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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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At least 73 records · Page 4
All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

Tuning the inductance of Josephson junction arrays without SQUIDs

It is customary to use arrays of superconducting quantum interference devices (SQUIDs) for implementing magnetic field-tunable inductors. Here, we demonstrate an equivalent tunability in a (SQUID-free) array of single Al/AlOx/Al Josephson tunnel junctions. With the proper choice of junction geometry, a perpendicularly applied magnetic field bends along the plane of the superconductor and focuses into the tunnel barrier region due to a demagnetization effect. Consequently, the Josephson inductance can be efficiently modulated by the Fraunhofer-type supercurrent interference. The elimination of SQUIDs not only simplifies the device design and fabrication but also facilitates a denser packing of junctions, and, hence, a higher inductance per unit length. As an example, we demonstrate a transmission line, the characteristic impedance of which is field-tuned in the range of 4–8 kΩ, centered around the important value of the resistance quantum h/(2e)2≈6.5 kΩ.

Physics↗
DOE OSTI2024

Segregation of Al and its effect on coercivity in Nd-Fe-B

Doping a small amount of Al can effectively enhance coercivity in Nd-Fe-B magnets. Here we investigated the partitioning behavior of Al and its effect on coercivity in Nd-Fe-B using first principles DFT (density functional theory) calculation and micromagnetic simulation. The calculated substitution energies of Fe by Al are negative at the crystallographic sites of 4c and 8j 2 while they are positive values at the other sites in Nd 2 Fe 14 B (2:14:1), implying a small solubility of Al in 2:14:1. Further, Al prefers to segregate at grain boundary (GB) and stabilize the Nd-Fe-Al phase with a Nd 6 Fe 13 Si-type tetragonal structure (6:13:1). The formation of the antiferromagnetic or weak ferrimagnetic 6:13:1-like phase depletes Fe and reduces the amount of ferromagnetic Nd–Fe type grain boundary phase (GBP), which weakens the inter-grain magnetic interaction. Micromagnetic simulations indicate that the 6:13:1-like GBP increases the pinning field of magnetic domain wall at GB and suppresses the nucleation of reversal magnetic domain on the grain surface of 2:14:1 during demagnetization process. The formation of Al-rich shell on 2:14:1 grain surface can further moderately increase the domain pinning field at GB and the nucleation field of 2:14:1 grain. Developing novel processing method to tailor Al segregation and promote formation of 6:13:1-like phase at GB can be a promising approach to improve coercivity in Nd-Fe-B magnet.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2026

Enhanced coercivity in Fe5C2/SiO2 core/shell nanocrystals

Rod-shaped Fe5C2 and core/shell Fe5C2/SiO2 nanocrystals were synthesized via a solution-based chemical method. Structural analysis confirmed the monoclinic phase of Fe5C2 with space group C2/c. Zero-field-cooling (ZFC) and field-cooling (FC) magnetization curves revealed distinct magnetic behaviors: uncoated Fe5C2 exhibited a low-temperature FC plateau indicative of strong dipolar interactions, while Fe5C2/SiO2 showed a monotonic increase in FC magnetization, suggesting reduced dipolar interactions due to SiO2 surface passivation. Isothermal remanent magnetization (IRM) and DC demagnetization (DCD) measurements supported this trend, with δM plots confirming weaker dipolar interactions in the coated sample. Bloch’s law fitting of temperature-dependent saturation magnetization showed a smaller Bloch’s constant for pure Fe5C2 and a larger value for Fe5C2/SiO2, reflecting enhanced surface disorder and reduced exchange coupling in the latter. Notably, Fe5C2/SiO2 demonstrated increased coercivity, attributed to decreased dipolar interaction and elevated surface anisotropy. Kneller’s law fitting yielded higher blocking temperatures for Fe5C2 (476 K) than Fe5C2/SiO2 (456 K), highlighting the impact of dipolar interactions on magnetic relaxation. These findings illustrate how SiO2 coatings effectively modulate dipolar interactions and enhance coercivity in Fe5C2 nanocrystals.

Joshi, Pramanand [Department of Physics, Universit↗
DOE OSTI2022

Topological magnetic hysteresis in single crystals of CeAgSb 2 ferromagnet

Closed-topology magnetic domains are usually observed in thin films and in an applied magnetic field. In this work, we report the observation of rectangular cross-section tubular ferromagnetic domains in thick single crystals of CeAgSb 2 in zero applied field. Relatively low exchange energy, small net magnetic moment, and anisotropic in-plane crystal electric fields lower the domain wall energy and allow for the formation of the closed-topology patterns. The tubular domain structure irreversibly transforms into a dendritic pattern upon cycling the magnetic field. This transition between closed and open topologies results in a ‘topological magnetic hysteresis’— the actual hysteresis in magnetization, not due to the imperfections and pinning, but due to the difference in the pattern morphology. Similar physics was suggested before in pure type-I superconductors and is believed to be a generic feature of other nonlinear single (present case), or two-phase (type-I superconductor) systems where the effects similar to demagnetization (shape-dependent macroscopic variation of properties) lead to pattern formation.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

On the trapped magnetic moment in type-II superconductors

In this article, measurements of the trapped (remanent) magnetic moment, M trap (H) , when a small magnetic field H is turned off after cooling below the superconducting transition temperature, T c , or ramping a magnetic field up and down after cooling in a zero magnetic field, offer substantial advantages in difficult cases of small samples and large field-dependent backgrounds, which is relevant for hydrogen-based ultra-high-T c superconductors (UHTS). Until recently, there was no need for a separate paper on the trapped magnetic flux for well-known critical-state models due to the simplicity of the physics involved. However, recent publications showed the need for such an analysis. This note summarizes the expectations for the Bean model with constant critical current density and the Kim model with field-dependent critical currents. It is shown that if the trapped moment is fitted to the power law, M trap $\propto$ H a , the fixed exponent α = 2 is exact for the Bean model, while Kim models show a wide interval of possible values, 2$\leqslant$a$\leqslant$4. Furthermore, accounting for reversible magnetization expands the range of possible exponents to 1$\leqslant$a$\leqslant$4. In addition, demagnetizing factors are essential and make the trapped moment orientation dependent even in isotropic materials. As a concrete application, it is shown that flux trapping experiments performed on H 3 S UHTS compounds can be well described using this generalized approach, lending further support to the type-II superconducting nature of H 3 S under ultra-high pressure.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Predicted strong coupling of solid-state spins via a single magnon mode

Abstract We propose an approach to realize a hybrid quantum system composed of a diamond nitrogen-vacancy (NV) center spin coupled to a magnon mode of the low-damping, low-moment organic ferrimagnet vanadium tetracyanoethylene. We derive an analytical expression for the spin-magnon cooperativity as a function of NV position under a micron-scale perpendicularly magnetized disk, and show that, surprisingly, the cooperativity will be higher using this magnetic material than in more conventional materials with larger magnetic moments, due to in part to the reduced demagnetization field. For reasonable experimental parameters, we predict that the spin-magnon-mode coupling strength is g ∼ 2π×10 kHz. For isotopically pure 12 C diamond we predict strong coupling of an NV spin to the unoccupied magnon mode, with cooperativity C λ = 15 for a wide range of NV spin locations within the diamond, well within the spatial precision of NV center implantation. Thus our proposal describes a practical pathway for single-spin-state-to-single-magnon-occupancy transduction and for entangling NV centers over micron length scales.

Candido, Denis R. (ORCID:0000000267083280)↗
DOE OSTI2022

Conservation of angular momentum in ultrafast spin dynamics

The total angular momentum of a closed system is a conserved quantity, which should remain constant in time for any excitation experiment once the pumping signal has been extinguished. Such conservation, however, is never satisfied in practice in any real-time first-principles description of the demagnetization process. Furthermore, there is growing experimental evidence that the same takes place in experiments. The missing angular momentum is usually associated to lattice vibrations, which are not measured experimentally and are never considered in real-time simulations. Here, in this study, we critically analyze the issue and conclude that current state-of-the-art simulations violate angular momentum conservation already at the electronic level of description. This shortcoming originates from an oversimplified description of the spin-orbit coupling, which includes atomic contributions but neglects completely that of itinerant electrons. We corroborate our findings with time-dependent simulations using model tight-binding Hamiltonians, and show that indeed such conservation can be reintroduced by an appropriate choice of spin-orbit coupling. The consequences of our findings on recent experiments are also discussed.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Structural and magnetic characterization of CeTa 7 ⁢O 19 and YbTa 7 ⁢ O 19 with a two-dimensional pseudospin-$\frac{1}{2}$ triangular lattice

Triangular lattice antiferromagnets are prototypes for frustrated magnetism and may potentially realize novel quantum magnetic states such as a quantum spin-liquid ground state. A recent work suggests NdTa 7 ⁢O 19 with rare-earth triangular lattice is a quantum spin-liquid candidate and highlights the large family of rare-earth heptatantalates as a framework for quantum magnetism investigation. Here, in this paper, we report the structural and magnetic characterization of CeTa 7 ⁢ O 19 and YbTa 7 ⁢ O 19 . Both compounds are isostructural to NdTa 7 ⁢O 19 with no detectable structural disorder. For CeTa 7 ⁢O 19 , the crystal field energy levels and parameters are determined by inelastic neutron scattering measurements. Based on the crystal field result, the magnetic susceptibility data could be well fitted and explained, which reveals that CeTa 7 ⁢O 19 is a highly anisotropic Ising triangular-lattice antiferromagnet (𝑔 𝑧 ⁡/𝑔 𝑥⁢𝑦 ∼ 3) with very weak exchange interaction (J ∼ 0.22 K). For YbTa 7⁢ O 19 , millimeter-sized single crystals could be grown. The anisotropic magnetization and electron spin resonance data show that YbTa 7⁢ O 19 has a contrasting in-plane magnetic anisotropy with 𝑔 𝑧 ⁡/𝑔 𝑥⁢𝑦 ∼ 0.67 similar as that of YbMgGaO 4 . The above results indicate that CeTa7⁢ O 19 and YbTa 7 ⁢O 19 with pseudospin-1/2 ground states might either be quantum spin-liquid candidate materials or find applications in adiabatic demagnetization refrigeration due to the weak exchange interaction.

Pan, Feihao↗
DOE OSTI2023

Ultrafast Dynamics of Intrinsic Anomalous Hall Effect in the Topological Antiferromagnet Mn 3 Sn

Here, we investigate ultrafast dynamics of the anomalous Hall effect (AHE) in the topological antiferromagnet Mn 3 ⁢Sn with sub-100 fs time resolution. Optical pulse excitations largely elevate the electron temperature up to 700 K, and terahertz probe pulses clearly resolve ultrafast suppression of the AHE before demagnetization. The result is well reproduced by microscopic calculation of the intrinsic Berry-curvature mechanism while the extrinsic contribution is clearly excluded. Our work opens a new avenue for the study of nonequilibrium AHE to identify the microscopic origin by drastic control of the electron temperature by light.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Inducing skyrmion flop transitions in Co 8 Zn 8 Mn 4 at room temperature

Magnetic skyrmions are topologically-protected spin textures that manifest in certain noncentrosymmetric ferromagnets under the right conditions of temperature and field. In thin-film skyrmion hosts, demagnetization effects combined with geometric confinement can result in two distinct types of spin textures: those with their axis of symmetry in the plane of the film (IP) and those with their axis pointing out-of-plane (OOP). Here we present Lorentz transmission electron microscopy evidence in conjunction with numerical modeling showing a flop transition between IP and OOP skyrmions in Co 8 ⁢Zn 8 ⁢Mn 4 at room temperature. Further, we show that this skyrmion flop transition is controllable via the angle of the external field relative to the film normal and we illustrate how this transition depends on thickness. Finally, we propose a skyrmion-writing device that utilizes the details of this transition.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Comparison of Candidate Designs and Performance Optimization for an Electric Traction Motor Targeting 50 kW/L Power Density

The continued expansion of the global electric vehicle fleet is accompanied by an unprecedented demand for high power density electric traction motors. With the ambitious U.S. DRIVE 2025 target of 50 kW/L power density and an equally aggressive cost reduction goal, innovative approaches have to be utilized in terms of both the design and manufacturing of electric traction motors. In this paper, six motor options are compared and the best design is picked for each option to investigate the drive and excitation requirements and the weighted power efficiency over multiple load points, with necessary mechanical stress and demagnetization checks. State-of-the-art winding technologies, including high slot fill die compressed windings and hairpin windings, and rotors both with permanent magnet (PM) and PM-free are incorporated. The design of high flux density and low harmonic content magnetic field is also demonstrated.

33 ADVANCED PROPULSION SYSTEMS↗
DOE OSTI2020

Magnetic Field Optimization of an In-Vacuum Undulator at NSLS-II Transactions on Applied Superconductivity

A 3.0 m long In-Vacuum Undulator (IVU) is employed at the National Synchrotron Light Source II (NSLS-II) for the Hard X-ray Nanoprobe (HXN) beamline providing structural and X-ray fluorescence imaging with world-leading spatial resolution. On July 1 st 2018 a gearbox shaft in the gap drive train assembly sheared due to corrosion fatigue caused by stress due to mechanical misalignment. Furthermore, the device was extracted from the storage ring and measured. The magnetic field measurements detected a degradation of the magnetic performance as well as a localized demagnetization of the magnetic modules of the first 3 upstream periods. We describe here the mechanical repairs and the magnetic phase shimming optimization implemented to restore the spectral performance of the device.

43 PARTICLE ACCELERATORS↗
DOE OSTI2020

Theoretical Correlation of Elemental Distribution of Nd and Pr in Ce-Fe-B Microstructure With Hard Magnetic Properties

Relatively resource-rich but property inferior-Ce-Fe-B magnet can be improved by partial replacement of Ce by Nd and/or Pr. In addition to the amount of Nd/Pr, their distribution profile in microstructure plays an important role. From our first principles density functional theory (DFT) calculation, the substitution energy of Ce by Pr/Nd is negative in Ce 2 Fe 14 B (2:14:1) while that for laves phase, CeFe 2 is positive, implying that Nd/Pr stabilize 2:14:1 and suppress the formation of the CeFe 2 phase. Further, micromagnetic simulation indicates that homogenized distribution of Nd/Pr improves squareness of demagnetization curve, while core (Ce-rich)-shell (Nd/Pr-rich) 2:14:1 grain structure enhances coercivity. Magnetic properties of Ce-Fe-B can be optimized by manipulating distribution profile of chemical element in microstructure based on their subtle difference in thermodynamic property, which is an effective pathway to design optimized chemical composition and processing route for high-performance magnet.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Initial in-orbit operation of the soft X-ray spectrometer Resolve onboard the X-ray imaging and spectroscopy mission satellite

The X-Ray Imaging and Spectroscopy Mission satellite was launched on September 6, 2023 (UT). Its Resolve instrument is a high-resolution X-ray spectrometer enabled by a microcalorimeter array thermally anchored to a 50-mK heat sink. Many sensitive, critical sub-systems comprise Resolve, including a multistage cryogenic cooling system, thin-film aperture filters, low-noise electronics, on-board signal processing, and several sources of X-rays for calibration. We summarize the initial on-orbit power-on and checkout of Resolve that commenced immediately after launch. Soon after launch, the cryocoolers were activated, and their operation was successfully established. On October 9, 2023, the first cycle of the adiabatic demagnetization refrigerator was carried out, bringing the sensors to their steady-state operational temperatures. Following this, the energy resolution at 5.9 keV was successfully measured. The energy scale of the system is highly sensitive to the thermal environment surrounding both the sensors and their analog electronics. Gain correction was performed using reference X-ray lines from onboard calibration sources. To optimize cooler frequency settings, noise spectra were collected across a range of frequencies, and the most suitable frequency pair was selected based on the in-orbit environment. During the final phase of the checkout, an attempt was made to open the gate valve, which is designed to protect the Dewar’s interior from external pressure during ground operations and launch. Unfortunately, this attempt was unsuccessful. As a result, the checkout process was temporarily paused, and a stable operational strategy was subsequently developed to enable Resolve to function effectively with the gate valve remaining closed.

X-ray micro-calorimeter↗
DOE OSTI2025

Pre- and post-launch operation of the Resolve soft X-ray spectrometer onboard the XRISM satellite

Resolve is a high-resolution X-ray spectrometer onboard the X-Ray Imaging and Spectroscopy Mission (XRISM), launched on September 6 (UT), 2023. The Resolve has performed better than its required spectral resolution (7 eV at full width at half maximum at 6 keV), both on the ground and in orbit, and has been confirmed to have comparable performance to the soft X-ray spectrometer onboard the ASTRO-H (Hitomi) satellite. The focal plane is composed of an array of microcalorimeter detectors operated at 50 mK to achieve the required energy resolution, and the cooling system is designed to satisfy the lifetime requirement of over 3 years. The focal plane and cooling system are contained in a vacuum-insulated dewar. The cooling system is equipped with a two-stage adiabatic demagnetization refrigerator (ADR) that uses superfluid liquid helium (LHe) as its heat sink. The system includes a third ADR stage that can be used to provide the heat sink when the helium is exhausted. A Joule–Thomson cooler and several two-stage Stirling coolers are used to reduce the heat load on the LHe. During pre-launch operations, we carried out a superfluid LHe top-off operation. The resultant amount of LHe onboard Resolve was over 35 L before launch, which is sufficient to meet the lifetime requirement. During post-launch operation, the LHe vent valve was opened 5 min after launch during rocket acceleration, and the cryocoolers were turned on after several orbits, as planned, which established stable cooling within the dewar. Pre- and post-launch operations for the Resolve instrument were planned around multiple constraints from launch vehicle operations; all were successfully completed, and the launch requirements were fully met.

X-Ray Imaging and Spectroscopy Mission↗
DOE OSTI2022

Nonequilibrium sub-10 nm spin-wave soliton formation in FePt nanoparticles

Magnetic nanoparticles such as FePt in the L1 0 phase are the bedrock of our current data storage technology. As the grains become smaller to keep up with technological demands, the superparamagnetic limit calls for materials with higher magnetocrystalline anisotropy. This, in turn, reduces the magnetic exchange length to just a few nanometers, enabling magnetic structures to be induced within the nanoparticles. Here, we describe the existence of spin-wave solitons, dynamic localized bound states of spin-wave excitations, in FePt nanoparticles. We show with time-resolved x-ray diffraction and micromagnetic modeling that spin-wave solitons of sub–10 nm sizes form out of the demagnetized state following femtosecond laser excitation. The measured soliton spin precession frequency of 0.1 THz positions this system as a platform to develop novel miniature devices.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2023

Light-induced ultrafast spin transport in multilayer metallic films originates from sp-d spin exchange coupling

Ultrafast interaction between the femtosecond laser pulse and the magnetic metal provides an efficient way to manipulate the magnetic states of matter. Numerous experimental advancements have been made on multilayer metallic films in the last two decades. However, the underlying physics remains unclear. Here, relying on an efficient ab initio spin dynamics simulation algorithm, we revealed the physics that can unify the progress in different experiments. We found that light-induced ultrafast spin transport in multilayer metallic films originates from the sp - d spin-exchange interaction, which can induce an ultrafast, large, and pure spin current from ferromagnetic metal to nonmagnetic metal without charge carrier transport. The resulting trends of spin demagnetization and spin flow are consistent with most experiments. It can explain a variety of ultrafast light-spin manipulation experiments with different systems and different pump-probe technologies, covering a wide range of work in this field.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Ultrafast electron microscopy for probing magnetic dynamics

Abstract The spatial features of ultrafast changes in magnetic textures carry detailed information on microscopic couplings and energy transport mechanisms. Electrons excel in imaging such picosecond or shorter processes at nanometer length scales. We review the range of physical interactions that produce ultrafast magnetic contrast with electrons, and specifically highlight the recent emergence of ultrafast Lorentz transmission electron microscopy. From the fundamental processes involved in demagnetization at extremely short timescales to skyrmion-based devices, we show that ultrafast electron imaging will be a vital tool in solving pressing problems in magnetism and magnetic materials where nanoscale inhomogeneity, microscopic field measurement, non-equilibrium behavior or dynamics are involved. Graphic abstract

36 MATERIALS SCIENCE↗