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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 145 records · Page 8
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Emerged Metallicity in Molecular Ferromagnetic Wires

Supramolecular engineering bridges molecular assembly with macromolecular charge-transfer salts, promising the design to construct supramolecular architectures that integrate cooperative properties difficult or impossible to find in conventional lattices. Here, we report the crystal engineering design and kinetic growth of one-dimensional supramolecular wires composed of bis(ethylenedithio)tetrathiafulvalene (ET + ) cation and polymeric Cu[N(CN) 2 ] 2 – anion. A bulk ferromagnetic order is discovered for filling up the gap where strong ferromagnetism is missing in such ET molecule-based charge-transfer salts. Metallicity is induced by electric current from the semiconducting wire, which is attributed to strain effect by tuning its close molecular contact. This structural feature is evidenced through the combination of various mechanistic spectroscopic studies. Further, electric dipole is established from the close molecular contacts and is suggestive to stabilize ferromagnetic spin interaction through anions bridging spin sites. The breakthrough shown here provides a pathway to explore low-dimensional supramolecular materials exhibiting strong electron correlation, metallicity, and ferromagnetism.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2022

Tunable Orbital Ferromagnetism at Noninteger Filling of a Moiré Superlattice

Here, the flat bands resulting from moiré superlattices exhibit fascinating correlated electron phenomena such as correlated insulators, ( Nature 2018, 556 (7699), 80–84), ( Nature Physics 2019, 15 (3), 237) superconductivity, ( Nature 2018, 556 (7699), 43–50), ( Nature 2019, 572 (7768), 215–219) and orbital magnetism. ( Science 2019, 365 (6453), 605–608), ( Nature 2020, 579 (7797), 56–61), ( Science 2020, 367 (6480), 900–903) Such magnetism has been observed only at particular integer multiples of n 0 , the density corresponding to one electron per moiré superlattice unit cell. Here, we report the experimental observation of ferromagnetism at noninteger filling (NIF) of a flat Chern band in a ABC-TLG/hBN moiré superlattice. This state exhibits prominent ferromagnetic hysteresis behavior with large anomalous Hall resistivity in a broad region of densities centered in the valence miniband at n = -2.3n 0 . We observe that, not only the magnitude of the anomalous Hall signal, but also the sign of the hysteretic ferromagnetic response can be modulated by tuning the carrier density and displacement field. Rotating the sample in a fixed magnetic field demonstrates that the ferromagnetism is highly anisotropic and likely purely orbital in character.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2026

Confined Room-Temperature Ferromagnetism in Kagome (Fe 3 Sn 2 /CoSn) Superlattices

We reveal the presence of robust ferromagnetism in topological kagome (Fe 3 Sn 2 /CoSn) ×n superlattices via polarized neutron reflectivity (PNR) and magnetization measurements. Using molecular beam epitaxy, we have synthesized superlattices of alternating ferromagnetic Fe 3 Sn 2 and paramagnetic CoSn layers, the interfacial integrity of which is confirmed using scanning transmission electron microscopy. Magnetization depth profiles, obtained by fitting the PNR data, confirm the presence of ferromagnetism in the confined Fe 3 Sn 2 layers at room temperature, with minimal thickness dependence down to 2 nm. A net magnetization is found in the near-surface region of CoSn, which we attribute to surface oxidation. Our results demonstrate that superlattice engineering is a means to confine ferromagnetism in kagome metals, enabling future studies of thickness-dependent anomalous Hall and Nernst effects as well as spin-dependent transport and tunneling in device structures.

kagome metals↗
DOE OSTI2021

Room-temperature intrinsic ferromagnetism in epitaxial CrTe 2 ultrathin films

While the discovery of two-dimensional (2D) magnets opens the door for fundamental physics and next-generation spintronics, it is technically challenging to achieve the room-temperature ferromagnetic (FM) order in a way compatible with potential device applications. Here in this paper, we report the growth and properties of single- and few-layer CrTe 2 , a van der Waals (vdW) material, on bilayer graphene by molecular beam epitaxy (MBE). Intrinsic ferromagnetism with a Curie temperature (TC) up to 300 K, an atomic magnetic moment of ~0.21 μ B /Cr and perpendicular magnetic anisotropy (PMA) constant (K u ) of 4.89 × 10 5 erg/cm 3 at room temperature in these few-monolayer films have been unambiguously evidenced by superconducting quantum interference device and X-ray magnetic circular dichroism. This intrinsic ferromagnetism has also been identified by the splitting of majority and minority band dispersions with ~0.2 eV at Г point using angle-resolved photoemission spectroscopy. The FM order is preserved with the film thickness down to a monolayer (T C ~ 200 K), benefiting from the strong PMA and weak interlayer coupling. The successful MBE growth of 2D FM CrTe 2 films with room-temperature ferromagnetism opens a new avenue for developing large-scale 2D magnet-based spintronics devices.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2020

A magnetic sensor using a 2D van der Waals ferromagnetic material

Two-dimensional (2D) van der Waals ferromagnetic materials are emerging as promising candidates for applications in ultra-compact spintronic nanodevices, nanosensors, and information storage. Our recent discovery of the strong room temperature ferromagnetism in single layers of VSe2 grown on graphite or MoS2 substrate has opened new opportunities to explore these ultrathin magnets for such applications. In this paper, we present a new type of magnetic sensor that utilizes the single layer VSe2 film as a highly sensitive magnetic core. The sensor relies in changes in resonance frequency of the LC circuit composed of a soft ferromagnetic microwire coil that contains the ferromagnetic VSe2 film subject to applied DC magnetic fields. We define sensitivity as the slope of the characteristic curve of our sensor, df0/dH, where f0 is the resonance frequency and H is the external magnetic field. The sensitivity of the sensor reaches a large value of 16×106Hz/Oe, making it a potential candidate for a wide range of magnetic sensing applications.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Non-Volatile Ferroelectric Switching of Ferromagnetic Resonance in NiFe/PLZT Multiferroic Thin Film Heterostructures

Magnetoelectric effect, arising from the interfacial coupling between magnetic and electrical order parameters, has recently emerged as a robust means to electrically manipulate the magnetic properties in multiferroic heterostructures. Challenge remains as finding an energy efficient way to modify the distinct magnetic states in a reliable, reversible and non-volatile manner. Here we report ferroelectric switching of ferromagnetic resonance in multiferroic bilayers consisting of ultrathin ferromagnetic NiFe and ferroelectric Pb 0.92 La 0.08 Zr 0.52 Ti 0.48 O 3 (PLZT) films, where the magnetic anisotropy of NiFe can be electrically modified by low voltages. Ferromagnetic resonance measurements confirm that the interfacial charge-mediated magnetoelectric effect is dominant in NiFe/PLZT heterostructures. Non-volatile modification of ferromagnetic resonance field is demonstrated by applying voltage pulses. The ferroelectric switching of magnetic anisotropy exhibits extensive applications in energy-efficient electronic devices such as magnetoelectric random access memories, magnetic field sensors and tunable radio frequency (RF)/microwave devices.

42 ENGINEERING↗
DOE OSTI2019

Strongly Enhanced Gilbert Damping in 3 d Transition-Metal Ferromagnet Monolayers in Contact with the Topological Insulator $\mathrm{Bi_2Se_3}$

Engineering Gilbert damping of ferromagnetic metal films is of great importance to exploit and design spintronic devices that are operated with an ultrahigh speed. Based on the scattering theory of Gilbert damping, we extend the torque method originally used in studies of magnetocrystalline anisotropy to theoretically determine Gilbert dampings of ferromagnetic metals. This method is utilized to investigate Gilbert dampings of 3d transition-metal ferromagnet iron, cobalt, and nickel monolayers that are contacted by the prototypical topological insulator Bi 2 Se 3 . Amazingly, we find that their Gilbert dampings are strongly enhanced by about one order of magnitude compared with dampings of their bulk forms and free-standing monolayers due to the strong spin-orbit coupling of $\mathrm{Bi_2Se_3}$. Finally, our work provides an attractive route for tailoring Gilbert damping of ferromagnetic metallic films by putting them in contact with topological insulators.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Magnetic properties of the itinerant ferromagnet LaCrGe 3 under pressure studied by La 139 NMR

139 La nuclear magnetic resonance (NMR) measurements under pressure (p = 0 – 2.64 GPa) have been carried out to investigate the static and dynamic magnetic properties of the itinerant ferromagnet LaCrGe 3 . 139 La -NMR spectra for all measured pressures in the ferromagnetically ordered state show a large shift due to the internal field induction |B int | ~ 4 T at the La site produced by Cr ordered moments. The change in B int by less than 5% with p up to 2.64 GPa indicates that the Cr 3d moments are robust under pressure. The temperature dependence of NMR shift and B int suggest that the ferromagnetic order develops below ~50 K under higher pressures in a magnetic field of ~7.2 T. Based on the analysis of NMR data using the self-consistent-renormalization (SCR) theory, the spin fluctuations in the paramagnetic state well above T C are revealed to be three-dimensional ferromagnetic throughout the measured p region.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Weak ferromagnetism and glassy state in κ - ( BEDT - TTF ) 2 Hg ( SCN ) 2 Br

SInce the first observation of weak ferromagnetism in the charge-transfer salt kappa-(BEDT-TTF)(2)Cu[N(CN)(2)]Cl [U. Welp et al., Phys. Rev. Lett. 69, 840 (1992)], no further evidence of a ferromagnetic state in this class of organic materials has been reported. Here, static and dynamic spin susceptibility measurements on kappa-(BEDT-TTF)(2)Hg(SCN)(2)Br exhibit weak ferromagnetism below 20 K on the geometrically frustrated background. Our experimental results suggest that frustrated spins in the molecular dimers suppress long-range antiferromagnetic order, forming a spin-glass-type ground state of the triangular lattice in the insulating phase which locally contains ferromagnetic polarons. Moreover, specific-heat data reveal an excess peak located around 5 K, indicating the glassy nature of the electrons as well.

Hemmida, M.↗
DOE OSTI2019

Insights into the evolution from ferromagnetism to antiferromagnetism: A doping-dependent study of NaCrSi x Ge 2 - x O 6 ( 0 ≤ x ≤ 2 )

NaCrGe 2 O 6 and NaCrSi 2 O 6 are isostructural compounds exhibiting different magnetic ground states. NaCrGe2O6 adopts a ferromagnetic ground state with Tc=6K, whereas NaCrSi 2 O 6 orders antiferromagnetically below T N =3.4K. Although it has been proposed that the intriguing magnetic behavior in Cr-based pyroxenes involves competition between antiferromagnetic direct exchange and ferromagnetic superexchange interactions, a delicate balance that is sensitive to Cr-Cr distance and local distortion, no spectroscopy study has been done to determine the microscopic interactions in these compounds. To delve deeper into the evolution from ferromagnetism to antiferromagnetism, we performed a doping-dependent study to investigate how the substitution of Ge by Si affects the magnetic properties of NaCrSi x Ge 2-x O 6 (x=0, 0.5, 1, 1.5, 2). Neutron diffraction and magnetization measurements show that replacing larger Ge with smaller Si simultaneously suppresses the ferromagnetic order. The lattice constants and the unit-cell volume contract, i.e., chemical pressure effect, and the Cr-Cr distance within the chain gradually decreases with increasing Si doping. High-resolution inelastic neutron-scattering studies of the spin waves of NaCrGe 2 O 6 and NaCrSi 2 O 6 indicate that replacing Ge with Si has profound effect on the intrachain coupling, whereas it has negligible effect on the interchain couplings. We compare our results, which indicate NaCrGe 2 O 6 is magnetic quasi-one-dimensional (1D) and NaCrSi 2 O 6 is three-dimensional (3D), with LiCr(Si,Ge) 2 O 6 , where LiCrSi 2 O 6 is proposed to be magnetic quasi-1D and LiCrGe 2 O 6 is 3D, and discuss the different behaviors in magnetic dimensionality crossover in the context of how substituting Ge with Si fine-tunes the relative ratio between the intrachain and interchain couplings that defines the magnetic dimensionality in these materials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Band structure evolution during the ultrafast ferromagnetic-paramagnetic phase transition in cobalt

The evolution of the electronic band structure of the simple ferromagnets Fe, Co, and Ni during their well-known ferromagnetic-paramagnetic phase transition has been under debate for decades, with no clear and even contradicting experimental observations so far. Using time- and spin-resolved photoelectron spectroscopy, we can make a movie on how the electronic properties change in real time after excitation with an ultrashort laser pulse. This allows us to monitor large transient changes in the spin-resolved electronic band structure of cobalt for the first time. We show that the loss of magnetization is not only found around the Fermi level, where the states are affected by the laser excitation, but also reaches much deeper into the electronic bands. We find that the ferromagnetic-paramagnetic phase transition cannot be explained by a loss of the exchange splitting of the spin-polarized bands but instead shows rapid band mirroring after the excitation, which is a clear signature of extremely efficient ultrafast magnon generation. Our result helps to understand band structure formation in these seemingly simple ferromagnetic systems and gives first clear evidence of the transient processes relevant to femtosecond demagnetization.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Reconfigurable ferromagnetic liquid droplets

Solid ferromagnetic materials are rigid in shape and cannot be reconfigured. Ferrofluids, although reconfigurable, are paramagnetic at room temperature and lose their magnetization when the applied magnetic field is removed. Here, we show a reversible paramagnetic-to-ferromagnetic transformation of ferrofluid droplets by the jamming of a monolayer of magnetic nanoparticles assembled at the water-oil interface. These ferromagnetic liquid droplets exhibit a finite coercivity and remanent magnetization. They can be easily reconfigured into different shapes while preserving the magnetic properties of solid ferromagnets with classic north-south dipole interactions. Their translational and rotational motions can be actuated remotely and precisely by an external magnetic field, inspiring studies on active matter, energy-dissipative assemblies, and programmable liquid constructs.

36 MATERIALS SCIENCE↗
DOE OSTI2024

On the Emergence of Ferromagnetism in LaCoO 3 Ultrathin Films

It is well known that the properties of a crystal evolve as it increases in size from a single atomic plane to that of the bulk. Such size-dependent transitions can stem from many different origins and depend on minute changes to crystal bonding and composition. Here, a model example is that of LaCoO 3 , which is non-magnetic in the bulk but can display ferromagnetism at the nanoscale. Here, the evolution of structure-property relationships is studied in the LaCoO 3-δ /SrTiO 3 (001) system as the thickness of LaCoO 3-δ is increased from a single plane to 10 unit cells. In situ synchrotron X-ray studies are performed during and post-deposition to probe changes in the interactions between structure, stoichiometry, and magnetic behavior. Structural quantification indicates that the oxygen octahedral rotation pattern evolves with thickness, due to inherent differences in crystal symmetry between the film and substrate. The change in rotation modifies the required energy barrier for the spin state transition via the Co–O bond length and Co–O–Co bond angle, affecting the appearance of ferromagnetism. Our results highlight the contributions of high spin Co 2+ and/or high spin Co 3+ to respective weak and robust ferromagnetism and the evolution of properties with size in ultrathin LaCoO 3-δ heterostructures.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Single-Domain Ferromagnet of Noncentrosymmetric Uniaxial Magnetic Ions and Magnetoelectric Interaction

The feasibility of a single-domain ferromagnet based on uniaxial magnetic ions was examined. For a noncentrosymmetric uniaxial magnetic ion of magnetic moment μ at a site of local electric dipole moment p , it is unknown to date whether μ prefers to be parallel or antiparallel to μ . The nature of this magnetoelectric interaction was probed in terms of analogical reasoning based on the Rashba effect and density functional theory (DFT) calculations. Here, we show that μ and p prefer an antiparallel arrangement, predict that Fe-doped CaZnOS is a single-domain ferromagnet like a bar magnet, and find the probable cause for the ferromagnetism and weak magnetization hysteresis in Fe-doped hexagonal ZnO and ZnS at very low dopant concentrations.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

d{sup 0} Half-Metallic Ferromagnetism in CaN and CaAs Pnictides: an Ab Initio Study

Conventional magnetism occurs in systems which contain transition metals or rare earth ions with partially filled d or f shells. It is theoretically predicted that compounds of groups IA and IIA with IV and V, in some structural phases, are ferromagnetic half-metals which made them new candidates for spintronics applications. Employing density functional theory (DFT), we investigate magnetism in binary compounds CaN and CaAs. Regarding the structure of analogous magnetic materials and experimental results of CaAs synthesis, we have considered two cubic structures: rocksalt (RS) and zincblende (ZB), and four hexagonal structures: NiAs, wurtzite (WZ), anti-NiAs, and NaO. The calculated results show that CaN in cubic, NiAs, and wurtzite structures, and CaAs only in zincblende phase have ferromagnetic ground states with a magnetic moment of 1 μ{sub B}. Electronic structure analysis of these materials indicates that magnetism originates from anion p states. Existence of flat p bands and consequently high density of states at the Fermi level of magnetic structures gives rise to Stoner spin splitting and spontaneous ferromagnetism.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Determination of the Transition Temperature of a Weak Ferromagnetic Thin Film by Means of an Evolution of the Method Based on the Arrott Plots

The ferromagnetic transition temperature (T{sub c}) of a weak ferromagnetic thin film Cu{sub 0.38}Ni{sub 0.62} has been estimated by using an evolution of the Belov, Goriaga, and Arrott (BGA) model. In fact, the typically used measurements of the zero-field-cooled-field-cooled magnetic moment (m) as a function of temperature (T), the magnetic hysteresis loops (m(H)) as a function of field, and the temperature dependence of the remanent magnetic moment, performed on our sample, did not allow us to individuate precisely the value of T{sub c}, mainly due to the influence of thermal effects and noise on the weak ferromagnetic response of the sample, resulting in a large uncertainty in the estimation of T{sub c}. On the other hand, also the commonly used method of determining the T{sub c} by means of a linear extrapolation of the high-field region of m{sup 2} as a function of the normalized magnetic field (H/m) at different temperatures (Arrott curves), working though for our analyzed material, could not completely fulfill the basic assumptions of the BGA model in general. For these reasons, we have considered the analysis of the derivatives of the Arrott curves, starting from observing that the T{sub c} of the sample corresponds to the temperature where the curvature of the Arrott curves at low fields inverts. In this way, the sensitivity in the determination of the Curie point from the inductive measurements of the magnetic moment is improved while the correct assumptions of the Arrott model are also fully respected.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Ferromagnetism in Fe-doped BaTiO{sub 3} Ceramics

Polycrystalline samples of BaTi{sub 1−x}Fe{sub x}O{sub 3} (x = 0.00–0.30) are prepared by solid-state reaction method and their structural and magnetic properties are studied. Detailed investigation of XRD patterns reveal the coexistence of tetragonal (space group P4mm) and hexagonal phases (space group P6{sub 3}/mmc) for x ≥ 0.1. Magnetic measurements reveal room-temperature ferromagnetism in x = 0.15–0.3 samples, and their ferromagnetic transition temperature increases from 397 K for x = 0.15 to 464 K for x = 0.3. The initial magnetization curves for x = 0.15–0.3 are analyzed in terms of bound magnetic polaron (BMP) model. The analysis of susceptibility data in the paramagnetic region by Curie-Weiss law confirms the ferromagnetic transition for x ≥ 0.15 and the effective magnetic moment systematically increases with increase in Fe concentration.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Spin Wave Resonance Frequency in the Bilayer Ferromagnetic Film

The dependence of spin wave resonance (SWR) frequency on the surface anisotropy, the interlayer and interfacial coupling, the ferromagnetic layer thickness and the external magnetic field in the bilayer ferromagnetic film have been investigated by using the linear spin wave approximation and Green’s function technique. We find that the SWR frequency of the ferromagnetic bilayer films are shifted to higher values corresponding to those of above factors, respectively. The mode of the energetically highest can be controlled by the interfacial coupling, the thickness and the symmetry of the film. The SWR frequency of mode number m = 1 is almost flat except for applied magnetic field. Analytical details and the validity of the method are shown and discussed.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗