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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 253 records · Page 14
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Low-Temperature Magnetic Properties of Vanadium-Doped ZnO Nanoparticles

Low-temperature magnetic study of transition metal-doped ZnO nanoparticles tends to spring ambiguities of magnetic phases due to defect states, environment of doped ions, and anisotropy. Interestingly, vanadium doping in ZnO brings out versatile magnetic ordering which offers more room for basic understanding of the underlying mechanism. Vanadium-doped ZnO nanoparticles are prepared using a simple cost-effective approach of solution combustion method. Macroscopic and microscopic aspects of the samples have been unraveled using structural and morphological studies with the implications on the exhibited magnetic behavior. Development of antiferromagnetic, ferromagnetic, and ferrimagnetic interactions in the material illustrated by low-temperature magnetic measurements preludes the possible interplay of dopant-initiated defect correlation and morphological and interfacial secondary phases. The grain boundary reformation due to vanadium in the ZnO nanograins is exploited through a systematic correlation of structural and morphological studies in order to gain more insight into the portrayed magnetic signatures.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Influence of Ga-concentration on the electrical and magnetic properties of magnetoelectric CoGa x Fe 2–x O 4 /BaTiO 3 composite

Multiferroic materials exhibit magnetoelectric (ME) coupling and promise new device applications including magnetic sensors, generators, and filters. An effective method for developing ME materials with enhanced ME effect is achieved by the coupling through the interfacial strain between piezoelectric and magnetostrictive materials. In this study, the electrical and magnetic properties of Ga doped magnetoelectric CoGa x Fe 2–x O 4 /BaTiO 3 composite are studied systematically. It is found that Ga doping improves the sensitivity of magnetoelastic response and stabilizes the magnetic phase of the composites. More importantly, Ga doping reduces the electrical conductivity of composite, as well as the dielectric loss. An enhancement of the electrostrain with doping Ga is also observed. Quantitative estimation indicates that magnetoelectric coupling is enhanced for Ga-doped CoGa x Fe 2–x O 4 /BaTiO 3 composites. As a result, the present work is beneficial to the practical application of composite CoFe 2 O 4 /BaTiO 3 -based multiferroic materials.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Effect of heavy metal layer thickness on spin-orbit torque and current-induced switching in Hf|CoFeB|MgO structures

We study the heavy metal layer thickness dependence of the current-induced spin-orbit torque (SOT) in perpendicularly magnetized Hf|CoFeB|MgO multilayer structures. The damping-like (DL) current-induced SOT is determined by vector anomalous Hall effect measurements. A non-monotonic behavior in the DL-SOT is found as a function of the thickness of the heavy-metal layer. The sign of the DL-SOT changes with increasing the thickness of the Hf layer in the trilayer structure. As a result, in the current-driven magnetization switching, the preferred direction of switching for a given current direction changes when the Hf thickness is increased above ∼7 nm. Although there might be a couple of reasons for this unexpected behavior in DL-SOT, such as the roughness in the interfaces and/or impurity based electric potential in the heavy metal, one can deduce a roughness dependence sign reversal in DL-SOT in our trilayer structure.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

Significant enhancement in structural, dielectric, piezoelectric and ferromagnetic properties of Ba0.9Sr0.1Zr0.1Ti0.9O3-CoFe2O4 multiferroic composites

Highlights: • (1-x)Ba{sub 0.9}Sr{sub 0.1}Zr{sub 0.1}Ti{sub 0.9}O{sub 3}-(x)CoFe{sub 2}O{sub 4} multiferroic composite has been synthesized using mechano-chemical activation technique. • An improvement in dielectric, ferroelectric and piezoelectric properties over the pristine BSZT sample has been observed. • The composite samples also show a substantial enhancement in ferromagnetic characteristics compare to bare BSZT sample. - Abstract: Multiferroic composites (1 − x)Ba{sub 0.9}Sr{sub 0.1}Zr{sub 0.1}Ti{sub 0.9}O{sub 3}-(x)CoFe{sub 2}O{sub 4} (x = 0.0, 0.05, 0.10, 0.20 & 0.30) were prepared using mechanochemical activation method. Rietveld refined XRD patterns confirm the diphasic nature of composite samples. All of the diffraction peaks of (1 − x)BSZT-(x)CFO can be identified for the separate perovskite (BSZT) and inverse spinal (CFO) phases and no other impurity phase were observed for the composites. (1 − x)BSZT-(x)CFO sample with x = 0.05 have shown significant improvement in microstructural, dielectric, ferroelectric and piezoelectric properties over the pristine BSZT sample. Such an improvement in electrical properties can be attributed to larger grain size, improved density and easier ferroelectric domain wall motion inside the grain. Dielectric breakdown strength of the composites also shows a similar trend and is observed to be maximum for the sample with x = 0.05 CFO concentration. The composite samples also show a substantial enhancement in ferromagnetic characteristics and increases progressively with increase in ferrite (CFO) content.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Absence of strain-mediated magnetoelectric coupling at fully epitaxial Fe/BaTiO{sub 3} interface (invited)

Interfacial MagnetoElectric coupling (MEC) at ferroelectric/ferromagnetic interfaces has recently emerged as a promising route to achieve electrical writing of magnetic information in spintronic devices. For the prototypical Fe/BaTiO{sub 3} (BTO) system, various MEC mechanisms have been theoretically predicted. Experimentally, it is well established that using BTO single crystal substrates MEC is dominated by strain-mediated mechanisms. In case of ferromagnetic layers epitaxially grown onto BTO films, instead, no direct evidence for MEC has been provided, apart from the results obtained on tunneling junction sandwiching a BTO tunneling barrier. In this paper, MEC at fully epitaxial Fe/BTO interface is investigated by Magneto-Optical Kerr Effect and magnetoresistance measurements on magnetic tunnel junctions fabricated on BTO. We find no evidence for strain-mediated MEC mechanisms in epitaxial systems, likely due to clamping of BTO to the substrate. Our results indicate that pure electronic MEC is the route of choice to be explored for achieving the electrical writing of information in epitaxial ferromagnet-ferroelectric heterostructures.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Interferometric properties of standing spin waves and the application to a phase comparator

We numerically studied the material and structural parameter dependence of interferometric properties of the standing spin wave resonance (SSWR) by micromagnetic simulations and demonstrate the feasibility of practical application to an integrated phase comparator. The micromagnetic configuration of the synthesized SSWR emitted from the two microwave currents flowing through the parallel strip lines depends on the phase difference Δϕ between them. Resultantly, the Δϕ is converted to the related output voltage with an overlaid phase detector, inductively coupled to the magnetic strip. Among the investigated various material systems, low damping (α ∼ 0.001) metallic ferromagnets are found to exhibit superior device performance due to the reduced viscous dissipation.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

Molecular Beam Epitaxy of Materials Interfaces with Atomic Precision

In this contribution a few selected examples to engineer material interfaces in nanostructured solids with atomic precision by means of molecular beam epitaxy (MBE) are presented. The examples include 2D electron gas systems for quantum transport and mesoscopic physics, quantum cascade lasers, Sb-based materials, ferromagnet-semiconductor heterostructures, as well as oxide materials for electronics and quantum physics. Finally, the prospects to fabricate novel van-der-Waals heterostructures are briefly discussed.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Is spin transport through molecules really occurring in organic spin valves? A combined magnetoresistance and inelastic electron tunnelling spectroscopy study

Molecular and organic spintronics is an emerging research field which combines the versatility of chemistry with the non-volatility of spintronics. Organic materials have already proved their potential as tunnel barriers (TBs) or spacers in spintronics devices showing sizable spin valve like magnetoresistance effects. In the last years, a large effort has been focused on the optimization of these organic spintronics devices. Insertion of a thin inorganic tunnel barrier (Al{sub 2}O{sub 3} or MgO) at the bottom ferromagnetic metal (FM)/organic interface seems to improve the spin transport efficiency. However, during the top FM electrode deposition, metal atoms are prone to diffuse through the organic layer and potentially short-circuit it. This may lead to the formation of a working but undesired FM/TB/FM magnetic tunnel junction where the organic plays no role. Indeed, establishing a protocol to demonstrate the effective spin dependent transport through the organic layer remains a key issue. Here, we focus on Co/Al{sub 2}O{sub 3}/Alq{sub 3}/Co junctions and show that combining magnetoresistance and inelastic electron tunnelling spectroscopy measurements one can sort out working “organic” and short-circuited junctions fabricated on the same wafer.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Exotic exchange bias at epitaxial ferroelectric-ferromagnetic interfaces

Multiferroics in spintronics have opened up opportunities for future technological developments, particularly in the field of ferroelectric (FE)-ferromagnetic (FM) oxide interfaces with functionalities. We find strong exchange bias shifts (up to 84 Oe) upon field cooling in metal-oxide (Fe/BaTiO{sub 3}) films combining FM and FE layers. The saturation magnetic moment of the FM layer is also significantly higher than in bulk (3.0 ± 0.2 μ{sub B}/atom) and the reversal mechanism occurs via a domain nucleation process. X-ray absorption spectroscopy at the Fe K-edge and Ba L3-edge indicate presence of few monolayers of antiferromagnetic FeO at the interface without the formation of any BaFeO{sub 3} layer. Polarized neutron reflectometry corroborates with our magnetization data as we perform depth profiling of the magnetic and structural densities in these bilayers. Our first principles density functional calculations support the formation of antiferromagnetic FeO layers at the interface along with an enhancement of Fe magnetic moments in the inner ferromagnetic layers.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

A FirstPrinciple Study on the Magnetic Properties of Ag, Al, Li, Mg, and NaDoped ReS{sub 2} Monolayers

Magnetic properties of ReS{sub 2} monolayer doped with nonmagnetic metals (Ag, Al, Li, Mg, and Na) are studied by the first-principle method Different dopants and doping sites are considered. Similar to transition metal (TM) atoms, magnetic behavior appears in Al and Mg-doped systems. On the other hand, the calculated binding energies show that the Al-doped system has a more stable-formed system than the Mg-doped system. Hence, we study the ferromagnetic interaction in two Al-doped ReS{sub 2} monolayers. Interestingly, as the Al–Al distance increases, both ferromagnetic (FM) and nonmagnetic (NM) states are found. Our results deem that the FM coupling originates from a p-d exchange-like p-p coupling interaction.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Magnetoelectric coupling and spin-dependent tunneling in Fe/PbTiO{sub 3}/Fe multiferroic heterostructure with a Ni monolayer inserted at one interface

We report on first-principles calculations of a Ni monolayer inserted at one interface in the epitaxial Fe/PbTiO{sub 3}/Fe multiferroic heterostructure, focusing on the magnetoelectric coupling and the spin-dependent transport properties. The results of magnetoelectric coupling calculations reveal an attractive approach to realize cumulative magnetoelectric effects in the ferromagnetic/ferroelectric/ferromagnetic superlattices. The underlying physics is attributed to the combinations of several different magnetoelectric coupling mechanisms such as interface bonding, spin-dependent screening, and different types of magnetic interactions. We also demonstrate that inserting a Ni monolayer at one interface in the Fe/PbTiO{sub 3}/Fe multiferroic tunnel junction is an efficient method to produce considerable tunneling electroresistance effect by modifying the tunnel potential barrier and the interfacial electronic structure. Furthermore, coexistence of tunneling magnetoresistance and tunneling electroresistance leads to the emergence of four distinct resistance states, which can be served as a multistate-storage device. The complicated influencing factors including bulk properties of the ferromagnetic electrodes, decay rates of the evanescent states in the tunnel barrier, and the specific interfacial electronic structure provide us promising opportunities to design novel multiferroic tunnel junctions with excellent performances.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Structural and magnetic properties of Y-doped La{sub 0.5}Sr{sub 0.5}MnO{sub 3} manganite system: Evidence of step-like magnetization

The magnetic and transport properties of bulk doped manganite, La{sub 0.5−y}Y{sub y}Sr{sub 0.5}MnO{sub 3} have been studied as a function of yttrium doping, y, for 0 ≤ y ≤ 0.25. In the absence of the yttrium this is a well studied half-doped manganite which is a metallic ferromagnet. The end members of this system (with y = 0 and 0.25) are found to be structurally different (rhombohedral and orthorhombic, respectively) due to the large distortions caused by the yttrium. These distortions make this series of compounds particularly interesting. The magnetization obtained in zero field cooled (ZFC) and field cooled (FC) studies show dramatic changes as the value of y is raised above 0.05. The magnetic behavior is further investigated by a study of the hysteresis loops, frequency dependent susceptibility and studies of the magnetic relaxation. The results are understood in terms of the average size of the cation on the A site, , and the variance, σ{sup 2} (= ∑y{sub i}{sup 2}r{sub i}{sup 2} - {sup 2}). The value of the variance, σ{sup 2}, increases rapidly with doping, y, induces melting of the long range ferromagnetic ordering, exhibiting a stepwise variation of magnetization (metamagnetic like). Based on the present results a magnetic phase diagram has been drawn demonstrating different magnetic phase sequences with the variation of σ{sup 2}.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Computational Research of Electronic and Magnetic Properties of Nonmetal Doping of Graphene-Like ZnO Monolayer

In this paper, the geometrical, electronic, and magnetic properties of nonmetal (B, C, N, F, Si, P, or S) atom doping of g-ZnO monolayer supercell have been comprehensively calculated using the first-principle method. The structural optimization implies that nonmetal atom doping of g-ZnO monolayer leads to the structural distortion around the doping atoms compared with pure g-ZnO sheet. We find that g-ZnO monolayer with one O atom per supercell substituted by one B or C atom is ferromagnetic half metal, and that substituted by one N, Si, or P atom exhibits a ferromagnetic semiconductor feature, while that substituted by one F or S atom appears to be nonmagnetic semiconductor. The total magnetic moments mainly originates from the spin polarization of the doping atom (B, C, N, Si, and P), and the rest comes from the nearest Zn and O atoms.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Study of Electronic, Mechanical, Magnetic, and Optical Properties of Mg{sub 0.75}TM{sub 0.25}S/Se (TM = Fe, Co, Ni): A First Principle Approach

A systematic study of the magnetism, mechanical, and optical behaviors of Mg{sub 0.75}TM{sub 0.25}S/Se (TM = Fe, Co, Ni) ordered alloys was conducted using the full-potential linearized augmented plane wave plus local orbital method. The ferromagnetic (FM) and antiferromagnetic (AFM) ground state energy differences and enthalpy of formation scrutinized their dynamical stability in the FM state. The elastic parameters decided their mechanical stability, ductile behavior, and partial ionic and covalent characters. Moreover, the calculated band structures and density of states revealed the type and origin of magnetism in terms of exchange splitting energies and exchange constants. The Fe- and Ni-doped MgS/Se alloys exhibited half metallic ferromagnetic (HMF) characteristics, while the Co-doped MgS/Se alloy depicted ferromagnetic semiconducting characters. A reduction in the magnetic moments of the transition metals was observed compared with their free space value, which is due to strong p-d hybridization. Furthermore, the optical spectrum showed the maximum light absorbed in the visible and in the UV region of the electromagnetic spectrum, which makes them potential candidates for optoelectronic and spintronic device applications.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Electrical-transport, magnetoresistance and magnetic properties of La{sub 0.7}Ca{sub 0.3}MnO{sub 3} and La{sub 0.7}Ca{sub 0.24}Sr{sub 0.06}MnO{sub 3} single crystals

We report the electrical-transport, magnetoresistance and magnetic properties of the hole doped La{sub 0.7}Ca{sub 0.3}MnO{sub 3} (LCMO) and La{sub 0.7}Ca{sub 0.24}Sr{sub 0.06}MnO{sub 3} (LCSMO) single crystals. It was prepared using floating zone technique. The resistivity data shows the metal to insulator transition (T{sub MI}) occurs at 211 K along c-axis and T{sub MI} = 185 K the ab-plane for LCMO and T{sub MI} = 290 K along the c-axis and T{sub MI} = 280 K along the ab-plane for LCSMO single crystals. It is observed that the T{sub MI} is higher along the c-axis as compared to that in the ab-plane, consequently signifying more favorable hoping of electrons is along the c-axis. The ac-susceptibility measurement shows that this material exhibits ferromagnetic to paramagnetic transition temperature (TC) at 206 K for LCMO and T{sub C} = 277 K for LCSMO single crystals. For magnetic memory device application point of view, the sample shows the maximum MR of 98% for LCMO and 80% for LCSMO single crystals at 8T applied magnetic field. Doping small amount of Sr (0.06%) reveals that the electronic and magnetic phase transition in CMR single crystal increases substantially and useful for device application. This is first time such type of comparative study in these manganite single crystals.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Role of ferroelectric/ferromagnetic layers on the ferroelectric properties of magnetoelectric composite films derived by chemical solution deposition

Highlights: • Multiferroic composite films with different components were prepared. • Individual layer effect on the properties of composite films was studied. • BTO-CFO film shows bigger ferroelectric polarization than BTO-BFO film. • Leakage current curves show BTO-CFO film has the smallest leakage current. • Leakage current is dominated by the space-charges-limited mechanisms. - Abstract: Composite thin films with BiFeO{sub 3} (BFO), CoFeO{sub 4} (CFO) and/or BaTiO{sub 3} (BTO) layers as the components are fabricated via CSD method. XRD and Raman patterns show all the composite films can be indexed to perovskite and spinel structures. Except the interfaces between BTO and CFO layers, the other two-phase interfaces can be clearly observed. Interestingly, the BTO-CFO film has bigger polarization than BTO-BFO, especially the thickness of CFO layer is smaller than the BFO layer, implying the ferromagnetic layer has a significant impact on the ferroelectric properties of the composite films, which is too thin to be fully polarized. The variation of leakage currents show BTO-CFO film has the smallest value, corroborating the results of ferroelectric measurements. The leakage current is dominated by the space-charges-limited mechanisms according to the fitting results, and the oxygen vacancies in the BFO layers play an important role for the variations of E{sub SCL} and E{sub TFL} voltages.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Fast strain wave induced magnetization changes in long cobalt bars: Domain motion versus coherent rotation

A high frequency (88 MHz) traveling strain wave on a piezoelectric substrate is shown to change the magnetization direction in 40 μm wide Co bars with an aspect ratio of 10{sup 3}. The rapidly alternating strain wave rotates the magnetization away from the long axis into the short axis direction, via magnetoelastic coupling. Strain-induced magnetization changes have previously been demonstrated in ferroelectric/ferromagnetic heterostructures, with excellent fidelity between the ferromagnet and the ferroelectric domains, but these experiments were limited to essentially dc frequencies. Both magneto-optical Kerr effect and polarized neutron reflectivity confirm that the traveling strain wave does rotate the magnetization away from the long axis direction and both yield quantitatively similar values for the rotated magnetization. An investigation of the behavior of short axis magnetization with increasing strain wave amplitude on a series of samples with variable edge roughness suggests that the magnetization reorientation that is seen proceeds solely via coherent rotation. Polarized neutron reflectivity data provide direct experimental evidence for this model. This is consistent with expectations that domain wall motion cannot track the rapidly varying strain.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Atomic Scale Studies of La/Sr Ordering in Colossal Magnetoresistant La 2-2x Sr 1+2x Mn 2 O 7 Single Crystals

To date, it is unclear whether chemical order (or disorder) is in any way connected to double exchange, electronic phase separation, or charge ordering (CO) in manganites. In this work, we carry out an atomic resolution study of the colossal magnetoresistant manganite La 2-2x Sr 1+2x Mn 2 O 7 (LSMO). We combine aberration-corrected electron microscopy and spectroscopy with spectroscopic image simulations, to analyze cation ordering at the atomic scale in real space in a number of LSMO single crystals. We compare three different compositions within the phase diagram: a ferromagnetic metallic material (x=0.36), an insulating, antiferromagnetic charge ordered (AF-CO) compound (x=0.5), which also exhibits orbital ordering, and an additional AF sample (x=0.56). Detailed image simulations are essential to accurately quantify the degree of chemical ordering of these samples. We find a significant degree of long-range chemical ordering in all cases, which increases in the AF-CO range. However, the degree of ordering is never complete nor can it explain the strongly correlated underlying ordering phenomena. Our results show that chemical ordering over distinct crystallographic sites is not needed for electronic ordering phenomena to appear in manganites, and cannot by itself explain the complex electronic behavior of LSMO.

36 MATERIALS SCIENCE↗