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At least 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Pseudospectral Landau-Lifshitz description of magnetization dynamics

Magnetic materials host a wealth of nonlinear dynamics, textures, and topological defects. This is possible due to the competition between strong nonlinearity and dispersion, which act at the atomic scale, as well as long-range interactions. However, these features are difficult to study analytically and numerically because of the vastly different temporal and spatial scales involved. Here, in this work, we present a pseudospectral approach for the Landau-Lifshitz equation that invokes energy and momentum conservation embodied in the magnon dispersion relation to accurately describe both atomic and continuum limits. Furthermore, this approach enables analytical study at every scale. We show the applicability of this model in both the continuum and atomic limit by investigating modulational instability and ultrafast evolution of magnetization due to transient grating, respectively, in a one-dimensional ferromagnetic chain with perpendicular magnetic anisotropy. This model provides the possibility of grid-independent multiscale numerical approaches that will enable the description of singularities within a single framework.

1-dimensional spin chains↗
DOE OSTI2015

Low-temperature magnetization dynamics of magnetic molecular solids in a swept field

The swept-field experiments on magnetic molecular solids such as Fe{sub 8} are studied using Monte Carlo simulations, and a kinetic equation developed to understand collective magnetization phenomena in such solids, where the collective aspects arise from dipole–dipole interactions between different molecules. Because of these interactions, the classic Landau–Zener–Stückelberg theory proves inadequate, as does another widely used model constructed by Kayanuma. It is found that the simulations provide a quantitatively accurate account of the experiments. The kinetic equation provides a similarly accurate account except at very low sweep velocities, where it fails modestly. This failure is attributed to the neglect of short-range correlations between the dipolar magnetic fields seen by the molecular spins. The simulations and the kinetic equation both provide a good understanding of the distribution of these dipolar fields, although analytic expressions for the final magnetization remain elusive.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

The role of Pt underlayer on the magnetization dynamics of perpendicular magnetic anisotropy Pt/Co{sub 2}FeAl{sub 0.5}Si{sub 0.5}/MgO

We investigate the role of Pt on the magnetization dynamics of Pt/Co{sub 2}FeAl{sub 0.5}Si{sub 0.5}/MgO with perpendicular magnetic anisotropy using the time resolved magneto-optic Kerr effect. Pt/Co{sub 2}FeAl{sub 0.5}Si{sub 0.5}/MgO shows ultrafast magnetization dynamics comparable to 3d ferromagnets and can be fully demagnetized. The demagnetization time τ{sub d} ∼ 0.27 ps and magnetic heat capacity are independent of the Pt underlayer, whereas the value of the electron-phonon coupling time τ{sub e} ∼ 0.77 ps depends on the presence of the Pt layer. We further measure the effective damping α{sub eff} ∼ 1 that does not scale as the inverse demagnetization time (1/τ{sub d}), but is strongly affected by the Pt layer.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Examination of the magnetism dynamics from intermixing effects in γ-Fe{sub 2}O{sub 3}/MnO core-shell nanoparticles

We have examined the effects of core-shell intermixing on the dynamical magnetism of γ-Fe{sub 2}O{sub 3}/MnO nanoparticles. The core and shell phases were identified using x-ray diffraction, and x-ray absorption spectroscopy identified Mn ions in both octahedral and tetrahedral sites, consistent with a significant amount of substitution at the core-shell interface to form an Fe/Mn-ferrite. The dynamical response was probed by Mössbauer spectroscopy, which decouples surface and core spins, and suggested a change in the relaxation behaviour among the spin populations within γ-Fe{sub 2}O{sub 3}/MnO relative to the γ-Fe{sub 2}O{sub 3} seed particles. Interestingly, the magnetic relaxation effects at the atomic scale, measured via Mössbauer spectroscopy, were enhanced, indicating that the addition of an MnO shell and intermixing affected the dynamical freezing process which altered the surface magnetism of the γ-Fe{sub 2}O{sub 3} core. Our results show that both the MnO shell and the interfacial intermixed layer are important in determining the core-shell nanoparticle magnetism.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Delineating magnetization dynamics in solution-processed doped yttrium iron garnet thin films

Here, in this work, thin films of ruthenium-doped and cerium-doped yttrium iron garnet were deposited on silicon using solgel chemistry. Doped YIG could be produced in phase pure form up to a precursor stoichiometry of Y 3 Ru 0.1 Fe 4.9 O 12 and Ce 0.7 Y 2.3 Fe 5 O 12 . Both dopants significantly increase the coercivity and anisotropy field of the materials either due to domain wall pinning or increased spin–orbit coupling from the dopant. To delineate these two effects, the dynamic magnetic properties were studied using strip line ferromagnetic resonance (FMR). The FMR linewidth was separated into intrinsic loss and inhomogeneous line broadening. Inhomogeneous line broadening was found to dominate the magnetic losses in all the films likely due to magnon scattering off grain boundaries, but the Gilbert damping remained fairly low. By comparing the two dopants, it was found that Gilbert damping increased more in Ce:YIG films than in Ru:YIG films. This finding was corroborated by changes in the anisotropy field of the films, indicating a larger contribution from spin–orbit coupling from cerium than from ruthenium. Surprisingly, while magnetic loss globally increased with higher substitution, adding a small amount of dopant actually reduced the inhomogeneous line broadening in both sets of films. This was corroborated by crystallite size. The damping in Ru:YIG also decreased with a small amount of the dopant, which has been predicted by Kittel for doped garnets. Thus, it follows that there is an ideal doping regime where solgel YIG can be doped at low levels without increasing magnetic loss.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Microwave soft x-ray microscopy for nanoscale magnetization dynamics in the 5–10 GHz frequency range

We present a scanning transmission x-ray microscopy setup combined with a novel microwave synchronization scheme for studying high frequency magnetization dynamics at synchrotron light sources. The sensitivity necessary to detect small changes in the magnetization on short time scales and nanometer spatial dimensions is achieved by combining the excitation mechanism with single photon counting electronics that is locked to the synchrotron operation frequency. Our instrument is capable of creating direct images of dynamical phenomena in the 5-10 GHz range, with high spatial resolution. When used together with circularly polarized x-rays, the above capabilities can be combined to study magnetic phenomena at microwave frequencies, such as ferromagnetic resonance (FMR) and spin waves. We demonstrate the capabilities of our technique by presenting phase resolved images of a ∼6 GHz nanoscale spin wave generated by a spin torque oscillator, as well as the uniform ferromagnetic precession with ∼0.1° amplitude at ∼9 GHz in a micrometer-sized cobalt strip.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2016

Europium mixed-valence, long-range magnetic order, and dynamic magnetic response in EuCu 2 ( Si x Ge 1 - x ) 2

In mixed-valence or heavy-fermion systems, the hybridization between local f orbitals and conduction band states can cause the suppression of long-range magnetic order, which competes with strong spin uctuations. Ce- and Yb-based systems have been found to exhibit fascinating physical properties (heavy-fermion superconductivity, non-Fermi-liquid states, etc.) when tuned to the vicinity of magnetic quantum critical points by use of various external control parameters (temperature, magnetic eld, chemical composition). Recently, similar effects (mixed-valence, Kondo uctuations, heavy Fermi liquid) have been reported to exist in some Eu-based compounds. Unlike Ce (Yb), Eu has a multiple electron (hole) occupancy of its 4f shell, and the magnetic Eu 2+ state (4f 7 ) has no orbital component in the usual LS coupling scheme, which can lead to a quite different and interesting physics. In the EuCu 2 (Si x Ge 1-x ) 2 series, where the valence can be tuned by varying the Si/Ge ratio, it has been reported that a significant valence uctuation can exist even in the magnetic order regime. This paper presents a detailed study of the latter material using different microscopic probes (XANES, Mossbauer spectroscopy, elastic and inelastic neutron scattering), in which the composition dependence of the magnetic order and dynamics across the series is traced back to the change in the Eu valence state. In particular, the results support the persistence of valence uctuations into the antiferromagnetic state over a sizable composition range below the critical Si concentration x c ≈ 0:65. In conclusion, the sequence of magnetic ground states in the series is shown to re ect the evolution of the magnetic spectral response.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Spatially resolved ultrafast magnetic dynamics initiated at a complex oxide heterointerface

Static strain in complex oxide heterostructures 1,2 has been extensively used to engineer electronic and magnetic properties at equilibrium 3 . In the same spirit, deformations of the crystal lattice with light may be used to achieve functional control across heterointerfaces dynamically 4 . Here, by exciting large-amplitude infrared-active vibrations in a LaAlO 3 substrate we induce magnetic order melting in a NdNiO 3 film across a heterointerface. Femtosecond resonant soft X-ray diffraction is used to determine the spatiotemporal evolution of the magnetic disordering. We observe a magnetic melt front that propagates from the substrate interface into the film, at a speed that suggests electronically driven motion. Lastly, light control and ultrafast phase front propagation at heterointerfaces may lead to new opportunities in optomagnetism, for example by driving domain wall motion to transport information across suitably designed devices.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

All electrical manipulation of magnetization dynamics in a ferromagnet by antiferromagnets with anisotropic spin Hall effects.

We investigate spin-orbit torques of metallic CuAu-I-type antiferromagnets using spin-torque ferromagnetic resonance tuned by a dc-bias current. The observed spin torques predominantly arise from diffusive transport of spin current generated by the spin Hall effect. We find a growth-orientation dependence of the spin torques by studying epitaxial samples, which may be correlated to the anisotropy of the spin Hall effect. The observed anisotropy is consistent with first-principles calculations on the intrinsic spin Hall effect. Our work suggests large tunable spin-orbit effects in magnetically-ordered materials.

Zhang, Wei↗
DOE OSTI2024

Magnetization dynamics driven by displacement currents across a magnetic tunnel junction

Understanding the high-frequency transport characteristics of magnetic tunnel junctions (MTJs) is crucial for the development of fast-operating spintronics memories and radio frequency devices. Here, we present the study of a frequency-dependent capacitive current effect in Co Fe B / Mg O -based MTJs and its influence on magnetization dynamics using a time-resolved magneto-optical Kerr effect technique. In our device, operating at gigahertz frequencies, we find a large displacement current of the order of mA, which does not break the tunnel barrier of the MTJ. Importantly, this current generates an Oersted field and spin-orbit torque, inducing magnetization dynamics. Our discovery holds promise for building robust MTJ devices operating under high current conditions, also highlighting the significance of capacitive impedance in high-frequency magnetotransport techniques. Published by the American Physical Society 2024

Safeer, C. K. (ORCID:0000000323969730)↗
DOE OSTI2015

Noise-induced bifurcations in magnetization dynamics of uniaxial nanomagnets

Stochastic magnetization dynamics in uniformly magnetized nanomagnets is considered. The system is assumed to have rotational symmetry as the anisotropy axis, the applied field, and the spin polarization are all aligned along an axis of symmetry. By appropriate integration of the Fokker-Planck equation associated to the problem, the stochastic differential equation governing the evolution of the angle between the magnetization orientation and the symmetry axis is derived. The drift terms present in this equation contain a noise-induced drift term, which, in combination with drift terms of deterministic origin, can be written as the derivative of an effective potential. Superparamagnetic-like transitions are studied in connections with the bifurcations of the effective potential as temperature and excitation conditions are varied.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Study of static and dynamic magnetic properties of Fe nanoparticles composited with activated carbon

Nanocomposite of Fe nanoparticles with activated carbon has been synthesized to alter the magnetic spin-spin interaction and hence study the dilution effect on the static and dynamic magnetic properties of the Fe nanoparticle system. Transmission electron microscopic (TEM) image shows the spherical Fe nanoparticles dispersed in carbon matrix with 13.8 nm particle size. Temperature dependent magnetization measurement does not show any blocking temperature at all, right up to the room temperature. Magnetic hysteresis curve, taken at 300 K, shows small value of the coercivity and this small hysteresis indicates the presence of an energy barrier and inherent magnetization dynamics. Langevin function fitting of the hysteresis curve gives almost similar value of particle size as obtained from TEM analysis. Magnetic relaxation data, taken at a temperature of 100 K, were fitted with a combination of two exponentially decaying function. This diluted form of nanoparticle system, which has particles size in the superparamagnetic limit, behaves like a dilute ensemble of superspins with large value of the magnetic anisotropic barrier.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2021

Magnetic dynamics with Weyl fermions

Transport of charge and valley (axial) degrees of freedom coupled to order-parameter dynamics in magnetic Weyl semimetals is studied in the framework of nonequilibrium thermodynamics. In addition to the established valley-related transport anomalies that are rooted in band-structure topology, we construct dissipative couplings between the three dynamic constituents of the system driven out of equilibrium by electromagnetic perturbations. We show how the valley degree of freedom mediates an effective coupling between the charge and magnetic sectors of the system, through a combination of the chiral anomaly, on the electric side, and the Onsager-paired valley torque and pumping, on the magnetic side. This work complements previous studies of magnetic Weyl semimetals by a more systematic analysis of collective dissipation. We discuss several concrete examples of the valley-mediated current-driven magnetic instabilities and charge pumping, and extend the theory to the antiferromagnetic case.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Tuning of the microwave magnetization dynamics in CoZr-based thin films by Nd-doping

To enhance the damping of microwave magnetization dynamics, we investigate the effect of Nd-doping (0–3.0 at. %) into CoZr thin films. The saturation magnetization decreases with Nd-doping. The coercivities generally decrease in the case of 1.5 at. % Nd-doping, while increase in the case of 3.0 at. % Nd-doping. The magnetization dynamics is characterized with the permeability spectra, which are determined by the ferromagnetic resonance behavior at the microwave range. The permeability spectra are analyzed through the fittings with Landau-Lifshitz-Gilbert equation. From the fittings, both the dynamic magnetic anisotropy field and the damping factor increase, due to the enhanced spin-orbital coupling by Nd-doping.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗