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At least 883 records · Page 49
All sourcesNASA NTRSDOE OSTI
DOE OSTI2011

Hidden magnetic configuration in epitaxial La1-rSrzMnO3 films

We present an unreported magnetic configuration in epitaxial La{sub 1-x}Sr{sub x}MnO{sub 3} (x {approx} 0.3) (LSMO) films grown on strontium titanate (STO). X-ray magnetic circular dichroism indicates that the remanent magnetic state of thick LSMO films is opposite to the direction of applied magnetic field. Spectroscopic and scattering measurements reveal that the average Mn valence varies from mixed Mn{sup 3+}/Mn{sup 4+} to an enriched Mn{sup 3+} region near the STO interface, resulting in a compressive lattice along a, b-axis and a possible electronic reconstruction in the Mn e{sub g} orbital (d{sub 3z{sup 2}-r{sup 2}}). This reconstruction may provide a mechanism for coupling the Mn{sup 3+} moments antiferromagnetically along the surface normal direction, and in turn may lead to the observed reversed magnetic configuration.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2017

Lower Emittance Lattice for the Advanced Photon Source Upgrade Using Reverse Bending Magnets

The Advanced Photon Source (APS) is pursuing an upgrade to the storage ring to a hybrid seven-bend-achromat design [1]. The nominal design provides a natural emittance of 67 pm [2]. By adding reverse dipole fields to several quadrupoles [3, 4] we can reduce the natural emittance to 41 pm while simultaneously providing more optimal beta functions in the insertion devices and increasing the dispersion function at the chromaticity sextupole magnets. The improved emittance results from a combination of increased energy loss per turn and a change in the damping partition. At the same time, the nonlinear dynamics performance is very similar, thanks in part to increased dispersion in the sextupoles. This paper describes the properties, optimization, and performance of the new lattice.

Borland, M.↗
DOE OSTI2014

Magnetic helicity of the global field in solar cycles 23 and 24

For the first time we reconstruct the magnetic helicity density of the global axisymmetric field of the Sun using the method proposed by Brandenburg et al. and Pipin et al. To determine the components of the vector potential, we apply a gauge which is typically employed in mean-field dynamo models. This allows for a direct comparison of the reconstructed helicity with the predictions from the mean-field dynamo models. We apply this method to two different data sets: the synoptic maps of the line-of-sight magnetic field from the Michelson Doppler Imager (MDI) on board the Solar and Heliospheric Observatory (SOHO) and vector magnetic field measurements from the Vector Spectromagnetograph (VSM) on the Synoptic Optical Long-term Investigations of the Sun (SOLIS) system. Based on the analysis of the MDI/SOHO data, we find that in solar cycle 23 the global magnetic field had positive (negative) magnetic helicity in the northern (southern) hemisphere. This hemispheric sign asymmetry is opposite to the helicity of the solar active regions, but it is in agreement with the predictions of mean-field dynamo models. The data also suggest that the hemispheric helicity rule may have reversed its sign during the early and late phases of cycle 23. Furthermore, the data indicate an imbalance in magnetic helicity between the northern and southern hemispheres. This imbalance seems to correlate with the total level of activity in each hemisphere in cycle 23. The magnetic helicity for the rising phase of cycle 24 is derived from SOLIS/VSM data, and qualitatively its latitudinal pattern is similar to the pattern derived from SOHO/MDI data for cycle 23.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS2004

Progress on the Plasmoid Thruster Experiment (PTX)

A plasmoid is a compact plasma structure with an integral magnetic field, that may be categorized according to the relative strength of the poloidal and toroidal magnetic field (B(sub p) and B(sub t), respectively). An object with B(sub p)/B(sub t), much much more than 1 is called a Field Reverse Configuration (FRC); if B(sub p) approximately equal to B(sub t), it is called a Spheromak. The thruster operates by repetitively producing plasmoids that are accelerated and ejected at high velocity. As this process is inductive, there are no electrodes. Also, the magnetic structure of the plasmoid should suppress thermal and mass losses to the wall, and improve detachment of the plasma exhaust from the thruster. This concept should be capable of producing an Isp in the range of 5,000 - 10,000 s with thrust densities of order 10(exp 5) N per square meters. The current experiment is designed to produce jet powers in the range of 5-10 kW, although the concept should be scalable into the MW range. In PTX, the plasmoid is formed inside of a single turn conical theta-pinch coil (17.58 cone angle). The coil is driven by a 640 nF, 35 kV capacitor bank, which rings at a frequency of 500 kHz. Previous experiments on PTX were conducted with a static-fill of propellant gas (6% H2 in He), and demonstrated reliable ionization over a pressure range of 40 - 200 mTorr. We are now adding a fast gas-puff valve to load the propellant, and a ringing pre-ionization circuit (f = 5 Mhz) to better control the plasmoid formation. An alternate coil (8.58 cone angle) will also be used, so as to investigate the effect of coil shape on performance. In addition, a variety of propellants will be used, including hydrogen, nitrogen, and argon. The plasmoid mass and velocity will be measured with a variety of diagnostics, including external B-dot probes and flux loops, a high-speed framing camera, and a HeNe laser interferometer. Internal B-dot probes and a quadruple Langmuir probe will provide additional measurements of the plasmoid shape and structure, as well as density, and will be described in a companion paper. The experimental results will be compared to calculations made using a 0-D coil-gun model and also a 2-D time dependent MHD code.

Martin, Adam↗
DOE OSTI2025

Hetero-Orbital Two-Component Fractional Quantum Hall States in Bilayer Graphene

A two-dimensional electron system exposed to a strong magnetic field produces a plethora of strongly interacting fractional quantum Hall (FQH) states, the complex topological orders of which are revealed through exotic emergent particles, such as composite fermions, and fractionally charged Abelian and non-Abelian anyons. Much insight has been gained by the study of multicomponent FQH states, where spin and pseudospin indices of the electron contribute additional correlation. Traditional multicomponent FQH states develop in situations where the components share the same orbital states and the resulting interactions are pseudospin independent; this homo-orbital nature is also crucial to their theoretical understanding. Here, we study “hetero-orbital” two-component FQH states, in which the orbital index is part of the pseudospin, rendering the multicomponent interactions strongly SU(2) anisotropic in the pseudospin space. Such states, obtained in bilayer graphene at the isospin transition between 𝑁 = 0 and 𝑁 = 1 electron Landau levels, are markedly different from previous homo-orbital two-component FQH states. In particular, we observe strikingly different behaviors for the parallel-vortex and reverse-vortex attachment composite fermion states, and an anomalously strong two-component 2/5 state over a wide range of magnetic field before it abruptly disappears at a high field. Our findings, combined with detailed theoretical calculations, reveal the surprising robustness of the hetero-orbital FQH effects, significantly enriching our understanding of FQH physics in this novel regime.

Bilayer graphene↗
DOE OSTI2025

Magnetic Solitons and Thickness‐Dependent Magnetization Reversal in Interconnected Helical Nanowire Arrays

By expanding magnetic nanostructures into the third dimension, it is possible to introduce new interactions and realize new forms of magnetic textures and emergent phenomena. Consequently, this unlocks new opportunities for applications in data storage, unconventional computing and sensing by utilizing 3D devices with enhanced functionalities. Connected magnetic nanowires offer a unique platform for applications such as neuromorphic computing due to their tunability and the presence of multiple transport pathways. However to realize this promise, it is necessary to further our understanding of how to locally control the magnetization in 3D, nanowire-based geometries. In this work we show the formation of magnetic domain walls, vortices, anti-vortices, and linked vortex-anti-vortex pairs in interconnected helical nanowire arrays. We show how wire diameter and 3D geometric design can control the states that form and reveal the magnetization reversal mechanism. Hence, we demonstrate this to be a highly tunable system, where the magnetization can be readily reconfigured by an external magnetic field.

3D Nanomagnetism↗
NASA NTRS2010

Electrical Switching of Perovskite Thin-Film Resistors

Electronic devices that exploit electrical switching of physical properties of thin films of perovskite materials (especially colossal magnetoresistive materials) have been invented. Unlike some related prior devices, these devices function at room temperature and do not depend on externally applied magnetic fields. Devices of this type can be designed to function as sensors (exhibiting varying electrical resistance in response to varying temperature, magnetic field, electric field, and/or mechanical pressure) and as elements of electronic memories. The underlying principle is that the application of one or more short electrical pulse(s) can induce a reversible, irreversible, or partly reversible change in the electrical, thermal, mechanical, and magnetic properties of a thin perovskite film. The energy in the pulse must be large enough to induce the desired change but not so large as to destroy the film. Depending on the requirements of a specific application, the pulse(s) can have any of a large variety of waveforms (e.g., square, triangular, or sine) and be of positive, negative, or alternating polarity. In some applications, it could be necessary to use multiple pulses to induce successive incremental physical changes. In one class of applications, electrical pulses of suitable shapes, sizes, and polarities are applied to vary the detection sensitivities of sensors. Another class of applications arises in electronic circuits in which certain resistance values are required to be variable: Incorporating the affected resistors into devices of the present type makes it possible to control their resistances electrically over wide ranges, and the lifetimes of electrically variable resistors exceed those of conventional mechanically variable resistors. Another and potentially the most important class of applications is that of resistance-based nonvolatile-memory devices, such as a resistance random access memory (RRAM) described in the immediately following article, Electrically Variable Resistive Memory Devices (MFS-32511-1).

Liu, Shangqing↗
DOE OSTI2015

Wavelength-dependent magnetic transitions of self-organized iron–aluminum stripes induced by pulsed laser irradiation

We investigate the laser wavelength dependence of structural and magnetic transitions on the surface of an iron–aluminum (FeAl) alloy induced by nanosecond pulsed laser irradiation. The formation of self-organized FeAl stripes with a wavelength-dependent period is observed in a local area on the (111)-oriented plane. Focused magneto-optical Kerr effect measurements reveal that the coercivity reaches up to 1.2 kOe with increasing the magnetic field rotation angle, which is estimated from the stripe direction, in FeAl stripes irradiated at 355 nm, and its magnetization reversal can be explained by the domain-wall motion model. On the other hand, the magnetization reversal agrees with the Stoner–Wohlfarth model in FeAl stripes irradiated at 1064 nm. This magnetic transition originates from the B2-to-A2 phase transition in stripe structures and bulk regions. These results indicate that the magnetic transition from the incoherent to coherent mode as well as the structural transformation of stripe patterns can be controlled by the incident laser wavelength.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2012

RANDOM BALLISTIC INTERPRETATION OF NONLINEAR GUIDING CENTER THEORY

Nonlinear guiding center (NLGC) theory has been used to explain the asymptotic perpendicular diffusion coefficient {kappa} of energetic charged particles in a turbulent magnetic field, which can be applied to better understand cosmic ray transport. Here we re-derive NLGC, replacing the assumption of diffusive decorrelation with random ballistic decorrelation (RBD), which yields an explicit formula for {kappa} . We note that scattering processes can cause a reversal of the guiding center motion along the field line, i.e., 'backtracking', leading to partial cancellation of contributions to {kappa}, especially for low-wavenumber components of the magnetic turbulence. We therefore include a heuristic backtracking correction (BC) that can be used in combination with RBD. In comparison with computer simulation results for various cases, NLGC with RBD and BC provides a substantially improved characterization of the perpendicular diffusion coefficient for a fluctuation amplitude less than or equal to the large-scale magnetic field.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS2018

Not an Oxymoron: Some X-Ray Binary Pulsars with Enormous Spin-Up Rates Reveal Weak Magnetic Fields

Three high-mass X-ray binaries have been discovered recently exhibiting enormous spin-up rates. Conventional accretion theory predicts extremely high-surface dipolar magnetic fields that we believe are unphysical. Instead, we propose quite the opposite scenario; some of these pulsars exhibit weak magnetic fields, so much so that their magnetospheres are crushed by the weight of inflowing matter. The enormous spin-up rate is achieved before inflowing matter reaches the pulsar's surface as the penetrating inner disc transfers its excess angular momentum to the receding magnetosphere, which, in turn, applies a powerful spin-up torque to the pulsar. This mechanism also works in reverse; it spins a pulsar down when the magnetosphere expands beyond corotation and finds itself rotating faster than the accretion disc, which then exerts a powerful retarding torque to the magnetic field and to the pulsar itself. The above scenaria cannot be accommodated within the context of neutron-star accretion processes occurring near spin equilibrium, thus they constitute a step towards a new theory of extreme (far from equilibrium) accretion phenomena.

Christodoulou, D. M.↗
DOE OSTI2011

PERIODIC RADIO EMISSION FROM THE M7 DWARF 2MASS J13142039+1320011: IMPLICATIONS FOR THE MAGNETIC FIELD TOPOLOGY

We present multi-epoch radio and optical observations of the M7 dwarf 2MASS J13142039+1320011. We detect a {approx}1 mJy source at 1.43, 4.86, 8.46, and 22.5 GHz, making it the most luminous radio emission over the widest frequency range detected from an ultracool dwarf to date. A 10 hr Very Large Array observation reveals that the radio emission varies sinusoidally with a period of 3.89 {+-} 0.05 hr, and an amplitude of {approx}30% at 4.86 GHz and {approx}20% at 8.46 GHz. The periodicity is also seen in circular polarization, where at 4.86 GHz the polarization reverses helicity from left- to right-handed in phase with the total intensity. An archival detection in the Faint Images of the Radio Sky at Twenty Centimeters survey indicates that the radio emission has been stable for at least a decade. We also detect periodic photometric variability in several optical filters with a period of 3.79 hr and measure a rotation velocity of vsin i = 45 {+-} 5 km s{sup -1}, in good agreement with the radio and optical periods. The subtle difference in radio and optical periods may be due to differential rotation, with {Delta}{Omega} {approx} 1 rad day{sup -1} between the equation and poles. The period and rotation velocity allow us to place a lower limit on the radius of the source of {approx}> 0.13R{sub sun}, about 30% larger than theoretical expectations. The properties of the radio emission can be explained with a simple model of a magnetic dipole misaligned relative to the stellar rotation axis, with the sinusoidal variations and helicity reversal due to the rotation of the magnetic poles relative to our line of sight. The long-term stability of the radio emission indicates that the magnetic field (and hence the dynamo) is stable on a much longer timescale than the convective turnover time of {approx}0.2 yr. If the radio emission is due to gyrosynchrotron emission the inferred magnetic field strength is {approx}0.1 kG, while the electron cyclotron maser process requires a field of at least 8 kG.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

Investigation of a V{sub 15} magnetic molecular nanocluster by the Monte Carlo method

Exchange interactions in a V{sub 15} magnetic molecular nanocluster are considered, and the process of magnetization reversal for various values of the set of exchange constants is analyzed by the Monte Carlo method. It is shown that the best agreement between the field dependence of susceptibility and experimental results is observed for the following set of exchange interaction constants in a V{sub 15} magnetic molecular nanocluster: J = 500 K, J Prime = 150 K, J Double-Prime = 225 K, J{sub 1} = 50 K, and J{sub 2} = 50 K. It is observed for the first time that, in a strong magnetic field, for each of the three transitions from low-spin to high-spin states, the heat capacity exhibits two closely spaced maxima.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2013

Shock formation processes due to interactions of two plasmas in a magnetic field and modified two-stream instabilities

The study of interactions of exploding and surrounding plasmas in an external magnetic field [K. Yamauchi and Y. Ohsawa, Phys. Plasmas 14, 053110 (2007)] is verified with two-dimensional (2D) electromagnetic particle simulations, for a case in which the initial velocity of the exploding plasma is perpendicular to the external magnetic field. The 2D simulations show essentially the same shock-formation processes as those in the previous one-dimensional simulation, including penetration of exploding ions into surrounding plasma, formation of a strong magnetic-field pulse due to deceleration of the exploding ions, ion reflection by the pulse, and subsequent splitting of the pulse into two magnetosonic pulses which then develop into forward and reverse shock waves. Furthermore, the 2D structure of electromagnetic fields in the region, where the exploding and surrounding ions overlap, is investigated with particular attention to the linear and nonlinear evolution of modified two-stream instabilities in the magnetic field that is being gradually compressed. The effects of these instabilities on ion reflection and on 2D magnetic fluctuations in the two generated pulses are also discussed.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2015

Large entropy change accompanying two successive magnetic phase transitions in TbMn{sub 2}Si{sub 2} for magnetic refrigeration

Structural and magnetic properties in TbMn{sub 2}Si{sub 2} are studied by variable temperature X-ray diffraction, magnetization, electrical resistivity, and heat capacity measurements. TbMn{sub 2}Si{sub 2} undergoes two successive magnetic transitions at around T{sub c1} = 50 K and T{sub c2} = 64 K. T{sub c1} remains almost constant with increasing magnetic field, but T{sub c2} shifts significantly to higher temperature. Thus, there are two partially overlapping peaks in the temperature dependence of magnetic entropy change, i.e., −ΔS{sub M} (T). The different responses of T{sub c1} and T{sub c2} to external magnetic field, and the overlapping of −ΔS{sub M} (T) around T{sub c1} and T{sub c2} induce a large refrigerant capacity (RC) within a large temperature range. The large reversible magnetocaloric effect (−ΔS{sub M}{sup peak} ∼ 16 J/kg K for a field change of 0–5 T) and RC (=396 J/kg) indicate that TbMn{sub 2}Si{sub 2} could be a promising candidate for low temperature magnetic refrigeration.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
NASA NTRS1993

Gasdynamics of relativistically expanding gamma-ray burst sources - Kinematics, energetics, magnetic fields, and efficiency

We calculate both analytically and numerically the evolution of highly relativistic fireballs through the stages of free expansion and coasting, and determine the dependence of the thermodynamic and radiation variables in the comoving and laboratory flames. The dynamics and the comoving geometry change at the (lab) expansion factors r/r(0) greater than eta and r/r(0) greater than eta-squared, respectively, where eta = E(0)/M(0)c-squared is the initial Lorentz factor. In the lab, the gas appears concentrated in a thin shell of width r(0) until r/r(0) of less than about eta-squared, and increases linearly after that. Magnetic fields may have been important in the original impulsive event. We discuss their effect on the fireball dynamics and also consider their effects on the radiation emitted when the fireball runs into an external medium and is decelerated. The inverse synchro-Compton mechanism can then yield high radiative efficiency in the reverse shock (and through turbulent instabilities and mixing also in the forward blast wave), producing a burst of nonthermal radiation mainly in the MeV to GeV range. The energy and duration depend on eta, the magnetic field strength, and the external density, and can match the range of properties observed in cosmic gamma-ray bursts.

Meszaros, P.↗
DOE OSTI2019

Magnetic and Superconducting Properties of the Heterogeneous Layered Structures V/Fe{sub 0.7}V{sub 0.3}/V/Fe{sub 0.7}V{sub 0.3}/Nb and Nb/Ni{sub 0.65(0.81)}Cu{sub 0.35(0.19)}

The heterogeneous ferromagnetic–superconducting layered heterostructures V/Fe{sub 0.7}V{sub 0.3}/V/Fe{sub 0.7}V{sub 0.3}/Nb and Nb/Ni{sub 0.65(0.81)}Cu{sub 0.35(0.19)}, which contain magnetic clusters and ferromagnetic domains, are studied. The magnetic and superconducting properties of the structures depend on the magnetic-layer thickness, the magnetic field, and the time elapsed from structure preparation. We detected the interaction of clusters with a domain structure, diamagnetism and magnetization reversal of the magnetic layer during the superconducting transition in a ferromagnetic–superconducting heterostructure, and a superconducting transition in the magnetic layer. The magnetic and resistive properties of the heterostructures changed in several weeks and months.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2011

In situ Lorentz TEM magnetization studies on a Fe-Pd-Co martensitic alloy.

Understanding domain wall pinning centers and the resultant mobility of ferroic (ferromagnetic and ferroelastic) walls under an applied magnetic field is of central importance to actuator applications of magnetic shape memory alloys. The movement of ferroic boundaries in a twinned Fe-Pd-Co martensite was analyzed by means of Lorentz mode transmission electron microscopy. An in situ magnetizing sample-holder was used to record the evolution of the magnetic domain structure as a function of the applied field. Fresnel images were recorded at different field values and a phase reconstruction algorithm was used to map the magnetization configuration inside the foil. The motion of magnetic domain walls was found to be strongly influenced by the presence of twin boundaries. Free-standing domain walls became mobile at fields as low as 15 Oe, whereas an order of magnitude higher field was required to depin domain walls that coincided with twin boundaries. The domain wall motion was completely reversible with a notable hysteresis.

36 MATERIALS SCIENCE↗
DOE OSTI2026

Spin Texture Control and Magnetic Gap Engineering in a Ferromagnetic Insulator–Topological Insulator Sandwiched Heterostructure

Quantum materials that combine magnetism with topological order are emerging as key platforms for next-generation spintronics and low-energy electronics. They enable the realization of emergent quantum phenomena, such as the quantum anomalous Hall effect and axion insulator states. The ferromagnetic insulator (FMI)/topological insulator (TI)/FMI sandwich structure of a single-septuple layer (1SL) MnBi 2 Te 4 /four-quintuple layer (4QL) Bi 2 Te 3 /1SL MnBi 2 Te 4 holds great potential to achieve such desirable quantum phenomena at an elevated temperature, owing to its large Dirac point band gap and high Curie temperature. Here, for this work, spin- and angle-resolved photoemission spectroscopy (spin-ARPES) is employed to directly verify that the band gap arises from broken time-reversal symmetry via proximity-driven magnetization. This study demonstrates direct control of the spin state via external magnetic fields and unambiguously confirms the exchange interaction as the gap-opening mechanism. The robust magnetic gap and controllable spin texture make this heterostructure a suitable candidate for spintronic applications and magnetic topological quantum phases.

exchange gap↗