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At least 1,603 records · Page 89
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

Observation of spin textures in La{sub 1−x}Sr{sub x}MnO{sub 3} (x = 0.175)

We have investigated topological spin textures in the ferromagnetic metallic phase of La{sub 0.825}Sr{sub 0.175}MnO{sub 3} with the centrosymmetric crystal structure by small-angle electron diffraction (SmAED) and low-temperature Lorentz transmission electron microscopy (TEM) experiments. In-situ Lorentz TEM and SmAED experiments revealed that type-I and type-II magnetic bubbles evolved from magnetic stripe domains with the Bloch-type domain wall by applying vertical magnetic field. Type-I magnetic bubbles with left-handed and right-handed spin helicity were randomly distributed and simultaneously type-II magnetic bubbles are formed locally. The important point about type-I and type-II magnetic bubbles is that their emergence depends strongly on whether perpendicular magnetic field is applied parallel to the magnetic easy axis along the [001] direction. Our experimental results suggested that the stabilization of magnetic bubbles should originate from the long-range dipole-dipole interactions, as opposed to the Dzyaloshinskii-Moriya interaction in helical magnets.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Quasi-one-dimensional Ising-like antiferromagnetism in the rare-earth perovskite oxide TbScO 3

The rare-earth perovskite TbScO 3 has been widely used as a substrate for the growth of epitaxial ferroelectric and multiferroic thin films, while its detailed low-temperature magnetic properties were rarely reported. In this paper, we performed detailed magnetization, specific heat, and single crystal neutron scattering measurements, along with the crystalline electric field calculations to study the low-temperature magnetic properties of TbScO 3. All our results suggest the magnetic Tb 3+ has an Ising-like pseudo-doublet ground state at low temperatures. Due to the constrain of local point symmetry, these Tb 3+ Ising moments are confined in the ab plane with a tilt angle of φ=±48° to the a axis. In zero field, the system undergoes an antiferromagnetic phase transition at T N =2.53 K, and forms a G x A y noncollinear magnetic structure below T N . Here, we find the dipole-dipole interactions play an important role to determine the magnetic ground state, which are also responsible for the quasi-one-dimensional magnetism in TbScO 3 . The significant anisotropic diffuse scatterings further confirm the quasi-one-dimensional magnetism along the c axis. The magnetic phase diagram with the field along the easy b axis is well established. In addition to the G x A y antiferromagnetic state, there is an exotic field-induced phase emerged near the critical field B c ≃0.7 T, where three-dimensional magnetic order is suppressed but strong one-dimensional correlations may still exist.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Muon Beam Helical Cooling Channel Design

The Helical Cooling Channel (HCC) achieves effective ionization cooling of the six-dimensional (6d) phase space of a muon beam by means of a series of 21st century inventions. In the HCC, hydrogen-pressurized RF cavities enable high RF gradients in strong external magnetic fields. The theory of the HCC, which requires a magnetic field with solenoid, helical dipole, and helical quadrupole components, demonstrates that dispersion in the gaseous hydrogen energy absorber provides effective emittance exchange to enable longitudinal ionization cooling. The 10-year development of a practical implementation of a muon-beam cooling device has involved a series of technical innovations and experiments that imply that an HCC of less than 300 m length can cool the 6d emittance of a muon beam by six orders of magnitude. We describe the design and construction plans for a prototype HCC module based on oxygen-doped hydrogen-pressurized RF cavities that are loaded with dielectric, fed by magnetrons, and operate in a superconducting helical solenoid magnet.

Johnson, Rolland↗
DOE OSTI2013

Effect of energy interlock on possible beam losses in the Booster to Storage Ring transfer line

Under normal operational conditions in NSLS-II the energy of the beam extracted from the Booster and transferred to and injected into the Storage Ring (SR) is 3 GeV. It was determined that for the commissioning purposes energy range of the beam reaching the SR is allowed to be 2 GeV - 3.15 GeV. While the upper limit of the beam energy is defined by the maximum possible settings of Booster dipoles at the top of the ramp, the lower energy limit has to be provided by magnet interlocks. Dynamic interlocks of the Booster dipoles would be an ideal solution, yet the constraints of time and resources do not allow providing such interlocks for commissioning stage of NSLS-II. Therefore, the static interlock of two bends in the Booster to SR transfer line (BSR) will be used - the bends B1 and B2 will be interlocked within 5% of their nominal values. The possible energy of the beam reaching each BSR magnet strongly depends on the details of interlock scheme. Thus, the possible beam losses in the BSR transfer line have to be calculated taking into account the particular energy interlock. The purpose of this exercise is to determine the potential beam loss locations, for further identification of the radiation risks they impose outside the existing bulk shielding, and specification of the supplemental shielding design necessary to mitigate those risks. We will present the beam systems description along with their planned operating ranges, which forms the basis of our analysis. Next, we will find the range of beam energy that can possibly be experienced by each BSR magnet. Finally, we will describe the method used to determine potential beam loss locations and we will present the results of our studies in a form of convenient to use table which includes all the information necessary for analysis of radiation risks induced by the determined beam losses. In the following analysis we assume, unless the opposite is indicated in the clearest possible terms, that the accelerator is configured in any feasible way that its design allows.

43 PARTICLE ACCELERATORS↗
DOE OSTI2017

Group of Quantum Bits Acting as a Bit Using a Single-Domain Ferromagnet of Uniaxial Magnetic Ions

Read/write operations with individual quantum bits (i.e., qbits) are a demanding concern to solve in quantum computing. To alleviate this difficulty, we considered the possibility of using a group of qbits that act collectively as a bit (hereafter, a group bit or a gbit, in short). A promising candidate for a gbit is a single-domain ferromagnet (SDF) independent of its size, which can be prepared as a magnet of well-separated uniaxial magnetic ions (UMIs) at sites of no electric dipole moment with their uniaxial axes aligned along one common direction. When magnetized, the UMIs of such a magnet have a ferromagnetic (FM) arrangement and the resulting SDF becomes a gbit with its two opposite moment orientations representing the |0⟩ and |1⟩ states of a bit. Here, we probed the requirements for such magnets and identified several 2H-perovskites as materials satisfying these requirements.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Self-replication with magnetic dipolar colloids

Colloidal self-replication represents an exciting research frontier in soft matter physics. Currently, all reported self-replication schemes involve coating colloidal particles with stimuli-responsive molecules to allow switchable interactions. In this paper, we introduce a scheme using ferromagnetic dipolar colloids and preprogrammed external magnetic fields to create an autonomous self-replication system. Interparticle dipole-dipole forces and periodically varying weak-strong magnetic fields cooperate to drive colloid monomers from the solute onto templates, bind them into replicas, and dissolve template complexes. We present three general design principles for autonomous linear replicators, derived from a focused study of a minimalist sphere-dimer magnetic system in which single binding sites allow formation of dimeric templates. We show via statistical models and computer simulations that our system exhibits nonlinear growth of templates and produces nearly exponential growth (low error rate) upon adding an optimized competing electrostatic potential. Lastly, we devise experimental strategies for constructing the required magnetic colloids based on documented laboratory techniques. We also present qualitative ideas about building more complex self-replicating structures utilizing magnetic colloids.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2010

R&D ERL: Magnetic measurements of the ERL magnets

The magnet system of ERL consists of G5 solenoids, 6Q12 quadrupoles with 0.58 T/m gradient, 3D60 dipoles with 0.4 T central field, 15 and 30 degree Z-bend injection line dipole/quadrupole combined function magnets, and extraction line magnets. More details about the magnets can be found in a report by G. Mahler. Field quality in all the 6Q12 quadrupoles, 3D60 dipoles and the injection line magnets has been measured with either a rotating coil, or a Hall probe mapper. This report presents the results of these magnetic measurements.

43 PARTICLE ACCELERATORS↗
DOE OSTI2020

Left-right canted-cosine-theta magnets

Disclosed herein are superconducting gantry magnets that include multiple quadrupole winding sections placed in sequence on a curve such that the effective current direction is reversed between sections. This produces alternating quadrupole field regions along the length of the bend whose individual integral strengths can be tuned by the location of the current polarity transitions. A simple transition scheme to reverse the current between sections can be implemented to allow for the use of one continuous winding and power supply. Dipole windings can be included in the superconducting gantry magnets so that the magnets produce superposed dipole and alternating quadrupole fields. The disclosed design for the windings and transition scheme to reverse current polarity can be implemented for higher order multipoles as well.

43 PARTICLE ACCELERATORS↗
DOE OSTI2018

Progress on HL-LHC Nb 3 Sn Magnets

The high-luminosity Large Hadron Collider (HL-LHC) project aims at allowing to increase the collisions in the LHC by a factor of ten in the decade 2025-2035. One essential element is the superconducting magnet around the interaction region points, where the large aperture magnets will be installed to allow to further reduce the beam size in the interaction point. The core of this upgrade is the Nb 3 Sn triplet, made up of 150-mm aperture quadrupoles in the range of 7-8 m. The project is being shared between the European Organization for Nuclear Research and the US Accelerator Upgrade Program, based on the same design, and on the two strand technologies. The project is ending the short model phase, and entering the prototype construction. We will report on the main results of the short model program, including the quench performance and field quality. A second important element is the 11 T dipole that replaces a standard dipole making space for additional collimators. The magnet is also ending the model development and entering the prototype phase. A critical point in the design of this magnet is the large current density, allowing increase of the field from 8 to 11 T with the same coil cross section as in the LHC dipoles. This is also the first two-in-one Nb 3 Sn magnet developed so far. We will report the main results on the test and the critical aspects.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2012

Non-diffusive resonant acceleration of electrons in the radiation belts

We describe a mechanism of resonant electron acceleration by oblique high-amplitude whistler waves under conditions typical for the Earth radiation belts. We use statistics of spacecraft observations of whistlers in the Earth radiation belts to obtain the dependence of the angle {theta} between the wave-normal and the background magnetic field on magnetic latitude {lambda}. According to this statistics, the angle {theta} already approaches the resonance cone at {lambda}{approx}15 Degree-Sign and remains close to it up to {lambda}{approx}30 Degree-Sign -40 Degree-Sign on the dayside. The parallel component of the electrostatic field of whistler waves often increases around {lambda}{approx}15 Degree-Sign up to one hundred of mV/m. We show that due to this increase of the electric field, the whistler waves can trap electrons into the potential well via wave particle resonant interaction corresponding to Landau resonance. Trapped electrons then move with the wave to higher latitudes where they escape from the resonance. Strong acceleration is favored by adiabatic invariance along the increasing magnetic field, which continuously transfers the parallel energy gained to perpendicular energy, allowing resonance to be reached and maintained. The concomitant increase of the wave phase velocity allows for even stronger relative acceleration at low energy <50keV. Each trapping-escape event of electrons of {approx}10keV to 100 keV results in an energy gain of up to 100 keV in the inhomogeneous magnetic field of the Earth dipole. For electrons with initial energy below 100 keV, such rapid acceleration should hasten their drop into the loss-cone and their precipitation into the atmosphere. We discuss the role of the considered mechanism in the eventual formation of a trapped distribution of relativistic electrons for initial energies larger than 100 keV and in microbursts precipitations of lower energy particles.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2011

Investigation of triple spin correlations and spin dynamics in ferromagnets

Data on the experimental detection and use of three-particle (chiral) spin dynamic correlations in ferromagnets are presented. The oblique-geometry method for investigating polarized neutron small-angle scattering is described, which gives the dependence that the scattering asymmetry has on the polarization P signs and the scattering angle {theta}. The following results of the dynamics investigation in the critical and ferromagnetic phases in the magnetic field are presented: the temperature dependence of the critical field H{sub c}, the factorization of the momentum transfer dependence of three-particle vertices, the corroboration of the 'hard' version of the dipole critical dynamics, and the dynamics of amorphous magnets and invars.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
NASA NTRS1993

Absence of magnetic trapping on closed field lines at Neptune

The large offset of Neptune's magnetic field in an offset tilted dipole (OTD) approximation allows some fraction of magnetospheric field lines to have monotonically increasing field magnitude over their entire length in the magnetosphere. The usual magnetic trapping is impossible on such field lines, meaning that all charged particles on these field lines will precipitate after mirroring at most once. These field lines can be thought of as the part of the global magnetic anomaly where no particles on the corresponding drift shells remain trapped for an entire rotation of the planet. In this paper we examine both the morphology and size of the affected surface area in the OTD2 and O8 magnetic field models. For regions where there are indications that part of the aurora could be due to monotonic field line precipitation we analyze whether these field lines could be sufficiently populated to produce auroral signatures by considering photoelectron production rates and radial diffusion. We conclude that monotonic field line precipitation may contribute to the observed Neptune aurora but does not explain the global phenomenon.

Paranicas, C.↗
DOE OSTI2017

Single-ion properties of the S eff = 1 2 XY antiferromagnetic pyrochlores Na A ' Co 2 F 7 ( A ' = Ca 2 + , Sr 2 + )

The antiferromagnetic pyrochlore material NaCaCo 2 F 7 is a thermal spin liquid over a broad temperature range (≈140 K down to T F =2.4 K), in which magnetic correlations between Co 2+ dipole moments explore a continuous manifold of antiferromagnetic XY states. The thermal spin liquid is interrupted by spin freezing at a temperature that is ~2% of the mean-field interaction strength, leading to short-range static XY clusters with distinctive relaxation dynamics. Here we report the low-energy inelastic neutron scattering response from the related compound NaSrCo 2 F 7 , confirming that it hosts the same static and dynamic spin correlations as NaCaCo 2 F 7 . We then present the single-ion levels of Co 2+ in these materials as measured by inelastic neutron scattering. An intermediate spin-orbit coupling model applied to an ensemble of trigonally distorted octahedral crystal fields accounts for the observed transitions. The single-ion ground state of Co 2+ is a Kramers doublet with a strongly XY-like g tensor ( gxy/gz ~3). The local disorder inherent from the mixed pyrochlore A sites (Na + /Ca 2+ and Na + /Sr 2+ ) is evident in these measurements as exaggerated broadening of some of the levels. A simple model that reproduces the salient features of the single-ion spectrum produces approximately 8.4% and 4.1% variation in the z and xy components of the g tensor, respectively. In conclusion, this study confirms that an S eff = 1/2 model with XY antiferromagnetic exchange and weak exchange disorder serves as a basic starting point in understanding the low-temperature magnetic behavior of these strongly frustrated magnets.

36 MATERIALS SCIENCE↗
NASA NTRS1976

An evaluation of recent quantitative magnetospheric magnetic field models

Magnetospheric field models involving dipole tilt effects are discussed, with particular reference to defined magnetopause models and boundary surface models. The models are compared with observations and with each other whenever possible. It is shown that models containing only contributions from magnetopause and tail current systems are capable of reproducing the observed quiet time field just in a qualitative way. The best quantitative agreement between models and observations take place when currents distributed in the inner magnetosphere are added to the magnetopause and tail current systems. One region in which all the models fall short is the region around the polar cusp. Obtaining physically reasonable gradients should have high priority in the development of future models.

Walker, R. J.↗
DOE OSTI2010

TRANSITION FROM REGULAR TO CHAOTIC CIRCULATION IN MAGNETIZED CORONAE NEAR COMPACT OBJECTS

Accretion onto black holes and compact stars brings material in a zone of strong gravitational and electromagnetic fields. We study dynamical properties of motion of electrically charged particles forming a highly diluted medium (a corona) in the regime of strong gravity and large-scale (ordered) magnetic field. We start our work from a system that allows regular motion, then we focus on the onset of chaos. To this end, we investigate the case of a rotating black hole immersed in a weak, asymptotically uniform magnetic field. We also consider a magnetic star, approximated by the Schwarzschild metric and a test magnetic field of a rotating dipole. These are two model examples of systems permitting energetically bound, off-equatorial motion of matter confined to the halo lobes that encircle the central body. Our approach allows us to address the question of whether the spin parameter of the black hole plays any major role in determining the degree of the chaoticness. To characterize the motion, we construct the recurrence plots (RPs) and we compare them with Poincare surfaces of section. We describe the RPs in terms of the recurrence quantification analysis, which allows us to identify the transition between different dynamical regimes. We demonstrate that this new technique is able to detect the chaos onset very efficiently and provide its quantitative measure. The chaos typically occurs when the conserved energy is raised to a sufficiently high level that allows the particles to traverse the equatorial plane. We find that the role of the black hole spin in setting the chaos is more complicated than initially thought.

79 ASTRONOMY AND ASTROPHYSICS↗