Higher Acoustic Modes Research Articles

Engineering optical mode ferromagnetic resonance in FeCoB films with ultrathin Ru insertion.

Ferromagnetic resonance (FMR) in soft magnetic films (SMFs) to a large extent determines the maximum working frequency of magnetic devices. The FMR frequency (fr) in an optical mode is usually much higher than that in the corresponding acoustic mode for exchange coupled ferromagnet/nonmagnet/ferromagnet (FM/NM/FM) trilayers. In this study, we prepared a 50 nm FeCoB film with uniaxial magnetic anisotropy (UMA), showing a high acoustic mode fr of 4.17 GHz. When an ultrathin Ru spacer was inserted in the very middle of the UMA-FeCoB film, the zero-field FMR was abruptly switched from an acoustic mode to an optical one with fr dramatically enhanced from 4.17 GHz to 11.32 GHz. Furthermore, the FMR mode can be readily tuned to optical mode only, acoustic mode only, or double mode by simply varying the applied filed, which provides a flexible way to design multi-band microwave devices.

PERTURBATIONS OF DARK MATTER GRAVITY

Until recently the study of the gravitational field of dark matter was primarily concerned with its local effects on the motion of stars in galaxies and galaxy clusters. On the other hand, the WMAP experiment has shown that the gravitational field produced by dark matter amplifies the higher acoustic modes of the CMBR power spectrum, more intensely than the gravitational field of baryons. Such a wide range of experimental evidences from cosmology to local gravity suggests the necessity of a comprehensive analysis of the dark matter gravitational field per se, regardless of any other attributes that dark matter may eventually possess. In this paper we introduce and apply Nash's theory of perturbative geometry to the study of the dark matter gravitational field alone, in a higher-dimensional framework. It is shown that the dark matter gravitational perturbations in the early universe can be explained by the extrinsic curvature of the standard cosmology. Together with the estimated presence of massive neutrinos, such geometric perturbation is compatible not only with the observed power spectrum in the WMAP experiment but also with the most recent data on the accelerated expansion of the universe. It is possible that the same structure formation exists locally, such as in the cases of young galaxies or in cluster collisions. In most other cases it seems to have ceased when the extrinsic curvature becomes negligible, leading to Einstein's equations in four dimensions. The slow motion of stars in galaxies and the motion of plasma substructures in nearly colliding clusters are calculated with the geodesic equation for a slowly moving object in a gravitational field of arbitrary strength.

An Experimental Investigation of Cavity Flow Oscillations and Tones of an In-Flow Microphone

An experiment was conducted to investigate the flow-induced nose-cone cavity oscillations of a B&K in-flow microphone. A 38.1-mm-diameter scale model of the B&K 12.7-mm UA 0386 nose cone was constructed and tested in a 177.8-mm-round free-jet in the Anechoic Chamber Wind Tunnel at NASA Ames Research Center at ReD up to 1.1 × 105. The results indicated that the B&K nose-cone cavity behaves much like a cavity without a screen. Detailed flow velocity measurements over the cavity screen have shown inflection points in the mean velocity profiles and high disturbance intensities in the vicinity of the cavity trailing edge. These results and the high coherence between the strong velocity and acoustic pressure oscillations at the cavity trailing edge implied that the cavity acoustic tones are the consequence of the cavity shear-layer impingement on the cavity trailing edge. The effect of cavity screen appears to be secondary in the velocity range tested. The screen seems mainly to modulate the Strouhal numbers of the higher acoustic modes.

On the interaction of higher duct modes with a perforated liner system with bias flow

We explore the three-dimensional interaction of higher acoustic modes with bias-flow perforated liners in cylindrical and annular ducts. Pressure fluctuations in the vicinity of the liners excite the production and shedding of vorticity from the rims of apertures in the liners. An effective liner compliance is used which accounts for this transfer of acoustical into vortical energy. The investigation is facilitated by a Green's function solution of the Helmholtz equation in a lined section of duct, allowing calculation of the amplitudes of exiting wave modes due to incoming acoustic disturbances. A system containing an arbitrary number of concentric liners and the hollow cavities formed between them can be modelled. The results for an incident plane wave are compared with those from our previously developed one-dimensional model, with excellent agreement. We demonstrate that results for all modes that travel parallel to the liner, including higher circumferential modes in narrow annular gaps, exhibit self-similar behaviour, and that liner design rules developed for planar duct modes can be adapted accordingly. The acoustic absorption can be strongly enhanced by downstream duct reflection for wavelengths larger than twice the liner length, but is less affected at higher frequencies that allow persistent pressure minima along the liner. Across larger frequency ranges, the liner systems are shown to permit two types of resonance associated with the duct and the cavities, respectively, and a third of Helmholtz type, associated with the system as a whole. The effect of these resonances on incident modes is demonstrated, and, in particular, we explore their enhancement of acoustic absorption.

Acoustic measurements for nanoscale test instruments

Acoustic methods have been used successfully for microscale contact nondestructive evaluation, but until very recently, were not available for the nanometer scale. Noncontact submicrometer resolution ultrasonic microscopy and scanning probe microscopy (SPM) operate at higher acoustic modes and have great potential to be qualitative but face data interpretation difficulties. Nanomechanical test instruments offer nanometer scale quantitative characterization. These tools coupled with in situ SPM-type imaging and simultaneous acoustic response monitoring open new instrumentation horizons for micro/nano fracture mechanics. Both active and passive acoustic methods can be utilized to characterize a large variety of substrates and coatings. Examples of AE monitoring of nanoindentation/scratch on data storage media will be presented. Plastic deformation induced events can be separated from contact friction events by identifying AE signatures. Active ultrasonic methods can be utilized for nanoscale characterization of tribological surfaces. A friction coefficient reduction of 20% was observed on an ultrasonically excited surface during nanoscratch testing and was further investigated using post-scratch SPM-type imaging. A synergy of localized ultrasonic monitoring and nanoindentation technique can lead to the development of new and promising instrumentation for characterizing in vivo/vitro biological tissues at the molecular level. Examples on evaluating ultrasonically transmitted signal through biological samples will be discussed.

The feasibility of using long-range acoustics as a measure of global ocean warming

The SOFAR channel in the ocean has been known for decades to be an efficient acoustic waveguide. Military applications of long-range acoustic transmissions using the SOFAR channel for communications and detection have been many, but scientific applications that capitalize on its acoustic properties have, until recently, been few. For ranges between source and receiver of order 1000 km, acoustic tomography yields the maximum return of information about both horizontal and vertical ocean structure but this sophisticated technique does not rely on the existence of the sound channel. Over megameter ranges, however, higher acoustic modes are scattered and absorbed so that only the lowest, slowest modes survive as near-axial waves in the sound channel. Following the reinterpretation in 1988 by Munk et al. of data from a long-range acoustic experiment conducted in 1960 (between Perth, Australia and Bermuda), Munk and Forbes (1989) proposed to further develop the technique for use in measuring path-averaged ocean temperatures. They intend to simply measure travel times of lowest mode signals simultaneously along many different megameter paths (refracted geodesics). They predict that, if sufficiently long time-series of such data are gathered, then many different spatial and time scales of ocean temperature variability will be captured and identified. These range from diurnal to decadal scales, including the signature of the often-predicted global warming. A feasibility test of this idea is scheduled for early 1991.