Ferromagnetic resonance in single vertices and 2D lattices macro-dipoles of elongated nanoelements: measurements and simulations.

We report broadband ferromagnetic resonance measurements of the in-plane magnetic field response of three- and four-fold symmetric vertices formed by non-contacting permalloy nano-ellipses together with extended lattices constructed from them. Complementing the experimental data with simulations, we are able to show that, as far as the most intense FMR responses are concerned, the spectra of vertices and lattices can largely be interpreted in terms of a superposition of the underlying hysteretic responses of the individual ellipses, as elemental building blocks of the system. This property suggest that it is possible to understand the orientation of the individual magnetic dipole moments in a dipole network in terms of dynamic measurements alone, thereby offering a powerful tool to analyze the alignment statistics in frustrated systems that are exposed to various magnetic histories.

Large Spin-Wave Bullet in a Ferrimagnetic Insulator Driven by the Spin Hall Effect.

Because of its transverse nature, spin Hall effects (SHE) provide the possibility to excite and detect spin currents and magnetization dynamics even in magnetic insulators. Magnetic insulators are outstanding materials for the investigation of nonlinear phenomena and for novel low power spintronics applications because of their extremely low Gilbert damping. Here, we report on the direct imaging of electrically driven spin-torque ferromagnetic resonance (ST-FMR) in the ferrimagnetic insulator Y_{3}Fe_{5}O_{12} based on the excitation and detection by SHEs. The driven spin dynamics in Y_{3}Fe_{5}O_{12} is directly imaged by spatially resolved microfocused Brillouin light scattering spectroscopy. Previously, ST-FMR experiments assumed a uniform precession across the sample, which is not valid in our measurements. A strong spin-wave localization in the center of the sample is observed indicating the formation of a nonlinear, self-localized spin-wave "bullet".

Controlled magnetic reversal in Permalloy films patterned into artificial quasicrystals.

We have patterned novel Permalloy thin films with quasicrystalline Penrose P2 tilings and measured their dc magnetization and ferromagnetic resonance absorption. Reproducible anomalies in the hysteretic, low-field data signal a series of abrupt transitions between ordered magnetization textures, culminating in a smooth evolution into a saturated state. Micromagnetic simulations compare well to experimental dc hysteresis loops and ferromagnetic resonance spectra and indicate that systematic control of magnetic reversal and domain wall motion can be achieved via tiling design, offering a new paradigm of magnonic quasicrystals.

Long-range surface plasmon polaritons propagating on a dielectric waveguide support.

In the traditional long-range surface plasmon geometry, an ultrathin metal film is sandwiched between two layers having identical dielectric constants. Here we demonstrate the long-range surface plasmon polariton (LRSPP) properties for a new structure where a thin layer with a dielectric constant exceeding that of the surroundings is inserted within the sandwich, provided the layer thickness d satisfies the condition k(⊥)d=mπ where k(⊥) is the component of the guide wavevector perpendicular to the layer and m is an integer. The resulting plasmon modes have smaller losses and nearly the same phase velocity as the original LRSPP. This provides a strategy to support silver films having thicknesses of 10's of nanometers to create plasmonic devices for sensor applications.

One-dimensional long-range plasmonic-photonic structures.

We have fabricated line gratings from periodically etched fused silica on which a thin silver film is deposited that is in turn covered with a silica index-matched fluid. This dielectrically symmetric geometry supports an independent long-range surface plasmon-polariton (LRSPP) and a short-range surface plasmon polariton, and the associated plasmonic band structure has been probed. Coupling to external light is achieved via the patterned grating, and an ultrasharp LRSPP linewidth of 4 nm is observed. The experimental results are compared with finite-difference time-domain simulations.

Perfect coupling of light to surface plasmons with ultra-narrow linewidths.

We examine the coupling of electromagnetic waves incident normal to a thin silver film that forms an oscillatory grating embedded between two otherwise uniform, semi-infinite half spaces. Two grating structures are considered, in one of which the midpoint of the Ag film remains fixed whereas the thickness varies sinusoidally, while in the other the mid point oscillates sinusoidally whereas the film thickness remains fixed. On reducing the light wavelength from the long wavelength limit, we encounter signatures in the transmission, T, and reflection, R, coefficients associated with: (i) the short-range surface plasmon mode, (ii) the long-range surface plasmon mode, and (iii) electromagnetic diffraction tangent to the grating. The first two features can be regarded as generalized (plasmon) Wood's anomalies whereas the third is the first-order conventional (electromagnetic) Wood's anomaly. The energy density at the film surface is enhanced for wavelengths corresponding to these three anomalies, particularly for the long-range plasmon mode in thin films. When exciting the silver film with a pair of waves incident from opposite directions, we find that by adjusting the grating oscillation amplitude and fixing the relative phase of the incoming waves to be even or odd, T+R can be made to vanish for one or the other of the plasmon modes; this corresponds to perfect coupling (impedance matching in the language of electrical engineering) between the incoming light and these modes.

Microwave absorption measurements using a broad-band meanderline approach.

We describe a technique that permits broad-band, field-dependent ferromagnetic and electron paramagnetic resonance absorption measurements that is applicable to thin films and patterned micro-/nanostructured arrays and is based on a wire-wound meanderline approach. Techniques to prepare meanderlines and perform microwave measurements are described along with some demonstrations involving an electron paramagnetic resonance calibration/test material, 2,2-diphenyl-1-picryl-hydrazyl, and a ferromagnetic cobalt thin film.

Enhancement of the supercurrent at a finite voltage in a sandwich-type ballistic SINIS junction.

A large, reentrant, Josephson current is observed in SINIS (Nb/Al/AlOx/Al/AlOx/Al/Nb) junctions at a finite voltage close to Delta/e (where Delta is the superconducting energy gap in S) and a bias current exceeding the zero-voltage Josephson current. The effect is studied using a multiterminal device configuration. A theoretical interpretation in terms of quantized electron states in the N layer is provided.

Osteoblast proliferation on hydroxyapatite thin coatings produced by right angle magnetron sputtering.

Right angle magnetron sputtering (RAMS) was used to produce hydroxyapatite (HA) film coatings on pure titanium substrates and oriented silicon wafer (Si(0 0 1)) substrates with flat surfaces as well as engineered surfaces having different forms. Analyses using synchrotron XRD, AFM, XPS, FTIR and SEM with EDS showed that as-sputtered thin coatings consist of highly crystalline hydroxyapatite. The HA coatings induced calcium phosphate precipitation when immersed in simulated body fluid, suggesting in vivo bioactive behavior. In vitro experiments, using murine osteoblasts, showed that cells rapidly adhere, spread and proliferate over the thin coating surface, while simultaneously generating strong in-plane stresses, as observed on SEM images. Human osteoblasts were seeded at a density of 2500 cells cm(-2) on silicon and titanium HA coated substrates by RAMS. Uncoated glass was used as a reference substrate for further counting of cells. The highest proliferation of human osteoblasts was achieved on HA RAMS-coated titanium substrates. These experiments demonstrate that RAMS is a promising coating technique for biomedical applications.

Direct study of the proximity effect in the normal layer inside of the stacked SINIS device.

We have observed a striking anisotropy in the electrical transport of layered multiterminal SINIS structures [where S, I, and N denote a superconductor (Nb), an insulator (AlOx), and a normal metal (Al), respectively]. We find that the lateral conductivity of the N layer is dissipative, but a superconducting current can flow normal to the structure, suggesting a direct Josephson coupling between the external S electrodes. A small coherent contribution to the lateral conductivity of the N layer is observed near zero voltage.

Evidence of electromagnetic absorption by collective modes in the heavy fermion superconductor UBe(13).

We present results of a microwave surface impedance study of the heavy fermion superconductor UBe(13). We clearly observe an absorption peak whose frequency and temperature dependence scales with the BCS gap function Delta(T). Resonant absorption into a collective mode, with energy approximately proportional to the superconducting gap, is proposed as a possible explanation. Fits to the data provide a simple relation between Delta(T) and the collective mode frequency.

Polarity inversion in polar-nonpolar-polar heterostructures.

We have observed an epilayer-thickness-dependent polarity inversion for the growth of CdTe on Sb(Bi)/CdTe(111)B. For films with Sb(Bi) thicknesses of less than 40 A (15 A), the CdTe layer shows a B (Te-terminated) face, but it switches to an A (Cd-terminated) face for thicker layers. On the other hand, a CdTe layer grown on Bi(Sb)/CdTe(111)A always shows the A face regardless of Sb or Bi layer thicknesses. In order to address the observations we have performed ab initio calculations, which suggest that the polarity of a polar material on a nonpolar one results from the binding energy difference between the two possible surface configurations.

Atomic force microscopy of the erythrocyte membrane skeleton.

The atomic force microscope was used to examine the cytoplasmic surface of untreated as well as fixed human erythrocyte membranes that had been continuously maintained under aqueous solutions. To assess the effects of drying, some membranes were examined in air. Erythrocytes attached to mica or glass were sheared open with a stream of isotonic buffer, which allowed access to the cytoplasmic membrane face without exposing cells to non-physiological ionic strength solutions. Under these conditions of examination, the unfixed cytoplasmic membrane face revealed an irregular meshwork that appeared to be a mixture largely of triangular and rectilinear openings with mesh sizes that varied from 35 to 100 nm, although few were at the upper limit. Fixed ghosts were similar, but slightly more contracted. These features represent the membrane skeleton, as when the ghosts were treated to extract spectrin and actin, these meshworks were largely removed. Direct measurements of the thickness of the membrane skeleton and of the lateral dimensions of features in the images suggested that, especially when air dried, spectrin can cluster into large, quite regularly distributed aggregates. Aggregation of cytoskeletal components was also favoured when the cells were attached to a polylysine-treated substrate. In contrast, the membrane skeletons of cells attached to substrates rendered positively charged by chemical derivatization with a cationic silane were much more resistant to aggregation. As steps were taken to reduce the possibility of change of the skeleton after opening the cells, the aggregates and voids were eliminated, and the observed structures became shorter and thinner. Ghosts treated with Triton X-100 solutions to remove the bilayer revealed a meshwork having aggregated components resembling those seen in air. These findings support the proposition that the end-to-end distance of spectrin tetramers in the cell in the equilibrium state is much shorter than the contour length of the molecule and that substantial rearrangements of the spectrin-actin network occur when it is expanded by low ionic strength extraction from the cell. This study demonstrates the applicability of AFM for imaging the erythrocyte membrane skeleton at a resolution that appears adequate to identify major components of the membrane skeleton under near-physiological conditions.

Redetermination of second-order susceptibility of zinc oxide single crystals.

We measured the second-harmonic signal generated by high-quality zinc oxide single crystals. Comparing this signal with that generated by single-crystal quartz, used as a standard, we found that the components of the second-order susceptibility tensor differ somewhat from the values reported earlier. Measurements of two zinc oxide single crystals with different orientations yield similar results.

Simple model for the observed plasmon conical radiation interference patterns in a kretschmann configuration.

In two previous papers we presented experimental results of measurements with a scanning plasmon optical microscope constructed in the Kretschmann configuration. The angular distribution of the conical radiation from the scanning tip was measured, and the multiple-scattering images of simple surface irregularities were detected. We make a simple model of the multiple-scattering images and compare it with our data, deriving excellent results.

Conical electromagnetic radiation in the Kretschmann attenuated total reflections configuration.

The polarization of the conical electromagnetic radiation produced by the scattering of surface plasmons by film roughness in the Kretschmann attenuated total reflection configuration was measured. The conical radiation was p polarized in the plane of observation. A tip of a scanning tunnel microscope was brought close to the film surface to act as a plasmon-scattering center. From a measurement of the conical radiation generated by this single-scattering center, it was found that plasmons tend to be forward scattered. Using a scanning plasmon optical microscope (SPOM), we imaged plasmon-scattering surface irregularities, which act in a first approximation as independent conical centers, along with interferences caused by these fixed conical radiation centers and the moving conical radiation center (the SPOM probe tip). The possibility of detecting localized surface plasmons with a SPOM is discussed.

Scanning plasmon optical microscope operation in atomic force microscope mode.

A scanning plasmon optical microscope has been operated with an atomic force microscope rather than a scanning tunnel microscope to scan a surface. The instrument produced a nanoscale optical image of dyemolecule crystals adsorbed on a silver film. Images of dye-molecule crystal islands are presented.

Buckytubes and derivatives: their growth and implications for buckyball formation.

Transmission electron microscopy (TEM) observations of graphite tubules (buckytubes) and their derivatives have revealed not only the previously reported buckytube geometries but also additional shapes of the buckytube derivatives. Detailed cross-sectional TEM images reveal the cylindrical cross section of buckytubes and the growth pattern of buckytubes as well as their derivatives. These observations of frozen growth stages of buckytubes and derivatives suggest a helical growth mechanism analogous to that of crystal growth via screw dislocations. The helicacy of buckytubes is analyzed by electron diffraction whereas the anisotropy of electronic structure is revealed by momentum transfer resolved electron energy loss spectrometry. Based on the TEM observations, it is proposed that buckytubes act as precursors to closed-shell fullerene (buckyball) formation and the possible steps in buckyball formation are outlined. In arc evaporation experiments in which residue rods (containing various amounts of buckytubes) were used as the starting anode for fullerene production, the amount of buckytubes in the rod was correlated with fullerene yield.

A novel technique to measure the group velocity of sound in dispersive media.

A technique is proposed for measuring the group velocity of sound by a frequency-modulated (FM) continuous wave (CW) technique. Detailed calculations are presented, showing that V(g)=(deltaomega(m)/deltaphi)d , where V(g), deltaomega(m), deltaphi, and d are, respectively, the group velocity, change of modulation frequency, phase change of the received signal detected by a biphase lock-in amplifier, and the distance traveled by the sound. The approach has the advantage of high resolution compared to the conventional time-of-flight measurement technique.

Novel technique for dynamic surface tension and viscosity measurements at liquid-gas interfaces.

A novel means of generating and detecting surface waves at liquid-gas interfaces has been successfully developed. Electrocapillarity is used to generate the waves which are detected via specular reflection of a laser beam from the fluid surface to a position sensitive photodiode. Such a scheme is compact, sensitive, and does not mechanically touch the fluid surface. A preliminary study of highly damped waves on the magnetically oriented liquid crystal MBBA is reported.