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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 595 records · Page 33
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Effect of Mechanical Stresses and Annealing on the Magnetic Structure and the Magnetic Impedance of Amorphous CoFeSiBCr Microwires

The structural and magnetic properties of amorphous ferromagnetic microwires can undergo significant measurements under the action of external mechanical stresses and heat treatment. The study of transformations occurring in this case is important for designing various sensors of mechanical stresses, loading, and temperature and also for inducing in the wires a certain type of magnetic anisotropy that plays a significant role in the realization of various effects in them. In this work, the influence of external stresses and annealing on the processes of the magnetization and the magnetic impedance of Co{sub 71}Fe{sub 5}B{sub 11}Si{sub 10}Cr{sub 3} microwires having a low positive magnetostriction (~10{sup -8}) in amorphous state has been studied. The influence of external stresses leads to a sharp change in the character of the magnetization reversal curve, which was due to the change in the sign of the magnetostriction and the type of magnetic anisotropy. The amplitude of higher harmonics and the value of the magnetic impedance, respectively, are sensitive to mechanical stresses. Elastic stresses in the wires with a partial crystallization do not lead to a marked change in the magnetic properties; however, annealing can lead to a substantial increase in the axial magnetic anisotropy of the wires existing in the stressed state. The experimental results are analyzed in the framework of a magnetostriction model of induced magnetic anisotropy.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Thermal Expansion of Co{sub 2}MAl (M = Ti, V, Cr, Mn, Fe, Ni) Band Ferromagnets

The thermal expansion coefficient (TEC) of Co{sub 2}MAl (M = Ti, V, Cr, Mn, Fe, Ni) band ferromagnets is measured in the temperature interval of 80–900 K. The magnetic-fluctuation contribution α{sub m}, {sub sf}(T) = α{sub m}(T) + α{sub sf}(T) to TEC comprised of negative Stoner α{sub m}(T) and positive spin-fluctuation α{sub sf}(T) contributions is presented. It is assumed that the contribution of α{sub m}(T) component is proportional to the square of spontaneous magnetization. The contribution of α{sub sf}(T) is stipulated by the effects of inducing local magnetic moments by spin fluctuations and is proportional to the Pauli susceptibility.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Thermoelectric Properties of a Ferromagnetic Semiconductor Based on a Dirac Semimetal (Cd{sub 3}As{sub 2}) under High Pressure

The pressure dependences of thermal emf (a parameter that ranks among the most sensitive to phase transformations) are studied for the purpose of identifying baric phase transitions in the 10–50 GPa interval in the Cd{sub 3}As{sub 2} + MnAs (44.7% MnAs) structure formed by ferromagnetic MnAs granules in a semiconductor Cd{sub 3}As{sub 2} matrix.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Low Temperature Magnetic Properties of Amorphous Ferromagnetic Fe–Si–B Glass-Coated and Glass Removed Microwire

This work presents results of investigations of low temperature magnetic properties of microwires, fabricated by the Ulitovsky–Taylor method from Fe–Si–B alloy. Influence of the glass coating presence on the magnetostatic properties was shown at room temperature and at 2 K. Conclusions about the peculiarities of temperature dependence of the magnetic moment, measured in low and high magnetic fields (10 Oe–5 kOe) in the temperature range from 2 to 300 K for a sample with partially reduced fabricated processinduced stress by removing the glass, were made.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Susceptibility Critical Exponent of a 1D Ising Ring-Type Ferromagnetic

Using the Monte Carlo method, critical behavior of the one-dimensional ferromagnetic Ising model has been investigated with allowance for the interaction of the second and third neighbors and four-particle interaction. The obtained results on the critical temperature were compared with the critical temperature of the quasi-one-dimensional Ising magnetic [(CH{sub 3}){sub 3}NH] · FeCl{sub 3} · 2H{sub 2}O and with the magnitude of the exchange interaction J/k{sub B} = 17.4 K. Within the scope of the finite-dimensional scaling theory, the critical susceptibility exponent has been calculated. It has been shown that values of the susceptibility exponent for the one-dimensional Ising model with periodic boundary conditions are considerably less than the known values of the exponents for three-dimensional systems. The critical susceptibility exponent strongly depends on energy parameters; namely, it decreases with an increase in them.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Interfacial Ferromagnetism in a Co/CdTe Ferromagnet/Semiconductor Quantum Well Hybrid Structure

The magnetization properties of a ferromagnet-semiconductor Co/CdMgTe/CdTe quantum well hybrid structure are investigated by several techniques. Exploiting the proximity effect between acceptor bound holes and magnetic ions we detect the magnetization curves by measuring the circular polarization of photoluminescence in an out-of-plane magnetic field. We show that magnetization originates from interfacial ferromagnet on Co-CdMgTe interface and the proximity effect is caused by magnetization of interfacial Co-CdMgTe ferromagnetic layer whose magnetic properties are very different from Co.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Features of Spin Waves Focusing in Ferromagnets

Abstract—Features of focusing spin waves in ferromagnets with magnetic moment exchange interaction with the closest neighbors and second neighbors are explored. It is shown that in the long-wave approximation, no spin wave focusing occurs: it is observed only in the wavevectors in the second half of the Brillouin zone (at aq ≥ π/2, where a is the lattice constant, q is the value of wavevector). Additionally, it is shown that magnons in such system are focused along directions [110], and are defocused in directions [100]. It is found that the external magnetic field and the magnetic anisotropy field do not cause changes in directions of magnon focusing. When the parameter of exchange interaction with the second neighbors near directions [110] and [111] is negative, magnons form a caustic, near which the intensity of the spin-wave field surges.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Quantum Fluctuations in Fe{sub x}Mn{sub 1 –} {sub x}Si with Inclusion of the LDA + U + SO Calculations of the Electronic Structure

Quantum spin fluctuations are studied in chiral helicoidal Fe{sub x}Mn{sub 1 –} {sub x}Si ferromagnets on the basis of the direct calculations of the electronic structure. The disappearance of the helicoidal long-range order is shown to be accompanied by the appearance of the crossover of the thermodynamic and quantum transitions with a sharp decrease in the local magnetization and the zero fluctuation amplitude. Suppression of the local magnetization by quantum spin fluctuations leads to the concentration–temperature magnetic transition with the disappearance of the helicoidal short-range order. The calculations of the magnetization of the Fe{sub x}Mn{sub 1 –} {sub x}Si compositions show that at 0.12 < x < 0.20 the magnetic state in the thermodynamic limit is characterized by the short-range (not long-range) helicoidal order.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Specific Features of Nonlinear Spin Wave Localization in a Layered Ferromagnet under the Action of the Magnetic Anisotropy of Layers

A trilayer structure model has been investigated, in which uniaxial ferromagnetic layers and thin-film interfaces between them are characterized by the magnetic anisotropy. The magnetization perturbation induced in this structure is described by the nonlinear Schrödinger equation with a nonlinear potential, which simulates the interfaces between ferromagnetic layers. It is shown that, in this structure, two types of nonlinear spin waves can propagate along the layers, which are induced by the nonlinearity of the interfaces caused by their magnetic anisotropy. The frequencies of such localized stationary states of spin waves have been obtained in the explicit analytical form. The conditions for the existence of these states, depending on the characteristics of the layers and interfaces between them, have been established.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Phase Transition in Cylindrical Ising Nanowires and Nanotubes: Approximation of the Molecular Field Theory

Within the approximation of the molecular field theory, the critical behavior of cylindrical Ising nanowires and nanotubes is studied. The model considered in this work consists of ferromagnetic spins S{sub c} situated in the core of the system and ferromagnetic spins S{sub s} situated on the surface wall; they are interconnected by the exchange coupling J{sub 1}. The transition temperature T{sub c} for such systems has been calculated as a function of exchange interaction parameters. The influence of the surface exchange interaction and surface coupling on the magnetodynamic behavior of the system have been studied. Some characteristic properties obtained on the phase diagram are related to the ratio of characteristic physical parameters on the surface and core of the abovementioned nanostructures.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Phase Transitions in Helicoidal Ferromagnets with Concentrational Fluctuations of Local Magnetization

A phenomenological approach to the theory of phase transitions induced by fluctuations in helicoidal ferromagnets with concentrational fluctuations is developed. For this purpose, random variables equal to one at a site ν occupied by a magnetic atom and zero otherwise are introduced into the Ginzburg–Landau functional. It is shown that, above the magnetic transition temperature (T{sub C}), due to concentrational effects, local magnetization is preserved, fluctuations of the helicoidal spin helix arise, and skyrmion states in the magnetic field are formed. The disappearance of the vortical states is due to the suppression of local magnetization by thermodynamic fluctuations at the temperature T{sub S} (>T{sub C}). The theoretical results explain the reasons for the significant expansion of the temperature range of skyrmion states in nonstoichiometric manganese monosilicide with manganese deficiency.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Simulation of the Interaction of a Magnetic Resonance Force Microscope Probe with a Ferromagnetic Sample

We propose an algorithm and present results of micromagnetic simulation of the resonant response of a probe pickup (cantilever) of a magnetic resonance force microscope. Simulation of induced oscillations of sample magnetization makes it possible to calculate the varying component of the force exerted on the probe by the sample and to construct spectra in the form of dependences of the amplitude of oscillations of the cantilever on the external magnetic field strength. Simulation of time dependences of all magnetization field components makes it possible to analyze the spatial distributions of spin-wave resonances of samples. For test objects in the form of rectangular permalloy microstrips, good agreement between model and experimental spectra is achieved.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Enhancing the Circular Polarization of Spin Light-Emitting Diodes by Processing in Selenium Vapor

Spin light-emitting diodes based on InGaAs/GaAs heterostructures with a CoPt ferromagnetic injector were fabricated. It was demonstrated that the processing of these structures in selenium vapor prior to the deposition of a CoPt contact provides an opportunity to enhance the circular polarization degree of diode emission. The observed increase in the polarization degree is attributed to the suppression of spin relaxation at the metal/semiconductor interface due to surface passivation and a reduction in the density of surface electron states as a result of processing in selenium vapor.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Exchange Spring in a Heterostructure with Pd{sub 0.96}Fe{sub 0.04} Low-Temperature Ferromagnet

The exchange bias of the magnetic hysteresis loop and the exchange-spring effect have been observed in thin-film heterostructures comprising layers of Pd{sub 0.96}Fe{sub 0.04} low-temperature ferromagnet, metallic cobalt, and cobalt monoxide (CoO) grown on single-crystalline sapphire by methods of magnetron sputtering and molecular beam epitaxy in an ultrahigh-vacuum system. It is established that the exchange anisotropy of Pd{sub 0.96}Fe{sub 0.04} layer in Pd{sub 0.96}Fe{sub 0.04}/CoO/Co/Al{sub 2}O{sub 3} thin-film heterostructure is absent and the exchange-spring effect in this system is not manifested. In the case of a Pd{sub 0.96}Fe{sub 0.04}/Co/CoO/Al{sub 2}O{sub 3} structure, it is shown that, according to the results of magnetometric measurements, the inversion of the order of antiferromagnetic and ferromagnetic layers leads to pinning of the magnetic moment of Pd{sub 0.96}Fe{sub 0.04} layer at the interface with cobalt and to realization of the exchange-spring effect. Additional evidence in favor of this interpretation was obtained using the micromagnetic modeling of magnetic hysteresis loops.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Quantum Coherence and Random Fields at Mesoscopic Scales

We seek to explore and exploit model, disordered and geometrically frustrated magnets where coherent spin clusters stably detach themselves from their surroundings, leading to extreme sensitivity to finite frequency excitations and the ability to encode information. Global changes in either the spin concentration or the quantum tunneling probability via the application of an external magnetic field can tune the relative weights of quantum entanglement and random field effects on the mesoscopic scale. These same parameters can be harnessed to manipulate domain wall dynamics in the ferromagnetic state, with technological possibilities for magnetic information storage. Finally, extensions from quantum ferromagnets to antiferromagnets promise new insights into the physics of quantum fluctuations and effective dimensional reduction. A combination of ac susceptometry, dc magnetometry, noise measurements, hole burning, non-linear Fano experiments, and neutron diffraction as functions of temperature, magnetic field, frequency, excitation amplitude, dipole concentration, and disorder address issues of stability, overlap, coherence, and control. We have been especially interested in probing the evolution of the local order in the progression from spin liquid to spin glass to long-range-ordered magnet.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2018

Testing the possibility of magnetic domain imaging based on circular & linear dichroism using photoemission electron microscopy

In recent years, an increasing number of memory and spintronic devices have been developed exploiting the combination of ferromagnetic (FM) and anti-ferromagnetic (aFM) materials. Consequently, magnetic imaging based on continuous-wave (CW) ultraviolet (UV) (λ = 266nm and longer wavelength) photoemission electron microscopy (PEEM) is gaining considerable attention due to the possibility of determining magnetizations for FM and aFM materials with 10 nm lateral resolution at video rate image acquisition. This PEEM-based approach exploits the polarization-dependent photoemission yield, which is subject to the polarization vector and the FM or aFM magnetization direction. Because of this unique attribute, magnetic imaging using PEEM when coupled to a laser with multiple illumination geometries allows for characterizing in-plane and out-of-plane magnetizations. This concept, however, has not been tested using a deep-UV laser (λ = 213nm), which has a much broader application space than the longer wavelength excitation used in previous reports. The purpose of this project in FY17 was to show the proof-of-concept of magnetic circular dichroism (MCD)-PEEM imaging using a λ = 210nm pulsed laser. Our results demonstrated the feasibility of in-plane and out-of-plane magnetic imaging with the limitations in the lateral resolution, data acquisition time, and signal-to-noise ratio anticipated for using a pulsed laser of moderate power. The project goal for FY18 is to construct the automated polarization-controlled data acquisition, and to establish the new lab facility in anticipation of acquiring a state-of-the-art high-power 213nm CW laser, planned to be installed in FY19. We successfully demonstrate the former by measuring dielectric stacks with polarization-dependent photoemission yield. Extrapolating from our result, we conclude that the capability of PEEM-based magnetic imaging using a CW deep UV laser could be a potential game-changer for scientific investigations and technological developments of magnetic materials and spintronic devices. In addition, polarization controlled PEEM imaging shows the potential for ellipsometry imaging of embedded nanomaterials exploiting their subtle differences in optical constants with respect to their surrounding dielectrics.

36 MATERIALS SCIENCE↗
NASA NTRS1981

Analysis of Barkhausen noise to assess shrink fit stresses

The possible application of Barkhausen Noise Analysis to assess shrink fit stresses in steel bushings was investigated. Ten specimens of four different size NAS steel bushings were used in experiments to determine the relationship between the Barkhausen signal amplitude and the hoop-stress in the bushings. This relationship was investigated for specimens which were loaded by a simple clamping fixture and specimens subjected to actual thermal interference fits. The results show that while the relationship of Barkhausen signal amplitude to hoop stress is predictable for specimens loaded by the simple clamping fixture, the relationship was less predictable for actual thermal fit bushings. The variation in results could possibly be due to variable levels of axial stress produced by the differential thermal expansion of the steel and aluminum.

Nondestructive testing↗