Ward L. Johnson

Characterization of magnetic properties at edges by edge-mode dynamics

We have used “trapped spin wave” or edge modes of magnetic precession to probe the magnetic environment near magnetic film edges magnetized perpendicular to the edge. Micromagnetic models of dynamics in stripes reveal that the edge mode frequency-field relationship depends on whether the edge surface is vertical or tapered, while the “bulk” modes are nearly unaffected. The models show the edge-mode frequency going to zero at the edge saturation field. This critical field becomes much less distinct for applied fields misaligned from the edge normal by as little as 1°. Ferromagnetic-resonance and Brillouin light-scattering measurements of the edge modes in an array of 480-nm-wide×12-nm-thick Ni80Fe20 stripes have a lower edge saturation field than the vertical edge models, but agree well with the model of 45°-tapered edges.

Spectroscopy of resonant torsional modes in cylindrical rods using electromagnetic‐acoustic transduction

Two ultrasonic techniques employing electromagnetic‐acoustic transduction are presented for performing measurements of the resonant torsional frequencies and Q of solid cylindrical metallic rods. One of these techniques uses long radio‐frequency pulses to drive the sample into resonance and the other uses continuous‐wave excitation. Measurements are performed on an aluminum alloy. Since the transduction involves no mechanical coupling, the background damping is low; the Q is 1.2×105 at 755 kHz with the sample simply supported on its side. The shear velocity is determined with an accuracy of better than 2 parts in 104 (limited by the uncertainty in the measurement of the sample radius).

Trapped torsional modes in solid cylinders

An approximate theory is presented for torsional modes in a solid cylinder having a larger‐diameter central section. Modes in a narrow frequency range just above the cutoff of each branch are ‘‘trapped’’ such that, in long samples, the amplitudes decay exponentially with distance from the central section. To confirm the theory, measurements were performed on aluminum alloy cylinders using noncontacting electromagnetic‐acoustic transduction. Vibrational amplitudes of a trapped mode measured as a function of position along the length of a sample are in good agreement with theoretical calculations. The resonant frequencies of three samples with different dimensions also closely match the theory for trapped modes. Additional generally weak resonances that are observed may be associated with torsional modes that vary sinusoidally along the entire length of the samples.

Ultrasonic spectroscopy of metallic spheres using electromagnetic‐acoustic transduction

An ultrasonic technique for studying vibrational resonant modes of metallic spheres is presented. The technique employs electromagnetic‐acoustic transduction with a configuration consisting of a sample surrounded by a coil in a static magnetic field. Resonance spectra from 0.5 to 4.5 MHz with the coil axis parallel and perpendicular to the magnetic field are measured for a 3.145‐mm‐diam sphere of polycrystalline 2024 aluminum. Elastic constants calculated from the resonant peak frequencies are consistent with results obtained using an ultrasonic pulse‐echo system. This new technique has advantages over pulse‐echo and conventional resonance techniques for experiments where high absolute accuracy is necessary or where samples are heated far above room temperature.

Ultrasonic damping in pure aluminum at elevated temperatures

Ultrasonic resonant damping was measured in polycrystalline 99.999% pure aluminum spheres between 22 and 465 °C at frequencies between 0.4 and 2.0 MHz. In fully annealed samples, the damping increases monotonically with temperature in a manner similar to that reported in numerous lower-frequency studies of a variety of materials. However, the activation energy of this increase is significantly less than values reported for aluminum at lower frequencies, and the frequency dependence is much different; these facts suggest that a different physical mechanism dominates the damping in the higher frequency range. The dependences on temperature, frequency, and vibrational amplitude are consistent with an anelastic dislocation mechanism. During recovery and recrystallization of cold-worked material, the damping drops irreversibly with a time dependence matching that expected for the dislocation density.

Anelastic loss in langatate

Anelastic loss in langatate (La/sub 3/Ga/sub 5.5/Ta/sub 0.5/O/sub 14/) was measured as a function of temperature from 80 to 1080 K at ultrasonic frequencies from 0.5 to 1.8 MHz. The specimens were monocrystalline cylinders with the trigonal axis parallel to the cylinder axis. Q/sup -1/ and the resonant frequencies were measured with inductive electromagnetic-acoustic and contacting transduction techniques. On cooling from room temperature to 80 K, Q/sup -1/ decreased by approximately an order of magnitude, with the most rapid drop occurring in the 220-260 K range. Part of this temperature dependence may result from the phonon-phonon interaction. However, the frequency dependence of Q/sup -1/ at room temperature is inconsistent with theories for the phonon-phonon interaction, and Qf at room temperature is much lower than values at higher frequencies reported by other researchers. These results suggest that Q/sup -1/ measured at and below room temperature is dominated by an anelastic defect contribution, rather than the intrinsic phonon-phonon damping. Dislocation damping is considered the most likely candidate for this contribution. At elevated temperatures, a relaxation peak appears with a maximum in the 750-810 K range for frequencies between 0.5 and 1.8 MHz. The activation energy of the relaxation is 1.1 eV, which is typical of point-defect relaxations. A second peak appears near 860 K during the first heating and disappears after heating to 1080 K. The observed peaks rise above a background that increases rapidly at the highest temperatures. By analogy with a similar effect observed in quartz, this background is assumed to arise from the anelastic relaxation of diffusing interstitial impurities.

Traction-free vibrations of finite trigonal elastic cylinders

The unrestrained, traction-free vibrations of finite elastic cylinders with trigonal material symmetry are studied using two approaches, based on the Ritz method, which formulate the weak form of the equations of motion in cylindrical and rectangular coordinates. Elements of group theory are used to divide approximation functions into orthogonal subsets, thus reducing the size of the computational problem and classifying the general symmetries of the vibrational modes. Results for the special case of an isotropic cylinder are presented and compared with values published by other researchers. For the isotropic case, the relative accuracy of the formulations in cylindrical and rectangular coordinates can be evaluated, because exact analytical solutions are known for the torsional modes. The calculation in cylindrical coordinates is found to be more accurate for a given number of terms in the series approximation functions. For a representative trigonal material, langatate, calculations of the resonant frequencies and the sensitivity of the frequencies on each of the elastic constants are presented. The dependence on geometry (ratio of length to diameter) is briefly explored. The special case of a transversely isotropic cylinder (with the elastic stiffness C14 equal to zero) is also considered.

Contributions to anelasticity in langasite and langatatea)

Maximization of the quality factors Q of langasite (LGS) and langatate (LGT) is necessary for optimal performance of acoustic resonators of these materials in frequency-control and high-temperature sensing applications. In this report, measurements and least-squares analysis of Q−1 as a function of ultrasonic frequency and temperature of undoped LGS (100 K to 750 K) and LGT (300 K to 760 K) reveal a superposition of physical effects, including point-defect relaxations and intrinsic phonon-phonon loss. In LGS, these effects are superimposed on a large temperature-dependent background with weak frequency dependence that is interpreted as arising from a relaxation process with a distribution of activation energies. This distributed relaxation is suggested to be a result of anelastic kink migration. No evidence for a significant background of this form is found in the LGT specimen, consistent with the lower measured dislocation etch-pit density of this crystal. The analysis of the dependence of Q−1 of LGT on ...

Elastic constants and dimensions of imprinted polymeric nanolines determined from Brillouin light scattering

Elastic constants and cross-sectional dimensions of imprinted nanolines of poly(methyl methacrylate) (PMMA) on silicon substrates are determined nondestructively from finite-element inversion analysis of dispersion curves of hypersonic acoustic modes of these nanolines measured with Brillouin light scattering. The results for the cross-sectional dimensions, under the simplifying assumption of vertical sides and a semicircular top, are found to be consistent with dimensions determined from critical-dimension small-angle x-ray scattering measurements. The elastic constants C(11) and C(44) are found to be, respectively, 11.6% and 3.1% lower than their corresponding values for bulk PMMA. This result is consistent with the dimensional dependence of the quasi-static Youngs modulus determined from buckling measurements on PMMA films with lower molecular weights. This study provides the first evidence of size-dependent effects on hypersonic elastic properties of polymers.

Acoustic loss in langasite and langanite

Resonant ultrasonic loss Q/sup -1/ in plano-convex unplated Y-cut disks of langasite and langanite was measured as a function of frequency and temperature with the aim of identifying dominant internal mechanisms that will degrade the performance of electronic oscillators. In both materials, the dependence on temperature is similar to that previously reported for langatate. An anelastic peak appears with maxima in the 150-280 K range at frequencies between 2 MHz and 14 MHz. The dependencies of this peak on temperature and frequency is consistent with a point-defect relaxation. The width of the peak is greater than that of a Debye peak, which indicates that the relaxation has a distribution of activation energies. The peak appears at higher temperatures in langasite than in langatate, and this may explain the lower room-temperature Q/sup -1/ reported thus far in langatate. Additional anelastic peaks appear at elevated temperatures. The peaks are superimposed on a component of the loss that increases monotonically with temperature according to an approximate Arrhenius expression.