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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 289 records · Page 16
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Microwave-Hydrothermal Synthesis of Y{sub 3}Fe{sub 5}O{sub 12} Nanoparticles: Sintering Temperature Effect on Structural, Magnetic and Dielectric Properties

Herein, we report a facile microwave-assisted hydrothermal method for the preparation of yttrium iron garnet (YIG, Y{sub 3}Fe{sub 5}O{sub 12})nanocrystals and realized the synergy between its structural and magnetic properties. The as-prepared powder thermal stability was characterized in detail by using thermogravimetric-differential thermal analysis (TG-DTA). The synthesized powder was microwave sintered at six different temperatures ranging from 973 to 1373 K for 60 min and characterized using X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM). Magnetic properties were investigated by vibrating-sample magnetometer (VSM) and ferromagnetic resonance (FMR) studies. The frequency variation of dielectric and magnetic properties was measured in the frequency range of 100 kHz–1.8 GHz. The observed magnetic and dielectric parameters such as saturation magnetization, coercivity, permeability, linewidth, dielectric constant and magnetodielectric losses are strongly affected by sintering temperature. The saturation magnetization and coercivity of YIG were found in the range 14.4–28.0 emu/g and 35–87 Oe, respectively. Furthermore, the FMR linewidth decreases from 844 to 204 Oe. This analysis undoubtedly demonstrates that the synthesis method and sintering temperature have the prominent effect on the electrical and magnetic properties of YIG and would be very useful for the garnet-based microwave device applications.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2013

Challenges and opportunities for multi-functional oxide thin films for voltage tunable radio frequency/microwave components

There has been significant progress on the fundamental science and technological applications of complex oxides and multiferroics. Among complex oxide thin films, barium strontium titanate (BST) has become the material of choice for room-temperature-based voltage-tunable dielectric thin films, due to its large dielectric tunability and low microwave loss at room temperature. BST thin film varactor technology based reconfigurable radio frequency (RF)/microwave components have been demonstrated with the potential to lower the size, weight, and power needs of a future generation of communication and radar systems. Low-power multiferroic devices have also been recently demonstrated. Strong magneto-electric coupling has also been demonstrated in different multiferroic heterostructures, which show giant voltage control of the ferromagnetic resonance frequency of more than two octaves. This manuscript reviews recent advances in the processing, and application development for the complex oxides and multiferroics, with the focus on voltage tunable RF/microwave components. The over-arching goal of this review is to provide a synopsis of the current state-of the-art of complex oxide and multiferroic thin film materials and devices, identify technical issues and technical challenges that need to be overcome for successful insertion of the technology for both military and commercial applications, and provide mitigation strategies to address these technical challenges.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2013

Effects of ruthenium seed layer on the microstructure and spin dynamics of thin permalloy films

The spin dynamics and microstructure properties of a sputtered 12 nm Ni{sub 81}Fe{sub 19} thin film have been enhanced by the use of a ruthenium seed layer. Both the ferromagnetic resonance field and linewidth are enhanced dramatically as the thickness of ruthenium seed layer is increased. The surface anisotropy energy constant can also be largely tailored from 0.06 to 0.96 erg/cm{sup -2} by changing the seed layer thickness. The changes to the dynamics magnetization properties are caused by both ruthenium seed layer induced changes in the Ni{sub 81}Fe{sub 19} structure properties and surface topography properties. Roughness induced inhomogeneous linewidth broadening is also seen. The damping constant is highly tunable via the ruthenium thickness. This approach can be used to tailor both the structure and spin dynamic properties of thin Ni{sub 81}Fe{sub 19} films over a wide range. And it may benefit the applications of spin dynamics and spin current based devices.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Magnetic and elastic anisotropy in magnetorheological elastomers using nickel-based nanoparticles and nanochains

Nickel (Ni) based nanoparticles and nanochains were incorporated as fillers in polydimethylsiloxane (PDMS) elastomers and then these mixtures were thermally cured in the presence of a uniform magnetic field. In this way, macroscopically structured-anisotropic PDMS-Ni based magnetorheological composites were obtained with the formation of pseudo-chains-like structures (referred as needles) oriented in the direction of the applied magnetic field when curing. Nanoparticles were synthesized at room temperature, under air ambient atmosphere (open air, atmospheric pressure) and then calcined at 400 °C (in air atmosphere also). The size distribution was obtained by fitting Small Angle X-ray Scattering (SAXS) experiments with a polydisperse hard spheres model and a Schulz-Zimm distribution, obtaining a size distribution centered at (10.0 ± 0.6) nm with polydispersivity given by σ = (8.0 ± 0.2) nm. The SAXS, X-ray powder diffraction, and Transmission Electron Microscope (TEM) experiments are consistent with single crystal nanoparticles of spherical shape (average particle diameter obtained by TEM: (12 ± 1) nm). Nickel-based nanochains (average diameter: 360 nm; average length: 3 μm, obtained by Scanning Electron Microscopy; aspect ratio = length/diameter ∼ 10) were obtained at 85 °C and ambient atmosphere (open air, atmospheric pressure). The magnetic properties of Ni-based nanoparticles and nanochains at room temperature are compared and discussed in terms of surface and size effects. Both Ni-based nanoparticles and nanochains were used as fillers for obtaining the PDMS structured magnetorheological composites, observing the presence of oriented needles. Magnetization curves, ferromagnetic resonance (FMR) spectra, and strain-stress curves of low filler's loading composites (2% w/w of fillers) were determined as functions of the relative orientation with respect to the needles. The results indicate that even at low loadings it is possible to obtain magnetorheological composites with anisotropic properties, with larger anisotropy when using nanochains. For instance, the magnetic remanence, the FMR field, and the elastic response to compression are higher when measured parallel to the needles (about 30% with nanochains as fillers). Analogously, the elastic response is also anisotropic, with larger anisotropy when using nanochains as fillers. Therefore, all experiments performed confirm the high potential of nickel nanochains to induce anisotropic effects in magnetorheological materials.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2015

Suppression of spin pumping between Ni{sub 80}Fe{sub 20} and Cu by a graphene interlayer

We compare ferromagnetic resonance measurements of Permalloy Ni{sub 80}Fe{sub 20} (Py) films sputtered onto Cu(111) films with and without a graphene (Gr) interlayer grown by chemical vapor deposition before Py deposition. A two-angle sputtering method ensured that neither Gr nor Py was degraded by the sample preparation process. We find the expected damping enhancement from spin pumping for the Py/Cu case and no detectable enhancement for the Py/Gr/Cu case. Since damping is sensitive to effects other than spin pumping, we used magnetometry to verify that differences in Py magnetostatic properties are not responsible for the difference in damping. We attribute the suppression of spin pumping in Py/Gr/Cu to the large contact resistance of the Gr/Cu interface.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Large spin-to-charge conversion in ultrathin gold-silicon multilayers

Investigation of the spin Hall effect in gold has triggered increasing interest over the past decade, since gold combines the properties of a large bulk spin-diffusion length and strong interfacial spin-orbit coupling. However, discrepancies between the values of the spin Hall angle of gold reported in the literature have brought into question the microscopic origin of the spin Hall effect in Au. Here, we investigate the thickness dependence of the effective spin-charge conversion efficiency in single Au films and ultrathin Au/Si multilayers by nonlocal transport and spin-torque ferromagnetic resonance measurements. We show that the inferred effective spin-charge conversion efficiency is strongly enhanced in ultrathin Au/Si multilayers, reaching exceedingly large values of 0.99 ± 0.34 when the thickness of the individual Au layers is scaled down to 2 nm. These findings reveal the coexistence of a strong interfacial spin-to-charge conversion effect, which becomes dominant in ultrathin Au, and bulk spin Hall effect with a relatively low bulk spin Hall angle of 0.012 ± 0.005. Furthermore, our experimental results suggest the key role of the intrinsic spin Hall effect enhancement along with a strong interfacial spin-orbit coupling-related effect in the large spin-to-charge conversion in ultrathin Au.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Non-proportionality of magnetic anisotropy and damping in CoFeB/MgO-based systems

We study the relationship between anisotropy K and damping α in MgO/CoFeB/Ta/CoFeB/MgO/cap films using vector network analyzer ferromagnetic resonance. Capping the stack with Ta and changing the thickness of the top MgO layer allow us to create significant variations in anisotropy while keeping the thickness and process conditions of the magnetic layer constant. The change in anisotropy can be attributed to the degradation of the CoFeB/MgO interface due to Ta intermixing with MgO upon deposition. This hypothesis is supported by measurements of similar samples with the bottom MgO thickness varied instead, which exhibit no significant change in anisotropy. This method of varying K allows identifying a regime where a spin reorientation transition occurs while α remains constant, proving the non-proportionality of K and α in perpendicular CoFeB/MgO systems. The effects of changing the capping layer material, notably Ru, are also discussed.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Annealing influence on the Gilbert damping parameter and the exchange constant of CoFeB thin films

We present a study of the influence of the annealing temperature T{sub a} on the magnetic properties of Co{sub 40}Fe{sub 40}B{sub 20} thin films. Using a vector network analyzer ferromagnetic resonance (VNA-FMR) setup and the magneto-optical Kerr effect, the dependence of the Gilbert damping parameter α, the exchange constant A, the saturation magnetization as well as the coercive field and the Kerr signal on T{sub a} is reported. Additionally, the correlation with the crystalline properties of the films studied by X-ray diffractometry is discussed. We found that while the damping parameter α and the coercive field show sharp changes starting at a certain T{sub a} value, the exchange constant A and the Kerr signal show a steady evolution. A differential modification of the film surface compared to the bulk is discussed as a possible reason. In any case, we found that the low damping values are preserved at the first onset of crystallization.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Tunable spin wave spectra in two-dimensional Ni{sub 80}Fe{sub 20} antidot lattices with varying lattice symmetry

Ferromagnetic antidot lattices are important systems for magnetic data storage and magnonic devices, and understanding their magnetization dynamics by varying their structural parameters is an important problems in magnetism. Here, we investigate the variation in spin wave spectrum in two-dimensional nanoscale Ni{sub 80}Fe{sub 20} antidot lattices with lattice symmetry. By varying the bias magnetic field values in a broadband ferromagnetic resonance spectrometer, we observed a stark variation in the spin wave spectrum with the variation of lattice symmetry. The simulated mode profiles showed further difference in the spatial nature of the modes between different lattices. While for square and rectangular lattices extended modes are observed in addition to standing spin wave modes, all modes in the hexagonal, honeycomb, and octagonal lattices are either localized or standing waves. In addition, the honeycomb and octagonal lattices showed two different types of modes confined within the honeycomb (octagonal) units and between two such consecutive units. Simulated internal magnetic fields confirm the origin of such a wide variation in the frequency and spatial nature of the spin wave modes. The tunability of spin waves with the variation of lattice symmetry is important for the design of future magnetic data storage and magnonic devices.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Enhanced magnetic damping in La{sub 0.7}Sr{sub 0.3}MnO{sub 3} capped by normal metal layer

La{sub 0.7}Sr{sub 0.3}MnO{sub 3}(LSMO) and Pt capped La{sub 0.7}Sr{sub 0.3}MnO{sub 3} 20 nm thick films have been grown on SrTiO{sub 3} substrates by pulsed laser deposition. Microstrip line ferromagnetic resonance (MS-FMR) technique is then used to investigate their magnetic dynamic properties and to particularly measure the damping constant based on the frequency dependence of microwave absorption linewidth. The results show that the effective damping constant of LSMO(20nm)/Pt(5.5nm) is three times larger than that of LSMO(20nm) and the films present weak in-plane uniaxial anistropy. The enhancement of the magnetic damping constant due to the capping of Pt is the manifestaction of the generation of spin current in Pt layer. Furthermore, the spin current induces an inverse spin Hall effect (ISHE) in LSMO(20nm)/Pt(5.5nm) system, measured using the FMR in cavity with 9.8 GHz excitation frequency. The linear dependence of ISHE on microwave power validates the mechenism of spin pumping in this bilayer system.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Moderate positive spin Hall angle in uranium

We report measurements of spin pumping and the inverse spin Hall effect in Ni{sub 80}Fe{sub 20}/uranium bilayers designed to study the efficiency of spin-charge interconversion in a super-heavy element. We employ broad-band ferromagnetic resonance on extended films to inject a spin current from the Ni{sub 80}Fe{sub 20} (permalloy) into the uranium layer, which is then converted into an electric field by the inverse spin Hall effect. Surprisingly, our results suggest a spin mixing conductance of order 2 × 10{sup 19} m{sup −2} and a positive spin Hall angle of 0.004, which are both merely comparable with those of several transition metals. These results thus support the idea that the electronic configuration may be at least as important as the atomic number in governing spin pumping across interfaces and subsequent spin Hall effects. In fact, given that both the magnitude and the sign are unexpected based on trends in d-electron systems, materials with unfilled f-electron orbitals may hold additional exploration avenues for spin physics.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2019

Layer-dependent spin-orbit torques generated by the centrosymmetric transition metal dichalcogenide β–MoTe 2

Single-crystal materials with sufficiently low crystal symmetry and strong spin-orbit interactions can be used to generate novel forms of spin-orbit torques on adjacent ferromagnets, such as the out-of-plane antidamping torque previously observed in WTe 2 /ferromagnet heterostructures. Here, we present measurements of spin-orbit torques produced by the low-symmetry material β-MoTe 2 , which, unlike WTe 2 , retains bulk inversion symmetry. Here, we measure spin-orbit torques on β-MoTe 2 /Permalloy heterostructures using spin-torque ferromagnetic resonance as a function of crystallographic alignment and MoTe 2 thickness down to the monolayer limit. We observe an out-of-plane antidamping torque with a spin-torque conductivity as strong as 1/3 of that of WTe 2 , demonstrating that the breaking of bulk inversion symmetry in the spin-generation material is not a necessary requirement for producing an out-of-plane antidamping torque. We also measure an unexpected dependence on the thickness of the β-MoTe 2 —the out-of-plane antidamping torque is present in MoTe 2 /Permalloy heterostructures when the β-MoTe 2 is a monolayer or trilayer thick, but goes to zero for devices with bilayer β-MoTe 2 .

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Spin Hall effects in metallic antiferromagnets – perspectives for future spin-orbitronics

We investigate angular dependent spin-orbit torques from the spin Hall effect in a metallic antiferromagnet using the spin-torque ferromagnetic resonance technique. The large spin Hall effect exists in PtMn, a prototypical CuAu-I-type metallic antiferromagnet. By applying epitaxial growth, we previously reported an appreciable difference in spin-orbit torques for c- and a-axis orientated samples, implying anisotropic effects in magnetically ordered materials. In this work we demonstrate through bipolar-magnetic-field experiments a small but noticeable asymmetric behavior in the spin-transfer-torque that appears as a hysteresis effect. We also suggest that metallic antiferromagnets may be good candidates for the investigation of various unidirectional effects related to novel spin-orbitronics phenomena.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Influence of the interface on the magnetic properties of ferromagnetic ultrathin films with various adjacent copper thicknesses

The interface and magnetic properties of two series of films with Ta(5 nm)/Fe{sub 20}Ni{sub 80}Nd{sub 0.017}(3 nm)/Cu(t nm) and Ta(5 nm)/Cu(t nm)/Fe{sub 50}Co{sub 50}Gd{sub 0.07}(3 nm)/Cu(2 nm) structures have been investigated by atomic force microscopy, vibrating sample magnetometer, and ferromagnetic resonance (FMR). The roughness of all films increases with increasing copper thickness, which causes the different grain sizes in the surface of films. The coercivity of FeCo-Gd films increases with increasing thickness of inserted Cu layer while decreases with increasing thickness of Cu capping layer for FeNi-Nd films. FMR linewidth exhibits huge dependence on the thickness of inserted Cu layer for FeCo-Gd films, increasing from 2270 to 3680 Oe, which comes from the additional contribution of effect of the two-magnon scattering. And the thickness of Cu capping layer shows also an influence on FMR linewidth of FeNi-Nd films, increasing from 190 to 320 Oe, which mainly comes from intrinsic FMR linewidth and plus minor inhomogeneous broadening. All of these extrinsic linewidth broadening are related to the interface roughness.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Optical detection of spin Hall effect in metals

Optical techniques have been widely used to probe the spin Hall effect in semiconductors. In metals, however, only electrical methods such as nonlocal spin valve transport, ferromagnetic resonance, or spin torque transfer experiments have been successful. These methods require complex processing techniques and measuring setups. We show here that the spin Hall effect can be observed in non-magnetic metals such as Pt and β-W, using a standard bench top magneto-optical Kerr system with very little sample preparation. Applying a square wave current and using Fourier analysis significantly improve our detection level. One can readily determine the angular dependence of the induced polarization on the bias current direction (very difficult to do with voltage detection), the orientation of the spin Hall induced polarization, and the sign of the spin Hall angle. This optical approach is free from the complications of various resistive effects, which can compromise voltage measurements. This opens up the study of spin Hall effect in metals to a variety of spin dynamic and spatial imaging experiments.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Magnonics: Selective heat production in nanocomposites with different magnetic nanoparticles

We theoretically study Ferromagnetic Resonance (FMR) in nanocomposites focusing on the analysis of heat production. It is demonstrated that at the FMR frequency, the temperature of nanoparticles can be raised at the rate of a few degrees per second at the electromagnetic (EM) irradiation power equivalent to the sunlight power. Thus, using FMR, one can initiate either surface or bulk reaction in the vicinity of a particular magnetic inclusion by purposely delivering heat to the nanoscale at a sufficiently fast rate. We examined the FMR features in (a) the film with a mixture of nanoparticles made of different materials; (b) the laminated films where each layer is filled with a particular type of magnetic nanoparticles. It is shown that different nanoparticles can be selectively heated at the different bands of EM spectrum. This effect opens up new exciting opportunities to control the microwave assisted chemical reactions depending on the heating rate.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Engineering Gilbert damping by dilute Gd doping in soft magnetic Fe thin films

By analyzing the ferromagnetic resonance linewidth, we show that the Gilbert damping constant in soft magnetic Fe thin films can be enhanced by ∼6 times with Gd doping of up to 20%. At the same time, the magnetic easy axis remains in the film plane while the coercivity is strongly reduced after Gd inclusion. X-ray magnetic circular dichroism measurements reveal a strong increase in the orbital-to-spin moment ratio of Fe with increasing Gd concentration, in full agreement with the increase in the Gilbert damping obtained for these thin films. Combined with x-ray diffraction and vibrating sample magnetometry, the results demonstrate that the FeGd thin films with dilute Gd doping of up to 20% are promising candidates for spin-transfer-torque applications in soft magnetic devices, in which an enhanced damping is required.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Enhancement of the spin–orbit torque efficiency in W/Cu/CoFeB heterostructures via interface engineering

Here, the spin-torque ferromagnetic resonance signal and the spin Hall magnetoresistance induced by the spin Hall effect of W/Cu/CoFeB heterostructures with different Cu layer thicknesses (t Cu ) have been systemically studied. The effective spin mixing conductance $g^{↑↓}_{eff}$, the interfacial spin transparency T, and the real spin–orbit torque efficiency ( J s J c ) real show a significant increase compared to the W/CoFeB heterostructure. ( J s J c ) real reaches its maximum of ~0.54, increased up to ~50% at the optimized tCu ~ 0.52 nm according to our theoretical prediction. More importantly, the intrinsic spin Hall angle of W, $θ^{int}_{SH}$ ~ 0.79 ± 0.20, has also been obtained after the correction of the inverse spin Hall effect and T. This suggests that the Cu insertion improves the interface quality and, therefore, assists the spin transport in the heterostructures, which potentially improves the performance of next-generation spintronic devices.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗