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Results for “MAGNETIC FIELD REVERSAL”

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 577 records · Page 32
All sourcesNASA NTRSDOE OSTI
DOE OSTI2017

Plasma fuelling with cryogenic pellets in the stellarator TJ-II

Cryogenic pellet injection is a widely used technique for delivering fuel to the core of magnetically confined plasmas. Indeed, such systems are currently functioning on many tokamak, reversed field pinch and stellarator devices. A pipe-gun-type pellet injector is now operated on the TJ-II, a low-magnetic shear stellarator of the heliac type. Cryogenic hydrogen pellets, containing between 3×10 18 and 4×10 19 atoms, are injected at velocities between 800 and 1200 m s -1 from its low-field side into plasmas created and/or maintained in this device by electron cyclotron resonance and/or neutral beam injection heating. In this paper, the first systematic study of pellet ablation, particle deposition and fuelling efficiency is presented for TJ-II. From this, light-emission profiles from ablating pellets are found to be in reasonable agreement with simulated pellet ablation profiles (created using a neutral gas shielding-based code) for both heating scenarios. In addition, radial offsets between recorded light-emission profiles and particle deposition profiles provide evidence for rapid outward drifting of ablated material that leads to pellet particle loss from the plasma. Finally, fuelling efficiencies are documented for a range of target plasma densities (~4×10 18 – ~2×10 19 m -3 ). These range from ~20%– ~85% and are determined to be sensitive to pellet penetration depth. Additional observations, such as enhanced core ablation, are discussed and planned future work is outlined.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
NASA NTRS1983

Magnetic structure of the boundary layer

While magnetospheric convection appears to be driven by both magnetic reconnection and the 'viscous' dragging of closed flux tubes acting in varying proportions but with reconnection predominating as a cause, the action of the closed flux tubes of the low latitude boundary layer seems to predominate in driving system 1 field aligned currents and discrete auroras. ISEE boundary layer data for the magnetic shear effects associated with field aligned currents has occasionally shown a reverse draping of boundary layer field lines whose cause may be plasma entry around the cusps, a north-south asymmetry in current flow to the polar regions, or magnetic reconnection at high latitudes. Boundary layer encounter character and distribution asymmetries suggest that the boundary layer and/or plasma mantle may differ substantially at the dawn and dusk sides of the magnetosphere, and that their properties may have seasonal dependencies.

Hones, E. W., Jr.↗
DOE OSTI2019

Effect of Exchange Interaction Constants on the Magnetization Reversal in a Hard/Soft Magnetic Bilayer Model

A model is developed to study a hard/soft magnetic bilayer in an external magnetic field by the Monte Carlo method. The influence of the constants of the exchange intralayer and interlayer interactions of the hard magnetic and soft magnetic layers on the magnetization reversal in the magnetic bilayer is investigated. The formation of a spin spring is shown to be mainly determined by the exchange interaction in the soft magnetic layer. Phase diagrams are plotted for the magnetic bilayer.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Chiral Majorana fermion modes in a quantum anomalous Hall insulator–superconductor structure

Majorana fermion is a hypothetical particle that is its own antiparticle. We report transport measurements that suggest the existence of one-dimensional chiral Majorana fermion modes in the hybrid system of a quantum anomalous Hall insulator thin film coupled with a superconductor. As the external magnetic field is swept, half-integer quantized conductance plateaus are observed at the locations of magnetization reversals, giving a distinct signature of the Majorana fermion modes. This transport signature is reproducible over many magnetic field sweeps and appears at different temperatures. This finding may open up an avenue to control Majorana fermions for implementing robust topological quantum computing.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Electron and Proton Heating in Transrelativistic Guide Field Reconnection

The plasma in low-luminosity accretion flows, such as the one around the black hole at the center of M87 or Sgr A* at our Galactic Center, is expected to be collisioness and of two temperatures, with protons hotter than electrons. Here, particle heating is expected to be controlled by magnetic reconnection in the transrelativistic regime ${\sigma }_{w}\sim 0.1\mbox{--}1$, where the magnetization ${\sigma }_{w}$ is the ratio of magnetic energy density to plasma enthalpy density. Using large-scale 2D particle-in-cell simulations, we explore for a fiducial ${\sigma }_{w}=0.1$ how the dissipated magnetic energy is partitioned between electrons and protons as a function of ${\beta }_{{\rm{i}}}$ (the ratio of proton thermal pressure to magnetic pressure) and of the strength of a guide field ${B}_{{\rm{g}}}$ perpendicular to the reversing field B 0. At low ${\beta }_{{\rm{i}}}\ (\lesssim 0.1)$, we find that the fraction of initial magnetic energy per particle converted into electron irreversible heat is nearly independent of ${B}_{{\rm{g}}}/{B}_{0}$, whereas protons are heated much less with increasing ${B}_{{\rm{g}}}/{B}_{0}$. As a result, for large ${B}_{{\rm{g}}}/{B}_{0}$, electrons receive the overwhelming majority of irreversible particle heating (~93% for ${B}_{{\rm{g}}}/{B}_{0}=6$). This is significantly different than the antiparallel case ${B}_{{\rm{g}}}/{B}_{0}=0$, in which irreversible electron heating accounts for only ~18% of the total particle heating (Rowan et al. 2017). At ${\beta }_{{\rm{i}}}\sim 2$, when both species start already relativistically hot (for our fiducial ${\sigma }_{w}=0.1$), electrons and protons each receive ~50% of the irreversible particle heating, regardless of the guide field strength. Our results provide important insights into the plasma physics of electron and proton heating in hot accretion flows around supermassive black holes.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2019

Study of Nucleation/Annihilation Process and Vortices Characteristics in Co/Py Rectangular Bilayers

Vortices are topological structures, which are characterized by circular organization of magnetic moments and by chirality. In this paper, we investigate the impact of film lateral dimensions and thicknesses on magnetization reversibility by applying an in-plane external magnetic field on Co(t{sub Co})/Py (3 nm) bilayers where the Py is the permalloy, a ferromagnetic alloy of nickel and iron (Ni{sub 80}Fe{sub 20}). The study is achieved by using appropriate micromagnetic simulations. We also show that the vortices number and their sizes can be controlled by playing on the lateral dimensions and the thickness of the bilayer.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Electric Field Control of Magnetic Skyrmion Helicity in a Centrosymmetric 2D van der Waals Magnet

Two-dimensional van der Waals magnets hosting topological magnetic textures, such as skyrmions, show promise for spintronics and quantum computing. Electrical control of these topological spin textures is crucial for enhancing operational performance and functionality. Here, using electron microscopy combined with in situ electric and magnetic biasing, we show that the skyrmion helicity in insulating Cr 2 Ge 2 Te 6 can be controlled by the direction of the external electric field applied during the field cooling process. The electric field-tuned helicity remains stable at low temperature, even amid variations in magnetic and electric fields. Our theoretical investigation reveals that nonzero Dzyaloshinskii-Moriya interactions between nearest neighbors, induced by the electric field, change their sign upon reversing the electric field direction, thereby facilitating helicity selection. The electrical control of magnetic helicity demonstrated in this study can be extended to other centrosymmetric skyrmion-hosting magnets, paving the way for future device designs in topological spintronics and quantum computing.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Proton switching molecular magnetoelectricity

Abstract The convergence of proton conduction and multiferroics is generating a compelling opportunity to achieve strong magnetoelectric coupling and magneto-ionics, offering a versatile platform to realize molecular magnetoelectrics. Here we describe machine learning coupled with additive manufacturing to accelerate the design strategy for hydrogen-bonded multiferroic macromolecules accompanied by strong proton dependence of magnetic properties. The proton switching magnetoelectricity occurs in three-dimensional molecular heterogeneous solids. It consists of a molecular magnet network as proton reservoir to modulate ferroelectric polarization, while molecular ferroelectrics charging proton transfer to reversibly manipulate magnetism. The magnetoelectric coupling induces a reversible 29% magnetization control at ferroelectric phase transition with a broad thermal hysteresis width of 160 K (192 K to 352 K), while a room-temperature reversible magnetic modulation is realized at a low electric field stimulus of 1 kV cm −1 . The findings of electrostatic proton transfer provide a pathway of proton mediated magnetization control in hierarchical molecular multiferroics.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Moiré-driven topological electronic crystals in twisted graphene

In a dilute two-dimensional electron gas, Coulomb interactions can stabilize the formation of a Wigner crystal. Although Wigner crystals are topologically trivial, it has been predicted that electrons in a partially filled band can break continuous translational symmetry and time-reversal symmetry spontaneously, resulting in a type of topological electron crystal known as an anomalous Hall crystal. Here we report signatures of a generalized version of the anomalous Hall crystal in twisted bilayer–trilayer graphene, whose formation is driven by the moiré potential. The crystal forms at a band filling of one electron per four moiré unit cells (ν = 1/4) and quadruples the unit-cell area, coinciding with an integer quantum anomalous Hall effect. The Chern number of the state is exceptionally tunable, and it can be switched reversibly between +1 and −1 by electric and magnetic fields. Several other topological electronic crystals arise in a modest magnetic field, originating from ν = 1/3, 1/2, 2/3 and 3/2. As a result, the quantum geometry of the interaction-modified bands is likely to be very different from that of the original parent band, which enables possible future discoveries of correlation-driven topological phenomena.

Electronic properties and materials↗
DOE OSTI2015

Antiferromagentic resonance detected by direct current voltages in MnF{sub 2}/Pt bilayers

We performed coplanar waveguide-based broadband ferromagnetic resonance experiments on the antiferromagnetic insulator MnF{sub 2}, while simultaneously recording the DC voltage arising in a thin platinum film deposited onto the MnF{sub 2}. The antiferromagnetic resonance is clearly reflected in both the transmission through the waveguide as well as the DC voltage in the Pt strip. The DC voltage remains largely unaffected by field reversal and thus presumably stems from microwave rectification and/or heating effects. However, we identify a small magnetic field orientation dependent contribution, compatible with antiferromagnetic spin pumping theory.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2019

Linear magnetoresistance in the low-field limit in density-wave materials

The magnetoresistance (MR) of a material is typically insensitive to reversing the applied field direction and varies quadratically with magnetic field in the low-field limit. Quantum effects, unusual topological band structures, and inhomogeneities that lead to wandering current paths can induce a cross-over from quadratic to linear MR with increasing magnetic field. Here we explore a series of metallic charge- and spin-density-wave systems that exhibit extremely large positive linear MR. By contrast to other linear MR mechanisms, this effect remains robust down to miniscule magnetic fields of tens of Oersted at low temperature. We frame an explanation of this phenomenon in a semiclassical narrative for a broad category of materials with partially gapped Fermi surfaces due to density waves.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Strain-induced chiral magnetic effect in Weyl semimetals

Here, we argue that strain applied to a time-reversal and inversion breaking Weyl semimetal in a magnetic field can induce an electric current via the chiral magnetic effect. A tight-binding model is used to show that strain generically changes the locations in the Brillouin zone but also the energies of the band touching points (tips of the Weyl cones). Since axial charge in a Weyl semimetal can relax via intervalley scattering processes, the induced current will decay with a time scale given by the lifetime of a chiral quasiparticle. Lastly, we estimate the strength and lifetime of the current for typical material parameters and find that it should be experimentally observable.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Thermodynamic signatures for the existence of Dirac electrons in ZrTe 5

We combine transport, magnetization, and torque magnetometry measurements to investigate the electronic structure of ZrTe 5 and its evolution with temperature. At fields beyond the quantum limit, we observe a magnetization reversal from paramagnetic to diamagnetic response, which is characteristic of a Dirac semi-metal. We also observe a strong non-linearity in the magnetization that suggests the presence of additional low-lying carriers from other low-energy bands. Finally, we observe a striking sensitivity of the magnetic reversal to temperature that is not readily explained by simple band-structure models, but may be connected to a temperature dependent Lifshitz transition proposed to exist in this material.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Zn-Doping Dependence of Stripe Order in La1.905Ba0.095CuO4

The effect of Zn-doping on the stripe order in La{sub 1.905}Ba{sub 0.095}CuO{sub 4} has been studied by means of x-ray and neutron diffraction as well as magnetization measurements. While 1% Zn leads to an increase of the spin stripe order, it unexpectedly causes a wipe out of the visibility of the charge stripe order. A magnetic field of 10 Tesla applied along the c-axis has no reversing effect on the charge order. We compare this observation with the Zn-doping dependence of the crystal structure, superconductivity, and normal state magnetism.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Tuning the metal-insulator crossover and magnetism in SrRuO 3 by ionic gating

Reversible control of charge transport and magnetic properties without degradation is a key for device applications of transition metal oxides. Chemical doping during the growth of transition metal oxides can result in large changes in physical properties, but in most of the cases irreversibility is an inevitable constraint. We report a reversible control of charge transport, metal-insulator crossover and magnetism in field-effect devices based on ionically gated archetypal oxide system - SrRuO 3 . In these thin-film devices, the metal-insulator crossover temperature and the onset of magnetoresistance can be continuously and reversibly tuned in the range 90–250 K and 70–100 K, respectively, by application of a small gate voltage. We infer that a reversible diffusion of oxygen ions in the oxide lattice dominates the response of these materials to the gate electric field. These findings provide critical insights into both the understanding of ionically gated oxides and the development of novel applications.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Reversal of the hall current and amplification of the magnetorotational instability in a weakly ionized plasma

A criterion for the development of a magnetorotational instability in a weakly ionized dusty plasma is considered. A dispersion relation for the wavenumber and the growth rate of an unstable perturbation is derived for an arbitrary angle between the wave vector and magnetic field. It is shown that the presence of dust grains can reverse the direction of the Hall current in the plasma and can shift the instability threshold to shorter wavelengths. Under certain conditions, Alfven fluctuations of arbitrary scale can be unstable. The Hall current reversal is found to have a strong effect on the development of a magnetorotational instability when the Alfven resonance frequency in a weakly ionized plasma is close to the rotation frequency of the accretion disk.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1987

On the reversal of the dipolar field of the sun and its possible implication for the reversal of the earth's field

Changes of the neutral line on the source surface (analogous to the magnetic dip equator of the earth) during the period between 1976 and 1983 are examined on the basis of the Stanford solar magnetic field data. Instead of the standard Mercator-like projection, the neutral line is shown on a spherical surface for 16 selected Carrington rotations. In spite of great complexity of the field variations, this presentation depicts clearly a fairly systematic rotational reversal of the dipolar field on the source surface during the sunspot maximum years. It is suggested that this solar situation is somewhat analogous to the planet earth in the sense that the core surface and the earth's surface may correspond to the photosphere and the source surface, respectively.

Saito, T.↗