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Results for “FERROMAGNETIC RESONANCE”

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At least 469 records · Page 26
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Enhanced Planar Antenna Efficiency Through Magnetic Thin-Films

This work proposes to use magnetic material as the substrate of planar antennas to overcome the platform effect caused by the conducting ground plane. The upper bound of the radiation efficiency of an electric-current-driven low-profile antenna is theoretically derived, which is inversely proportional to the Gilbert damping factor of the magnetic material. Meanwhile, the improvement of radiation due to the use of magnetic material is demonstrated by a three-dimensional (3D) multiphysics and multiscale time-domain model. The simulation results match the theoretical derivation, showing 25% radiation efficiency from a planar antenna backed by a FeGaB thin film with 2.56 m thickness. Furthermore, for conductive ferromagnetic materials, it is shown that the eddy current loss can be well suppressed by laminating the thin film into multiple layers. Here, the radiation efficiency of the modeled antenna with a conductive ferromagnetic substrate is improved from 2.2% to 11.8% by dividing the substrate into 10 layers, with a ferromagnetic material fill factor of 93%.

42 ENGINEERING↗
DOE OSTI2014

Nanometer-Thick Yttrium Iron Garnet Films With Extremely Low Damping

Yttrium iron garnet (YIG) films that are in the nanometer thickness range and show extremely low damping are reported. The films were deposited via sputtering at room temperature and were then annealed in O 2 at high temperature. A 22-nm-thick YIG film showed a Gilbert damping constant α = (8.58 ± 0.21) × 10 -5 , which represents the lowest damping ever reported for nanometer-thick magnetic films. The film had a gyromagnetic ratio of |γ| = 2.83 MHz/Oe and a saturation induction of 4πM s = 1766 G, which are both very close to those of single-crystal YIG bulk materials. The film had a very smooth surface, with an rms surface roughness of about 0.13 nm.

current topics↗
DOE OSTI2016

HIDE-AND-SEEK WITH THE FUNDAMENTAL METALLICITY RELATION

We use ∼83,000 star-forming galaxies at 0.04 < z < 0.3 from the Sloan Digital Sky Survey to study the so-called fundamental metallicity relation (FMR) and report on the disappearance of its anti-correlation between metallicity and star formation rate (SFR) when using the new metallicity indicator recently proposed by Dopita et al. In this calibration, metallicity is primarily sensitive to the emission line ratio [N ii] λ 6584/[S ii] λλ 6717, 6731 that is insensitive to dilution by pristine infalling gas that may drive the FMR anti-correlation with SFR. Therefore, we conclude that the apparent disappearance of the FMR (using this new metallicity indicator) does not rule out its existence.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

Magnetic characteristics of a high-layer-number NiFe/FeMn multilayer

We report the static and dynamic magnetic characteristics of a high-layer-number NiFe/FeMn multilayer test structure with potential applications in broadband absorber and filter devices. To allow fine control over the absorption linewidths and to understand the mechanisms governing the resonances in a tailored structure similar to that expected to be used in real world applications, the multilayer was intentionally designed to have layer thickness and interface roughness variations. Magnetometry measurements show that the sample has complex hysteresis loops with features consistent with single ferromagnetic film reversals. Characterisation by transmission electron microscopy allows us to correlate the magnetic properties with structural features, including the film widths and interface roughnesses. Analysis of resonance frequencies from broadband ferromagnetic resonance measurements as a function of field magnitude and orientation provide values of the local exchange bias, rotatable anisotropy, and uniaxial anisotropy fields for specific layers in the stack and explain the observed mode softening. The linewidths of the multilayer are adjustable around the bias field, approaching twice that seen at larger fields, allowing control over the bandwidth of devices formed from the structure.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Self-oscillation of standing spin wave in ring resonator with proportional-integral-derivative control

We report on numerical analysis on self-oscillation of standing spin wave excited in a nanostructured active ring resonator, consists of a ferromagnetic nanowire with perpendicular anisotropy. The confined resonant modes are along the nanowire length. A positive feedback with proportional-integral-derivative gain control was adopted in the active ring. Stable excitation of the 1st order standing spin wave has been demonstrated with micromagnetic simulations, taking into account the thermal effect with a random field model. The stationary standing spin wave with a pre-determined set variable of precession amplitude was attained within 20 ns by optimizing the proportional-integral-derivative gain control parameters. The result indicates that a monochromatic oscillation frequency f{sub osc} is extracted from the initial thermal fluctuation state and selectively amplified with the positive feedback loop. The obtained f{sub osc} value of 5.22 GHz practically agrees with the theoretical prediction from dispersion relation of the magneto static forward volume wave. It was also confirmed that the f{sub osc} change due to the temperature rise can be compensated with an external perpendicular bias field H{sub b}. The observed quick compensation time with an order of nano second suggests the fast operation speed in the practical device application.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Magnetization Dynamics in Two-Dimensional Arrays of Square Microelements

The collective magnetization motion in an array of magnetostatically interacting ferromagnetic square elements is studied theoretically and experimentally. Dispersion laws are obtained for collective modes for some particular cases of the distribution of the topological charges π{sub T} of magnetic vortices in particles. Resonance curves are plotted with allowance for dissipation for various values of π{sub T}. An experimental investigation of the ferromagnetic resonance in a two-dimensional array of particles qualitatively supports the calculation results.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Current control of magnetic anisotropy via stress in a ferromagnetic metal waveguide

In this work, we demonstrate that in-plane charge current can effectively control the spin precession resonance in an Al 2 O 3 /CoFeB/Ta heterostructure. Brillouin light scattering was used to detect the ferromagnetic resonance field under microwave excitation of spin waves at fixed frequencies. The current control of spin precession resonance originates from modification of the in-plane uniaxial magnetic anisotropy field $H_k$, which changes symmetrically with respect to the current direction. Numerical simulation suggests that the anisotropic stress introduced by joule heating plays an important role in controlling $H_k$. These results provide new insight into current manipulation of magnetic properties and have broad implications for spintronic devices.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Giant resonances in topological spin Hall effect due to electron-skyrmion scattering in two-dimensional Rashba spin-orbit ferromagnets

Here we study a topological spin Hall effect where conduction electrons are scattered by Néel-type skyrmions in two-dimensional Rashba spin-orbit ferromagnets. We find the resonance structures in direct and Hall electric conductivities. The resonances strongly depend on the skyrmion size, the Rashba spin-orbit coupling vector value, and the relative direction between magnetization and Rashba vectors. The antiparallel arrangement of the Rashba spin-orbit coupling vector with respect to the magnetization determines the resonance structure and increases the direct conductivity by about two orders of magnitude. For the parallel arrangement the electric conductivities decrease by one to three orders of magnitude and the Hall conductivity changes the sign. The Rashba spin-orbit coupling also modifies electron bands depending on the ratio ε R /J, where ε R is Rashba energy and J is an exchange integral between conduction and localized electrons. If ε R /J ≤ 1, each energy band has the single minimum while for ε R /J > 1 the lower band has the “Mexican-hat” shape with the maximum at k = 0. We focus on the dependencies of direct and Hall electric conductivities on Fermi energy, ε F , skyrmion sizes, and Rashba spin-orbit coupling constant values and its relative direction with the magnetization. Analyzing both types of the energy bands, in general, we find two types of resonances: at the minimum of the upper band and behaviors, we employ the scattering pattern analysis. The resonance dependencies on skyrmion sizes and Rashba, a spin-orbit coupling contestant, can be qualitatively explained in terms of the Ramsauer-Townsend scattering of the upper band electrons by the skyrmion quantum well. The resonance properties can be used in spin transistors. To discover the resonances it is necessary to know the specific range of the parameters. At some values of the parameters the electric conductivity changes by about two orders of magnitude in the narrow range of ε F (< 0.01 eV). To detect the resonances it is also important to identify the relative direction of the Rashba spin-orbit coupling vector with respect to the magnetization.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Room-temperature ferromagnetism of Fe-doped TiO{sub 2} nanoparticles driven by oxygen vacancy

Graphical abstract: A series of Ti{sub 1−x}Fe{sub x}O{sub 2−δ} (x ≤ 0.03) nanoparticles (NPs) were synthesized by sol–gel route. The room temperature ferromagnetism (RTFM) has been explained in terms of vacancy induced bound magnetic polaron (BMP) model, where the Fe{sup 3+} ions ferromagnetically coupled mediated by oxygen vacancies. Display Omitted Highlights: ► The Fe-doped NPs have been synthesized by sol–gel method. ► Ferromagnetism of Fe-doped TiO{sub 2} NPs are intrinsic property. ► Room temperature ferromagnetism observed in all the Fe-doped samples. ► Variation in M{sub s} is attributed due to oxygen vacancies and defect formation. -- Abstract: A series of Ti{sub 1−x}Fe{sub x}O{sub 2−δ} (0 ≤ x ≤ 0.03) nanoparticles (NPs) were synthesized by sol–gel route. The NPs had a size distribution in the range of 15–40 nm and were identified as the anatase TiO{sub 2} by X-ray diffraction (XRD) and Raman analysis. XRD, selected area electron diffraction, Raman and Mössbauer analysis ruled out the signature of Fe-cluster or any other oxides of Fe. The redshifting of the band edge emission peak observed in UV–vis absorption studies further confirmed the doping of Fe ions in the TiO{sub 2} lattice. Raman studies show the shifting and broadening in E{sub g}(1) and E{sub g}(3) modes with Fe doping. It suggested that the activation of ferromagnetism with increasing Fe doping concentration was related to the oxygen vacancy defects. The presence of such defects was further confirmed from electron paramagnetic resonance (EPR) measurements. The observed ferromagnetism is interpreted in terms of bound magnetic polaron (BMP) model.

36 MATERIALS SCIENCE↗
DOE OSTI2026

Low-Field EMR Studies of Permalloy Films and Gratings

Flat and profile-modulated permalloy films have been studied by the electron magnetic resonance (EMR) method. In addition to ferromagnetic and spin-wave resonances, the structures demonstrate low-field EMR signals of an unusual shape, which form a hysteresis loop in sweeping fields. The low-field signals are attributed to a fast reorientation of magnetic domains. The low-field EMR behavior is comparable to the behavior in magneto-dependent photovoltage previously observed in the optical experiments. The shapes of the loops and typical values of the switching fields depend on the profile modulation parameters confirming the possibility of controlling magnetic properties and the coupling of magnetic and optical effects with nanoscale geometry.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Deconvolution of mixed magnetism in multilayer graphene

Magnetic properties of graphite modified at the edges by KCl and exfoliated graphite in the form of twisted multilayered graphene (<4 layers) are analyzed to understand the evolution of magnetic behavior in the absence of any magnetic impurities. The mixed magnetism in multilayer graphene is deconvoluted using Low field-high field hysteresis loops at different temperatures. In addition to temperature and the applied magnetic field, the density of edge state spins and the interaction between them decides the nature of the magnetic state. By virtue of magnetometry and electron spin resonance studies, we demonstrate that ferromagnetism is intrinsic and is due to the interactions among various paramagnetic centers. The strength of these magnetic correlations can be controlled by modifying the structure.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2014

Structural and magnetic properties of irradiated SiC

We present a comprehensive structural characterization of ferromagnetic SiC single crystals induced by Ne ion irradiation. The ferromagnetism has been confirmed by electron spin resonance, and possible transition metal impurities can be excluded to be the origin of the observed ferromagnetism. Using X-ray diffraction and Rutherford backscattering/channeling spectroscopy, we estimate the damage to the crystallinity of SiC, which mutually influences the ferromagnetism in SiC.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Excitation of Nonlinear Spin Oscillations in an Antiferromagnet under the Action of Pulsed Terahertz Pumping

We present a model of excitation of nonlinear spin oscillations in an antiferromagnet under the action of terahertz pulses of an electromagnetic field. It is shown that, with increasing pumping pulse amplitude, the spin system response increases linearly in the fundamental (resonant) quasi-ferromagnetic mode and quadratically in the second harmonic. We have compared the theoretically obtained dependences of the intensities of the sample response amplitudes for the fundamental mode and the second harmonic with the experimental ones. The comparison demonstrates the applicability of the developed method of analyzing dynamic processes in antiferromagnets occurring under the action of terahertz electromagnetic pulses.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2013

Low-frequency resonance increase in the resistance of a microcontact between ferromagnetic and nonmagnetic metals

The magnetic dynamics of a mesoscopic three-dimensional magnet has been studied by measuring the resistance of a nanodimensional (point) microcontact between paramagnetic and ferromagnetic metals. The resistance was measured by a modulation technique under conditions where a significant role was played by dipole-dipole interaction, magnetic field, and dissipation. It was found that the resistance of the microcontact exhibits resonance growth at low frequencies ({approx}10{sup 3} s{sup -1}). The properties of resonances are described by a model of microcontact gyromagnetic oscillations (MCGMOs) based on mutual transformation of spin and mechanical angular momentum. Experimental techniques, basic properties, and the MCGMO model are described. The passage of an electric current through the interface between paramagnetic and ferromagnetic metals leads to nonequilibrium magnetization localized at the interface. A high current density in the microcontact determines the strong excitation of magnetization (high density of magnons) at which the interaction between magnons becomes significant. In a uniaxial magnet, the attraction of magnons leads to the formation of a spatially localized configuration of gapless long-wavelength magnons (magnetic soliton). At a given excitation of magnetization, the vector structure of a magnetic soliton possesses a minimum free energy (configuration energy minimum). The configuration energy minimum of a magnetic soliton is responsible for the radical increase in the soliton spin relaxation time, which determines the fundamental possibility of exciting stationary low-frequency MCGMOs.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Magnetoelastic resonance as a probe for exchange springs at antiferromagnet-ferromagnet interfaces

In prototype ferromagnet-antiferromagnet interfaces we demonstrate that surface acoustic waves can be used to identify complex magnetic phases arising upon evolution of exchange springs in an applied field. Applying sub-GHz surface acoustic waves to study the domain structure of the ferromagnetic layer in exchange-biased bilayers of Ir 20 Mn 80 -Co 60 Fe 20 B 20 , we are able to associate the magnetoelastic resonance with the presence of the exchange spin-spirals in both the ferromagnetic and antiferromagnetic layer. Our findings offer a complementary, integrative insight into emergent magnetic materials for applications of noncollinear spin textures in view of low-energy-consumption spintronic devices.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗