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At least 937 records · Page 52
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

Amplification of collective magnetic fluctuations in magnetized bi-maxwellian plasmas for parallel wave vectors. I. Electron-proton plasma

The general electromagnetic fluctuation theory is a powerful tool to analyze the magnetic fluctuation spectrum of a plasma. Recent works utilizing this theory for a magnetized non-relativistic isotropic Maxwellian electron–proton plasma have demonstrated that the equilibrium ratio of |δB|/B{sub 0} can be as high as 10{sup −12}. This value results from the balance between spontaneous emission of fluctuations and their damping, and it is considerably smaller than the observed value |δB|/B{sub 0} in the solar wind at 1 au, where 10{sup −3}≲|δB|/B{sub 0}≲10{sup −1}. In the present manuscript, we consider an anisotropic bi-Maxwellian distribution function to investigate the effect of plasma instabilities on the magnetic field fluctuations. We demonstrate that these instabilities strongly amplify the magnetic field fluctuations and provide a sufficient mechanism to explain the observed value of |δB|/B{sub 0} in the solar wind at 1 au.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Nonlinear Transverse Cascade and Sustenance of MRI Turbulence in Keplerian Disks with an Azimuthal Magnetic Field

We investigate magnetohydrodynamic turbulence driven by the magnetorotational instability (MRI) in Keplerian disks with a nonzero net azimuthal magnetic field using shearing box simulations. As distinct from previous studies, we analyze turbulence dynamics in Fourier (k-) space to understand its sustenance. The linear growth of the MRI with the azimuthal field has a transient character and is anisotropic in Fourier space, leading to anisotropy of nonlinear processes in Fourier space. As a result, the main nonlinear process appears to be a new type of angular redistribution of modes in Fourier space—the nonlinear transverse cascade—rather than the usual direct/inverse cascade. We demonstrate that the turbulence is sustained by the interplay of the linear transient growth of the MRI (which is the only energy supply for the turbulence) and the transverse cascade. These two processes operate at large length scales, comparable to the box size (disk scale height); the corresponding small wavenumber area, called the vital area in Fourier space, is crucial for the sustenance, while outside the vital area, direct cascade dominates. The interplay of the linear and nonlinear processes in Fourier space is generally too intertwined for a vivid schematization. Nevertheless, we reveal the basic subcycle of the sustenance that clearly shows the synergy of these processes in the self-organization of the magnetized flow system. This synergy is quite robust and persists for the considered different aspect ratios of the simulation boxes. The spectral characteristics of the dynamical processes in these boxes are qualitatively similar, indicating the universality of the sustenance mechanism of the MRI turbulence.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2024

Gradient Technique Theory: Tracing Magnetic Field and Obtaining Magnetic Field Strength

Abstract The gradient technique is a promising tool with theoretical foundations based on the fundamental properties of MHD turbulence and turbulent reconnection. Its various incarnations use spectroscopic, synchrotron, and intensity data to trace the magnetic field and measure the media magnetization in terms of Alfvén Mach number. We provide an analytical theory of gradient measurements and quantify the effects of averaging gradients along the line of sight and over the plane of the sky. We derive analytical expressions that relate the properties of gradient distribution with the Alfvén Mach number M A . We show that these measurements can be combined with measures of sonic Mach number or line broadening to obtain the magnetic field strength. The corresponding technique has advantages to the Davis–Chandrasekhar–Fermi way of obtaining the magnetic field strength.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2024

Data for "Integrated ab initio modelling of atomic order and magnetic anisotropy for rare-earth-free magnet design: effects of alloying additions in L1 0 FeNi."

We describe an integrated modelling approach to accelerate the search for novel, single-phase, multicomponent materials with high magnetocrystalline anisotropy (MCA). For a given system we predict the nature of atomic ordering, its dependence on the magnetic state, and then proceed to describe the consequent MCA, magnetisation, and magnetic critical temperature (Curie temperature). Crucially, within our modelling framework, the same ab initio description of a material’s electronic structure determines all aspects. We demonstrate this holistic method by studying the effects of alloying additions in FeNi, examining systems with the general stoichiometries Fe 4 Ni 3 X and Fe 3 Ni 4 X, for additives including X = Pt, Pd, Al, and Co. The atomic ordering behaviour predicted on adding these elements, fundamental for determining a material’s MCA, is rich and varied. Equiatomic FeNi has been reported to require ferromagnetic order to establish the tetragonal L1 0 order suited for significant MCA. Our results show that when alloying additions are included in this material, annealing in an applied magnetic field and/or below a material’s Curie temperature may also promote tetragonal order, along with an appreciable effect on the predicted hard magnetic properties.

36 MATERIALS SCIENCE↗
NASA NTRS2018

Global High Resolution Crustal Magnetic Field at the Surface of the Moon from Low-Altitude Lunar Prospector Magnetic Gradient Data

We derived new vector gradients based models of crustal magnetic field at the lunar surface with data from the Lunar Prospector (LP) satellite using two model parameterization approaches: a global set of 35820 1° spaced (~30 km) equal area monopoles at 20 km below the surface (O’Brien and Parker, 1994; Olsen et al., 2017) and combined results of subsets of 100000 0.66° spaced monopoles at the same depth. We use the scheme of iteratively reweighted least-squares inversion to compute the initial model. Then the amplitudes of these monopoles are determined by minimizing the misfit to the components together with the global average of |Br| at the ellipsoid surface (i.e. applying a L1 model regularization of Br). In previous approaches using vector fields for modeling, we found that external field contamination leads to spurious anomalies in the downward continued field models even with stringent data selection criteria and ad-hoc noise removal techniques (e.g., satellite’s position in the Moon’s wake w.r.t. the solar wind and in the Earth’s magnetotail, internal/external dipoles fields removal, low-order polynomial removal, joint equivalent source cross-validation technique and visually removing remaining anomalous segments). On the other hand, with the use of gradients-only data (along-track first finite differences), we were able to completely bypass the ad-hoc techniques. Similar processing of Kaguya magnetic data, which have only higher altitude coverage over the most of the Moon except in the region of the South Pole–Aitken (SPA) basin, completely misses some of the anomalies seen in the Lunar Prospector data. The combined Lunar Prospector and Kaguya gradient-based models also severely degrade the derivation of the anomaly fields in many regions. Euler analysis of isolated anomaly features from the Reiner Gamma swirl suggests top depths of about 0.3 to 1.5 km and center depths of 10-14 km; in the region between Stein and Vallier craters north of the SPA basin our analysis suggests top depths of around 1.5 km and center depths of 13-15 km. With the spectral depth determination techniques, the SPA basin region yields depths to the base of magnetization ranging between 15 and 40 km. Three-dimensional modeling and the bulk magnetization determinations of the sources constrained by the Euler and spectral methods is underway.

Gradient↗
NASA NTRS2023

Extragalactic Magnetism with SOFIA (SALSA Legacy Program). V. First Results on the Magnetic Field Orientation of Galaxies

We present the analysis of the magnetic field ( B -field) structure of galaxies measured with far-infrared (FIR) and radio (3 and 6 cm) polarimetric observations. We use the first data release of the Survey of extragALactic magnetiSm with SOFIA of 14 nearby (<20 Mpc) galaxies with resolved (5"-18" ; 90 pc–1 kpc) imaging polarimetric observations using SOFIA/HAWC+ from 53 to 214 μm . We compute the magnetic pitch-angle (𝜓 B ) profiles as a function of the galactocentric radius. We introduce a new magnetic alignment parameter ( ζ ) to estimate the disordered-to-ordered ratio of spiral B -fields. We find FIR and radio wavelengths to not generally trace the same B -field morphology in galaxies. The 𝜓 B profiles tend to be more ordered across all galactocentric radii in radio ( ζ 6cm = 0.93 + 0.03) than in FIR ( ζ 154μm = 0.84 + 0.14). For spiral galaxies, FIR B -fields are 2%–75% more turbulent than the radio B -fields. For starburst galaxies, we find that FIR polarization is a better tracer of the B -fields along the galactic outflows than radio polarization. Our results suggest that the B -fields associated with dense, dusty, turbulent star-forming regions (those traced at FIR) are less ordered than warmer, less dense regions (those traced at radio) of the interstellar medium. The FIR B -fields seem to be more sensitive to the activity of the star-forming regions and molecular clouds within a vertical height of a few hundred parsecs in the disk of spiral galaxies than the radio B -fields.

magnetic field (B-field)↗
DOE OSTI2014

Methods for characterizing magnetic footprints of perpendicular magnetic recording writer heads

In this work, the magnetic footprints, along with some of its dynamic features in recording process, of perpendicular magnetic recording writer heads have been characterized by using three different techniques. Those techniques are the spin-stand stationary footprint technique, the spin-stand dynamic footprint technique, and the coherent writing technique combined with magnetic force microscope imaging method. The characteristics of those techniques have been compared to one another. It was found experimentally that the spin-stand stationary method could not precisely catch some peculiar recording dynamics of the write heads in certain conditions. The advantages and disadvantages among all those techniques are also examined and discussed in detail.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Effect of substrate rotation on domain structure and magnetic relaxation in magnetic antidot lattice arrays

Microdimensional triangular magnetic antidot lattice arrays were prepared by varying the speed of substrate rotation. The pre-deposition patterning has been performed using photolithography technique followed by a post-deposition lift-off. Surface morphology taken by atomic force microscopy depicted that the growth mechanism of the grains changes from chain like formation to island structures due to the substrate rotation. Study of magnetization reversal via magneto optic Kerr effect based microscopy revealed reduction of uniaxial anisotropy and increase in domain size with substrate rotation. The relaxation measured under constant magnetic field becomes faster with rotation of the substrate during deposition. The nature of relaxation for the non-rotating sample can be described by a double exponential decay. However, the relaxation for the sample with substrate rotation is well described either by a double exponential or a Fatuzzo-Labrune like single exponential decay, which increases in applied field.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Correlating spin transport and electrode magnetization in a graphene spin valve: Simultaneous magnetic microscopy and non-local measurements

Using simultaneous magnetic force microscopy and transport measurements of a graphene spin valve, we correlate the non-local spin signal with the magnetization of the device electrodes. The imaged magnetization states corroborate the influence of each electrode within a one-dimensional spin transport model and provide evidence linking domain wall pinning to additional features in the transport signal.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Magnetic properties in MnBi alloy of small crystallites for permanent magnet devices

A rare-earth free alloy like MnBi is a potential candidate for developing small magnets and devices. In a commercially viable method, a MnBi alloy was prepared by arc melting Mn and Bi metals in a 1:1 ratio. In terms of the X-ray diffraction a single crystalline MnBi phase is formed of the as prepared alloy. FESEM images delineate thin MnBi layers (25 – 40 nm thickness) of average EDX composition throughout the specimen. A large coercivity 5.501 kOe (6.5 emu/g magnetization) observed in an M-H at 300 K is decreased to 0.171 (9.0 emu/g magnetization) at 100 K in decreasing upon cooling.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Efficient expulsion of magnetic flux in superconducting radiofrequency cavities for high Q{sub 0} applications

Even when cooled through its transition temperature in the presence of an external magnetic field, a superconductor can expel nearly all external magnetic flux. This paper presents an experimental study to identify the parameters that most strongly influence flux trapping in high purity niobium during cooldown. This is critical to the operation of superconducting radiofrequency cavities, in which trapped flux degrades the quality factor and therefore cryogenic efficiency. Flux expulsion was measured on a large survey of 1.3 GHz cavities prepared in various ways. It is shown that both spatial thermal gradient and high temperature treatment are critical to expelling external magnetic fields, while surface treatment has minimal effect. For the first time, it is shown that a cavity can be converted from poor expulsion behavior to strong expulsion behavior after furnace treatment, resulting in a substantial improvement in quality factor. Microscopic investigations are performed to study the relevant changes in the material from this treatment. Future plans are described to build on this result in order to optimize treatment for future cavities.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Effect of Mg Content in Melt-Spun Mn–Bi–Mg–Sb–In Alloys on the Structure and Properties of Field-Annealed Magnets

Mn-Bi alloys modified with Mg and other elements were recently developed into promising rare-earth-free permanent magnets through successive melt-spinning, warm compaction, and short magnetic-field annealing. This letter reports herein on the crystal structures, texture, and magnetic properties observed in Mn 50 Bi 49-x Mg x Sb 0.5 In 0.5 alloys (x = 0, 1.5, 3.0, 4.5) throughout these processing steps. The Mg substitution favors MnBi-based intermetallic phases over the Bi- and Mn-based solid solutions and the stable α intermetallic phase over the metastable β' intermetallic phase. Magnesium increases the c/a ratio of the α phase lattice parameters for all studied x values and it increases the degree of the field-annealing-induced texture for x ≤ 3. The effect of Mg on coercivity is explained as combination of a positive one via the α phase magnetocrystalline anisotropy and a negative one via the development of texture. The effect of the substitution on the maximum energy product additionally involves the decreasing density of the alloys; the optimum x is found to be ≈3. Additionally reported are the thermal properties of selected alloys. The mechanism of field-annealing effect on the coercivity is briefly discussed.

36 MATERIALS SCIENCE↗
DOE OSTI2015

On the size-dependent magnetism and all-optical magnetization switching of transition-metal silicide nanostructures

Theoretical investigations of the electronic structure, synthesis, and all-optical magnetization switching of transition-metal silicide nanostructures are reported. The magnetic moment of the nanostructures is studied as a function of the silicide cluster size and configuration. The experimentally demonstrated magnetization switching of nanostructured nickel silicide by circularly polarized light makes it possible to create high-speed storage devices with high density data recording.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Contributions of Magnetic Phases to the FORC Diagram in (NdDy)(FeCo)B Magnets

The relationship of the micromagnetic structure, the contributions of various crystalline phases and hysterons obtained by the first order reversal curves (FORC) method in sintered (NdDy)(FeCo)B magnets is discussed. The distribution map of parameters of magnetic hysteresis partial loops that allows the separation of various spin ensembles to the magnetization has been built.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Universal Electronic‐Structure Relationship Governing Intrinsic Magnetic Properties in Permanent Magnets

An electronic-structure-centered perspective is presented on permanent-magnet (PM) design, highlighting two key levers, that is, saturation magnetization (M s ), governed by 3d-band filling and exchange physics, and magnetocrystalline anisotropy energy (MAE), arising from spin-orbit coupling (SOC) on anisotropic orbital populations. Reviewing current practices, including DFT-based MAE/J ij extraction, atomistic-spin and micromagnetic modeling, and high-throughput machine learning (ML) pipelines, three bottlenecks limiting predictive discovery is identified that is i) electronic-structure accuracy for small MAE (sensitive to functional choice, Hubbard U, and many-body effects), ii) finite-temperature and kinetic realism (phonon/magnon renormalization, ordering kinetics), and iii) descriptor and multiscale decoupling (lack of SOC-weighted and orbital-resolved fingerprints). Deep dives into the electronic-structure of Nd─Fe─B and Fe─N show how these fingerprints govern magnetic performance, motivating DFT- and quantum-mechanics-based descriptors for discovery. Unbiased, structure-driven exploration, coupled with high-throughput simulations, ML, generative AI, and reasoning models, accelerates candidate identification and propagates insights across scales. Addressing supply-chain risks, on future needs of designing “critical-element-free” magnets with tailored microstructure and high energy products is emphasized. By integrating electronic fingerprints, AI reasoning, and multiscale modeling, a practical roadmap is provided for rare-earth-lean or rare-earth free, high-performance, sustainable PMs.

Singh, Prashant [Ames Laboratory, and Iowa State U↗
DOE OSTI2015

Influence of Guest Exchange on the Magnetization Dynamics of Dilanthanide Single‐Molecule‐Magnet Nodes within a Metal–Organic Framework

Abstract Multitopic organic linkers can provide a means to organize metal cluster nodes in a regular three‐dimensional array. Herein, we show that isonicotinic acid N ‐oxide (HINO) serves as the linker in the formation of a metal–organic framework featuring Dy 2 single‐molecule magnets as nodes. Importantly, guest solvent exchange induces a reversible single‐crystal to single‐crystal transformation between the phases Dy 2 (INO) 4 (NO 3 ) 2 ⋅2 solvent (solvent=DMF (Dy 2 ‐DMF), CH 3 CN (Dy 2 ‐CH 3 CN)), thereby switching the effective magnetic relaxation barrier (determined by ac magnetic susceptibility measurements) between a negligible value for Dy 2 ‐DMF and 76 cm −1 for Dy 2 ‐CH 3 CN. Ab initio calculations indicate that this difference arises not from a significant change in the intrinsic relaxation barrier of the Dy 2 nodes, but rather from a slowing of the relaxation rate of incoherent quantum tunneling of the magnetization by two orders of magnitude.

Zhang, Xuejing↗
DOE OSTI2015

Influence of Guest Exchange on the Magnetization Dynamics of Dilanthanide Single‐Molecule‐Magnet Nodes within a Metal–Organic Framework

Abstract Multitopic organic linkers can provide a means to organize metal cluster nodes in a regular three‐dimensional array. Herein, we show that isonicotinic acid N ‐oxide (HINO) serves as the linker in the formation of a metal–organic framework featuring Dy 2 single‐molecule magnets as nodes. Importantly, guest solvent exchange induces a reversible single‐crystal to single‐crystal transformation between the phases Dy 2 (INO) 4 (NO 3 ) 2 ⋅2 solvent (solvent=DMF (Dy 2 ‐DMF), CH 3 CN (Dy 2 ‐CH 3 CN)), thereby switching the effective magnetic relaxation barrier (determined by ac magnetic susceptibility measurements) between a negligible value for Dy 2 ‐DMF and 76 cm −1 for Dy 2 ‐CH 3 CN. Ab initio calculations indicate that this difference arises not from a significant change in the intrinsic relaxation barrier of the Dy 2 nodes, but rather from a slowing of the relaxation rate of incoherent quantum tunneling of the magnetization by two orders of magnitude.

Zhang, Xuejing↗