Volume 117, Issue 21, 07 June 2015
Index of content:
- Photonics, Plasmonics, Lasers, and Optical Phenomena
117(2015); http://dx.doi.org/10.1063/1.4921873View Description Hide Description
A new approach to achieve VO2-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 VO2-type one.
117(2015); http://dx.doi.org/10.1063/1.4921874View Description Hide Description
We investigate the mode properties of planar dielectric aluminum-quinoline (Alq3) multilayer waveguides comprising one single or three equally spaced embedded nanometer-thin (∼10 nm thick) Alq3-Mg0.9 :Ag 0.1 composite metal-island layers. The plasmonic waveguides were fabricated by organic molecular beam deposition. Transverse magnetic (TM) and transverse electric (TE) modes were selectively excited using the m-line method. The symmetric plasmonic TM0 mode was launched in all waveguides and—in addition—two higher order plasmonic TM1 and TM2 modes were generated in waveguides comprising three metal layers. Other TM modes have hybrid dielectric-plasmonic characters, showing an increased effective refractive index when one electric field antinode is close to a metallic layer. TM modes which have all their antinode(s) in the dielectric layers propagate essentially like dielectric modes. TE modes with antinode(s) at the position of the metal layer(s) are strongly damped while the losses are low for TE modes comprising a node at the position of the composite metal film(s). The possibility to control the effective refractive index and the losses for individual hybrid plasmonic-dielectric TM and dielectric TE modes opens new design opportunities for mode selective waveguides and TM-TE mode couplers.
117(2015); http://dx.doi.org/10.1063/1.4921929View Description Hide Description
We consider the possibility of observing continuous third-harmonic generation using a strongly driven, single-band one-dimensional metal. In the absence of scattering, the quantum efficiency of frequency tripling for such a system can be as high as 93%. Combining the Floquet quasi-energy spectrum with the Keldysh Green's function technique, we derive a semiclassical master equation for a one-dimensional band of strongly and rapidly driven electrons in the presence of weak scattering by phonons. The power absorbed from the driving field is continuously dissipated by phonon modes, leading to a quasi-equilibrium in the electron distribution. We use the Kronig-Penney model with varying effective mass to establish the growth parameters of an InAs/InP nanowire near optimal for third harmonic generation at terahertz frequency range.
Vectorial magnetometry with second-harmonic generation effect in studies of implantation induced inhomogeneity in garnet films117(2015); http://dx.doi.org/10.1063/1.4921888View Description Hide Description
The magnetization-induced second-harmonic generation (MSHG) effect was applied to study changes of magnetization distribution caused by ions implantation in magnetic garnet film of (111) symmetry. The evolution of the magnetization vector in perpendicular magnetic field H was studied as a function of coherently rotated polarizers by an angle . The intensities exhibit completely different character as compared to the unimplanted film. The experimental results were explained in the frame of a phenomenological model of the MSHG effect, developed for the structure of 3m symmetry, composed of implanted and unimplanted sublayers. The theoretical approach allowed to determine the amplitudes and phases of nonlinear optical susceptibility tensor elements as well as the vector components. In contrast to the linear magneto-optical Faraday effect, application of nonlinear MSHG method allows for simultaneous determination of all components of the magnetization vector in single experiment. It was found that contributions from the sublayers associated with mz components enter to the MSHG effect with opposite signs due to difference in phases of . It is shown that vector in the implanted sublayer undergoes spatial evolution vs H during the magnetization process, with a complex trajectory corresponding to the reorientation of the from sample plane to the normal direction. The developed methods can be useful in studies of other magnetic materials with intrinsic or artificially introduced inhomogeneities.
- Electrical Discharges, Plasmas, and Plasma-Surface Interactions
Effect of N2 and Ar gas on DC arc plasma generation and film composition from Ti-Al compound cathodes117(2015); http://dx.doi.org/10.1063/1.4921952View Description Hide Description
DC arc plasma from Ti, Al, and Ti1−xAlx (x = 0.16, 0.25, 0.50, and 0.70) compound cathodes has been characterized with respect to plasma chemistry (charged particles) and charge-state-resolved ion energy for Ar and N2 pressures in the range 10−6 to 3 × 10−2 Torr. Scanning electron microscopy was used for exploring the correlation between the cathode and film composition, which in turn was correlated with the plasma properties. In an Ar atmosphere, the plasma ion composition showed a reduction of Al of approximately 5 at. % compared to the cathode composition, while deposited films were in accordance with the cathode stoichiometry. Introducing N2 above ∼5 × 10−3 Torr, lead to a reduced Al content in the plasma as well as in the film, and hence a 1:1 correlation between the cathode and film composition cannot be expected in a reactive environment. This may be explained by an influence of the reactive gas on the arc mode and type of erosion of Ti and Al rich contaminations, as well as on the plasma transport. Throughout the investigated pressure range, a higher deposition rate was obtained from cathodes with higher Al content. The origin of generated gas ions was investigated through the velocity rule, stating that the most likely ion velocities of all cathode elements from a compound cathode are equal. The results suggest that the major part of the gas ions in Ar is generated from electron impact ionization, while gas ions in a N2 atmosphere primarily originate from a nitrogen contaminated layer on the cathode surface. The presented results provide a contribution to the understanding processes of plasma generation from compound cathodes. It also allows for a more reasonable approach to the selection of composite cathode and experimental conditions for thin film depositions.
- Magnetism, Spintronics, and Superconductivity
117(2015); http://dx.doi.org/10.1063/1.4921828View Description Hide Description
A trilayer micromagnetic model based on the Landau-Lifshitz-Bloch equation of motion is utilized to study the properties of L10-FePt/TiN/L10-FePt pseudo spin valves (PSVs) in direct comparison with experiment. Theoretical studies give an insight on the crystallographic texture, magnetic properties, reversal behavior, interlayer coupling effects, and magneto-transport properties of the PSVs, in particular, with varying thickness of the top L10-FePt and TiN spacer. We show that morphological changes in the FePt layers, induced by varying the FePt layer thickness, lead to different hysteresis behaviors of the samples, caused by changes in the interlayer and intralayer exchange couplings. Such effects are important for the optimization of the PSVs due to the relationship between the magnetic properties, domain structures, and the magnetoresistance of the device.
117(2015); http://dx.doi.org/10.1063/1.4921887View Description Hide Description
In this work, a spintronic sensor based microwave holographic imaging system is developed, demonstrating the feasibility of microwave holographic imaging applications using a spintronic microwave sensor. The high sensitivity of the microwave phase measurement allows the coherent imaging of the target reconstructed in noise environments. Adapting the broadband measurement, not only the shape but also the distance of target can be determined, which implies that a three-dimensional imaging is achievable using a spintronic device.
117(2015); http://dx.doi.org/10.1063/1.4921902View Description Hide Description
In this work, the feasibility condition of Powder-In-Tube (PIT) processed wires of Fe(Se,Te) superconductor has been investigated. We faced several technical issues that are extensively described and discussed. In particular, we tested different metals and alloys as external sheaths (Cu, Ag, Nb, Ta, Ni, Fe, cupronickel, brass) concluding that the only sheath that does not affect substantially the Fe(Se,Te) phase is Fe. On the other hand, Fe sheath introduces excess iron in the Fe(Se,Te) phase, which affects the superconducting properties; we investigated the effects of the thermal treatments and of the powder composition in order to avoid it. The maximum Jc value obtained in our samples is 4 × 102 A/cm2, comparable to other published values of PIT conductors of the 11 family. We conclude that the fabrication of Fe(Se,Te) wires by PIT method is quite challenging and other approaches should be developed.
117(2015); http://dx.doi.org/10.1063/1.4921908View Description Hide Description
We investigate in details the inertial dynamics of a uniform magnetization in the ferromagnetic resonance context. Analytical predictions and numerical simulations of the complete equations within the Inertial Landau-Lifshitz-Gilbert (ILLG) model are presented. In addition to the usual precession resonance, the inertial model gives a second resonance peak associated to the nutation dynamics provided that the damping is not too large. The analytical resolution of the equations of motion yields both the precession and nutation angular frequencies. They are function of the inertial dynamics characteristic time τ, the dimensionless damping α, and the static magnetic field H. A scaling function with respect to ατγH is found for the nutation angular frequency, also valid for the precession angular frequency when ατγH ≫ 1. Beyond the direct measurement of the nutation resonance peak, we show that the inertial dynamics of the magnetization has measurable effects on both the width and the angular frequency of the precession resonance peak when varying the applied static field. These predictions could be used to experimentally identify the inertial dynamics of the magnetization proposed in the ILLG model.
Electrical field modification of dynamic magnetic properties in FeCo films grown onto [Pb(Mg1/3Nb2/3)O3]0.68-[PbTiO3]0.32(011) piezoelectric substrates with Ru underlayers117(2015); http://dx.doi.org/10.1063/1.4921909View Description Hide Description
A detailed investigation of electrical tuning of dynamic magnetization of the FeCo magnetic thin film grown onto a PMN-PT piezoelectric substrate was carried out based on the measurement of the zero-field permeability spectra under the application of a voltage across the thickness of the substrate. The resonance frequency can be tuned from 2.95 GHz to 5.9 GHz upon the application of a voltage on the sample in unpoled state. After poling, the resonance frequency of the sample can be tuned in the range from 4.75 GHz to 5.9 GHz. In addition, it was found that after poling the peak of the permeability spectra is broadened compared to before poling, which can be tentatively attributed to the magnetic anisotropy dispersion arising from the presence of the stress-induced anisotropy. The result is also discussed in conjunction with the angular measurement of the static hysteresis loops of the sample before and after poling.
117(2015); http://dx.doi.org/10.1063/1.4921973View Description Hide Description
The spin orientation dependence of magnetic hysteresis and microwave ferromagnetic resonance data are investigated in FeCoBSi amorphous thin films. Demagnetization effect allows the weak interface-rooted out-of-plane anisotropy to build up local spin orientation domains under the dominant in-plane anisotropy. As a result, two phase magnetization reversal and double-peak ferromagnetic resonance traces with varying damping behavior are observed. Due to the distribution of in-plane and out-of-plane spin orientations, the ferromagnetic resonance bandwidth has been extensively expanded with the full width at half maximum increased from 1.2 GHz to 3.5 GHz.
117(2015); http://dx.doi.org/10.1063/1.4921425View Description Hide Description
We compare ferromagnetic resonance measurements of Permalloy Ni80Fe20 (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.
117(2015); http://dx.doi.org/10.1063/1.4922126View Description Hide Description
We have developed a novel ferromagnetic resonance (FMR) measurement technique using the magneto-optical Kerr effect. The measurement technique uses microwave-frequency, intensity-modulated light to stroboscopically measure the Kerr angle due to the magnetization precession. We demonstrate that this stroboscopic magneto-optical Kerr effect provides a frequency domain and phase sensitive FMR measurement. The measurement is sensitive enough to detect the precessing magnetization with the precession cone angle below 1°.
Tunable configurational anisotropy in collective magnetization dynamics of Ni80Fe20 nanodot arrays with varying dot shapes117(2015); http://dx.doi.org/10.1063/1.4921976View Description Hide Description
We present broadband ferromagnetic resonance measurements of tunable spin wave anisotropy in arrays of nanodots with different dot shapes. Magnetization dynamics of the circular dot array shows two modes, while square, diamond, and triangular dot arrays show three, three, and four modes, respectively. Various distinct rotational symmetries in the configurational anisotropy of the nanodot arrays are observed with the variation of dot shape. The observed spin wave modes are reproduced by micromagnetic simulations and the calculated mode profiles show different collective modes determined by internal and stray magnetic fields. Effects of dot shapes are observed in combination with the effects of lattice symmetry and the shape of the boundary of the array. The collective behaviour is observed to be weakest in the diamond shaped dots and strongest in circular shaped dots. This is further confirmed by the stray field calculation. The large variation of spin wave mode frequencies and their configurational anisotropies with dot shapes are important for selection of suitable basis structures for future magnonic crystals.
117(2015); http://dx.doi.org/10.1063/1.4921977View Description Hide Description
A spin-polarized electrical current leads to a variety of periodical magnetic structures in nanostripes. In the presence of the Ørsted field, which always assists an electrical current, the basic types of magnetic structures, i.e., a vortex-antivortex crystal and cross-tie domain walls, survive. The Ørsted field prevents saturation of the nanostripe and a longitudinal domain wall appears instead. Possible magnetization structures in stripes with different geometrical and material properties are studied numerically and analytically.
117(2015); http://dx.doi.org/10.1063/1.4921979View Description Hide Description
Dysprosium(Dy)-doped zinc oxide (Dy:ZnO) thin films were fabricated on c-oriented sapphire substrate by pulsed-laser deposition with doping concentration ranging from 1 to 10 at. %. X-ray diffraction (XRD), Raman-scattering, optical transmission spectroscopy, and spectroscopic ellipsometry revealed incorporation of Dy into ZnO host matrix without secondary phase. Solubility limit of Dy in ZnO under our deposition condition was between 5 and 10 at. % according to XRD and Raman-scattering characteristics. Optical transmission spectroscopy and spectroscopic ellipsometry also showed increase in both transmittance in ultraviolet regime and band gap of Dy:ZnO with increasing Dy density. Zinc vacancies and zinc interstitials were identified by photoluminescence spectroscopy as the defects accompanied with Dy incorporation. Magnetic investigations with a superconducting quantum interference device showed paramagnetism without long-range order for all Dy:ZnO thin films, and a hint of antiferromagnetic alignment of Dy impurities was observed at highest doping concentration—indicating the overall contribution of zinc vacancies and zinc interstitials to magnetic interaction was either neutral or toward antiferromagnetic. From our investigations, Dy:ZnO thin films could be useful for spin alignment and magneto-optical applications.
- Dielectrics, Ferroelectrics, and Multiferroics
Influence of parameters of molecular mobility on formation of structure in ferroelectric vinylidene fluoride copolymers117(2015); http://dx.doi.org/10.1063/1.4921851View Description Hide Description
The low-temperature molecular mobility has been studied by dielectric relaxation spectroscopy on the bulk films of statistic copolymers of vinylidene fluoride (VDF) with tetra- (TFE) and trifluoroethylene obtained by crystallization from a solution in acetone. The results show that activation energy of the kinetic units in the glassy state depends significantly on the concentration of polar groups. The cooperative mobility above the glass transition temperature is described by Vogel-Tamman-Fulcher equation; the lowest value of the effective activation energy was found in the copolymer with highest amount of non-polar TFE content. It has been shown that the parameters of the dynamics are related to the structural parameters obtained by infrared spectroscopy, wide-angle X-ray Spectroscopy, and small-angle X-ray scattering. In particular, the largest crystals and the highest value of the “large” period have been detected in VDF/TFE copolymer with lowest activation parameters of microbrownian and local mobility. Number of lamellar crystals in stacks is determined by the concentration of VDF polar groups in copolymer chain.
Improvement of the piezoelectric properties in (K,Na)NbO3-based lead-free piezoelectric ceramic with two-phase co-existing state117(2015); http://dx.doi.org/10.1063/1.4921860View Description Hide Description
Two phases of (K,Na)NbO3 (KNN) co-exist in a KNN-based composite lead-free piezoelectric ceramic 0.910(K1− x Na x )0.86Ca0.04Li0.02Nb0.85O3−δ–0.042K0.85Ti0.85Nb1.15O5–0.036BaZrO3–0.0016Co3O4– 0.0025Fe2O3–0.0069ZnO system, over a wide range of Na fractions, where 0.56 ≤ x ≤ 0.75. The crystal systems of the two KNN phases are identified to tetragonal and orthorhombic by analyzing the synchrotron powder X-ray diffraction (XRD) data, high-resolution transmission electron microscopy (HR-TEM), and selected-area electron diffraction (SAD). In the range 0.33 ≤ x ≤ 0.50, the main component of the composite system is found to be single-phase KNN with a tetragonal structure. Granular nanodomains of the orthorhombic phase dispersed in the tetragonal matrix have been identified by HR-TEM and SAD for 0.56 ≤ x ≤ 0.75. Only a trace amount of the orthorhombic phase has been found in the SAD patterns at the composition x = 0.56. However, the number of orthorhombic nanodomains gradually increases with increasing Na content up to x < 0.75, as observed from the HR-TEM images. An abrupt increase and agglomeration of the nanodomains are observed at x = 0.75, where weak diffraction peaks of the orthorhombic phase have also become detectable from the XRD data. The maximum value of the electromechanical coupling coefficient, k p = 0.56, has been observed at the composition x = 0.56.
Strategy for stabilization of the antiferroelectric phase (Pbma) over the metastable ferroelectric phase (P21ma) to establish double loop hysteresis in lead-free (1−x)NaNbO3-xSrZrO3 solid solution117(2015); http://dx.doi.org/10.1063/1.4921876View Description Hide Description
A new lead-free antiferroelectric solid solution system, (1−x)NaNbO3-xSrZrO3, was rationalized through noting the crystal chemistry trend, of decreasing the tolerance factor and an increase in the average electronegativity of the system. The SrZrO3 doping was found to effectively stabilize the antiferroelectric (P) phase in NaNbO3 without changing its crystal symmetry. Preliminary electron diffraction and polarization measurements were presented which verified the enhanced antiferroelectricity. In view of our recent report of another lead-free antiferroelectric system (1−x)NaNbO3-xCaZrO3 [H. Shimizu et al. “Lead-free antiferroelectric: xCaZrO3 - (1−x)NaNbO3 system (0 ≤ x ≤ 0.10),” Dalton Trans. (published online)], the present results point to a general strategy of utilizing tolerance factor to develop a broad family of new lead-free antiferroelectrics with double polarization hysteresis loops. We also speculate on a broad family of possible solid solutions that could be identified and tested for this important type of dielectric.
- Physics of Nanoscale, Mesoscale, and Low-Dimensional Systems
117(2015); http://dx.doi.org/10.1063/1.4921814View Description Hide Description
Nanostructured tin (Sn) is a promising high-capacity electrode for improved performance in lithium-ion batteries for electric vehicles. In this work, Sn nanoisland growth for nanostructured electrodes assisted by the pulse laser irradiation has been investigated based on a mesoscale modeling formalism. The influence of pertinent processing conditions, such as pulse duration, heating/cooling rates, and atom flux, on the Sn nanostructure formation is specifically considered. The interaction between the adsorbed atom and the substrate, represented by the adatom diffusion barrier, is carefully studied. It is found that the diffusion barrier predominantly affects the distribution of Sn atoms. For both α-Sn and β-Sn, the averaged coordination number is larger than 3 when the diffusion barrier equals to 0.15 eV. The averaged coordination number decreases as the diffusion barrier increases. The substrate temperature, which is determined by heating/cooling rates and pulse duration, can also affect the formation of Sn nanoislands. For α-Sn, when applied low heating/cooling rates, nanoislands cannot form if the diffusion barrier is larger than 0.35 eV.