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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 433 records · Page 24
All sourcesNASA NTRSDOE OSTI
DOE OSTI2013

Room-temperature perpendicular magnetic anisotropy of MgO/Fe/MgO ultrathin films

We used the anomalous Hall effect to study the magnetic properties of MgO/Fe(t)/MgO(001) structures in which the Fe thickness t ranged from 4 Å to 14 Å. For the iron deposited at 140 K, we obtained perpendicular magnetization at room temperature below the critical thickness of t{sub c} = (9 ± 1) Å. In the vicinity of t{sub c}, the easy magnetization axis switched from an out-of-plane orientation to an in-plane orientation, and the observed spin-reorientation transition was considered in terms of the competition among different anisotropies. The perpendicular magnetization direction was attributed to magnetoelastic anisotropy. Finally, the temperature-dependent spin-reorientation transition was analyzed for Fe thicknesses close to t{sub c}.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2010

Electronic and magnetic structure of GaxFe1-x thin films

The electronic as well as magnetic properties of Ga{sub x}Fe{sub 1-x} films were studied by soft x-ray measurements. Using x-ray magnetic circular dichroism the Fe majority-spin band was found to be completely filled for x {approx} 0.3. With further enhanced Ga content, the Fe moment as well as the angular dependence of the x-ray magnetic linear dichroism decrease strongly, which we attribute to the formation of D0{sub 3} precipitates. Moreover, the magnetocrystalline anisotropy drops significantly.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Spin-orbit interaction tuning of perpendicular magnetic anisotropy in L1{sub 0} FePdPt films

The dependence of perpendicular magnetic anisotropy K{sub u} on spin-orbit coupling strength ξ is investigated in L1{sub 0} ordered FePd{sub 1−x}Pt{sub x} films by time-resolved magneto-optical Kerr effect measurements and ab initio density functional calculations. Continuous tuning of K{sub u} over a wide range of magnitude is realized by changing the Pt/Pd concentration ratio, which strongly modifies ξ but keeps other leading parameters affecting K{sub u} nearly unchanged. Ab initio calculations predict a nearly quadratic dependence of K{sub u} on ξ, consistent with experimental data. K{sub u} increases with increasing chemical order and decreasing thermal spin fluctuations, which becomes more significant for samples with higher Pt concentration. The results demonstrate an effective method to tune K{sub u} utilizing its sensitivity on ξ, which will help fabricate magnetic systems with desirable magnetic anisotropy.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Probing nonlinear magnetization dynamics in Fe/MgO(001) film by all optical pump-probe technique

An all-optical pump-probe technique has been employed to investigate the nonlinear magnetization dynamics of a 10 nm Fe/MgO(001) thin film in time domain. The magnetization precession was excited by pump-laser pulses and modulated by laser fluence variations. With increasing the laser fluence up to 7.1 mJ/cm{sup 2}, in addition to the uniform precession mode, a second harmonic signal was detected. The time evolution of the second harmonic signal was obtained in time-frequency domain. Based on the Landau-Lifshitz-Gilbert equation, the numerical simulation was performed to reproduce the observed the frequency doubling behaviors in Fe/MgO(001) film.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2013

Growth temperature dependent structural and magnetic properties of epitaxial Co{sub 2}FeAl Heusler alloy films

The structural and magnetic properties of a series of Co{sub 2}FeAl Heusler alloy films grown on GaAs(001) substrate by molecular beam epitaxy have been studied. The epitaxial Co{sub 2}FeAl films with an ordered L{sub 21} structure have been successfully obtained at growth temperature of 433 K, with an in-plane cubic magnetic anisotropy superimposed with an unusual uniaxial magnetic anisotropy. With increasing growth temperature, the ordered L{sub 21} structure degrades. Meanwhile, the uniaxial anisotropy decreases and eventually disappears above 673 K. The interfacial bonding between As and Co or Fe atom is suggested to be responsible for the additional uniaxial anisotropy.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Contribution from Ising domains overlapping out-of-plane to perpendicular magnetic anisotropy in Mn 4 N thin films on MgO(001)

Single phase ε-Mn4N thin and ultrathin films are grown on MgO(001) using molecular beam epitaxy. Reflection high-energy electron diffraction and outof-plane x-ray diffraction measurements are taken for each sample in order to determine the in- and out-of-plane strain for each sample. Vibrating sample magnetometry and superconducting quantum interference device measurements, which are performed on the thin and ultrathin films respectively, are used to plot the magnetization of each sample versus both in- and out-of-plane $\vec H$-fields and to determine the magnitude of perpendicular magnetic anisotropy in these films. Three significant components of perpendicular magnetic anisotropy are observed in these films and are attributed to sample strain (1 component) and shape (2 components). Among these components, the most significant component (0.8 to 4.9 $\frac{Merg}{cm^3}$) is identified as a second term of shape anisotropy, which possesses a negative linear relationship with sample thickness over the range from 9 nm to 310 nm. Atomic (magnetic) force microscopy measurements show the presence of a surface localized magnetic polarization (22% to 82%), which increases with decreasing thickness, when the net magnetization of the films is zero. The second term of shape anisotropy as well as the surface localized polarization, which each depend on sample thickness, are each regarded as a consequence of Ising domains overlapping out-of-plane in these films.

Materials Science↗
DOE OSTI2015

Angular dependence of the spin textures in two-dimensional chiral magnets

The angular dependence of spin textures in thin helimagnetic films is investigated by a Monte Carlo simulation. When an external field is applied at an angle relative to the film normal, we find that the skyrmion states with broken axis-symmetric structure are able to persist over a wide range of angles by changing the spin orientation. In addition, the uniaxial anisotropy is able to stabilize the distorted skyrmions. This behavior reflects the robust topological stability of skyrmion states in helimagnets and favors their application in spintronic devices.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Stabilization of magnetic bubbles in [Ni/Co]n multilayers on an oxygen-reconstructed Nb(110) surface via an ultra-thin Cu interlayer

Magnetic thin films hosting topological spin textures, such as magnetic skyrmions, hold high potential for breakthroughs in the field of spintronics, due to good scalability and energy efficiency. Novel computational architectures such as memory-in-logic devices rely on material platforms able to host those topological spin textures. Furthermore, recently proposed designs of novel quantum information technologies are based on heterostructures where topological spin textures are in direct proximity to a superconducting layer. Here, we demonstrate the stabilization of out-of-plane magnetic bubbles in highly ordered [Ni/Co]n multilayers on a Nb(110) single crystal. This is achieved without the need for the removal of the well-known Nb(110)-oxide surface reconstruction, due to the introduction of an ultra-thin Cu interlayer in between the Nb substrate and the magnetic multilayer. The Cu interlayer generates a well-ordered hexagonal surface, which is key for the epitaxial growth of the [Ni/Co]n multilayers hosting the desired out-of-plane anisotropy. The magnetic ground state of the prepared material stacks is directly imaged via spin-polarized low-energy electron microscopy, revealing the presence of magnetic bubble domains with lateral sizes as small as 450 nm.

Dibajeh, Ahmad↗
DOE OSTI2021

Synthesis and characterization of amorphous Fe2.75Dy-oxide thin films demonstrating room-temperature semiconductor, magnetism, and optical transparency

Recently, amorphous/disordered oxide thin films made from Fe and lanthanides like Dy and Tb have been reported to have a rich set of magnetic, optical, and electronic properties, as well as room-temperature magneto-electric coupling with multiferroics [A. Malasi et al., Sci. Rep. 5, 18157 (2015); H. Taz et al., Sci. Rep. 6, 27869 (2016); and H. Taz et al., Sci. Rep. 10, 1–10 (2020)]. Here, we report the synthesis and detailed characterization of Fe2.75Dy-oxide thin films prepared on various substrates using electron beam co-evaporation. The structure, chemistry, electric, magnetic, and optical properties were studied for the as-prepared and annealed (373 K, in air, 1 h) films of thickness 40 nm. High resolution transmission electron microscopy and electron diffraction study showed that the films were amorphous in both the as-prepared and annealed states. The electron energy-loss spectroscopy studies quantified that metal oxygen stoichiometry changed from Fe2.75Dy-O1.5 to Fe2.75Dy-O1.7 upon annealing. Synchrotron-based x-ray absorption spectroscopy investigation confirmed that the as-prepared films were highly disordered with predominantly metallic Fe and Dy states that became slightly oxidized with annealing in air. The as-prepared amorphous films demonstrated significantly high value of ordinary (∼10 cm2/V s) and anomalous (∼102 cm2/V s) Hall mobility and high electrical conductivity of ∼103 S/cm at room temperature. The cryogenic magnetic property measurement shows two-step magnetization below 200 K, suggesting exchange-spring magnetic interaction. The nature of the field cooled and zero-field cooled curves suggested a spin-glass like transition between 78 K and 80 K, with a characteristic broad peak. The Tauc plot analysis from optical transmission spectra confirms the existence of an optical bandgap of ∼2.42 eV that increased slightly to ∼2.48 eV upon annealing. This rich set of transport, optical, and magnetic properties in these thin films is very exciting and points to potential applicability in low-cost multifunctional devices requiring a combination of transparent, semiconducting, and magnetic responses, such as in spintronics.

Prasad Koirala, Krishna (ORCID:000000029137917X)↗
DOE OSTI2011

Magnetometry and transport data complement polarized neutron reflectometry in magnetic depth profiling

Exchange coupled magnetic hard layer/soft layer thin films show a variety of complex magnetization reversal mechanisms depending on the hierarchy of interaction strengths within and between the films. Magnetization reversal can include uniform rotation, soft layer biasing, as well as exchange spring behavior. We investigate the magnetization reversal of a CoPt/Permalloy/Ta/Permalloy heterostructure. Here, Stoner-Wohlfarth-type uniform magnetization rotation of the virtually free Permalloy layer and exchange spring behavior of the strongly pinned Permalloy layer are found in the same sample. We investigate the complex magnetization reversal by polarized neutron reflectometry, magnetometry, and magneto-transport. The synergy of combining these experimental methods together with theoretical modeling is key to obtain the complete quantitative depth resolved information of the magnetization reversal processes for a multilayer of mesoscopic thickness.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Quantitative control of magnetic ordering in FeRh thin films using 30 keV Ga ion irradiation from a focused ion beam system

We investigated a low-energy ion-beam irradiation process for the magnetic modification of FeRh thin films using a focused ion beam system. Low-energy ion-beam irradiation induced ferromagnetic states in the FeRh thin films at low temperatures, that originally exhibited antiferromagnetism, as effectively as high-energy ion-beam irradiation. Because the energy deposited by the elastic collisions caused by the irradiation determined the magnetic properties of the samples, the magnetic state of the FeRh thin films could be quantitatively controlled. The low-energy ion-beam irradiation using a focused ion beam system is a potential technique to modify the magnetic properties of materials on the nano- and micro-scales, which may lead to a variety of novel spin devices and applications.

36 MATERIALS SCIENCE↗
DOE OSTI2010

Magnetic Structure of Multiferroic BiFeO3 Film with Engineered Ferroelectric Domains

We report on neutron scattering and piezoresponse force microscopy studies of multiferroic BiFeO{sub 3} films epitaxially grown on vicinal SrTiO{sub 3} substrates. We find that these BiFeO{sub 3} films exhibit a G-type antiferromagnetic structure modulated with cycloidal spiral magnetic ordering with the features of magnetic domains depending upon the ferroelectric domain states: a single antiferromagnetic domain for the films with a ferroelectric monodomain while two- or multiantiferromagnetic domains existing in the films displaying two- or four-ferroelectric variants, respectively. These results imply the correlation between ferroelectric and magnetic order parameters in the BiFeO{sub 3} films and the expected strong magnetoelectric coupling makes these multiferroic films promising candidates for device applications with the capability to tune the magnetism using an electric field.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2017

Magnetic resonance studies of three-layer FeNi/Bi/FeNi films

The interlayer coupling in three-layer FeNi/Bi/FeNi films is studied by electron magnetic resonance. The magnetic anisotropy at the permalloy–bismuth interface is shown to play a significant role in the formation of the magnetic state of the film structure. The interlayer coupling oscillation period is found to be about 8 nm. The interlayer coupling and the interface anisotropy and their temperature dependences are determined.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Magnetism in curved geometries

Extending planar two-dimensional structures into the three-dimensional space has become a general trend in multiple disciplines, including electronics, photonics, plasmonics and magnetics. This approach provides means to modify conventional or to launch novel functionalities by tailoring the geometry of an object, e.g. its local curvature. In a generic electronic system, curvature results in the appearance of scalar and vector geometric potentials inducing anisotropic and chiral effects. In the specific case of magnetism, even in the simplest case of a curved anisotropic Heisenberg magnet, the curvilinear geometry manifests two exchange-driven interactions, namely effective anisotropy and antisymmetric exchange, i.e. Dzyaloshinskii–Moriya-like interaction. As a consequence, a family of novel curvature-driven effects emerges, which includes magnetochiral effects and topologically induced magnetization patterning, resulting in theoretically predicted unlimited domain wall velocities, chirality symmetry breaking and Cherenkov-like effects for magnons. The broad range of altered physical properties makes these curved architectures appealing in view of fundamental research on e.g. skyrmionic systems, magnonic crystals or exotic spin configurations. In addition to these rich physics, the application potential of three-dimensionally shaped objects is currently being explored as magnetic field sensorics for magnetofluidic applications, spin-wave filters, advanced magneto-encephalography devices for diagnosis of epilepsy or for energy-efficient racetrack memory devices. Finally, these recent developments ranging from theoretical predictions over fabrication of three-dimensionally curved magnetic thin films, hollow cylinders or wires, to their characterization using integral means as well as the development of advanced tomography approaches are in the focus of this review.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Structural and magnetic properties of FeNi thin films fabricated on amorphous substrates

FeNi films were fabricated by sputtering and rapid thermal annealing on thermally amorphous substrates to realize the formation of an L1{sub 0}-FeNi phase by a simple method. Structural and magnetic properties of FeNi films were investigated by varying the annealing temperature. L1{sub 0}-FeNi superlattice peaks were not observed in X-ray diffraction patterns, indicating no formation of L1{sub 0}-ordered phase, however, the surface structure systematically changed with the annealing temperature. Magnetization curves also revealed a drastic change depending on the annealing temperature, which indicates the close relation between the morphology and magnetic properties of FeNi films fabricated on amorphous substrates.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Rotational magnetic anisotropy in polycrystalline FePt films fabricated by solid-state synthesis

It is shown that annealing 550°C of the Fe/Pt bilayer films fabricated by layer-by-layer deposition onto polycrystalline Al{sub 2}O{sub 3} substrates in an atomic ratio of Fe: Pt = 71: 29 leads to the formation of the inplane rotational magnetic anisotropy with the easy axis that can be aligned by magnetic field in any direction in the film plane. The increase in the Pt content in the investigated film system to 50 at % leads to an increase in the anisotropy constant to 6.5 × 10{sup 6} erg/cm{sup 3}, which exceeds the value in the Fe{sub 71}Pt{sub 29} sample by a factor of 6. In addition to the rotational anisotropy, the Fe50Pt50 films are characterized by the perpendicular rotational magnetic anisotropy, which is higher than the film shape anisotropy. Therefore, the easy axis in these samples can be aligned by magnetic field in any spatial direction. According to the X-ray and magnetic measurement data, the synthesized samples have a polycrystalline structure consisting of two ordered phases: magnetically hard L1{sub 0}–FePt and magnetically soft L1{sub 2}–Fe{sub 3}Pt. This gives us grounds to suggest that the rotational magnetic anisotropy originates from the epitaxial intergrowth and exchange coupling of these two phases.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

High tunneling magnetoresistance ratio in perpendicular magnetic tunnel junctions using Fe-based Heusler alloys

Heulser alloys Fe{sub 2}Cr{sub 1−x}Co{sub x}Si (FCCS) with different Co compositions x have been predicted to have high spin polarization. High perpendicular magnetic anisotropy (PMA) has been observed in ultra-thin FCCS films with magnetic anisotropy energy density up to 2.3 × 10{sup 6 }erg/cm{sup 3}. The perpendicular magnetic tunnel junctions (p-MTJs) using FCCS films with different Co compositions x as the bottom electrode have been fabricated and the post-annealing effects have been investigated in details. An attractive tunneling magnetoresistance ratio as high as 51.3% is achieved for p-MTJs using Fe{sub 2}CrSi (FCS) as the bottom electrode. The thermal stability Δ can be as high as 70 for 40 nm dimension devices using FCS, which is high enough to endure a retention time of over 10 years. Therefore, Heusler alloy FCS is a promising PMA candidate for p-MTJ application.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Influence of boron diffusion on the perpendicular magnetic anisotropy in Ta|CoFeB|MgO ultrathin films

We have studied structural and magnetic properties of Ta|CoFeB|MgO heterostructures using cross-section transmission electron microscopy (TEM), electron energy loss spectrum (EELS) imaging, and vibrating sample magnetometry. From the TEM studies, the CoFeB layer is found to be predominantly amorphous for as deposited films, whereas small crystallites, diameter of ∼5 nm, are observed in films annealed at 300 °C. We find that the presence of such nanocrystallites is not sufficient for the occurrence of perpendicular magnetic anisotropy. Using EELS, we find that boron diffuses into the Ta underlayer upon annealing. The Ta underlayer thickness dependence of the magnetic anisotropy indicates that ∼0.2 nm of Ta underlayer is enough to absorb the boron from the CoFeB layer and induce perpendicular magnetic anisotropy. Boron diffusion upon annealing becomes limited when the CoFeB layer thickness is larger than ∼2 nm, which coincides with the thickness at which the saturation magnetization M{sub S} and the interface magnetic anisotropy K{sub I} drop by ∼20%. These results show the direct role which boron plays in determining the perpendicular magnetic anisotropy in CoFeB|MgO heterostructures.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗