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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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Driving plasmas to extreme magnetizations using strong laser compression and high initial magnetic field
The goal of the project is to develop a platform for the OMEGA-60 laser facility to study MHD effects in cylindrical implosions at regimes of large magnetic pressure and magnetization.
Development of Turbulent Magnetic Reconnection in a Magnetic Island
In this paper, with two-dimensional particle-in-cell simulations, we report that the electron Kelvin–Helmholtz instability is unstable in the current layer associated with a large-scale magnetic island, which is formed in multiple X-line guide field reconnections. The current sheet is fragmented into many small current sheets with widths down to the order of the electron inertial length. Secondary magnetic reconnection then occurs in these fragmented current sheets, which leads to a turbulent state. The electrons are highly energized in such a process.
Current and Future Constraints on Primordial Magnetic Fields
We present new limits on the amplitude of potential primordial magnetic fields (PMFs) using temperature and polarization measurements of the cosmic microwave background (CMB) from Planck, the BICEP2/Keck Array, Polarbear, and SPTpol. We reduce twofold the 95% confidence upper limit on the CMB anisotropy power due to a nearly scale-invariant PMF, with an allowed B-mode power at ℓ = 1500 of D{sub ℓ=1500}{sup BB}<0.071 μK{sup 2} for Planck versus D{sub ℓ=1500}{sup BB}<0.034 μK{sup 2} for the combined data set. We also forecast the expected limits from soon-to-deploy CMB experiments (like SPT-3G, Adv. ACTpol, or the Simons Array) and the proposed CMB-S4 experiment. Future CMB experiments should dramatically reduce the current uncertainties by one order of magnitude for the near-term experiments and two orders of magnitude for the CMB-S4 experiment. The constraints from CMB-S4 have the potential to rule out much of the parameter space for PMFs.
LOCAL INTERSTELLAR MAGNETIC FIELD DETERMINED FROM THE INTERSTELLAR BOUNDARY EXPLORER RIBBON
The solar wind emanating from the Sun interacts with the local interstellar medium (LISM), forming the heliosphere. Hydrogen energetic neutral atoms (ENAs) produced by the solar-interstellar interaction carry important information about plasma properties from the boundaries of the heliosphere, and are currently being measured by NASA's Interstellar Boundary Explorer (IBEX). IBEX observations show the existence of a “ribbon” of intense ENA emission projecting a circle on the celestial sphere that is centered near the local interstellar magnetic field (ISMF) vector. Here we show that the source of the IBEX ribbon as a function of ENA energy outside the heliosphere, uniquely coupled to the draping of the ISMF around the heliopause, can be used to precisely determine the magnitude (2.93 ± 0.08 μG) and direction (227.°28 ± 0.°69, 34.°62 ± 0.°45 in ecliptic longitude and latitude) of the pristine ISMF far (∼1000 AU) from the Sun. We find that the ISMF vector is offset from the ribbon center by ∼8.°3 toward the direction of motion of the heliosphere through the LISM, and their vectors form a plane that is consistent with the direction of deflected interstellar neutral hydrogen, thought to be controlled by the ISMF. Our results yield draped ISMF properties close to that observed by Voyager 1, the only spacecraft to directly measure the ISMF close to the heliosphere, and give predictions of the pristine ISMF that Voyager 1 has yet to sample.
H-Mediated Magnetic Interactions between Layers in a 2D Mn II -Dicyanamide Polymer: Neutron Diffraction, DFT, and Quantum Monte Carlo Calculations: H-Mediated Magnetic Interactions between Layers in a 2D Mn II -Dicyanamide Polymer: Neutron Diffraction, DFT, and Quantum Monte Carlo Calculations
We report neutron-diffraction investigations of the quasi-2D Mn-II(dca)(2)(pym)(H2O) (pym = N2C4H4) compound, where high-spin Mn-II ions are bridged by dicyanamide anions, [N(CN)(2)](-) (herein abbreviated dca). Inside the layers, Mn2+ ions are connected by single or double dca bridges. The magnetic phase diagram was established by neutron diffraction on a single crystal. In the low-field phase, the Mn-II ions are antiferro-magnetically ordered in the layers, with moments nearly parallel to the c axis, and the layers are antiferromagnetically coupled. The spin-flop phase corresponds to ferromagnetic coupling between the antiferromagnetic layers, in which the Mn-II moments are nearly perpendicular to the c axis. The induced spin-density distribution in the paramagnetic phase, determined by polarized neutron diffraction, visualizes the super-exchange pathways through the dca ligands within the layers and through H bonding between neighboring layers. The theoretical spin density obtained by bidimensional periodic DFT calculations is compared with the experimental results. Furthermore, quantum Monte Carlo simulations have been performed to compare the DFT results with experimental susceptibility measurements.
Integrated SQUID/Sensor Metallic Magnetic Microcalorimeter for Gamma-Ray Spectroscopy
Metallic magnetic microcalorimeters (MMCs) achieve energy resolution comparable to transition-edge sensors (TESs) but rely on different measurement physics that may allow MMCs to surpass TESs in some future applications. We have recently completed fabrication of new MMC γ-ray detector arrays using several exploratory sensor designs. All designs integrate the SQUID and sensor on the same chip and use a superconducting cap layer on the paramagnet, but explore different combinations of combined/separate sensing and magnetization coils and direct/flux transformer coupling to the input SQUIDs. This report describes the design and initial testing of one of these devices, which has so far demonstrated an energy resolution of 38 eV at 60 keV near 10 mK using natural-abundance silver–erbium paramagnet.
Effect of Thermoelectric Magnetic Convection on Shrinkage Porosity at the Final Stage of Solidification of GCr18Mo Steel Under Axial Static Magnetic Field
Adopting effective strategies to eliminate the shrinkage porosity at the final stage of steel solidification is one of the most attractive research subjects for metallurgists. Despite the progress in understanding the thermoelectric magnetic convection (TEMC) in the solidification process, utilization of TEMC for modifying the solidification feeding of steel has been ignored. The effects of TEMC on the shrinkage porosity at the final stage of solidification of GCr18Mo steel were investigated under an axial static magnetic field (ASMF). The morphology and porosity of GCr18Mo steel were investigated quantitatively by X-ray microtomography. We found that the amount of porosity first decreases and then increases with increasing intensity of the ASMF. Meanwhile, the shape of the porosity changed from tortuous to elliptic under the ASMF. On the basis of both the experimental and simulated results, the decrease in porosity is attributed to an adequate development of the TEMC leading to sufficient feeding. This study sheds light on an alternative technique for eliminating the shrinkage porosity at the final stage of steel solidification in industrial applications.
Pr{sub 1.33}Pt{sub 4}Ga{sub 10}: Superstructure and magnetism
Pr{sub 1.33}Pt{sub 4}Ga{sub 10} crystals were prepared by Ga-flux method. The superstructure of this compound was studied by single-crystal X-ray diffraction (XRD), transmission electron microscopy (TEM), and diffuse X-ray scattering. Pr{sub 1.33}Pt{sub 4}Ga{sub 10} adopts the P6{sub 3}/mmc space group with a=b=4.3227(5) Å, c=16.485(3) Å: the structure features Pr{sub 2}Ga{sub 3} layers alternating with Pt{sub 2}Ga{sub 4} layers along the c-axis. TEM studies and pair distribution function (PDF) analysis of X-ray total scattering data show that Pr{sub 2}Ga{sub 3} layers possess an ordered superstructure (of dimension a′=a√(3)) in which Pr vacancies and Ga atoms are ordered within the ab-plane but disordered along the c-direction. PDF analysis also shows temperature-dependent structural features local to the Pr{sup 3+} ion. Magnetic measurements reveal that Pr{sup 3+} ions order ferrimagnetically below 12.5(2) K. - Graphical abstract: Left: Crystal structure of Pr{sub 1.33}Pt{sub 4}Ga{sub 10} showing Pr and Ga vacancies in the Pr{sub 2}Ga{sub 3} plane. Right: Tunneling electron microscopy (TEM) image of Pr{sub 1.33}Pt{sub 4}Ga{sub 10}. These vacancies have been studied using TEM and pair distribution function analysis. Magnetic measurements reveal that Pr{sup 3+} ions order ferrimagnetically below 7.8(2) K. - Highlights: • TEM studies indicate a superstructure from Pr and Ga vacancies. • Pair distribution function analyses may point to structural relaxation of vacancies. • Pr{sub 1.33}Pt{sub 4}Ga{sub 10} behaves as a ferromagnet and exhibits a metamagnetic transition.
Measurements of the chiral magnetic effect with background isolation in 200 GeV Au+Au collisions at STAR
Using two novel methods, pair invariant mass (m inv ) and comparative measurements with respect to reaction plane (Ψ RP ) and participant plane (Ψ PP ), we isolate the possible chiral magnetic effect (CME) from backgrounds in 200 GeV Au+Au collisions at STAR. The invariant mass method identifies the resonance background contributions, coupled with the elliptic flow (ν 2 ), to the charge correlator CME observable (Δγ). At high mass (m inv > 1.5 GeV/c 2 ) where resonance contribution is small, we obtain the average Δγ magnitude. In the low mass region (m inv > 1.5 GeV/c 2 ), resonance peaks are observed in Δγ(m inv ). An event shape engineering (ESE) method is used to model the background shape in m inv to extract the potential CME signal at low m inv . In the comparative method, the Ψ RP is assessed by spectator neutrons measured by the ZDCs, and the Ψ PP by the 2 nd -harmonic event plane measured by the TPC. The ν 2 is stronger along Ψ PP and weaker along Ψ RP ; in contrast, the magnetic field, mainly from spectator protons, is weaker along Ψ PP and stronger along Ψ RP . As a result, the Δγ measured with respect to Ψ RP and Ψ PP contain different amounts of CME and background, and can thus determine these two contributions. It is found that the possible CME signals with background isolation by these two novel methods are small, on the order of a few percent of the inclusive Δγ measurements.
Single-Layer Magnet Phase in Intrinsic Magnetic Topological Insulators, [MnTe][Bi 2 Te 3 ] n , Far beyond the Thermodynamic Limit
The intrinsic magnetic topological insulator (IMTI) family [MnTe][Bi 2 Te 3 ]n has demonstrated magneto-topological properties dependent on n, making it a promising platform for advanced electronics and spintronics. However, due to technical barriers in sample synthesis, their properties in the large n limit remain unknown. To overcome this, we utilized the atomic layer-by-layer molecular beam epitaxy (ALL-MBE) technique and achieved IMTIs with n as large as 15, far beyond that previously reported in bulk crystals or thin films. Then, we discover that the “single-layer magnet (SLM)” phase, primarily determined by intralayer ferromagnetic coupling, emerges for n > ∼4 and remains little affected up to n = 15. Nonetheless, still, nonzero, interlayer ferromagnetic coupling is necessary to stabilize the SLM phase, suggesting that the SLM phase eventually disappears in the n → ∞ limit. This study uncovers the secrets of IMTIs beyond the thermodynamic limit and opens a door to diverse magneto-topological applications.
Structure and Magnetism Evolution from FeCo Nanoparticles to Hollow Nanostructure Conversion for Magnetic Applications
Hollow nanostructures have witnessed an increasing interest in emerging fields, including energy storage electrodes, nanocatalysis, photonics, and biomedical carriers, etc. The fundamental mechanisms for hollowing conversion chemistry of complex nanoalloys is indispensable to grow such nanostructures with controlled compositions and functionalities. In this contribution, the Kirkendall effect on magnetic Fe-Co bimetallic alloys with the formation of hollow nanostructures for exchange interactions was studied. The rich diffusion kinetics of binary Fe-Co alloys accompanied with its conversion from solid, core-shell to hollow nanostructures were been captured through X-ray spectroscopy and electron microscopy. We further report the cation exchange improved magnetic performance of hollow nanostructures with increasing the crystallinity.
A Simple Analytical Model for Magnetization and Coercivity of Hard/Soft Nanocomposite Magnets
Here, we present a simple analytical model to estimate the magnetization (σ s ) and intrinsic coercivity (Hci) of a hard/soft nanocomposite magnet using the mass fraction. Previously proposed models are based on the volume fraction of the hard phase of the composite. But, it is difficult to measure the volume of the hard or soft phase material of a composite. We synthesized Sm 2 Co 7 /Fe-Co, MnAl/Fe-Co, MnBi/Fe-Co, and BaFe 12 O 19 /Fe-Co composites for characterization of their σs and Hci. The experimental results are in good agreement with the present model. Therefore, this analytical model can be extended to predict the maximum energy product (BH) max of hard/soft composite.
Static magnetic order with strong quantum fluctuations in spin-1/2 honeycomb magnet Na 2 Co 2 TeO 6
Kitaev interactions, arising from the interplay of frustration and bond anisotropy, can lead to strong quantum fluctuations and, in an ideal case, to a quantum-spin-liquid state. However, in many nonideal materials, spurious non-Kitaev interactions typically promote a zigzag antiferromagnetic order in the d-orbital transition-metal compounds. Here, by combining neutron scattering with muon-spin rotation and relaxation techniques, we provide mechanism insights into the exotic properties of Na 2 Co 2 TeO 6 , a candidate material of the Kitaev model. Below T N , the zero-field muon-spin relaxation rate becomes almost constant (~0.45 μs –1 ). We attribute this temperature-independent relaxation rate to the strong quantum fluctuations, as well as to the frustrated Kitaev interactions. As the magnetic field increases, neutron scattering data indicate a broader spin-wave excitation at the K-point. Therefore, quantum fluctuations seem not only robust but are even enhanced by the applied magnetic field. Our findings provide valuable hints for understanding the onset of the quantum-spin-liquid state in Kitaev materials.
Magnetic transitions and isotropic versus anisotropic magnetic behaviour of [CH 3 NH 3 ][M(HCOO) 3 ] M = Mn 2+ , Co 2+ , Ni 2+ , Cu 2+ metal–organic perovskites
We show unconventional magnetic features of the [CH 3 NH 3 ][M(HCOO) 3 ] perovskite-like family such as the isotropic/anisotropic response unmasked by high magnetic fields.
Improved inertial control for permanent magnet synchronous generator wind turbine generators
With increasing integrations of large‐scale systems based on permanent magnet synchronous generator wind turbine generators (PMSG‐WTGs), the overall inertial response of a power system will tend to deteriorate as a result of the decoupling of rotor speed and grid frequency through the power converter as well as the scheduled retirement of conventional synchronous generators. Thus, PMSG‐WTGs can provide value to an electric grid by contributing to the system's inertial response through the inherent kinetic energy stored in their rotating masses and fast power converter control. In this study, an improved inertial control method based on the maximum power point tracking operation curve is introduced to enhance the overall frequency support capability of PMSG‐WTGs in the case of large supply–demand imbalances. Moreover, this method is implemented in the CART2‐PMSG integrated model in MATLAB/Simulink to investigate its impact on the wind turbine's structural loads during the inertial response process. Simulation results indicate that the proposed method can effectively reduce the frequency nadir, arrest the rate of change of frequency, and alleviate the secondary frequency dip while imposing no negative impact on the major mechanical components of the wind turbine.
Current-induced switching of magnetic tunnel junctions: Effects of field-like spin-transfer torque, pinned-layer magnetization orientation, and temperature
We study current-induced switching in magnetic tunnel junctions in the presence of a field-like spin-transfer torque and titled pinned-layer magnetization in the high current limit at finite temperature. We consider both the Slonczewski and field-like torques with coefficients a{sub J} and b{sub J}, respectively. At finite temperatures, σ=b{sub J}/a{sub J}=±1 leads to a smaller mean switching time compared that with σ=0. The reduction of switching time in the presence of the field-like term is due to the alignment effect (for σ>0) and the initial torque effect.
Influence of hydrogen patterning gas on electric and magnetic properties of perpendicular magnetic tunnel junctions
To identify the degradation mechanism in magnetic tunnel junctions (MTJs) using hydrogen, the properties of the MTJs were measured by applying an additional hydrogen etch process and a hydrogen plasma process to the patterned MTJs. In these studies, an additional 50 s hydrogen etch process caused the magnetoresistance (MR) to decrease from 103% to 14.7% and the resistance (R) to increase from 6.5 kΩ to 39 kΩ. Moreover, an additional 500 s hydrogen plasma process decreased the MR from 103% to 74% and increased R from 6.5 kΩ to 13.9 kΩ. These results show that MTJs can be damaged by the hydrogen plasma process as well as by the hydrogen etch process, as the atomic bonds in MgO may break and react with the exposed hydrogen gas. Compounds such as MgO hydrate very easily. We also calculated the damaged layer width (DLW) of the patterned MTJs after the hydrogen etching and plasma processes, to evaluate the downscaling limitations of spin-transfer-torque magnetic random-access memory (STT-MRAM) devices. With these calculations, the maximum DLWs at each side of the MTJ, generated by the etching and plasma processes, were 23.8 nm and 12.8 nm, respectively. This result validates that the hydrogen-based MTJ patterning processes cannot be used exclusively in STT-MRAMs beyond 20 nm.