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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 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

Electronic and magnetic properties of manganite thin films with different compositions and its correlation with transport properties: An X-ray resonant magnetic scattering study

Here, we present x-ray resonant magnetic dichroism and x-ray resonant magnetic scattering measurements of the temperature dependence of magnetism in Pr-doped La-Ca-Mn-O films grown on (110) NdGaO{sub 3} substrates. We observed thermal hysteresis of the ferromagnetism in one film that also showed large thermal hysteresis of ∼18 K in transport measurements. While in a second film of a different nominal chemistry, which showed very small thermal hysteresis ∼3 K in transport measurements, no thermal hysteresis of the ferromagnetism was observed. These macroscopic properties are correlated with evolution of surface magnetization across metal insulator transition for these films as observed by soft x-ray resonant magnetic scattering measurements.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Structures and magnetic properties of Co-Zr-B magnets studied by first-principles calculations

The structures and magnetic properties of Co-Zr-B alloys near the composition of Co{sub 5}Zr with B at. % ≤6% were studied using adaptive genetic algorithm and first-principles calculations. The energy and magnetic moment contour maps as a function of chemical composition were constructed for the Co-Zr-B magnet alloys through extensive structure searches and calculations. We found that Co-Zr-B system exhibits the same structure motif as the “Co{sub 11}Zr{sub 2}” polymorphs, and such motif plays a key role in achieving strong magnetic anisotropy. Boron atoms were found to be able to substitute cobalt atoms or occupy the “interruption” sites. First-principles calculations showed that the magnetocrystalline anisotropy energies of the boron-doped alloys are close to that of the high-temperature rhombohedral Co{sub 5}Zr phase and larger than that of the low-temperature Co{sub 5.25}Zr phase. Our calculations provide useful guidelines for further experimental optimization of the magnetic performances of these alloys.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Understanding temperature and magnetic-field actuated magnetization polarity reversal in the Prussian blue analogue Cu 0.73 Mn 0.77 [Fe(CN) 6 ]. z H 2 O, using XMCD

We have investigated the microscopic origin of temperature and magnetic-field actuated magnetization reversal in Cu0.73Mn0.77[Fe(CN)(6)]center dot zH(2)O, using XMCD. Our results show a fair deviation from the mean-field-theory in the form of different ordering temperatures of Fe and Mn sublattices. A preferential sign reversal of Mn spin under magnetic field and different spin cant angles for the two sublattices have also been observed. An antiferromagnetic coupling between the Fe and Mn sublattices along with different ordering temperatures (sublattice decoupling) for these sublattices explain the temperature-dependent magnetization reversal. Whereas, Mn spin reversal alone (under external magnetic field) is responsible for the observed field-dependent magnetization reversal. The dissimilar magnetic behavior of Fe and Mn sublattices in this cubic 3d-orbital system has been understood by invoking disparity and competition among inter-sublattice magnetic control parameters, viz. magnetic Zeeman energy, exchange coupling constant and magnetic anisotropy constant. Our results have significant design implications for future magnetic switches, by optimizing the competition among these magnetic control parameters.

Lahiri, Debdutta↗
DOE OSTI2015

Simulation of magnetic hysteresis loops and magnetic Barkhausen noise of α-iron containing nonmagnetic particles

Here, the magnetic hysteresis loops and Barkhausen noise of a single α-iron with nonmagnetic particles are simulated to investigate into the magnetic hardening due to Cu-rich precipitates in irradiated reactor pressure vessel (RPV) steels. Phase field method basing Landau-Lifshitz-Gilbert (LLG) equation is used for this simulation. The results show that the presence of the nonmagnetic particle could result in magnetic hardening by making the nucleation of reversed domains difficult. The coercive field is found to increase, while the intensity of Barkhausen noise voltage is decreased when the nonmagnetic particle is introduced. Simulations demonstrate the impact of nucleation field of reversed domains on the magnetization reversal behavior and the magnetic properties.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Surface-Engineered MXenes: Electric Field Control of Magnetism and Enhanced Magnetic Anisotropy

Controlling magnetism in two-dimensional (2D) materials via electric fields and doping enables robust long-range order by providing an external mechanism to modulate magnetic exchange interactions and anisotropy. Here in this report, we predict that transition metal carbide and nitride MXenes are promising candidates for controllable magnetic 2D materials. The surface terminations introduced during synthesis act as chemical dopants that influence the electronic structure, enabling controllable magnetic order. We show ground-state magnetic ordering in Janus M 2 XO x F 2–x (M is an early transition metal, X is carbon or nitrogen, and x = 0.5, 1, or 1.5) with asymmetric surface functionalization, where local structural and chemical disorder induces magnetic ordering in some systems that are nonmagnetic or weakly magnetic in their pristine form. The resulting magnetic states of these noncentrosymmetric structures can be robustly switched and stabilized by tuning the interlayer exchange couplings with small applied electric fields. Furthermore, bond directionality is enhanced by Janus functionalization, resulting in improved magnetic anisotropy, which is essential to stable 2D magnetic ordering. The mixed termination-induced anisotropy leads to robust Ising ferromagnetism with an out-of-plane easy axis over the full range of relevant termination compositions for Janus Mn 2 N. Janus Cr 2 C, V 2 C, and Ti 2 C were found to be robustly antiferromagnetic. Our results provide a strategy for exploiting asymmetric surface functionalization to achieve room-temperature nanoscale magnetism under ambient conditions in MXenes with currently available synthesis techniques.

2D materials↗
DOE OSTI2015

Particle acceleration and plasma dynamics during magnetic reconnection in the magnetically dominated regime

Magnetic reconnection is thought to be the driver for many explosive phenomena in the universe. The energy release and particle acceleration during reconnection have been proposed as a mechanism for producing high-energy emissions and cosmic rays. We carry out two- and three-dimensional (3D) kinetic simulations to investigate relativistic magnetic reconnection and the associated particle acceleration. The simulations focus on electron–positron plasmas starting with a magnetically dominated, force-free current sheet (σ≡B{sup 2}/(4πn{sub e}m{sub e}c{sup 2})≫1). For this limit, we demonstrate that relativistic reconnection is highly efficient at accelerating particles through a first-order Fermi process accomplished by the curvature drift of particles along the electric field induced by the relativistic flows. This mechanism gives rise to the formation of hard power-law spectra f∝(γ−1){sup −p} and approaches p = 1 for sufficiently large σ and system size. Eventually most of the available magnetic free energy is converted into nonthermal particle kinetic energy. An analytic model is presented to explain the key results and predict a general condition for the formation of power-law distributions. The development of reconnection in these regimes leads to relativistic inflow and outflow speeds and enhanced reconnection rates relative to nonrelativistic regimes. In the 3D simulation, the interplay between secondary kink and tearing instabilities leads to strong magnetic turbulence, but does not significantly change the energy conversion, reconnection rate, or particle acceleration. This study suggests that relativistic reconnection sites are strong sources of nonthermal particles, which may have important implications for a variety of high-energy astrophysical problems.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

Magnetic self-assembly for the synthesis of magnetically exchange coupled MnBi/Fe–Co composites

Exchange coupled hard/soft MnBi/Fe–Co core/shell structured composites were synthesized using a magnetic self-assembly process. MnBi particles were prepared by arc-melting, and Fe–Co nanoparticles were synthesized by an oleic acid assisted chemical reduction method. Grinding a mixture of micron-sized MnBi and Fe–Co nanoparticles in hexane resulted in MnBi/Fe–Co core/shell structured composites. The MnBi/Fe–Co (95/5 wt%) composites showed smooth magnetic hysteresis loops, enhanced remanent magnetization, and positive values in the ΔM curve, indicating exchange coupling between MnBi and Fe–Co particles. - Graphical abstract: Both MnBi and Fe–Co particles were dispersed in hexane for grinding. Because of the oleic acid used during the Fe–Co nanoparticle synthesis, they could be well dispersed in hexane. During the grinding, the size of MnBi particles was decreased, hexane was evaporated, and the Fe–Co nanoparticles were concentrated in the solvent and magnetically attracted by MnBi particles, forming a core/shell structure. - Highlights: • Exchange coupled MnBi/Fe–Co composites are synthesized through magnetic selfassembly. • Magnetic exchange coupling is demonstrated by smooth magnetic hysteresis loops, enhanced remanent magnetization, and dominant positive peak in the ΔM curve. • The experimental results in magnetic properties are close to the theoretical calculation results.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Electrically Tunable Magnetism in Magnetic Topological Insulators

The external controllability of the magnetic properties in topological insulators would be important both for fundamental and practical interests. Here we predict the electric-field control of ferromagnetism in a thin film of insulating magnetic topological insulators. The decrease of band inversion by the application of electric fields results in a reduction of magnetic susceptibility, and hence in the modification of magnetism. Remarkably, the electric field could even induce the magnetic quantum phase transition from ferromagnetism to paramagnetism. We further propose a transistor device in which the dissipationless charge transport of chiral edge states is controlled by an electric field. In particular, the field-controlled ferromagnetism in a magnetic topological insulator can be used for voltage based writing of magnetic random access memories in magnetic tunnel junctions. The simultaneous electrical control of magnetic order and chiral edge transport in such devices may lead to electronic and spintronic applications for topological insulators.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Strong surface magnetic field generation in relativistic short pulse laser–plasma interaction with an applied seed magnetic field

Abstract While plasma often behaves diamagnetically, we demonstrate that the laser irradiation of a thin opaque target with an embedded target-transverse seed magnetic field B seed can trigger the generation of an order-of-magnitude stronger magnetic field with opposite sign at the target surface. Strong surface field generation occurs when the laser pulse is relativistically intense and results from the currents associated with the cyclotron rotation of laser-heated electrons transiting through the target and the compensating current of cold electrons. We derive a predictive scaling for this surface field generation, B gen ∼ −2 πB seed Δ x / λ 0 (in the large spot size limit), where Δ x is the target thickness and λ 0 is the laser wavelength, and conduct 1D and 2D particle-in-cell simulations to confirm its applicability over a wide range of conditions. We additionally demonstrate that both the seed and surface-generated magnetic fields can have a strong impact on application-relevant plasma dynamics, for example substantially altering the overall expansion and ion acceleration from a μ m-thick laser-irradiated target with a kilotesla-level seed magnetic field.

Weichman, K. (ORCID:0000000234877922)↗
DOE OSTI2022

Second-order magnetic responses in quantum magnets: Magnetization under ac magnetic fields

Here we investigate second-order magnetic responses of quantum magnets against ac magnetic fields. We focus on the case where the z component of the spin is conserved in the unperturbed Hamiltonian and the driving field is applied in the xy plane. We find that linearly polarized driving fields induce a second-harmonic response, while circularly polarized fields generate only a zero-frequency response, leading to a magnetization with a direction determined by the helicity. Employing an unbiased numerical method, we demonstrate the nonlinear magnetic effect driven by the circularly polarized field in the XXZ model and show that the magnitude of the magnetization can be predicted by the dynamical spin structure factor in the linear response regime.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Quantum Imaging of Magnetic Phase Transitions and Spin Fluctuations in Intrinsic Magnetic Topological Nanoflakes

Topological materials featuring exotic band structures, unconventional current flow patterns, and emergent organizing principles offer attractive platforms for the development of next-generation transformative quantum electronic technologies. The family of MnBi 2 T e4 (Bi 2 Te 3 )n materials is naturally relevant in this context due to their nontrivial band topology, tunable magnetism, and recently discovered extraordinary quantum transport behaviors. Despite numerous pioneering studies, to date, the local magnetic properties of MnBi 2 Te 4 (Bi 2 Te 3 )n remain an open question, hindering a comprehensive understanding of their fundamental material properties. Exploiting nitrogen-vacancy (NV) centers in diamond, we report nanoscale quantum imaging of magnetic phase transitions and spin fluctuations in exfoliated MnBi 2 Te 4 (Bi 2 Te 3 )n flakes, revealing the underlying spin transport physics and magnetic domains at the nanoscale. Furthermore, our results highlight the unique advantage of NV centers in exploring the magnetic properties of emergent quantum materials, opening new opportunities for investigating the interplay between topology and magnetism.

Antiferromagnetic materials↗
DOE OSTI2019

Tunable Magnetic Transition to a Singlet Ground State in a 2D van der Waals Layered Trimerized Kagomé Magnet

Incorporating magnetism into two-dimensional (2D) van der Waals (vdW) heterostructures is crucial for the development of functional electronic and magnetic devices. Here, we show that Nb 3 X 8 (X = Cl, Br) is a family of 2D layered trimerized kagomé magnets that are paramagnetic at high temperatures and undergo a first-order phase transition on cooling to a singlet magnetic state. X-ray diffraction shows that a rearrangement of the vdW stacking accompanies the magnetic transition, with high- and low-temperature phases consistent with scanning transmission electron microscopy images of the end members α-Nb 3 Cl 8 and β-Nb 3 Br 8 . The temperature of this transition is systematically varied across the solid solution Nb 3 Cl 8–x Br x (x = 0–8), with x = 6 having transitions near room temperature. Furthermore, the solid solution also varies the optical properties, which are further modulated by the phase transition. As such, they provide a platform on which to understand and exploit the interplay between dimensionality, magnetism, and optoelectronic behavior in vdW materials.

2D materials↗
DOE OSTI2016

G-mode magnetic force microscopy: Separating magnetic and electrostatic interactions using big data analytics

In this work, we develop a full information capture approach for Magnetic Force Microscopy (MFM), referred to as generalized mode (G-Mode) MFM. G-Mode MFM acquires and stores the full data stream from the photodetector, captured at sampling rates approaching the intrinsic photodiode limit. The data can be subsequently compressed, denoised, and analyzed, without information loss. Here, G-Mode MFM is implemented and compared to the traditional heterodyne-based MFM on model systems, including domain structures in ferromagnetic Yttrium Iron Garnet and the electronically and magnetically inhomogeneous high entropy alloy, CoFeMnNiSn. We investigate the use of information theory to mine the G-Mode MFM data and demonstrate its usefulness for extracting information which may be hidden in traditional MFM modes, including signatures of nonlinearities and mode-coupling phenomena. Finally, we demonstrate detection and separation of magnetic and electrostatic tip-sample interactions from a single G-Mode image, by analyzing the entire frequency response of the cantilever. G-Mode MFM is immediately implementable on any atomic force microscopy platform and as such is expected to be a useful technique for probing spatiotemporal cantilever dynamics and mapping material properties, as well as their mutual interactions.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗