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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 1,225 records · Page 68
All sourcesNASA NTRSDOE OSTI
DOE OSTI2019

Topological Magnetic-Spin Textures in Two-Dimensional van der Waals Cr 2 Ge 2 Te 6

Two-dimensional (2D) van der Waals (vdW) materials show a range of profound physical properties that can be tailored through their incorporation in heterostructures and manipulated with external forces. The recent discovery of long-range ferromagnetic order down to atomic layers provides an additional degree of freedom in engineering 2D materials and their heterostructure devices for spintronics, valleytronics, and magnetic tunnel junction switches. In this work, using direct imaging by cryo-Lorentz transmission electron microscopy we show that topologically nontrivial magnetic-spin states, skyrmionic bubbles, can be realized in exfoliated insulating 2D vdW Cr 2 Ge 2 Te 6 . Due to the competition between dipolar interactions and uniaxial magnetic anisotropy, hexagonally packed nanoscale bubble lattices emerge by field cooling with magnetic field applied along the out-of-plane direction. Despite a range of topological spin textures in stripe domains arising due to pair formation and annihilation of Bloch lines, bubble lattices with single chirality are prevalent. In conclusion, our observation of topologically nontrivial homochiral skyrmionic bubbles in exfoliated vdW materials provides a new avenue for novel quantum states in atomically thin insulators for magneto-electronic and quantum devices.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Analysis of ultrafast magnetization switching dynamics in exchange-coupled ferromagnet–ferrimagnet heterostructures

Magnetization switching in ferromagnets has so far been limited to the current-induced spin–orbit–torque effects. Recent observation of helicity-independent all-optical magnetization switching (HI-AOS) in an exchange-coupled ferromagnet–ferrimagnet (FM-FEM) heterostructures expanded the range and applicability of such ultrafast heat-driven magnetization switching. Here we report the element-resolved HI-AOS dynamics of such an exchange-coupled system, using a modified microscopic three-temperature model. We have studied the effect of (i) the Curie temperature of the FM, (ii) FEM composition, (iii) the long-range Ruderman–Kittel–Kasuya–Yosida (RKKY) exchange-coupling strength, and (iv) the absorbed optical energy on the element-specific time-resolved magnetization dynamics. The phase-space of magnetization illustrates how the RKKY coupling strength and the absorbed optical energy influence the switching time. Our analysis demonstrates that the threshold switching energy depends on the composition of the FEM and the switching time depends on the Curie temperature of the FM as well as RKKY coupling strength. Importantly, this simulation anticipates new insights into developing faster and more energy-efficient spintronics devices.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Enhancing ferromagnetic resonance absorption for very thin insulating magnetic films with spin plasmonics

We consider enhancing the ferromagnetic resonance (FMR) absorption of very thin insulating magnetic films by placing it on top of a dielectric. We find that the signal is enhanced by at least an order of magnitude due to a new nonreciprocal interface resonance that is a mixture of the magnetic surface plasmon mode and a wave guide mode. This resonance occurs over a wide range of thicknesses of the dielectric that is still much less than the wavelength and is made possible by the negative magnetic susceptibility of the magnetic layer. The line width of absorption is reduced by an order of magnitude less than the Gilbert damping parameter. At some frequency, the group velocity of this resonance is negative. Experimentally, very thin yttrium iron garnet (YIG) films are grown on a Gadolinium Gallium Garnet (GGG) substrate which can be considered the dielectric. Our model applies to experiments performed in the YIG/GGG system. Indeed, our picture resolves the disagreement on the magnitude of the spin diffusion lengths obtained with the FMR and the Brillouin scattering techniques. It also provides for a way to make new adaptive thin film miniaturized photonic nonreciprocal devices with low loss.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

RKKY Exchange Bias Mediated Ultrafast All-Optical Switching of a Ferromagnet

The discovery of ultrafast helicity-independent all-optical switching (HI-AOS), as well as picosecond all-electrical switching of a ferrimagnet, has inspired the ultrafast spintronics community to explore ultrafast switching of a ferromagnet to achieve practical ultrafast storage and memory devices. Two explored mechanisms of HI-AOS of a ferromagnet in ferromagnet-ferrimagnet heterostructure are: a) exploiting the indirect exchange coupling with and b) injection of non-local spin current originated from a switching ferrimagnet. Here, in this manuscript, exchange mediated HI-AOS of a Ruderman–Kittel–Kasuya–Yosida (RKKY) exchange coupled “[Co/Pt]-multilayers/Pt spacer/CoGd” heterostructure is demonstrated. The authors have measured layer-resolved static magnetic properties, single-shot HI-AOS, and magnetization dynamics of the ferromagnetic Co/Pt multilayers (MLs), that are ferromagnetically or antiferromagnetically coupled with ferrimagnetic CoGd layers. Time-resolved magnetization dynamics reveal a 3.5 ps switching time of the Co/Pt MLs, which is the fastest switching of a ferromagnet reported to date. Employing an extended microscopic three-temperature model, the temporal dynamics of the exchange coupled ferromagnet–ferrimagnet heterostructure are simulated, qualitatively and quantitatively explaining the experimental switching phenomena. This work experimentally as well as theoretically establishes the mechanism of exchange mediated all-optical switching of ferromagnet-ferrimagnet heterostructures, which can be integrated with a magnetic tunnel junction for efficient reading after ultrafast energy-efficient switching.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Ferromagnetism of Fe 3 Sn and alloys

Hexagonal Fe 3 Sn has many of the desirable properties for a new permanent magnet phase with a Curie temperature of 725 K, a saturation moment of 1.18 MA/m. and anisotropy energy, K 1 of 1.8 MJ/m 3 . However, contrary to earlier experimental reports, we found both experimentally and theoretically that the easy magnetic axis lies in the hexagonal plane, which is undesirable for a permanent magnet material. One possibility for changing the easy axis direction is through alloying. We used first principles calculations to investigate the effect of elemental substitutions. The calculations showed that substitution on the Sn site has the potential to switch the easy axis direction. Transition metal substitutions with Co or Mn do not have this effect. We attempted synthesis of a number of these alloys and found results in accord with the theoretical predictions for those that were formed. However, the alloys that could be readily made all showed an in-plane easy axis. The electronic structure of Fe 3 Sn is reported, as are some are magnetic and structural properties for the Fe 3 Sn 2 , and Fe 5 Sn 3 compounds, which could be prepared as mm-sized single crystals.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Study of structural and ferromagnetic resonance properties of spinel lithium ferrite (LiFe{sub 5}O{sub 8}) single crystals

The effect of B-site cation ordering on the room temperature structural and ferromagnetic resonance (FMR) properties of single crystal spinel lithium ferrite (LiFe{sub 5}O{sub 8}, LFO) have been investigated. A detailed microstructural analysis is done through X-ray diffraction, polarized Raman spectroscopy, and transmission electron microscopy (TEM) to examine the effect of post-annealing on the B-site cation ordering. The X-ray diffraction pattern of the as-grown crystal indicates a disordered state of the crystal. However, the annealed sample shows additional superlattice reflections corresponding to the ordered phase. This ordering is further confirmed by Raman spectra and TEM images, which reveal ordering of Li and Fe ions at the octahedral sites contrasting with the relatively high degree of octahedral site disorder in the as-grown crystal. To study the effect of B-site ordering on the magnetic properties and FMR linewidth, vibrating sample magnetometry and broadband FMR measurements have been performed for both the ordered and disordered phases of lithium ferrite. The value of saturation magnetization for both phases is ∼290 emu/cm{sup 3}. A single mode FMR profile is observed for both phases with little distortion. The linewidth characteristics of the ordered and disordered phases of lithium ferrite phases are compared, and it is observed that the linewidth is independent of the cation ordering. Both the phases exhibit a low linewidth (∼26 Oe at 30 GHz) and the effective damping parameter for the as-grown and annealed samples is determined to be 0.0021 ± 0.0001.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Exchange bias effect in Ti doped nanocrystalline SrFeO{sub 3-δ}

Materials of Ti doped nanocrystalline SrFeO{sub 3-δ} were synthesized through solid state reaction. Detailed magnetization measurements were carried out in zero field cooled (ZFC) and field cooled (FC) conditions. Compounds of SrFe{sub 1-x}Ti{sub x}O{sub 3-δ} (x = 0.1 to 0.3) are found to be spin glass and parent compound is a helical antiferromagnet. Non magnetic Ti{sup 4+} reduces the strength of exchange interactions and the curvature of hysteresis is changed towards concave nature. Exchange bias is observed below the peak temperature (irreversibility in magnetization (T{sub Irr})) in ZFC-FC of SrFe{sub 1-x}Ti{sub x}O{sub 3-δ} (x = 0 to 0.3). The coercivity and exchange bias field values are found to be decreases with increase in temperature. Observed exchange bias effect is attributed to competition between antiferromagnetic superexchange and ferromagnetic double exchange interactions.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Absence of a Dirac gap in ferromagnetic Cr x (Bi 0.1 Sb 0.9 ) 2-x Te 3

Magnetism breaks the time-reversal symmetry expected to open a Dirac gap in 3D topological insulators that consequently leads to the quantum anomalous Hall effect. The most common approach of inducing a ferromagnetic state is by doping magnetic 3d elements into the bulk of 3D topological insulators. In Cr 0.15 (Bi 0.1 Sb 0.9 ) 1.85 Te 3 , the material where the quantum anomalous Hall effect was initially discovered at temperatures much lower than the ferromagnetic transition, T C , the scanning tunneling microscopy studies have reported a large Dirac gap of ~20–100 meV. The discrepancy between the low temperature of quantum anomalous Hall effect (< C ) and large spectroscopic Dirac gaps (>>T C ) found in magnetic topological insulators remains puzzling. Here, we used angle-resolved photoemission spectroscopy to study the surface electronic structure of the pristine and potassium doped surface of Cr 0.15 (Bi 0.1 Sb 0.9 ) 1.85 Te 3 . Upon potassium deposition, the p-type surface state of the pristine sample was turned into an n-type, allowing the spectroscopic observation of Dirac point. Finally, we find a gapless surface state, with no evidence of a large Dirac gap reported in tunneling studies.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Ultrafast optically induced spin transfer in ferromagnetic alloys

The vision of using light to manipulate electronic and spin excitations in materials on their fundamental time and length scales requires new approaches in experiment and theory to observe and understand these excitations. The ultimate speed limit for all-optical manipulation requires control schemes for which the electronic or magnetic subsystems of the materials are coherently manipulated on the time scale of the laser excitation pulse. In our work, we provide experimental evidence of such a direct, ultrafast, and coherent spin transfer between two magnetic subsystems of an alloy of Fe and Ni. Our experimental findings are fully supported by time-dependent density functional theory simulations and, hence, suggest the possibility of coherently controlling spin dynamics on subfemtosecond time scales, i.e., the birth of the research area of attomagnetism.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Identifying the sources of ferromagnetism in sol-gel synthesized Zn{sub 1−x}Co{sub x}O (0≤x≤0.10) nanoparticles

We have carefully investigated the structural, optical and electronic properties and related them with changes in the magnetism of sol-gel synthesized Zn{sub 1−x}Co{sub x}O (0≤x≤0.10) nanoparticles. Samples with x≤0.05 were free of spurious phases. Samples with x≤0.03 were found to be with only high spin Co{sup 2+} ions into ZnO structure, whereas sample with x=0.05, exhibited the presence of high spin Co{sup 2+} and low spin Co{sup 3+}. We found that the intensity of the main EPR peak associated with Co{sup 2+} varies with the nominal Co content in a similar manner as the saturation magnetization and coercive field do. These results point out that the ferromagnetism in these samples should directly be correlated with the presence of divalent cobalt ions. Bound magnetic polaron (BMP) model and the charge transfer model are insufficient to explain the ferromagnetic properties of Zn{sub 1−x}Co{sub x}O nanoparticles. The room temperature ferromagnetism (RTFM) may be originated from a combination of several factors such as the interaction of high spin Co{sup 2+} ions, perturbation/alteration and/or changes in the electronic structure of ZnO close to the valence band edge and grain boundary effects. - Graphical abstract: The intensity of the main EPR peak associated with Co{sup 2+} varies with the nominal Co content in a similar manner as the saturation magnetization and coercive field do. These results point out that the ferromagnetism in these samples should directly be correlated with the presence of Co{sup 2+} ions. Display Omitted - Highlights: • Systematic and carefully study of physical-chemical properties of Zn{sub 1−x}Co{sub x}O nanoparticles. • Samples with x=0.01 and 0.03 were found to be with only high spin Co{sup 2+}. • Sample with x=0.05, exhibited the presence of high spin Co{sup 2+} and low spin Co{sup 3+}. • The BMP and charge transfer models seem not explain the ferromagnetic properties. • RTFM: high spin Co{sup 2+} ions, defects close to the valence band and grain boundary effects.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Effects of composition and crystallographic ordering on the ferromagnetic transition in Ni–Co–Mn–In magnetic shape memory alloys

In the present study, the effects of composition and the level of crystallographic ordering were studied in Ni 45 Co 5 Mn 50-X In X magnetic shape memory alloys with X = 13.55, 13.71 and 13.92, in order to reveal the relationship between the Curie temperature (T C ) of the austenite phase and the degree of L2 1 ordering. Various samples were prepared using secondary heat treatments at different temperatures below the order-disorder transition temperature and for different durations, after solution heat treatment, in order to control the degree of ordering. T C s were determined using calorimetric and magnetometry measurements. The relationship between the T C and the degree of order was used to predict the evolution of the order during selected heat treatments. It was shown that the T C of austenite exhibits a monotonous non-linear dependence on the fraction of the L2 1 phase. This relationship was corroborated via ab initio numerical calculations within a mean field approximation. Dependence of the crystallographic order parameter on temperature and duration of secondary heat treatments showed that ordering kinetics is fast, the total degree of order depends only on the annealing temperature and cooling rate for long enough treatments, and maximum achievable degree of order is limited by the low thermal energy available for atomic diffusion at low temperatures. Lastly, small changes in composition were observed to have a significant impact on martensitic transformation temperatures, and a significant but smaller effect on the ferromagnetic transition in these systems.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Twist Engineering of Anisotropic Excitonic and Optical Properties of a Two-Dimensional Magnetic Semiconductor

Two-dimensional (2D) van der Waals (vdW) magnetic semiconductors are a new class of quantum materials for studying the emergent physics of excitons and spins in the 2D limit. Twist engineering provides a powerful tool to manipulate the fundamental properties of 2D vdW materials. Here, in this work, we show that twist engineering of the anisotropic ferromagnetic monolayer semiconductor CrSBr leads to bilayer magnetic semiconductors with continuously tunable magnetic moment, dielectric anisotropy, exciton energy, and linear dichroism. We furthermore provide a model for exciton energy in the media with tunable anisotropy. These results advance fundamental studies of 2D vdW materials and open doors to applications to nano-optics, twistronics, and spintronics.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Lithiation bridged molecular conducting magnets

Combining ferromagnetism and metallic conductivity has been extensively studied in contemporary materials science. However, building such multifunctionalities in molecular chemistry is particularly challenging, on which two collective properties are long considered as mutually exclusive. Here, we report molecular conducting magnet consisting of two-dimensional graphene and molecular planar magnetic networks bridged by electrochemical lithiation, exhibiting the co-existence of distinct magnetism and conductivity and magneto-electric coupling effects. Here, the room temperature magnetoresistance and magnetoelectric coupling coefficient of such complex solid reach 10.7% and 1.77 V/Oe cm, respectively, which can be further tailored by modulating the interaction between graphene and molecular magnetic network. In addition, the graphene passivation simultaneously introduces environmental stability of molecular mag-nets. The electrochemical lithiation offers a pathway towards coupling magnetism and conductivity in molecular complexes.

2D organic complexes↗
DOE OSTI2014

Magnetization switching and inverted hysteresis in perpendicular antiferromagnetic superlattices

The magnetization of antiferromagnetic (AFM) superlattices as a function of applied field was investigated using Monte Carlo simulations. The simulated hysteresis loops of systems with N magnetic layers with AFM coupling between the layers exhibit distinct steps with magnetization that decreases with increasing N . Systems with odd N exhibit 3 steps and inverted hysteresis for N > 3, whereas systems with even N exhibit 4 steps, for N > 2, and their microscopic switching sequence is non-deterministic and can take two distinct pathways, even though the switching of the global magnetization is exactly reversible. Here, magnetic superlattice structures are attractive for the development of meta-materials with unique properties. In antiferromagnetic (AFM) multilayer structures, where ferromagnetic (FM) layers are coupled antiferromagnetically and the strength of the inter-layer interaction is on the order of the Zeeman energy, meta-magnetic transitions can be induced by moderate magnetic fields. In such systems, the properties can be easily tuned by modifying the inter-layer interactions.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Defect engineered d{sup 0} ferromagnetism in tin-doped indium oxide nanostructures and nanocrystalline thin-films

Origin of unexpected defect engineered room-temperature ferromagnetism observed in tin-doped indium oxide (ITO) nanostructures (Nanowires, Nano-combs) and nanocrystalline thin films fabricated by pulsed laser deposition has been investigated. It is found that the ITO nanostructures prepared under argon environment exhibit strongest ferromagnetic signature as compared to that nanocrystalline thin films grown at oxygen. The evidence of singly ionized oxygen vacancy (V{sub 0}{sup +}) defects, obtained from various spectroscopic measurements, suggests that such V{sub 0}{sup +} defects are mainly responsible for the intrinsic ferromagnetic ordering. The exchange interaction of the defects provides extensive opportunity to tune the room-temperature d{sup 0} ferromagnetism and optical properties of ITOs.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Chemical disorder determines the deviation of the Slater–Pauling rule for Fe 2 MnSi-based Heusler alloys: evidences from neutron diffraction and density functional theory

Fe 2 MnSi fails to follow the Slater–Pauling rule. This phenomenon is thought to originate from either: (i) an antiferromagnetic arrangement of Mn ions at low temperature and/or (ii) chemical disorder. An important insight on this issue could be achieved by considering Fe 2 MnSi 1-x Ga x compounds, thoroughly studied here by means of magnetization, neutron diffraction and density functional calculations (DFT). Our results indicate that chemical disorder (and not the antiferromagnetic arrangement) is responsible for the deviation of the Slater–Pauling rule on Fe 2 MnSi-based Heusler alloys. Furthermore, evidences suggest that Ga substitution into Si site favors the Fe/Mn disorder, further enhancing the observed deviation.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Effect of Mn doping and charge transfer on LaTi 1- x Mn x O 3

Here, we report the magnetic and electronic transport properties of Mn-doped LaTi 1-x Mn x O 3 (x= 0, 0.1, 0.3, 0.5) as a function of temperature and an applied magnetic field. It was found that the Mn-doped samples show a magnetic transition which is not present in the parent LaTiO 3 . The Mn-doped samples showed fluctuations in magnetization at low fields below their Néel transition temperature indicating electronic phase separation in the material. Increased Mn content in the sample strengthens the ferromagnetic-like moment while maintaining G-type antiferromagnetic phase by charge transfer from Mn to Ti and influencing orbital ordering of the Ti 3+ t 2g orbitals. The results are discussed in parallel with transport and bulk magnetization measurements detailing the electronic behavior. An additional context for the mechanism is supported by first-principles density-function theory calculations.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Propagation and dispersion of shock waves in magnetoelastic materials

Previous studies examining the response of magnetoelastic materials to shock waves have predominantly focused on applications involving pulsed power generation, with limited attention given to the actual wave propagation characteristics. This study provides detailed magnetic and mechanical measurements of magnetoelastic shock wave propagation and dispersion. Laser generated rarefacted shock waves exceeding 3 GPa with rise times of 10 ns were introduced to samples of the magnetoelastic material Galfenol. The resulting mechanical measurements reveal the evolution of the shock into a compressive acoustic front with lateral release waves. Importantly, the wave continues to disperse even after it has decayed into an acoustic wave, due in large part to magnetoelastic coupling. The magnetic data reveal predominantly shear wave mediated magnetoelastic coupling, and were also used to noninvasively measure the wave speed. The external magnetic field controlled a 30% increase in wave propagation speed, attributed to a 70% increase in average stiffness. Lastly, magnetic signals propagating along the sample over 20× faster than the mechanical wave were measured, indicating these materials can act as passive antennas that transmit information in response to mechanical stimuli.

36 MATERIALS SCIENCE↗