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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 217 records · Page 12
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Perpendicular magnetic tunnel junctions with double barrier and single or synthetic antiferromagnetic storage layer

The magnetic properties of double tunnel junctions with perpendicular anisotropy were investigated. Two synthetic antiferromagnetic references are used, while the middle storage magnetic layer can be either a single ferromagnetic or a synthetic antiferromagnetic FeCoB-based layer, with a critical thickness as large as 3.0 nm. Among the different achievable magnetic configurations in zero field, those with either antiparallel references, and single ferromagnetic storage layer, or parallel references, and synthetic antiferromagnetic storage layer, are of particular interest since they allow increasing the efficiency of spin transfer torque writing and the thermal stability of the stored information as compared to single tunnel junctions. The latter configuration can be preferred when stray fields would favour a parallel orientation of the reference layers. In this case, the synthetic antiferromagnetic storage layer is also less sensitive to residual stray fields.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

A review of high magnetic moment thin films for microscale and nanotechnology applications

Here, the creation of large magnetic fields is a necessary component in many technologies, ranging from magnetic resonance imaging, electric motors and generators, and magnetic hard disk drives in information storage. This is typically done by inserting a ferromagnetic pole piece with a large magnetisation density M S in a solenoid. In addition to large M S , it is usually required or desired that the ferromagnet is magnetically soft and has a Curie temperature well above the operating temperature of the device. A variety of ferromagnetic materials are currently in use, ranging from FeCo alloys in, for example, hard disk drives, to rare earth metals operating at cryogenic temperatures in superconducting solenoids. These latter can exceed the limit on M S for transition metal alloys given by the Slater-Pauling curve. This article reviews different materials and concepts in use or proposed for technological applications that require a large M S , with an emphasis on nanoscale material systems, such as thin and ultra-thin films. Attention is also paid to other requirements or properties, such as the Curie temperature and magnetic softness. In a final summary, we evaluate the actual applicability of the discussed materials for use as pole tips in electromagnets, in particular, in nanoscale magnetic hard disk drive read-write heads; the technological advancement of the latter has been a very strong driving force in the development of the field of nanomagnetism.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Theoretical model study of dynamic ferromagnetic susceptibility in mono-layer graphene

We report here a microscopic theoretical study of dynamic ferromagnetic spin susceptibility of electrons for graphene systems, which deal with a tight-binding model Hamiltonian consisting of the hopping of electrons up to third-nearest-neighbors, impurity and substrate effects besides Coulomb interaction of electrons at A-and B- sub- lattices. The spin susceptibility involves four two-particle Green’s functions, which are calculated by Zubarev’s Green’s function technique. The up and down electron occupancies at A and B sub-lattices are computed numerically and self-consistently. The temperature dependent susceptibility shows a pronounced peak at Curie temperature for critical Coulomb interaction U{sub c} = 2.2t{sub 1}.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Comparison of catalytic activities for photocatalytic and sonocatalytic degradation of organic dye in the presence of ternary Fe{sub 3}O{sub 4}/ZnO/CuO magnetic heteregenous nanocatalyst

The Fe{sub 3}O{sub 4}/ZnO/CuO nanocatalyst with various CuO loading were synthesized by sol-gel method and were characterized by powder X-ray diffraction, field emission scanning electron microscopy, energy-dispersive X-ray spectroscopy, UV-Vis spectroscopy, and vibrating sample magnetometry. The findings demonstrate that all samples exhibit ferromagnetic behavior at room temperatureand containa well-crystalline ternary oxide nanocatalyst. Methylene blue was taken as the model of organic dye to evaluate its photocatalytic and sonocatalytic degradation in the presence of Fe{sub 3}O{sub 4}/ZnO/CuO nanocatalyst. The observed degradation activity indicate that the order of degradation of methylene blue issonocatalysis> photocatalysis. Fe{sub 3}O{sub 4}/ZnO/CuO nanocatalyst with the lowest CuO loading exhibit the highest rate of degradation of methylene blue during the sono- and photocatalytic processes. The experimental data shows that holes are the predominant oxidative species involved in the sono- and photodegradation of methylene blue.

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

Spin-torque ferromagnetic resonance measurements utilizing spin Hall magnetoresistance in W/Co 40 Fe 40 B 20 /Mg O structures

Here, we study the magnetic properties of W/Co 40 Fe 40 B 20 (CoFeB)/MgO films using the spin-torque ferromagnetic resonance (ST-FMR) technique. This study takes the advantage of the spin Hall magnetoresistance (SMR) for generating an oscillating resistance, which is one of the necessary requirements for obtaining mixing voltage in the ST-FMR technique. We have measured both the as-grown and the annealed samples with different CoFeB layer thicknesses, which include the in-plane and out-of-plane magnetic anisotropies. The spectra for these two types of anisotropies show distinct signatures. By analyzing the ST-FMR spectra, we extract the effective anisotropy field for both types of samples. In addition, we investigate the influence of CoFeB thickness and annealing on the Gilbert damping constant. Our experiments show that by taking advantage of SMR, the ST-FMR measurement acts as an effective tool with high sensitivity for studying the magnetic properties of ultrathin magnetic films.

42 ENGINEERING↗
DOE OSTI2018

Bragg coherent diffractive imaging of ferromagnetic nickel nanoparticles

We synthesized Ni nanoparticles with an atmospheric-thermal chemical vapor deposition method and investigated their lattice distortions and defects using Bragg coherent diffractive imaging. Because the strain can alter the magnetic properties of ferromagnetic materials, it is important to study the lattice distortion and structural imperfection. As a result of the Bragg coherent diffractive imaging, we found that the Ni nanocubes have a deformation-free zone in the middle. In addition, the defects such as dislocation and twin domain defects were detected in other Ni nanoparticles based on the Bragg diffraction images and patterns. The imaging of nanoparticles can contribute to a better understanding of their growth mechanisms and improve the design and synthesis of nanoparticles.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2019

Phase detection of spin waves in yttrium iron garnet and metal induced nonreciprocity

Herein we report experiments which characterize spin wave propagation in a thin (111) yttrium iron garnet film for arbitrary angles between the in-plane magnetic field and the mode wavevectors. By measuring the magnetic field evolution of the phase of the wave traveling across the film, we deduce the frequency dependence of the wavevector, the dispersion relation, from which the mode velocity follows. Additionally, we observe multiple nodes in the regime of the propagating Damon-Eshbach mode; these arise from avoided crossings associated with the higher, exchange split, standing wave modes along the film normal, the positions of which correlate with the direct absorption measurements of their positions. This information allows a determination of the exchange parameter. Using this technique, we examine the nonreciprocity in spin wave propagation that results from an adjacent metal layer.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2021

Nanoscale imaging of Gilbert damping using signal amplitude mapping

Ferromagnetic resonance force microscopy (FMRFM) is a powerful scanned probe technique that uses sub-micrometer-scale, spatially localized standing spin wave modes (LMs) to perform local ferromagnetic resonance (FMR) measurements. In this work, we show the spatially resolved imaging of Gilbert damping in a ferromagnetic material (FM) using FMRFM. Typically damping is measured from the FMR linewidth. We demonstrate an approach to image the spatial variation of Gilbert damping utilizing the LM resonance peak height to measure the LM resonance cone angle. This approach enables determination of damping through field-swept FMRFM at a single excitation frequency. The extreme force sensitivity of similar to 2 fN at room temperature can resolve changes of Gilbert damping as small as ~2 x 10 -4 at 2GHz, corresponding to ~0.16Oe in FMR linewidth resolution. This high sensitivity, high spatial resolution, and single frequency imaging of Gilbert damping creates the opportunity to study spin interactions at the interface between an insulating FM and a small volume of nonmagnetic material such as atomically thin two-dimensional materials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Proximity-induced magnetism and the enhancement of damping in ferromagnetic/heavy metal systems

The relationship between proximity-induced magnetism (PIM) at the heavy metal/ferromagnet interface and spin-transport across such interfaces has generated significant debate. To investigate the link between the two, element specific x-ray magnetic circular dichroism and ferromagnetic resonance measurements were made on the same CoFe/Au/Pt and NiFe/Au/Pt thin film samples with varying Au thickness, with complementary SIMS analysis, which shows evidence of Ni diffusion from NiFe into the Pt. An approximately linear relationship is observed between the magnitude of Pt PIM and magnitude of damping enhancement in both systems. The results demonstrate that electronic hybridization of the heavy metal and ferromagnet is required for a full understanding of damping enhancement and interfacial spintransport for spintronic devices.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2022

Effects of thin metal contacts on few-layer van der Waals ferrielectric CuInP 2 S 6

Out-of-plane polarized ferroelectric materials in a capacitive structure provide a key component for several technological applications. Furthermore, two-dimensional materials are expected to aid in the quest for both ultrathin and flexible electronics. Of the various two-dimensional ferroelectrics with out-of-plane polarization, CuInP 2 S 6 is special in that the Cu atoms are highly mobile and it has been shown to possess both low- and high-polarization states. Here, using density-functional-theory calculations, we explore the stabilization of the ferroelectric state for several prototypical metal contacts (Gr, Ni, Cu, Au, and Ag). In all cases, we find that the ferroelectric state can be stabilized at fewer layers than in the freestanding case. For all of the considered conventional metal contacts, we also find the existence of a quasi-ferroelectric state that stabilizes a polar phase for thicknesses greater than two layers of CIPS. In the cases of Au and Ag, interfacial alignment and strain can be used to stabilize ferroelectricity at the bilayer limit. Furthermore, we find that the strength of the interaction between the contact and CuInP 2 S 6 also leads to stabilization of the high-polarization state when ferroelectricity is stabilized. Lastly, energy-barrier calculations show that the system is still switchable in the presence of contact doping from the metal contacts.

2D materials↗
DOE OSTI2025

Ice sculpting: An artificial spin ice Tutorial on controlling microstate and geometry for magnonics and neuromorphic computing

Artificial spin ice, arrays of strongly interacting nanomagnets, are complex magnetic systems with many emergent properties, rich microstate spaces, intrinsic physical memory, high-frequency dynamics in the GHz range, and compatibility with a broad range of measurement approaches. This Tutorial article aims to provide the foundational knowledge needed to understand, design, develop, and improve the dynamic properties of artificial spin ice. Special emphasis is placed on introducing the theory of micromagnetics, which describes the complex dynamics within these systems, along with their design, fabrication methods, and standard measurement and control techniques. The article begins with a review of the historical background, introducing the underlying physical phenomena and interactions that govern artificial spin ice. We then explore the standard experimental techniques used to prepare the microstate space of the nanomagnetic array and to characterize magnetization dynamics, both in artificial spin ice and more broadly in ferromagnetic materials. Finally, we introduce the basics of neuromorphic computing applied to the case of artificial spin ice systems with a goal to help researchers new to the field grasp these exciting new developments.

Sultana, Rawnak [Univ. of Delaware, Newark, DE (Un↗
DOE OSTI2023

Strong ultrafast demagnetization due to the intraband transitions

Abstract Demagnetization in ferromagnetic transition metals driven by a femtosecond laser pulse is a fundamental problem in solid state physics, and its understanding is essential to the development of spintronic devices.Ab initiocalculation of time-dependent magnetic moment in the velocity gauge so far has not been successful in reproducing the large amount of demagnetization observed in experiments. In this work, we propose a method to incorporate intraband transitions within the velocity gauge through a convective derivative in the crystal momentum space. Our results for transition-element bulk crystals (bcc Fe, hcp Co and fcc Ni) based on the time-dependent quantum Liouville equation show a dramatic enhancement in the amount of demagnetization after the inclusion of an intraband term, in agreement with experiments. We also find that the effect of intraband transitions on each ferromagnetic material is distinctly different because of their band structure and spin property differences. Our finding has a far-reaching impact on understanding of ultrafast demagnetization.

Physics↗
DOE OSTI2016

Stoner versus Heisenberg: Ultrafast exchange reduction and magnon generation during laser-induced demagnetization

Understanding how the electronic band structure of a ferromagnetic material is modified during laser-induced demagnetization on femtosecond timescales has been a long-standing question in condensed matter physics. Here, we use ultrafast high harmonics to measure time-, energy-, and angle-resolved M-edge magnetic asymmetry spectra for Co lms after optical pumping to induce ultrafast demagnetization. This provides a complete data set that we can compare with advanced ab initio magneto-optical calculations. Here, tur analysis identifies that the dominant mechanisms contributing to ultrafast demagnetization on timescales up to several picoseconds are a transient reduction in the exchange splitting and the excitation of ultrafast magnons. Surprisingly, we nd that the magnon contribution to ultrafast demagnetization is already strong on sub-picosecond timescales, while the reduction in exchange splitting persists to several picoseconds.

36 MATERIALS SCIENCE↗
DOE OSTI2017

First-order reversal curve of the magnetostructural phase transition in FeTe

We apply the first-order reversal curve (FORC) method, adapted from studies of ferromagnetic materials, to the magnetostructural phase transition of Fe 1+y Te. FORC measurements reveal two features in the hysteretic phase transition, even in samples where traditional temperature measurements display only a single transition. For Fe 1.13 Te, the influence of magnetic field suggests that the main feature is primarily structural while a smaller, slightly higher-temperature transition is magnetic in origin. By contrast, Fe 1.03 Te has a single transition which shows a uniform response to magnetic field, indicating a stronger coupling of the magnetic and structural phase transitions. We also introduce uniaxial stress, which spreads the distribution width without changing the underlying energy barrier of the transformation. Finally, the work shows how FORC can help disentangle the roles of the magnetic and structural phase transitions in FeTe.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Competition of lattice and spin excitations in the temperature dependence of spin-wave properties

The interplay of magnons and phonons can induce strong temperature variations in the magnetic exchange interactions, leading to changes in the magnetothermal response. This is a central mechanism in many magnetic phenomena, and in the new field of Spin Caloritronics, which focuses on the combination of heat and spin currents. Boson model systems have previously been developed to describe the magnon-phonon coupling but, until recently, studies rely on empirical parameters. In this paper, we propose a first-principles approach to describe the dependence of the magnetic exchange integrals on phonon renormalization, leading to changes in the magnon dispersion as a function of temperature. The temperature enters into the spin dynamics (by introducing fluctuations) as well as in the magnetic exchange itself. Depending on the strength of the coupling, these two temperatures may or may not be equilibrated, yielding different regimes. We test our approach in typical and well-known ferromagnetic materials: Ni, Fe, and Permalloy. Finally, we compare our results to recent experiments on the spin-wave stiffness, and discuss departures from Bloch’s law and parabolic dispersion.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Hydrodynamic description of long-distance spin transport through noncollinear magnetization states: Role of dispersion, nonlinearity, and damping

Nonlocal compensation of magnetic damping by spin injection has been theoretically shown to display dynamic, noncollinear magnetization states that carry spin currents over micrometer distances. Such states can be generically referred to as dissipative exchange flows (DEFs) because spatially diffusing spin currents are established by the mutual exchange torque exerted by neighboring spins. Analytical studies to date have been limited to the weak spin injection assumption whereby the equation of motion for the magnetization is mapped to hydrodynamic equations describing spin flow and then linearized. In this work, we analytically and numerically study easy-plane ferromagnetic channels subject to spin injection of arbitrary strength at one extremum under a unified hydrodynamic framework. We propose that DEFs generally exhibit a nonlinear profile along the channel accompanied by a nonlinear frequency tunability. At large injection strengths, we fully characterize a magnetization state we call a contact-soliton DEF (CS-DEF) composed of a stationary soliton at the injection site, which smoothly transitions into a DEF and exhibits a negative frequency tunability. The transition between a DEF and a CS-DEF occurs at the maximum precessional frequency and coincides with the Landau criterion: a subsonic to supersonic flow transition. Leveraging the hydraulic-electrical analogy, the current-voltage characteristics of a nonlinear DEF circuit are presented. Micromagnetic simulations of nanowires that include magnetocrystalline anisotropy and nonlocal dipole fields are in qualitative agreement with the analytical results. The magnetization states found here along with their characteristic profile and spectral features provide quantitative guidelines to pursue an experimental demonstration of DEFs in ferromagnetic materials and establish a unified description for long-distance spin transport.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Continuum Modeling of Inductor Hysteresis and Eddy Current Loss Effects in Resonant Circuits

This paper presents experimental validation of a high-fidelity toroid inductor modeling technique. The aim of this research is to accurately model the instantaneous magnetization state and core losses in ferromagnetic materials. Quasi–static hysteresis effects are captured using a Preisach model. Eddy currents are included by coupling the associated quasi-static Everett function to a simple finite element model representing the inductor cross sectional area. The modeling technique is validated against the nonlinear frequency response from two different series RLC resonant circuits using inductors made of electrical steel and soft ferrite. The method is shown to accurately model shifts in resonant frequency and quality factor. The technique also successfully predicts a discontinuity in the frequency response of the ferrite inductor resonant circuit.

Pries, Jason L.↗
DOE OSTI2017

Analytical Modeling of a Double-Sided Flux Concentrating E-Core Transverse Flux Machine with Pole Windings

In this paper, a nonlinear analytical model based on the Magnetic Equivalent Circuit (MEC) method is developed for a double-sided E-Core Transverse Flux Machine (TFM). The proposed TFM has a cylindrical rotor, sandwiched between E-core stators on both sides. Ferrite magnets are used in the rotor with flux concentrating design to attain high airgap flux density, better magnet utilization, and higher torque density. The MEC model was developed using a series-parallel combination of flux tubes to estimate the reluctance network for different parts of the machine including air gaps, permanent magnets, and the stator and rotor ferromagnetic materials, in a two-dimensional (2-D) frame. An iterative Gauss-Siedel method is integrated with the MEC model to capture the effects of magnetic saturation. A single phase, 1 kW, 400 rpm E-Core TFM is analytically modeled and its results for flux linkage, no-load EMF, and generated torque, are verified with Finite Element Analysis (FEA). The analytical model significantly reduces the computation time while estimating results with less than 10 percent error.

30 DIRECT ENERGY CONVERSION↗