Bias-field-induced spiral dynamic domains induced in iron garnet films

The induction of an Anger state (AS) in a multidomain magnetic medium by a static bias magnetic field is observed. The properties of the spiral dynamic domains in the induced AS are substantially different from those of previously studied spiral domains. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 1, 60–64 (10 January 1997)     

Dynamic self-organization of magnetic domains in amorphous films with perpendicular anisotropy

Dynamic self-organization of magnetic domains is observed in amorphous gadolinium-cobalt films in narrow temperature intervals on both sides of the magnetic compensation point. Spiral dynamic domains form in a limited range of ac magnetic field amplitudes and frequencies. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 11, 688–692 (10 December 1997)     

Evolution of spiral dynamic magnetic domains in Anger states of iron garnet films

Dynamic domain structures in the Anger state of iron garnet films are analyzed by a video micrographic technique. Data are obtained on the evolution of individual spiral domains over the lifetime of each domain. A new mechanism for the destruction of this state is discovered. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 6, 453–456 (25 March 1996)     

Microinhomogeneities in the distribution of oxygen concentration in ferrite-garnet films

Artificial microinhomogeneities were created in iron-garnet (TmBiCa)₃(FeGa)₅O_12−γ films with various degrees of oxygen nonstoichiometry, changes in the individual magnetic characteristics of these inhomogeneities were tracked during chemical oxidation and reduction, and the interactions of these inhomogeneities with planar domain walls and spiral domains were investigated. The resulting experimental data were interpreted by analyzing how the saturation magnetization, domain-wall surface-energy density, and uniaxial anisotropy constants depended on the concentrations of different-valency iron ions, cationic vacancies, and anionic vacancies. Fiz. Tverd. Tela (St. Petersburg) 41, 2016–2019 (November 1999)     

Spectral dependence of the creation and erasure of electrical structure in ruby during nonlaser irradiation. Domain conductivity of ruby

The dependence of the rate of formation and erasure of photoinduced electrical domain structure in ruby on the spectral composition of nonlaser illumination is studied. Near-ultraviolet irradiation is found to cause rapid erasure of the domain structure. The photocurrent density and photoconductivity are determined as functions of the electric field strength of the domains during irradiation with different spectral compositions at 77 K and 300 K assuming a “capacitor” model for the domains. The dark conductivity of ruby is estimated. Fiz. Tverd. Tela (St. Petersburg) 39, 295–301 (February 1997)     

Surface polaritons in a dielectric at a boundary with a metal in a static electric field

The existence of surface polaritons in a dielectric at a boundary with an ideal metal or superconductor in a static electric field is predicted. The frequency regions in which polaritons exist are substantially different for opposite orientations of the electric field, so that a change in the direction of the field signifies “switching on” or “switching off” of surface polaritons with a fixed frequency. Pis'ma Zh. éksp. Teor. Fiz. 68, No. 12, 876–880 (25 December 1998)     

Anomalous absorption of ultrasound in gadolinium in a magnetic field

The absorption coefficient α _k for longitudinal ultrasonic (15 MHz) waves propagating transverse to the direction of a magnetic field H is measured in single crystal gadolinium. It is found that in fields H⩽600 Oe, the peak in α _k is shifted toward lower temperatures, while the absolute magnitude of the absorption rises with increasing H. It is shown on the basis of dynamic scaling that the anomalous behavior of α _k in fields H⩽600 Oe can be explained by introducing a magnetic field analog of the Landau-Khalatnikov relaxation mechanism. Fiz. Tverd. Tela (St. Petersburg) 39, 339–340 (February 1997)     

Real-time dynamic vocal tract imaging using an accelerated spiral GRE sequence and low rank plus sparse reconstruction

PurposeTo develop a real-time dynamic vocal tract imaging method using an accelerated spiral GRE sequence and low rank plus sparse reconstruction.MethodsSpiral k-space sampling has high data acquisition efficiency and thus is suited for real-time dynamic imaging; further acceleration can be achieved by undersampling k-space and using a model-based reconstruction. Low rank plus sparse reconstruction is a promising method with fast computation and increased robustness to global signal changes and bulk motion, as the images are decomposed into low rank and sparse terms representing different dynamic components. However, the combination with spiral scanning has not been well studied. In this study an accelerated spiral GRE sequence was developed with an optimized low rank plus sparse reconstruction and compared with L1-SPIRiT and XD-GRASP methods. The off-resonance was also corrected using a Chebyshev approximation method to reduce blurring on a frame-by-frame basis.ResultsThe low rank plus sparse reconstruction method is sensitive to the weights of the low rank and sparse terms. The optimized reconstruction showed advantages over other methods with reduced aliasing and improved SNR. With the proposed method, spatial resolution of 1.3*1.3 mm² with 150 mm field-of-view (FOV) and temporal resolution of 30 frames-per-second (fps) was achieved with good image quality. Blurring was reduced using the Chebyshev approximation method.ConclusionThis work studies low rank plus sparse reconstruction using the spiral trajectory and demonstrates a new method for dynamic vocal tract imaging which can benefit studies of speech disorders.     

Dynamic magnetic susceptibility of a superconductor with a relaxing magnetic moment

Within the framework of the diffusion approximation for the dynamic magnetic flux, a model is constructed for the dynamic magnetic susceptibility of a superconductor with a nonuniform and time-dependent magnetic field distribution in the sample. The possibility of appearance of peaks in the temperature (or frequency) dependence of χ″ in a structurally homogeneous superconducting sample in the presence of a magnetic-field-induced nonuniformity of the diffusion parameter is demonstrated. The character of the temporal evolution of the magnetic susceptibility during relaxation of the magnetic moment of the sample is predicted and its properties are investigated. Fiz. Tverd. Tela (St. Petersburg) 39, 811–815 (May 1997)     

Barkhausen effect in stepped motion of a plane domain boundary in gadolinium molybdate

Barkhausen pulses generated in stepped motion of a single plane domain boundary (PDB) are investigated experimentally in single-crystalline wafers of the extrinsic ferroelastic ferroelectric gadolinium molybdate containing artificial pinning centers of the “field inhomogeneity” type near the edges of the sample. Two scenarios of the evolution of a PDB in interaction with “defects” are proposed on the basis of analyzing the shape of the pulses in a linearly increasing field: small changes of the pulse shape in a weak field and the generation of wedge domains in a strong field. The proposed mechanism of PDB motion due to the generation of steps near the edge of the sample and their longitudinal motion provides a means for explaining the experimentally observed linear field dependence of the PDB velocity and for determining the velocity of the steps. Fiz. Tverd. Tela (St. Petersburg) 41, 301–305 (February 1999)     

Resonance enhancement of diffuse scattering of x-rays in a waveguide heterostructure

The diffuse scattering of x rays in a four-layer waveguide-type heterostructure is investigated. The dynamic enhancement of diffuse scattering is detected experimentally in regions corresponding to the excitation of waveguide modes for both the incident and scattered waves. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 4, 219–223 (25 August 1997)     

Possibility of the appearance of spin autowaves in a model of a ferromagnet with nonuniform dissipation

A model of a ferromagnet with nonuniform dissipation is introduced for the Landau-Lifshitz equations. It is shown that in this model a ferromagnet can be regarded as an oscillating active medium where the formation of autowave structures — spin autowaves, pacemakers, and spiral waves — is possible. Their wave characteristics, expressed in terms of the parameters of the medium, are found for a special case. Fiz. Tverd. Tela (St. Petersburg) 39, 513–515 (March 1997)     

Dielectric relaxation in deuterated triglycine sulfate crystals

A study has been made of the slow relaxation of the dielectric permittivity of deuterated triglycine sulfate (DTGS) initiated by application of a dc electric field. The field and temperature dependences of the relaxation time associated with domain-wall motion were obtained. The effect of the internal electric field on relaxation processes is seen in different behavior of the field and temperature dependences under different orientations of the external field. The existence in DTGS crystals of two temperature regions of domain-structure rearrangement lying 7–8 and 15–18 °C below the Curie point has been established. Fiz. Tverd. Tela (St. Petersburg) 40, 1542–1545 (August 1998)     

Propagation of a heat pulse in a bounded conducting medium: Thermoelectric detection

The one-dimensional, non-steady thermal field on the surface of a bounded conducting sample arising from the absorption of a square heat pulse of arbitrary duration is calculated. It is shown that the spatial temperature distributions for long and short pulses are fundamentally different: After a short heat pulse is “switched off” regions of local heating arise; such regions do not occur in the case of a long pulse. The thermoelectric response is calculated and it is shown that data on the thermal conductivity and thermal diffusivity can be obtained in a single experiment. Fiz. Tverd. Tela (St. Petersburg) 41, 606–611 (April 1999)     

Observation of diamagnetic domains in beryllium

We have observed diamagnetic domains (Condon domains) in a beryllium single crystal in magnetic fields H⩽3 T (H∥[0001]) at liquid-helium temperatures. The formation of the domain structure was determined according to magnetic-breakdown quantum oscillations of the resistance thermoelectric power as well as according to the splitting of the resonance peak of the free spin precession frequency of muons (μSR). The alternation of a uniform state (with one μSR peak) and a state with domain structure (with two peaks) is consistent as regards the periodicity with the de Haas-van Alphen effect, the period is ΔH≅78 Oe, and the range of existence of domains and the difference in their magnetizations are ΔB=4πΔM=B ₂−B ₁≅30 Oe. Fiz. Tverd. Tela (St. Petersburg) 40, 524–526 (March 1998)     

Capabilities of a muon method for studying the diamagnetic domains accompanying the de Haas-van Alphen effect

This paper analyzes the capabilities of a muon (μSR) method for studying the de Haas-van Alphen effect and the diamagnetic domain structure accompanying it. It is shown that, unlike the NMR method, the μSR method makes it possible to observe the formation of a diamagnetic domain structure in all metals. It is not currently known what type of domain structure accompanies the de Haas-van Alphen effect: one-dimensional (laminar) or two-dimensional. It is shown that the line shape of the Fourier spectrum of the signal makes it possible to determine both the character of the domain structure (two-dimensional or laminar) and the magnetic field distribution in the domains. Zh. éksp. Teor. Fiz. 111, 250–261 (January 1997)     

Vierbein walls in condensed matter

The effective field, which plays the part of the vierbein in general relativity, can have topologically stable surfaces, vierbein domain walls, at which the effective contravariant metric is degenerate. We consider vierbein walls separating domains with flat spacetime which are not causally connected at the classical level. The possibility of a quantum mechanical connection between the domains is discussed. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 11, 705–710 (10 December 1999) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Magnetic lines of force in a Tornado trap

Numerical methods are used to analyze the structure of the magnetic field of an ideal Tornado trap; i.e., a trap consisting of linear currents whose magnitude and location in space are such as to produce a spherical separatrix with a radius η _c between the helices of the trap and partition the magnetic field into two regions so that the lines of force from the working volume η<η _c do not pass beyond its boundary. It is found that the magnetic lines of force of the working volume form a series of regions, each with its own properties, and do not penetrate from one region to another. In particular, one of the regions is a layer of magnetic toroidal surfaces adjacent to the inner surface of the separatrix. Zh. Tekh. Fiz. 67, 30–34 (January 1997)     

Features of the resonant interaction of moving neutral atoms and clusters with superlattice surface

Features of the interaction of moving neutral atoms, molecules, and clusters with a superlattice field (for example, the system of linear magnetic and electric domains) are considered. It is shown that the character of the particle motion depends on the ratio of the frequency ω₂₁ of the internal electromagnetic resonance to the bounce frequency Ω _s determined by the superlattice period, the velocity of the particle motion, and the possible moments of the particle in the ground d ₁₁ and excited d ₂₂ states. The conditions for regimes of attraction and repulsion of particles by the superlattice are considered. The preconditions for formation of a one-dimensional potential well located far from the superlattice and for stable channeling of neutral and charged particles in this well are also considered. Depending on the ratio of ω₂₁ to Ω _s , particle sorting and beam separation occur during interaction of the multicomponent beam consisting of different particles with the superlattice field.     

Dynamic response of a mesoscopic antiferromagnetic particle to an ac magnetic field

Calculations are made for the dynamic susceptibility of a weakly ferromagnetic mesoparticle in the macroscopic quantum coherence region and the probability of conservation of the initial polarized state in a pulsed magnetic field. Fiz. Tverd. Tela (St. Petersburg) 39, 1240–1243 (July 1997)