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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 901 records · Page 50
All sourcesNASA NTRSDOE OSTI
DOE OSTI2019

Long lifetime of thermally excited magnons in bulk yttrium iron garnet

Spin currents are generated within the bulk of magnetic materials due to heat flow, an effect called intrinsic spin Seebeck. This bulk bosonic spin current consists of a diffusing thermal magnon cloud, parametrized by the magnon chemical potential (μ m ), with a diffusion length of several microns in yttrium iron garnet (YIG). Transient optothermal measurements of the spin-Seebeck effect (SSE) as a function of temperature reveal the time evolution of μ m due to intrinsic SSE in YIG. The interface SSE develops at times <2 ns while the intrinsic SSE signal continues to evolve at times >500 μs, dominating the temperature dependence of SSE in bulk YIG. Time-dependent SSE data are fit to a multitemperature model of coupled spin/heat transport using the finite-element method (FEM), where the magnon spin lifetime (τ) and magnon-phonon thermalization time (τ mp ) are used as fit parameters. From 300 to 4 K, τ mp ) varies from 1 to 10 ns, whereas tau varies from 2 to 60 μ s with the spin lifetime peaking at 90 K. At low temperature, a reduction in τ is observed consistent with impurity relaxation reported in ferromagnetic resonance measurements. Finally, these results demonstrate that the thermal magnon cloud in YIG contains extremely low-frequency magnons (similar to 10 GHz), providing spectral insight to the microscopic scattering processes involved in magnon spin/heat diffusion.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2013

Electron Doping by Charge Transfer at LaFeO 3 /Sm 2 CuO 4 Epitaxial Interfaces

The breakdown of the lattice translational invariance symmetry that occurs at complex oxide interfaces may profoundly modify their electronic structure, leading to interfacial states with properties drastically different from those of the superlattice individual components. The appearance of a conducting two dimensional (2D) electron gas at the interface between two insulating oxides and induced magnetism in a non-magnetic material are just two among many fascinating examples. [ 1 8 ] One of the key factors underlying novel properties is the modifi cation of the doping and orbital occupancy near those interfaces, which may result from charge transfer processes. [ 3 , 9 11 ] If materials used in heterostructures have different work functions, a non-equilibrium situation will be created at the interface and charge will be transferred until the chemical potential levels off. [ 12 ] The use of such phenomena to modify doping in heterostructures has been proposed theoretically as a new route to avoid the quenched disorder that inevitably accompanies the chemical doping. At the interface between a Mott insulating parent compound of the high critical temperature superconductor (HTSC) family and a suitable material that would act as the charge donor, electron doped phases could be stabilized which would eventually turn metallic and perhaps superconducting. [ 12 , 13 ] Such charge transfer processes have been observed at interfaces involving copper oxides such as La 0.7 Ca 0.3 MnO 3 / YBa 2 Cu 3 O 7 , [ 14 ] La 2 x Sr x CuO 4 /La 2 CuO 4 [ 15 ] and SrTi 1 x Nb x O 3 / Sm 2 CuO 4 . [ 16 ] While a novel 2D superconducting state was found at the La 2-x Sr x CuO 4 /La 2 CuO 4 interface, [ 17 ] the effect of doping by charge transfer could not be examined in the other two cases due to the detrimental effect on the YBa 2 Cu 3 O 7 superconductivity of the spin polarized electrons from La 0.7 Ca 0.3 MnO 3 in one case and due to the conducting nature of the SrTi 1 x Nb x O 3 in the other case, which obscures changes in the conducting properties of the interface layer. In this paper, the structural and

Bruno, Flavio Y.↗
DOE OSTI2025

Mesoscale Magnetostructural Phase Separation in Fe‐deficient Fe 5 GeTe 2

Two-dimensional van der Waals ferromagnet Fe 5-x GeTe 2 (F5GT) is promising for spintronic applications due to its high Curie temperature, layered structure, and ability to host complex magnetic textures. However, the origin of its sample-dependent magnetic anisotropy remains unclear, hindering control of its magnetic behavior. Here, we use spatially resolved cryogenic scanning transmission electron microscopy (STEM) to correlatively map magnetism, lattice structure, and chemistry across atomic-to-micron scales. We reveal that only mesoscale, not nanoscale, inclusions of a Fe-deficient secondary phase significantly modify magnetic behavior, establishing a previously unrecognized critical length scale. This phase separation, induced by quenching, leads to in-plane magnetic anisotropy, while slow cooling confines separation to a few nanometers and preserves out-of-plane anisotropy. These findings reconcile prior inconsistencies and establish a predictive framework for tuning magnetism in F5GT through thermal processing, with broader implications for controlling anisotropy in other two-dimensional magnetic materials.

2D ferromagnets↗
DOE OSTI2025

Revealing Robust Room Temperature Ferromagnetism in Gd‐Doped Few‐Layered MoS 2 Thin Films

2D MoS 2 holds great promise for spintronics, yet is limited by intrinsic diamagnetism. This study demonstrates inducing ferromagnetic behavior in MoS 2 films doped with 0.47% Gd, achieving an ultrahigh saturation magnetization of 454 emu/cm 3 in a few‐layered film over 11‐times higher than bulk films (40 nm). Raman spectroscopy, X‐ray photoelectron spectroscopy, X‐ray magnetic circular dichroism, and density functional theory (DFT) calculations reveal an interplay between Gd dopants and Mo, S vacancies (V 1Mo+2S ), leading to the formation of bound magnetic polarons (BMPs) that drive ferromagnetic ordering. H 2 S annealing and DFT calculations reveal that defect healing reduces the saturation magnetization by 83%. High sulfur migration barrier in few‐layered films helps preserve BMPs, thereby sustaining ferromagnetism, whereas lower migration barriers in bulk films lead to suppression. These findings highlight the synergy between Gd doping and defect engineering in achieving ultrahigh room‐temperature ferromagnetism, offering a scalable strategy for developing high‐performance 2D magnetic materials for spintronic applications.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Nonvolatile Electric Control of the Anomalous Hall Effect in an Ultrathin Magnetic Metal

Abstract Low‐dimensional magnetism has been boosted by the recent discovery of the ferromagnetism in layered two‐dimensional (2D) semiconductors. Although the macroscopic magnetic moments of 2D ferromagnets are weak, the anomalous Hall effect (AHE) can serve as a versatile electric probe to their magnetic properties. Here, the nonvolatile electric‐field manipulation of the AHE in an ultrathin metallic ferromagnet with perpendicular magnetic anisotropy at room temperature is reported, which is achieved by the electrostatic modulation of the longitudinal resistivity via a ferroelectric substrate without varying magnetization. Therefore, this work demonstrates an electric‐field‐controlled room‐temperature memory device based on the zero‐magnetic‐field anomalous Hall resistance of an ultrathin ferromagnet. More importantly, the experimental results disentangle magnetization and anomalous Hall resistance. As a result, the study reveals a linear decrease of anomalous Hall conductivity with normal conductivity, which is distinct from previous scaling relations. Accordingly, this work manifests a universe avenue to harnessing the AHE in low‐dimensional magnetic materials.

Feng, Zexin↗
DOE OSTI2016

On the Extended Series of Quaternary Zintl Phases Ca13REMnSb11 (RE = La–Nd, Sm, Gd–Dy)

Eight new compounds with a general formula Ca 14– x RE x MnSb 11 ( RE = La–Nd, Sm, Gd–Dy; x ≈ 1) were synthesized by the molten flux method and their structures were established via single‐crystal X‐ray diffraction. The overarching goal of this study was to investigate the effect of electron doping, via substitution of RE 3+ at Ca 2+ sites, on the crystal structure and physical properties of Ca 14 MnSb 11 , which is a candidate for thermoelectric applications. All studied phases are isostructural, and crystallize in the tetragonal body‐centered space group I 4 1 / acd (Ca 14 AlSb 11 structure type, Pearson index tI 208). The structure is made up of MnSb 4 10– tetrahedra, randomly mixed Ca 2+ and RE 3+ cations, Sb 3– anions, and linear Sb 3 7– polyanions. Comprehensive structural work confirms high rare‐earth metal content with Ca 2+ / RE 3+ randomly mixed on all four cation sites, with the caveat that the larger RE atoms (La–Nd) prefer to occupy the Ca2 site, while the smaller RE atoms (Sm, Gd, Tb, and Dy) preferably occupy the Ca1 site. Nearly phase‐pure polycrystalline samples have been synthesized using solid‐state reactions, and have been used for physical property measurements. The resistivities of the bulk Ca 14– x RE x MnSb 11 ( RE = La–Nd, Sm, Gd; x ≈ 1) samples reveal semiconducting behavior, consistent with the notion of electron doping in the parent p ‐type Ca 14 MnSb 11 material. Magnetic susceptibilities suggest complex magnetic ordering at temperatures below ca. 50 K, likely originating from coupling between two or more magnetic sub‐lattices.

Prakash, Jai↗
DOE OSTI2017

Analysis of Magnetization as a Function of Temperature for CoMn{sub 1−x}Fe{sub x}Ge

Our experimental measurements for the magnetization M as a function of temperature are analyzed for the multilayer structure of CoMn{sub 1−x}Fe{sub x}Ge for x = 0.085 and x = 0.12 at constant magnetic fields of 3, 5, and 7 T. For this analysis, a power-law formula is used below the transition temperature T{sub C} for these compositions and the values of the critical exponent β for the magnetization M (order parameter) are determined. Our analysis indicates that a first order transition at lower concentrations (x = 0.085) changes toward a second order at higher concentrations (x = 0.12) at constant magnetic fields (3, 5, and 7 T) for the multilayer CoMn{sub 1−x}Fe{sub x}Ge, as observed experimentally.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Evolution of Intrinsic and Magnetic Field-Induced Magnetic Anisotropies in Strongly Phase-Separated Manganite Thin Films

Single crystalline thin film (∼ 100 nm) of La{sub 0.18}Pr{sub 0.40}Ca{sub 0.42}MnO{sub 3} is grown on (001) oriented LaAlO{sub 3} substrates and the evolution of anisotropy associated with the supercooling behaviour of the magnetic liquid is studied as a function of temperature and magnetic field. The angle-dependent magnetization measurements ascertain that the easy magnetic axis lies in the plane of the film while the hard axis is along the plane normal. The ratio of the easy and hard axes magnetizations (M{sub ||} and M{sub ⊥}) measured at 10 K, viz., M{sub ||}/M{sub ⊥} (10 K) = 2.6, confirms the strongly anisotropic nature of the film. The easy axis ferromagnetic (FM) transition temperature (T{sub C}) is smaller than that along the hard axis. The giant hysteresis in FCC–FCW M–T, which manifests the magnetic liquid behaviour of the strongly phase-separated manganites is appreciably narrowed along the hard axis. The strain glass state is also less dominant along the easy axis. The analysis of the M–H data brings out that the easy axis remanence (M{sub r||}) shows a nonlinear temperature dependence, while the one corresponding to the hard axis (M{sub r⊥}) shows a nearly linear behaviour. The coercivity (H{sub C}) follows a law of the type H{sub C}(T)=H{sub C}(0)(1−AT{sup b}). The value of exponent b ∼ 0.5 for single-domain particles, but in the present case, the best fit to the experimental data yields the exponent b ≈ 0.25 for both H{sub C||} and H{sub C⊥}. The smaller value of the exponent is attributed to the non-canonical nature of the ferromagnetic state and the associated strongly phase-separated nature of the LPCMO thin film.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Macro- and Nanoscale Magnetic Anisotropy of FeNi(P) Micropillars in Polycarbonate Membrane

Macroscopic and local magnetic anisotropy of the FeNi(P) pillars deposited using electroless plating in the pores of the nuclear track-etched polycarbonate membrane has been studied. The alloy fills the pores and forms a nail-shaped pillar. The macroscopic easy magnetization axis was found to be perpendicular to the membrane plane due to magnetic shape anisotropy. The macroscopic magnetic anisotropy constant decreases with decreasing pillar diameter from 0.4 to 0.1 μm supposedly due to increase of the pillar cap contribution. Approach to magnetic saturation analysis indicates that the order in local easy magnetization axis is localized on a nanoscale. The correlation length of the local easy axis and the local magnetic anisotropy field in (Fe{sub 100−x}Ni{sub x}){sub 98}P{sub 2} pillars of different compositions with diameter of 0.4 μm are studied.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2019

Structural and Magnetic Properties Study of Fe{sub 50}Si{sub 50} Powders with Different Microparticle Sizes

The structural properties and magnetic behavior of the Fe{sub 50}Si{sub 50} system were determined using X-ray diffraction (XRD), Mössbauer spectrometry (MS), vibration sample magnetometry (VSM), and scanning electron microscopy (SEM). Samples of Fe{sub 50}Si{sub 50} were melted using an arc furnace, then powder was obtained through a diamond file, and this was passed through different sieves. Finally, nanoparticles of this system were produced by means of surfactant-assisted mechanical alloying. XRD confirmed the presence of the FeSi (CS) and Fe{sub 5}Si{sub 3} structural phases. By transmission MS, it was observed that at room temperature the spectrum is formed by three general components, a doublet associated with the FeSi (CS) phase and two sextets associated with the Fe{sub 5}Si{sub 3} phase; this gives the idea of the distribution of the sizes of particles present in the sample. By VSM, the soft magnetic behavior of the system was ratified and it was determined that the biggest saturation magnetization corresponds to the smallest particles.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2019

Study of Critical Behavior and Magnetocaloric Effect in Nd{sub 1− x} Sr{sub x}MnO{sub 3} Compounds

In this paper, we have investigated the influence of substitution of Nd by Sr on the structural, magnetic, and magnetocaloric properties of Nd{sub 1 − x}Sr{sub x}MnO{sub 3} (x = 0.3 and x = 0.4). These compounds were prepared using the solid state method. The structural, morphological, and magnetic properties of our system were characterized by XRD, MEB techniques, and a magnetometer for magnetic characterization. The Rietveld refinement has revealed the coexistence of both Pnma orthorhombic and R-3c rhombohedral phases. The magnetic data indicate that the compounds exhibit a continuous paramagnetic (PM) to ferromagnetic (FM) phase transition. In addition, the critical behavior in both compounds associated with the magnetic phase transition has been studied by the magnetization isotherms. The critical exponents are deduced using various techniques such as the modified Arrott plot, Kouvel–Fisher plot, and critical isotherm technique. Finally, the results obtained of the magnetocaloric effect are in agreement with the reported values of previous researches. These results show that our samples can be used as a magnet refrigerant at room temperature.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Influence of Solidification Velocity on Partitioning and Precipitate Evolution in a Multicomponent, Soft Magnetic Alloy Processed by Laser Melting

Multicomponent alloys have recently been shown to overcome the structural-functional barrier associated with soft magnetic materials. Here, single laser tracks have been used to investigate the effects of solidification velocity on the Al 7 Fe 32.6 Co 27.7 Ni 27.7 Ta 5 (at pct) alloy. Ta-rich cell wall partitioning was shown to affect the final distribution and size of the precipitating L1 2 phase, after aging. These findings demonstrate that solidification rate control offers the potential to tailor localized performance in alloys with size-dependent precipitation properties.

Brookins, Jeffrey M. [Oak Ridge National Laborator↗
DOE OSTI2017

Mechanism of Texture and Precipitates Evolution in CGO Silicon Steel During High-Temperature Annealing Process

Grain-oriented silicon steel is an important soft magnetic material for electrical applications. A conventional grain-oriented silicon steel with final thicknesses of 0.27, 0.23 and 0.20 mm was prepared separately by a two-stage cold rolling process. Texture evolution, precipitates distribution and Goss textures formed during secondary recrystallization were analyzed by electron backscattered diffraction and field emission scanning electron microscopy. The results show that plate thickness influences the distribution density of secondary phase particles and the content of favorable texture components such as {111} <112>, {111} <110> and {112} <110> texture. Decreasing plate thickness increases the areal density of secondary phase particles, the temperature of secondary recrystallization, which also increases the content of favorable texture component, and the strength of Goss texture in secondary recrystallization. Goss grains have higher particle density than the grains with other orientations, which facilitates their abnormal growth by swallowing surrounding grains with lower particle density.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Disorder-order transformation in Fe-50%Co fabricated via laser powder bed fusion

Fe-Co alloys are an important class of soft magnetic materials that often pose challenges in their fabrication because of the brittle B2-ordered phase. We show that laser beam powder bed fusion (PBF-LB), owing to its rapid cooling rates, offers an avenue for the fabrication of these alloys by suppressing the disorder →order phase transformation at room temperature. We use neutron diffraction to understand the phase transformations in a Fe-50%Co alloy fabricated via PBF-LB. We report that the disorder→order phase transformation in this alloy occurs concurrently via homogeneous ordering and classical nucleation and growth.

36 MATERIALS SCIENCE↗
DOE OSTI2023

A high-performance implementation of atomistic spin dynamics simulations on x86 CPUs

Atomistic spin dynamics simulations provide valuable information about the energy spectrum of magnetic materials in different phases, allowing one to identify instabilities and the nature of their excitations. However, the time cost of evaluating the dynamical correlation function S(q,t) increases quadratically as the number of spins N, leading to significant computational effort, making the simulation of large spin systems very challenging. In this work, here, we propose to use a highly optimized general matrix multiply (GEMM) subroutine to calculate the dynamical spin-spin correlation function that can achieve near-optimal hardware utilization. Furthermore, we fuse the element-wise operations in the calculation of S(q,t) into the in-house GEMM kernel, which results in further performance improvements of 44\% - 71\% on several relatively large lattice sizes when compared to the implementation that uses the GEMM subroutine in OpenBLAS, which is the state-of-the-art open source library for Basic Linear Algebra Subroutine (BLAS).

97 MATHEMATICS AND COMPUTING↗
DOE OSTI2025

Record acceleration of the two-dimensional Ising model using a high-performance wafer-scale engine

The versatility and wide-ranging applicability of the Ising model, originally introduced to study phase transitions in magnetic materials, have made it a cornerstone in statistical physics and a valuable tool for evaluating the performance of emerging computer hardware. Here, we present a novel implementation of the two-dimensional Ising model on Cerebras Wafer-Scale Engine (WSE) – a revolutionary processor that is opening new frontiers in computing. In our deployment of the checkerboard algorithm, we optimized the Ising model to take advantage of the unique WSE architecture. Specifically, we employed a compressed bit representation storing 16 spins on each int16 word, and efficiently distributed the spins over the processing units enabling seamless weak scaling and limiting communications to only immediate neighboring units. Our implementation can handle up to 754 simulations in parallel, achieving an aggregate of over 61.8 trillion flip attempts per second for Ising models with up to 200 million spins. This represents a gain of up to 148 times over previously reported single-devices with a highly optimized implementation on NVIDIA V100 and up to 88 times in productivity compared to NVIDIA H100. Our findings highlight the significant potential of the WSE in scientific computing, particularly in the field of materials modeling.

Ising model↗
DOE OSTI2019

An overview of the first principles studies of doped RE-TM5 systems for the development of hard magnetic properties

In this study, we compiled all the first principles calculations performed on RE-TM 5 (RE: rare earth, TM: transition metal) system to address the following question: Has everything been tried to improve the hard magnetic properties of this special intermetallic compound, and are there any venues that are worth pursuing? As it is the case with the first principles studies in any field, they are driven mainly by experimental work: (i) to validate their results, and (ii) to extend our understanding of the underlying physical phenomena so that the theory in hand can be used as a predictive tool to shed light on certain what-if scenarios. Our analysis indicates that three major compounds are investigated more than others: (1) YCo 5 , because it can potentially be an intermediate-performance hard magnet, and the presence of yttrium with no localized 4f electrons simplifies calculations significantly, (2) SmCo 5 , because it has the largest magnetic anisotropy energy among all RE hard magnets as well as very impressive high temperature performance, (3) CeCo 5 , because Ce is the most abundant rare earth element with a curious 4f electron behavior that gives rise to pronounced deviations in its structural and magnetic properties across the lanthanides series. This is followed by a brief analysis of several ab initio approaches that were developed to treat these rather complicated systems both at 0 K and at finite temperatures. Towards the end, we elucidate the role of exchange-correlation approximations such as local density approximations (LDA) and generalized gradient approximation (GGA) formulation in determining the MAE and M s by analyzing their corresponding density of states (DOS), and providing our results on a rather overlooked hard-magnetic material: LaCo 5 .

36 MATERIALS SCIENCE↗