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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 73 records · Page 4
All sourcesNASA NTRSDOE OSTI
DOE OSTI2019

Competing effects of Kerr nonlinearity and K-photon absorptions on continuous-wave laser inscriptions

Material processing with laser offers a reliable tool in a large variety of micromachining technology, ranging from on-disc inscriptions to cell ablations through DNA combing and imprinting for fabrications of micro and nanofluidic devices. In these applications lasers are designed to operate in specific regimes characterized by their powers and wavelengths, so understanding characteristic properties of the distinct possible laser operation regimes turns out to be a relevant step toward optimization of their uses as well as improvement of the technology. In this work we examine the stability of a model of femtosecond laser intended for laser inscriptions in nonlinear transparent media, taking into consideration the laser-induced material damage and multiphoton ionization. The model approximates the laser dynamics by a complex Ginzburg–Landau equation with a Kerr plus a high-order nonlinear term accounting for K-photon ionization, coupled to a Drude equation for time evolution of the electron plasma density. Following the Benjamin–Feir instability theory it is shown that depending on characteristic parameters of the model, multiphoton ionization processes can stabilize continuous-wave or pulse regimes. These parameters include the group-delay dispersion, the linear and nonlinear gains, the phase shift per roundtrip and the plasma ionization rate.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2025

Multimodal correlative study of Hall transport and magnetic phases in Fe/Gd multilayer systems

The Fe/Gd multilayer system hosts a number of magnetic phases, such as stripe, mixed stripe and skyrmion, skyrmion lattice, and isolated skyrmions for a wide range of temperature and magnetic field. We report different Hall transport signals in a Fe/Gd system through multimodal correlative resonant soft x-ray scattering (RSXS), Hall effect, magneto-optic Kerr effect, and transmission x-ray microscopy measurements. The simultaneous nature of the RSXS and Hall transport measurements allowed us to accurately connect various features in the transport data with the specific magnetic phases. We found that the topological Hall effect (THE) shows peaks with opposite signs, which we attribute to two different mechanisms. Our multimodal correlative study indicates that the sign reversal in THE occurs when the system transforms to and from a skyrmion lattice and low density isolated skyrmion phases. We propose that the skyrmion lattice contributes to the THE through a Berry phase induced emergent magnetic field mechanism in one case, and a skew scattering mechanism corresponding to the isolated low density skyrmion state.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Controlling the anisotropy and domain structure with oblique deposition and substrate rotation

Effect of substrate rotation on anisotropy and domain structure for a thin ferromagnetic film has been investigated in this work. For this purpose Co films with 10 nm thickness have been prepared by sputtering with oblique angle of incidence for various substrate rotations. This method of preparation induces a uniaxial anisotropy due to shadow deposition effect. The magnetization reversal is studied by magneto-optic Kerr effect (MOKE) based microscope in the longitudinal geometry. The Co films prepared by rotating the substrate with 10 and 20 rpm weakens the anisotropy but does not completely give isotropic films. But this leads to high dispersion in local grain anisotropy resulting in ripple and labyrinth domains. It is observed that the substrate rotation has moderate effect on uniaxial anisotropy but has significant effect on the magnetization reversal process and the domain structure.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Fine-tuning of canted magnetization in stepped Fe films through thickness variation, Au capping, and quantum confinement

Here, we present a joint experimental and theoretical study that demonstrates how to efficiently control a canted state of magnetization in Fe films grown on Ag(001) vicinal surface and precisely characterize it with the magneto-optical Kerr effect. It is shown that by employing different mechanisms to tune the magnetization tilting angle, any magnetization orientation within the plane perpendicular to the step edges can be achieved. In particular, increasing the Fe film thickness leads to continuous rotation of the magnetization easy axis toward the film surface and the sense of this rotation in uncovered films is opposite to that in films covered with Au. Another tuning mechanism is provided by oscillatory changes of the tilting angle at low temperatures due to formation of quantum well states in Fe films. The observed canting of magnetization is explained within a phenomenological model by an interplay of the shape anisotropy and two magnetocrystalline anisotropy terms, perpendicular and step-induced anisotropies, which results in an effective uniaxial magnetic anisotropy. The fitted thickness dependencies of the anisotropy constants accurately reproduce experimental variations of the tilting angle with both Fe and Au thicknesses as well as transient changes of the magnetization orientation in ultrathin Fe films upon submonolayer Au coverage, observed with spin-polarized low-energy electron microscopy.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Modulation of interlayer exchange coupling strength in magnetic tunnel junctions via strain effect

Interlayer exchange coupling of two ferromagnetic electrodes separated by a thin MgO tunnel barrier is investigated using magneto-optical Kerr effect. We find that the coupling field can be reduced by more than 40% as the thickness of a top Ta capping layer increases from 0.5 to 1.2 nm. In contrast, a similar film stack with an additional 3 nm Ru capping layer displays no such dependence on Ta thickness. Transmission electron microscopy study shows that the oxidation of the exposed Ta capping layer induces changes in the crystalline structures of the underlying films, giving rise to the observed reduction of the interlayer coupling field.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Interplay between magnetism and superconductivity in UTe 2

Time-reversal symmetry breaking (TRSB) in UTe 2 was inferred from observations of a spontaneous Kerr response in the superconducting state after cooling in zero magnetic field, while a finite c-axis magnetic field training was further used to determine the nature of the nonunitary composite order parameter of this material. Here, we present an extensive study of the magnetic-field-trained Kerr effect, which unveils a unique critical state of pinned “ferromagnetic vortices.” We show that a remanent Kerr signal appears following the removal of a training magnetic field, which reflects the response to the TRSB order parameter and the external magnetic field through the paramagnetic susceptibility. Furthermore, this unambiguously demonstrates the importance of the magnetic fluctuations and their intimate relation to the composite order parameter. Focusing the beam on the center of the sample, we are able to accurately determine the maximum field that is screened by the critical state and the respective critical current. Measurements in the presence of magnetic field show the tendency of the superconductor to produce shielding currents that oppose the increase in vortex-induced magnetization due to the diverging paramagnetic susceptibility.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Entanglement concentration and purification of two-mode squeezed microwave photons in circuit QED

We present a theoretical proposal for a physical implementation of entanglement concentration and purification protocols for two-mode squeezed microwave photons in circuit quantum electrodynamics (QED). First, we give the description of the cross-Kerr effect induced between two resonators in circuit QED. Then we use the cross-Kerr media to design the effective quantum nondemolition (QND) measurement on microwave-photon number. By using the QND measurement, the parties in quantum communication can accomplish the entanglement concentration and purification of nonlocal two-mode squeezed microwave photons. We discuss the feasibility of our schemes by giving the detailed parameters which can be realized with current experimental technology. Our work can improve some practical applications in continuous-variable microwave-based quantum information processing.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Tunable magnetization dynamics in disordered FePdPt ternary alloys: Effects of spin orbit coupling

The magnetization dynamics of disordered Fe₀.₅(Pd{sub 1–x}Pt{sub x})₀.₅ alloy films was studied by time-resolved magneto-optical Kerr effect and ferromagnetic resonance. The intrinsic Gilbert damping parameter α₀ and the resonance linewidth change linearly with the Pt atomic concentration. In particular, the induced in-plane uniaxial anisotropy constant K{sub U} also increases for x increasing from 0 to 1. All these results can be attributed to the tuning effect of the spin orbit coupling. For the disordered ternary alloys, an approach is proposed to control the induced in-plane uniaxial anisotropy, different from conventional thermal treat methods, which is helpful to design and fabrications of spintronic devices.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Hydrogen-mediated long-range magnetic ordering in Pd-rich alloy film

The effect of hydrogenation on a 14 nm Co{sub 14}Pd{sub 86}/Al{sub 2}O{sub 3}(0001) thin film was investigated on the basis of the magnetooptical Kerr effect. After exposure to H{sub 2} gas, the squareness of the hysteresis loop showed a large transition from approximately 10% to 100% and the saturation Kerr signal was reduced to nearly 30% of the pristine value. The reversibility of the transition was verified and the response time was within 2–3 s. These observations indicate that the hydride formation transformed the short-range coupled and disordered magnetic state of the Co{sub 14}Pd{sub 86} film to a long-range-ordered ferromagnetic state and induced appreciable decrease in the magnetic moment. The enhanced long-range-ordering and the reduction of the magnetic moment were attributed to the change of electronic structure in Co{sub 14}Pd{sub 86} with hydrogen uptake.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
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 OSTI2021

Electrical and optical characterizations of spin-orbit torque

To further reduce the energy consumption in spin–orbit torque devices, it is crucial to precisely quantify the spin–orbit torque (SOT) in different materials and structures. In this work, heavy metal/ferromagnet and heavy metal/ferrimagnet heterostructures are employed as the model systems to compare the electrical and optical methods for the SOT characterization, which are based on the anomalous Hall effect and the magneto-optical Kerr effect, respectively. It is found that both methods yield the consistent SOT strength for the current-driven magnetization switching measurements and the harmonic measurements. Our results suggest that the optical method is a feasible and reliable tool to investigate SOT, which is a powerful way to develop insulator-based magnetic systems in the future.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

VO{sub 2}-like thermo-optical switching effect in one-dimensional nonlinear defective photonic crystals

A new approach to achieve VO{sub 2}-like thermo-optical switching in a one-dimensional photonic crystal by the combination of thermo-optical and optical Kerr effects was proposed and numerically demonstrated in this study. The switching temperature and the hysteresis width can be tuned in a wide temperature range. Steep transition, high optical contrast, and low pumping power can be achieved at the same time. This kind of one-dimensional photonic crystal-based bistable switch will be low-cost, easy-to-fabricate, and versatile in practical applications compared with traditional VO{sub 2}-type one.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Effect of annealing on exchange stiffness of ultrathin CoFeB film with perpendicular magnetic anisotropy

The effect of annealing on the exchange stiffness of ultrathin CoFeB films with perpendicular magnetic anisotropy was investigated through the observation of magnetic domain structures by magneto-optic Kerr-effect microscopy. A significant reduction of the exchange stiffness after an annealing process was observed, which is in striking contrast to a previous report that studied thick CoFeB films with in-plane magnetic anisotropy. Our results suggest that interdiffusion of non-magnetic atoms from the adjacent layer into CoFeB layer reduces the exchange stiffness, which explains the difference between the annealing effect on ultrathin and the thick CoFeB films. Thus, it is critical to prevent annealing-induced interdiffusion in order to suppress undesired sub-volume switching that degrades thermal stability of a free-layer in spin-transfer torque magnetic random access memory.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

The in-plane anisotropic magnetic damping of ultrathin epitaxial Co{sub 2}FeAl film

The in-plane orientation-dependent effective damping of ultrathin Co{sub 2}FeAl film epitaxially grown on GaAs(001) substrate by molecular beam epitaxy (MBE) has been investigated by employing the time-resolved magneto-optical Kerr effect (TR-MOKE) measurements. It is found that the interface-induced uniaxial anisotropy is favorable for precession response and the anisotropy of precession frequency is mainly determined by this uniaxial anisotropy, while the magnetic relaxation time and damping factor exhibit the fourfold anisotropy at high-field regime. The field-independent anisotropic damping factor obtained at high fields indicates that the effective damping shows an intrinsic fourfold anisotropy for the epitaxial Co{sub 2}FeAl thin films.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Magneto-optical spectroscopy of ferromagnetic shape-memory Ni-Mn-Ga alloy

Magneto-optical properties of single crystal of Ni{sub 50.1}Mn{sub 28.4}Ga{sub 21.5} magnetic shape memory alloy in martensite and austenite phase were systematically studied. Crystal orientation was approximately along (100) planes of parent cubic austenite. At room temperature, the sample was in modulated 10M martensite phase and transformed to cubic austenite at 323 K. Spectral dependence of polar magneto-optical Kerr effect was obtained by generalized magneto-optical ellipsometry with rotating analyzer in the photon energy range from 1.2 to 4 eV, and from room temperature to temperature above the Curie point. The Kerr rotation spectra exhibit prominent features typical for complexes containing Mn atoms. Significant spectral changes during transformation to austenite can be explained by different optical properties caused by changes in density of states near the Fermi energy.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Multiple soliton self-frequency shift cancellations in a temporally tailored photonic crystal fiber

We report the generation of multiple soliton self-frequency shift cancellations in a temporally tailored tellurite photonic crystal fiber (PCF). The temporally regulated group velocity dispersion (GVD) is generated in the fiber by soliton induced optical Kerr effect. Two red-shifted dispersive waves spring up when two Raman solitons meet their own second zero-dispersion-wavelengths in the PCF. These results show how, through temporally tailored GVD, nonlinearities can be harnessed to generate unexpected effects.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Effect of Ga{sup +} irradiation in molecular-beam epitaxy grown Pt/Co/Pt thin films studied by magneto-optic spectroscopy

In-depth profile changes induced by Ga{sup +} ion irradiation in Pt(5 nm)/Co(3.3 nm)/Pt(20 nm)/Mo(20 nm) sandwiches MBE grown on Al{sub 2}O{sub 3} substrates are deduced from complex magneto-optic polar Kerr effect (PMOKE) measurements at photon energies between 1 and 5 eV. The Ga{sup +} irradiation stimulates a redistribution of Pt and Co and leads to broadening of alloyed regions at Pt-Co and Co-Pt interfaces, which is evaluated using PMOKE spectra. The effect of four Ga{sup +} fluences D between zero and 6 × 10{sup 15} Ga{sup +}/cm{sup 2} was studied. The observed PMOKE azimuth rotation peak centered at 4.5 eV reaches the maximum of 0.42° at D = 1 × 10{sup 15} Ga{sup +}/cm{sup 2} and becomes thus enhanced by a factor of 3.2 with respect to that in the non-irradiated sample. At D = 6 × 10{sup 15} Ga{sup +}/cm{sup 2} the peak amplitude falls to 0.05°. To find the in-depth profile of Co concentration s in the sandwiches as a function of D, the PMOKE azimuth rotation and ellipticity spectra are compared with a multilayer model, where ideal flat interfaces are replaced by sequences of Co{sub s}Pt{sub 1−s} layers. The dependence on D is compared with that evaluated by simulations of the structural effects of ion irradiation. At the highest D, the irradiation produces an almost complete erosion of the top Pt and Co accompanied by mixing at the Pt-Mo interface.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Optical quantum bit string comparator

Quantum computation has attracted much attention and several quantum algorithms have been proposed in the literature. However, the hardware able to implement such algorithms is still a challenge. In this work, we provide an optical setup for implementation of a quantum bit string comparator, QBSC, for polarization-based qubit, using the non-linear Kerr effect. The QBSC is an important structure for implementation of conditional statements in quantum algorithms.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗