGS nanocomposite which enhances the bandwidth.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Evaluation of the critical behavior near ferromagnetic to paramagnetic phase transition in CrTe 1-x Se x alloys: An experimental study

For this study, we used the conventional solid-state reaction method to prepare stoichiometric samples of CrTe 1-x Se x , where 0 ≤ x ≤ 0.10, and investigated the structural and critical behavior of the prepared samples. Room temperature powder X-ray diffraction, along with Rietveld refinement, revealed the emergence of the NiAs structure with P6 3 /mmc (194) space group with increasing Se substitution. The high-temperature linear fit to inverse of the dc-susceptibility versus temperature for the mother sample resulted in an effective moment of 3.65μ B Cr with Curie-Weiss temperature near 335K, which is slightly higher than the Tc of ~332K obtained from the inflection point of magnetization versus temperature. Magnetization isotherms were employed to investigate the critical behavior of ferromagnetic CrTe 1-x Se x with 0.0 ≤ x ≤ 0.10 near their Curie temperatures (Tc). The magnetic behavior near Tc was found to follow 3D mean-field critical exponents with a second-order phase transition in all samples investigated. We fine-tuned the critical exponents (β, γ, and δ) using a combination of an iteration technique, the Kouvel-Fisher method, and modified Arrott plots. All samples follow a mean field behavior, with Tc ranging from 298 to 340K. The acquired values exhibit self-consistency, as indicated by the results from the Widom scaling relation. Furthermore, the magnetization isotherms exhibit a universal scaling behavior, providing additional credence to the calculated critical exponents.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Exact intrinsic localized excitation of an anisotropic ferromagnetic spin chain in external magnetic field with Gilbert damping, spin current and PT -symmetry

In this work, we obtain the exact one-spin intrinsic localized excitation in an anisotropic Heisenberg ferromagnetic spin chain in a constant/variable external magnetic field with Gilbert damping included. We also point out how an appropriate magnitude spin current term in a spin transfer nano-oscillator (STNO) can stabilize the tendency towards damping. Further, we show how this excitation can be sustained in a recently suggested PT -symmetric magnetic nanostructure. We also briefly consider more general spin excitations.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Perspectives on spin hydrodynamics in ferromagnetic materials

The field of spin hydrodynamics aims to describe magnetization dynamics from a fluid perspective. For ferromagnetic materials, there is an exact mapping between the Landau-Lifshitz equation and a set of dispersive hydrodynamic equations. This analogy provides ample opportunities to explore novel magnetization dynamics and magnetization states that can lead to potential applications that rely entirely on magnetic materials, for example, long-distance transport of information. Additionally, this article provides an overview of the theoretical foundations of spin hydrodynamics and their physical interpretation in the context of spin transport. We discuss other proposed applications for spin hydrodynamics as well as our view on challenges and future research directions.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2023

Exact hopfion vortices in a 3D Heisenberg ferromagnet

Here, we find exact static soliton solutions for the unit spin vector field of an inhomogeneous, anisotropic three-dimensional Heisenberg ferromagnet. Each soliton is labeled by two integers n and m. It is a (modified) skyrmion in the z = 0 plane with winding number n, which twists out of the plane m times in the z-direction to become a 3D soliton. Here m arises due to the periodic boundary condition at the z-boundaries. We use Whitehead’s integral expression to find that the Hopf invariant of the soliton is an integer H = nm. It represents a hopfion vortex. Plots of the preimages of this topological soliton show that they are either unknots or nontrivial knots, depending on n and m. Any pair of preimage curves links H times, corroborating the interpretation of H as a linking number. We also calculate the exact energy of the hopfion vortex, and show that its topological lower bound has a sublinear dependence on H. Using Derrick’s scaling analysis, we demonstrate that the presence of a spatial inhomogeneity in the anisotropic interaction, which in turn introduces a characteristic length scale in the system, leads to the stability of the hopfion vortex.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Electrons matter in ferromagnetic criticality

When elemental iron is cooled from high temperatures, magnetic moments carried by its d-electrons begin a continuous transition to a state of parallel aligned moments below its Curie temperature TC of about 1040 K. As established in the mid-1900s, the statistical physics of this second order phase transition is universal, independent of the ordering temperature, material structure and chemical composition as well as the character of moment-bearing electrons. Finally, the thermodynamics of the phase transition is determined solely by symmetries of the transition’s order parameter – magnetization in the case of a ferromagnet.

36 MATERIALS SCIENCE↗
DOE OSTI2019

STEM Study of Structure and Local Short-Range Orders in the Fe 5- x GeTe 2 Crystals with Ferromagnetism Near Room Temperature

Van der Waals (vdW) bonded materials have historically attracted intense attention, due to their promise for advanced electronic and optical functionalities. Recently, magnetically-active vdW materials have been recognized as an important class of materials for realizing new physics and functionalities. Of the magnetic vdW materials, ferromagnetic Fe 3 GeTe 2 has garnered significant attention for its high Curie temperature T C ~230 K. Recently, a Fe 5-x GeTe 2 compound with thicker Fe-Ge slabs and stronger magnetism has been successfully obtained. As viewed by bulk diffraction, the material has a relatively large unit cell and contains intrinsic disorder. The nature of the local structure is anticipated to impact the Curie temperature, which ranges from 270–310K depending on processing conditions. As such, an atomic-scale probe of the local structures and compositions is essential for understanding the intrinsic magnetism in this vdW material.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Single Crystal Growth and Study of the Ferromagnetic Superconductor RbEuFe 4 As 4

RbEuFe 4 As 4 exhibits both superconducting order 0.05 in the FeAs layers and ferromagnetic order in the Eu layers, providing a good platform to study the interaction and microscopic coexistence of these two traditionally incompatible orders. Growing high-quality RbEuFe 4 As 4 single crystals is essential to investigate these phenomena at a deeper level. Here in this paper we report the successful growth of the RbEuFe 4 As 4 single crystals with millimeter-size dimensions using a RbAs flux. The high-quality crystals were characterized via resistivity, magnetization, and heat capacity measurements. Single crystal X-ray diffraction data refinements reveal almost regular FeAs 4 tetrahedra (As1-Fe-As1 = 108.60(7)degrees, As2-Fe-As2 = 109.01 (8)degrees, and As1-Fe-As2 = 109.81(2)degrees) in RbEuFe 4 As 4 , providing structural support for the highest superconducting transition temperature (T c = 36.8 K) among all known AAeFe 4 As 4 (A = K, Rb, Cs; Ae = Ca, Sr, Eu) compounds. Finally, our flux method using RbAs can also be employed to grow other desired transition metal compounds targeting the same crystal structure.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗