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At least 685 records · Page 38
All sourcesNASA NTRSDOE OSTI
DOE OSTI2017

Mo-Terminated Edge Reconstructions in Nanoporous Molybdenum Disulfide Film

The catalytic and magnetic properties of molybdenum disulfide (MoS 2 ) are significantly enhanced by the presence of edge sites. One method to obtain a high density of edge sites in a two-dimensional (2D) film is by introducing porosity. Yet, the large-scale bottom-up synthesis of a porous 2D MoS 2 film remains challenging and the correlation of growth conditions to the atomic structures of the edges is not well understood. In this work, using molecular beam epitaxy, we prepare wafer-scale nanoporous MoS 2 films under conditions of high Mo flux and study their catalytic and magnetic properties. Atomic-resolution electron microscopy imaging of the pores reveals two new types of reconstructed Mo-terminated edges, namely, a distorted 1T (DT) edge and the Mo-Klein edge. Nanoporous MoS 2 films are magnetic up to 400 K, which is attributed to the presence of Mo-terminated edges with unpaired electrons, as confirmed by density functional theory calculation. The small hydrogen adsorption free energy at these Mo-terminated edges leads to excellent activity for the hydrogen evolution reaction.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Magnetic and Resonant Properties of Fe–Bi Films

Film structures in the Fe–Bi system have been studied experimentally. The magnetic state of the two-layer structures is shown by electron magnetic resonance to be dependent on the order of depositing magnetic and nonmagnetic layers. The three-layer structures demonstrate the effect of the exchange bias, the value of which is dependent on the bismuth interlayer thickness.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Surface magnetism and proximity effects in hexaboride thin films

Here, the unexpected ferromagnetism in alkaline hexaborides has attracted extensive research efforts. The exceptionally high Curie temperature sustained by very low density of magnetic moments is yet to be clearly understood. Here, we report the magnetic properties studied in CaB 6 and SrB 6 thin films and their heterostructures with SmB 6 grown by pulsed laser deposition. A coexistence of ferromagnetic and superparamagnetic orders with 2D-type magnetic anisotropy was observed in CaB 6 and SrB 6 . Boron deficiency and closely correlated local oxidization were found at the film surface that contacts air and its interface with the oxide substrate. These results suggest that the magnetism observed is likely generated by two thin surface/interface layers with high concentration of boron vacancies which are energetically stabled by oxygen cation. Additionally, clear suppression of the superconductivity proximity effect in SmB 6 film was observed when SmB 6 was grown on top of the magnetic CaB 6 . Such results demonstrate the interesting perspective of engineering multifunctional heterostructures by coupling the surface magnetic layer with the diversity of other hexaboride materials.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Acoustic spin pumping in magnetoelectric bulk acoustic wave resonator

We present the generation and detection of spin currents by using magnetoelastic resonance excitation in a magnetoelectric composite high overtone bulk acoustic wave (BAW) resonator (HBAR) formed by a Al-ZnO-Al-GGG-YIG-Pt structure. Transversal BAW drives magnetization oscillations in YIG film at a given resonant magnetic field, and the resonant magneto-elastic coupling establishes the spin-current generation at the Pt/YIG interface. Due to the inverse spin Hall effect (ISHE) this BAW-driven spin current is converted to a dc voltage in the Pt layer. The dependence of the measured voltage both on magnetic field and frequency has a resonant character. The voltage is determined by the acoustic power in HBAR and changes its sign upon magnetic field reversal. We compare the experimentally observed amplitudes of the ISHE electrical field achieved by our method and other approaches to spin current generation that use surface acoustic waves and microwave resonators for ferromagnetic resonance excitation, with the theoretically expected values.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Magnetization in Nanocrystalline PLD Zinc Ferrite Thin Films Deposited on Fused Quartz Substrate

The nanocrystalline zinc ferrite (ZnFe{sub 2}O{sub 4}) thin films were grown on the fused quartz substrate using pulsed laser deposition. The effects of atmospheric conditions on structural, magnetic, and morphological properties have been systematically studied using X-ray diffraction, vibrating sample magnetometry, and atomic force microscopy. The films showed textured growth of zinc ferrite for the substrate temperatures at ≥ 600 °C irrespective of the ambient environment. Interestingly, thin films of zinc ferrite showed a magnetic hysteresis loop in contrast to the paramagnetic nature of their bulk counterpart. The value of saturation magnetization (4πM{sub s}) increased from 130 to 360 G when the substrate temperature was increased from room temperature (RT) to 800 °C in the oxygen atmosphere. In the absence of oxygen pressure, the value of 4πM{sub s} is observed to be 330 G for the film deposited at RT and found to increase to a value of 670 G when the film is deposited at 400 °C and the maximum value of 2580 G is observed for the film deposited at 700 °C. However, the value of 4πM{sub s} of 800 °C dropped down to 2000 G.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2010

Room-Temperature Ferromagnetism of Cu-Doped ZnO Films Probed by Soft X-Ray Magnetic Circular Dichroism

We report direct evidence of room-temperature ferromagnetic ordering in O-deficient ZnO:Cu films by using soft x-ray magnetic circular dichroism and x-ray absorption. Our measurements have revealed unambiguously two distinct features of Cu atoms associated with (i) magnetically ordered Cu ions present only in the oxygen-deficient samples and (ii) magnetically disordered regular Cu{sup 2+} ions present in all the samples. We find that a sufficient amount of both oxygen vacancies (V{sub O}) and Cu impurities is essential to the observed ferromagnetism, and a non-negligible portion of Cu impurities is uninvolved in the magnetic order. Based on first-principles calculations, we propose a microscopic 'indirect double-exchange' model, in which alignments of localized large moments of Cu in the vicinity of the V{sub O} are mediated by the large-sized vacancy orbitals.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2011

Effect of nonmagnetic alkaline-earth dopants on magnetic properties of BiFeO{sub 3} thin films

Bi{sub 0.95}A{sub 0.05}FeO{sub 3} (A = Ba, Sr, Ca) thin films were deposited on (111) Pt/Ti/SiO{sub 2}/Si substrates via chemical solution deposition. The magnetization of the doped specimens increases sharply as compared to that of pure BiFeO{sub 3}, and it increases as the doped ionic radius gets larger. X-ray diffraction analysis reveals that the films are single phase, and no other extra magnetic phases are detected. X-ray photoelectron spectroscopy analysis suggests that the content of Fe{sup 3+} increases and that that of Fe{sup 2+} decreases. According to the x-ray diffraction and Raman analyses, there are corresponding changes in the lattice parameters and the Raman spectra as the doped ionic radius gets larger. These results indicate that neither the magnetic coupling of Fe{sup 2+} nor the parasitic magnetic phases could be the origin of the magnetization improvement, and we consider that the magnetization improvement mainly originates from the doped ionic radius size effect.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Control of magnetization reversal in oriented strontium ferrite thin films

Oriented Strontium Ferrite films with the c axis orientation were deposited with varying oxygen partial pressure on Al{sub 2}O{sub 3}(0001) substrate using Pulsed Laser Deposition technique. The angle dependent magnetic hysteresis, remanent coercivity, and temperature dependent coercivity had been employed to understand the magnetization reversal of these films. It was found that the Strontium Ferrite thin film grown at lower (higher) oxygen partial pressure shows Stoner-Wohlfarth type (Kondorsky like) reversal. The relative importance of pinning and nucleation processes during magnetization reversal is used to explain the type of the magnetization reversal with different oxygen partial pressure during growth.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Deposition of high permeability FeCoN films on mica substrates

In this study, sheet mica was explored as an alternative substrate for thin film deposition due to its unique layered structure and atomically flat surface. Static magnetic properties and microwave permeability of sputtered FeCoN films on mica with and without SiO{sub 2} under-layer were examined. In-plane isotropy and a large coercivity of 70 Oe were observed for the film directly deposited on mica substrate. With the insertion of 40 nm SiO{sub 2} under-layer between the film and mica substrate, well-defined in-plane anisotropy is formed due to the significant reduce in film residual stress, associated with a 3 Oe coercive force along easy axis. As a result, excellent microwave properties have been achieved and are comparable with the same film on Si substrate. Moreover, the permeability spectra are surprisingly stable from room temperature up to 390 K. Mica is proved as a promising substrate for magnetic films with high microwave performance, which is a great advantage for practical applications. Furthermore, our results provide an effective way to reduce the residual stress in sputtered films.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Unconventional magnetisation texture in graphene/cobalt hybrids

Magnetic domain structure and spin-dependent reflectivity measurements on cobalt thin films intercalated at the graphene/Ir(111) interface are investigated using spin-polarised low-energy electron microscopy. We find that graphene-covered cobalt films have surprising magnetic properties. Vectorial imaging of magnetic domains reveals an unusually gradual thickness-dependent spin reorientation transition, in which magnetisation rotates from out-of-the-film plane to the in-plane direction by less than 10° per cobalt monolayer. During this transition, cobalt films have a meandering spin texture, characterised by a complex, three-dimensional, wavy magnetisation pattern. In addition, spectroscopy measurements suggest that the electronic band structure of the unoccupied states is essentially spin-independent already a few electron-Volts above the vacuum level. These properties strikingly differ from those of pristine cobalt films and could open new prospects in surface magnetism.

36 MATERIALS SCIENCE↗
DOE OSTI2018

The finite size effect on the transport and magnetic properties of epitaxial Fe 3 O 4 thin films

Here, magnetite (Fe 3 O 4 ) has great potential for use in the new field of spintronics due to its interesting physical properties, e.g., half-metallic ferromagnetic nature and metal–insulator transition (Verwey transition). Therefore, a basic understanding of these properties is essential for applications in spintronics devices, especially as the film thickness is reduced. In this work, the transport and magnetic properties of stoichiometric ultrathin epitaxial Fe 3 O 4 films have been investigated. The Fe 3 O 4 films were grown on MgO (001) substrates using molecular beam epitaxy under optimal growth conditions. Low energy electron diffraction and X-ray photoemission spectroscopy confirmed that the films are single phase Fe 3 O 4 . The Verwey transition has been investigated using both transport and magnetization measurements. The magnetization measurements show a sharp Verwey transition in all of these films, which indicates that the films have properties comparable to the bulk. Furthermore, the magnetization measurements at room temperature show that the ultrathin films with thickness t < 20 nm are ferromagnetic with magnetization values greater than those for bulk magnetite. Such enhanced magnetization in ultrathin Fe 3 O 4 films is very promising for spin injection and other applications.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2011

Effects of CrRu-SiO{sub x} underlayer with MgO intermediate layer on the microstructure and magnetic properties of FePt-C thin film

The effect of the CrRu-SiO{sub x} underlayer with different doping concentrations and the thickness of the CrRu underlayer on the microstructure and magnetic properties of FePt-C films were investigated. FePt films exhibited L1{sub 0} (001) texture at various SiO{sub x} doping concentrations. The coercivities were as large as 28 kOe and the slope of M-H loop at coercivity was approximately equal to 1, suggesting that FePt grains were well exchange decoupled. Grain size was only slightly reduced after introducing the CrRu-SiO{sub x} underlayer. But the contact angle between the FePt grains and the MgO intermediate layer around 135 deg. indicated the a MgO intermediate layer was not favored for smaller grains to obtain good L1{sub 0} (001) texture. X-ray photoelectron spectroscopy in-depth profile showed that Si diffused into a whole FePt-C layer and C diffused to the film surface.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Tuning 2D magnetism in Fe 3+X GeTe 2 films by element doping

Two-dimensional (2D) ferromagnetic materials have been discovered with tunable magnetism and orbital-driven nodal-line features. Controlling the 2D magnetism in exfoliated nanoflakes via electric/magnetic fields enables a boosted Curie temperature (T C ) or phase transitions. One of the challenges, however, is the realization of high T C 2D magnets that are tunable, robust and suitable for large scale fabrication. Here, we report molecular-beam epitaxy growth of wafer-scale Fe 3+X GeTe 2 films with T C above room temperature. By controlling the Fe composition in Fe 3+X GeTe 2 , a continuously modulated T C in a broad range of 185–320 K has been achieved. This widely tunable T C is attributed to the doped interlayer Fe that provides a 40% enhancement around the optimal composition X = 2. We further fabricated magnetic tunneling junction device arrays that exhibit clear tunneling signals. Our results show an effective and reliable approach, i.e. element doping, to producing robust and tunable ferromagnetism beyond room temperature in a large-scale 2D Fe 3+X GeTe 2 fashion.

2D ferromagnetic material↗
DOE OSTI2016

Structural, mechanical, and magnetic properties of GaFe{sub 3}N thin films

Using the density-functional theory, the structural, mechanical, and magnetic properties were investigated for different GaFe{sub 3}N configurations: ferromagnetic, ferrimagnetic, paramagnetic, and nonmagnetic. Ferrimagnetic and high-spin ferromagnetic states exhibit the lowest energy and are the competing ground states as the total energy difference is 0.3 meV/atom only. All theoretically predicted values could be fully confirmed by experiments. For this, the authors synthesized phase pure, homogeneous, and continuous GaFe{sub 3}N films by combinatorial reactive direct current magnetron sputtering. Despite the low melting point of gallium, the authors succeeded in the growth of GaFe{sub 3}N films at a temperature of 500 °C. Those thin films exhibit a lattice parameter of 3.794 Å and an elastic modulus of 226 ± 20 GPa. Magnetic susceptibility measurements evidence a magnetic phase transitions at 8.0 ± 0.1 K. The nearly saturated magnetic moment at ±5 T is about 1.6 μB/Fe and is close to the theoretically determined magnetic moment for a ferrimagnetic ordering (1.72 μB/Fe).

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Observation of Quantum Anomalous Hall Effect and Exchange Interaction in Topological Insulator/Antiferromagnet Heterostructure

Integration of a quantum anomalous Hall insulator with a magnetically ordered material provides an additional degree of freedom through which the resulting exotic quantum states can be controlled. Here, an experimental observation is reported of the quantum anomalous Hall e?ect in a magnet-ically-doped topological insulator grown on the antiferromagnetic insulator Cr?O?. The exchange coupling between the two materials is investigated using ?eld-cooling-dependent magnetometry and polarized neutron re?ec-tometry. Both techniques reveal strong interfacial interaction between the antiferromagnetic order of the Cr?O? and the magnetic topological insulator, manifested as an exchange bias when the sample is ?eld-cooled under an out-of-plane magnetic ?eld, and an exchange spring-like magnetic depth pro?le when the system is magnetized within the ?lm plane. These results identify antiferromagnetic insulators as suitable candidates for the manipula-tion of magnetic and topological order in topological insulator ?lms.

Pan, Lei↗
DOE OSTI2014

Magnetic properties of Mn{sub 3}O{sub 4} film with a coexistence of two preferential orientations

A Mn{sub 3}O{sub 4} film with a coexistence of two preferential orientations has been grown on a Pt(111)//Si(100) substrate by plasma-assisted molecular beam epitaxy. The structural characteristics and chemical compositions of the film are investigated by using X-ray diffraction, Raman, and X-ray photoelectron spectra in detail. Together with the magnetic tests, the film is demonstrated to be a polycrystalline hausmannite Mn{sub 3}O{sub 4} with no other impurities. Moreover, the hysteresis loops of the film are found to display a step or a characteristic shrinking at low fields. On the other hand, similar magnetic characteristics have also been discovered on the film with two phases grown on a MgAl{sub 2}O{sub 4}(001) substrate. In our opinion, considering the large magnetocrystalline anisotropy and shape anisotropy of the single crystal Mn{sub 3}O{sub 4} film reported in previous works, the special structures and phases of the two films result in both of them as soft+hard magnetic composites, in agreement with some other reports.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Tuning the Magnetic Properties of CrI 3 Using Ni Thin Film Deposition for Applications in Spintronic Devices

Chromium triiodide (CrI 3 ), a van der Waals magnet, has recently been shown to host Ising ferromagnetism down to the monolayer limit. It is a potentially important material in 2D magnet-based applications, such as magnetic sensors and spintronic devices. Prior studies have revealed the coexistence of two different types of interlayer magnetic coupling, the antiferromagnetic (AFM) coupling near the surface and ferromagnetic (FM) coupling in the deep bulk layers, in pristine CrI 3 crystals below the Curie temperature. In this study, we used Ni thin film deposition to tune the surface magnetic states in bulk CrI 3 . A nanometer thickness (4 nm thick) of Ni was deposited on the surface of CrI 3 using electron-beam evaporation to form a Ni/CrI 3 heterostructure. The deposited Ni thin layer forms nanoclusters that completely cover the CrI 3 surface. Magnetic states of CrI 3 before and after Ni deposition are probed by ultralow-frequency magneto-Raman spectroscopy. Instead of seeing three spin wave branches in Raman scattering below 2 T, as in pristine bulk CrI 3 , in Ni/CrI 3 , we observe only a single spin wave branch, which is softened compared to that in pristine CrI 3 and displays a Zeeman shift under an out-of-plane magnetic field up to 7 T. This observation reveals that the AFM layers on pristine CrI 3 crystal surfaces transit into FM layers, so the entire CrI 3 crystal is in the FM state after Ni deposition. First-principles calculations show that Ni atoms tend to diffuse into the CrI 3 lattice, and the FM interlayer coupling has a much lower energy than AFM coupling in the presence of Ni atoms. As a result, our studies show that the magnetic state of CrI 3 can be modified through deposition of a thin metal layer on the surface, offering a route for controlling the spin degree of freedom in van der Waals magnets.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Anisotropy of electrical and magnetic transport properties of epitaxial SrRuO{sub 3} thin films

SrRuO{sub 3} (SRO) thin films with different thickness (2–70 nm) have been grown on (001), (110), and (111) SrTiO{sub 3} substrates. The (001)-SRO films (2–8 nm) exhibited smooth flat surfaces whereas the (110)- and (111)-SRO films featured a faceted island structure. Room temperature resistivity and residual resistivity are the lowest for the (111)-SRO films (30–70 nm). Over all thicknesses, we observed enhanced magnetization in the (111)-SRO films (∼4 μ{sub B}/Ru) compared with that for the (001)- and (110)-SRO films (∼2 μ{sub B}/Ru and ∼3 μ{sub B}/Ru, respectively), suggesting a low-spin state t{sub 2g}(3↑,1↓), high-spin state t{sub 2g}(3↑)e{sub g}(1↑), and mixed low- and high-spin states for the (001)-, (111)-, and (110)-SRO films, respectively. The dependence of resistivity on temperatures near T{sub C} follows a power law with exponent β = 0.312 and β = 0.363 for the (110)- and (111)-SRO films, respectively. These critical exponents are consistent with magnetic data with scaling law M = C (T{sub C} − T){sup β}. At low temperatures, dM/dT and dρ/dT show a linear relationship in the temperature range for the Fermi liquid. These results suggest that the intrinsic electrical and magnetic transport properties are coupled.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗