卷 81, 编号 8 (2017)

Physical, mathematical, and engineering problems related to creating two fundamentally different types of tomographs (so-called linear and nonlinear tomographs) are considered. The tomographs under development are designed for diagnosing pathological tumors of soft biological tissues at early stages of their growth.

Two theoretical approaches for simulating nonlinear focused ultrasound fields generated by a diagnostic convex array are compared. The first model is based on the three-dimensional Westervelt equation and describes the full structure of the array field with high accuracy. However, it requires great computational resources and is technically difficult. The second model is based on an axially symmetric form of the parabolic KZK equation for estimating the strength of nonlinear effects in the focal region of a beam, which reduces the computational time by a factor of several hundreds. To establish the boundary conditions to the KZK model, the radius and the focal length of a circular piston source are defined such that the simulated field on the beam axis in the linear case fits the real structure of the field in the focal region. It is shown that the parabolic model can be used to accurately describe the spatial and temporal structure of the field generated by a diagnostic transducer in the focal region of the beam along its axis and in the plane of the beam’s electronic focusing.

A robust algorithm for sound source localization in shallow sea based on a 2D Fourier transform of source motion interference patterns is developed. A spectrogram in time-frequency coordinates obtained after a Fourier transform contains localized areas of spectral density intensity of constructively interfering modes and a distributed area of noise spectral density. Signal spectral density is localized in an area whose linear sizes are determined by the lowest frequency and spatial scales of field variability. The peak input signal-to-noise ratio is estimated for a single receiver when robust detection is ensured and estimates of speed and initial distance to the receiver are close to the actual values. The high robustness of the algorithm is based on the coherent summation of mode amplitudes occurring at different times. Experimental results demonstrating the effectiveness of the algorithm are presented. For higher than peak signal-to-noise ratios, random estimates of the parameters are close to their statistically average values.

Ultrasonic techniques allow examination of internal structure and the detection of discontinuities at the interface of various joints. Contact joints obtained via diffusion welding, sintering, and other adhesive methods are investigated using impulse acoustic microscopy. It is shown that short probing pulses of focused ultrasound with frequencies of 50–100 MHz reveal areas with different adhesion strengths, areas of partial contact, peeling or air bubbles, and buffer layer thicknesses. Mechanisms of acoustic contrast at such interfaces are discussed. The results are of interest to specialists in the field of high-resolution ultrasonic nondestructive testing. They are needed to predict the failure mechanisms of composite products, from carbon fiber–reinforced plastics for the aviation industry to high-density ceramics used in medical prosthetics.

A new way of generating a pure spin current using a magnetoelectric composite high-overtone bulk acoustic-wave resonator (HBAR) based on a layered Al–ZnO–Al–Gd₃Ga₅O₁₂–Y₃Fe₅O₁₂–Pt structure is proposed. It is established that the efficiency of generating the rf magnetic field driving the spin current by this method exceeds that of sources which use surface acoustic waves for the excitation of magnetic dynamics and is not inferior to the efficiency of usual electromagnetic ways of exciting a pure spin current that employ microwave cavities.

Indicatrices (isofrequency curves) of magnetostatic surface waves are studied with allowance for wave attenuation. A bias magnetic field is applied tangentially to a ferrite film. Arbitrary angles between the bias magnetic field and the wave vector are examined.

Theoretical dependences of the maximum bending of a strip domain’s borders on the parameters of a magnetic elliptical inhomogeneity are obtained. These relations can be used for quantitative estimates of changes in domain configuration in the presence of an elliptical magnetic inhomogeneity, and for conducting different experiments and theoretical investigations.

The total magnetic potential generated in the far field of a ferrite slab as a result of a plane noncollinear surface spin wave incident on a slit in an opaque screen is investigated. The dependence of the potential amplitude from the polar angle in the plane of a ferrite slab is calculated for different orientations of the screen with relative to an external magnetic field magnetizing the slab. It is found that two diffractive beams are generated in the slab as a result of spin diffraction on the slit. The angular width of each beam depends largely on the slit’s orientation and can be greater or smaller than value λ_n/D (where D is the slit’s width and λ_n is the length of the spin wave corresponding to each beam).

Time dependences of the azimuthal component of the torque T_φ(t) acting on magnetization are calculated to understand the nature of the delayed magnetization acceleration effect observed during the 90° pulsed magnetization of real ferrite–garnet films, in which biaxial anisotropy exists alongside with in-plane anisotropy. A calculation technique based on analyzing an operating point trajectory is used. Calculations show that if the effective anisotropy field H_K2 is comparable to the magnetizing pulse amplitude H_ma, abruptly ascending regions at characteristic times t* in curves T_φ(t) arise, in the limit of which nonlinear magnetization oscillations formed. The shape of these regions depends weakly on the magnetizing pulse front duration τ_f. This explains the reason of the weak dependence of the nonlinear magnetization oscillations on duration of the magnetizing pulse front. Calculations also show that the main features of the delayed acceleration effect are less clear upon an increase of the pulse amplitude: the behavior of curves T_φ(t) becomes smoother near times t*, and an increase in the pulse front duration is accompanied by a stronger drop in the intensity of magnetization oscillations.

Results from an experimental study of self-excitation in ferrite single crystals upon nonlinear ferromagnetic resonance are considered. The dependences of the frequency and amplitude of self-oscillations on the microwave power and the intensity of oscillation anisotropy are obtained. Self-excitation is of interest because it is the only effect not induced by noise in strongly excited matter.

Multilayered (CoFeB)_m(SiO₂)_100−m nanogranular film composites are probed via ferromagnetic resonance (FMR) spectroscopy. When the layer thickness is 1.0 to 2.6 nm and magnetic phase concentrations remain in the percolation range, the FMR field in these nanosystems reveals no dependence on the magnetic composite layer thickness. The observed phenomena are explained in the context of the FMR theory, and the relationship between the magnetic granule shape and absorbed electromagnetic radiation is established as well.

A theoretical distribution of the magnetic fields near rectangular cutouts in strip conductors of different widths is derived. An experimental investigation of the magnetic fields of the strip conductors is performed using devices based on magnetoresistive and Hall sensors. A way of detecting strip conductor defects using the above sensors is developed, based on the agreement between the experimental results and the theoretical calculations.

An continuing search for primary particles is performed using a new means of finding and tracking particles in nuclear emulsions. Particle interactions registered in stratospheric X-ray emulsion chambers (XECs) are attributed to nucleon–nuclei interactions recorded in the Russian–Japanese RUNJOB experiment. According to the data from processing the interactions between galactic particles and matter in XECs RUNJOB-3B, 6A, and RUNJOB-11A, 11B with energy thresholds ΣE_γ ≥ 3 and ΣE_γ ≥ 5 TeV, respectively, around 50% of all proton tracks went unidentified, which coincides with the initial results from the RUNJOB experiment.