Drift of domain walls in a harmonic magnetic field

It is shown that a two-step form of the dynamic magnetization curve (and the hysteresis loop) established for a multiaxial ferrite-garnet wafer with a low quality factor (Q < 1) and considerable anisotropy in the plane (K p /K u = 14) in the frequency range of 25-1000 Hz is explained by the reconstruction of the dynamic domain structure, particularly by the established features of the drift of domain boundaries in the harmonic magnetic field. © 2013 Allerton Press, Inc

Unidirectional motion of magnetic domain walls: The experiment and numerical simulation

The results of study of unidirectional motion of topologically different domain structures under the influence of periodic bipolar and unipolar magnetic field pulses applied perpendicular to the sample plane of (111) iron garnet single crystal plate are presented. The response of the domain structure to the field pulses was studied by direct observations utilizing the stroboscopic technique. Experimentally obtained dependences of the speed of unidirectional motion of stripe domains on the parameters of external bipolar pulsed magnetic field are compared with the results of numerical simulations. © Published under licence by IOP Publishing Ltd.Ministry of Science and Higher Education of the Russian Federation: 3.6121.2017The work was performed within the framework of the basic part of the state assignment of the Ministry of Science and Higher Education of the Russian Federation (project 3.6121.2017)

Experimental and Theoretical Study of Stripe Magnetic Domain Structure Drift in Iron Garnet Crystals

The results of experimental and theoretical study of magnetic domain structure drift in low frequency oscillating magnetic field oriented perpendicular to the sample plate are presented. Experimental study was performed on uniaxial iron garnet (TbErGd)₃(FeAl)₅O₁₂ (111) plate with rhombic anisotropy for the case when orientation of domain walls of stripe domains is preserved. Dynamic domain structure was revealed by means of magnetooptic Faraday effect and registered by high speed digital camera at the speed equal to 1200 fps. Theoretical model based on the motion equations for coupled harmonic oscillators that takes into account attenuation and field inhomogeneity along the plate is proposed

The two hybrid B-type pulsators: Nu Eridani and 12 Lacertae

The rich oscillation spectra determined for the two stars, Nu Eridani and 12 Lacertae, present an interesting challenge to stellar modelling. The stars are hybrid objects showing a number of modes at frequencies typical for Beta Cep stars but also one mode at frequency typical for SPB stars. We construct seismic models of these stars considering uncertainties in opacity and element distribution. We also present estimate of the interior rotation rate and address the matter of mode excitation. We use both the OP and OPAL opacity data and find significant difference in the results. Uncertainty in these data remains a major obstacle in precise modelling of the objects and, in particular, in estimating the overshooting distance. We find evidence for significant rotation rate increase between envelope and core in the two stars. Instability of low-frequency g-modes was found in seismic models of Nu Eri built with the OP data, but at frequencies higher than those measured in the star. No such instability was found in models of 12 Lac. We do not have yet a satisfactory explanation for low frequency modes. Some enhancement of opacity in the driving zone is required but we argue that it cannot be achieved by the iron accumulation, as it has been proposed.Comment: 9 pages, 8 figures, accepted for publication in MNRA

SPB stars in the open SMC cluster NGC 371

Pulsation in beta Cep and SPB stars are driven by the kappa mechanism which depends critically on the metallicity. It has therefore been suggested that beta Cep and SPB stars should be rare in the Magellanic Clouds which have lower metallicities than the solar neighborhood. To test this prediction we have observed the open SMC cluster NGC 371 for 12 nights in order to search for beta Cep and SPB stars. Surprisingly, we find 29 short-period B-type variables in the upper part of the main sequence, many of which are probably SPB stars. This result indicates that pulsation is still driven by the kappa mechanism even in low metallicity environments. All the identified variables have periods longer than the fundamental radial period which means that they cannot be beta Cep stars. Within an amplitude detection limit of 5 mmag no stars in the top of the HR-diagram show variability with periods shorter than the fundamental radial period. So if beta Cep stars are present in the cluster they oscillate with amplitudes below 5 mmag, which is significantly lower than the mean amplitude of beta Cep stars in the Galaxy. We see evidence that multimode pulsation is more common in the upper part of the main sequence than in the lower. We have also identified 5 eclipsing binaries and 3 periodic pulsating Be stars in the cluster field.Comment: 8 pages, 11 figures. Accepted for publication in MNRA

Micromagnetic Structures near a Second Order Phase Transition in Monocrystalline Ferrite Garnet Plates

The main parameters of micromagnetic structure formation in a vicinity of a second order phase transition were determined experimentally and theoretically. The theoretical study was performed using micromagnetic approach. External magnetic field $H_{c}$ of appearance of micromagnetic structure and micromagnetic structure period $L_{c}$ were determined for (001)-oriented plate with uniaxial $K_{u}$ and cubic K₁ magnetic anisotropy. The plate was saturated by the field applied in its plane. In the model we assumed that magnetization vector undergoes small deviations from equilibrium if magnetic field is slightly reduced. These deviations are periodic in nature: micromagnetic structure has a form of a plane wave. Dependences of $H_{c}$ and $L_{c}$ on an azimuthal angle of external magnetic field and on anisotropy constants $K_{u}$ and K₁ were derived analytically in this work. Experimental studies of micromagnetic structure near the second order phase transition were conducted on (EuEr)₃(FeGa)₅O₁₂ (001)-oriented 50 μm thick ferrite-garnet plate with $K_{u}$ = 5700 erg/cm³ and K₁=-3700 erg/cm³. Micromagnetic structure was revealed by means of magnetooptic Faraday effect. The in-plane field was increased up to 2000 Oe. Experimentally determined values of $H_{c}$ and $L_{c}$ were compared with theoretical estimates

Motion and Interaction of Magnetic Dislocations in Alternating Magnetic Field

Abstract The behavior of magnetic dislocations (MDs) in an alternating harmonic magnetic field in iron garnets has been experimentally investigated. The results are presented for single-crystal plates in which the drift of domain walls is observed in fields of sound frequencies. It is found that MDs in a stripe domain structure are able to move not only along but also across domain walls. A pairwise interaction between magnetic dislocations when they approach each other to distances on the order of the sizes of the cores of MDs is revealed. The processes of the annihilation, mutual passing of magnetic dislocations through each other and overtaking are found. The features of the dynamic behavior of MDs are explained using a mechanism based on the presence of vertical Bloch lines in a structure of domain walls. MDs are formed at nucleation centers, and their nucleation field is lower than the drift-starting field, which corresponds to previously proposed dislocational mechanism of the drift. The dependencies of quantitative parameters of the drift and MDs on amplitude and frequency of the pumping field are determined. The behavior of MDs should be considered when analyzing the mechanisms for magnetization and temperature-dependent phase transitions in magnetic layers

Discovery and analysis of p-mode and g-mode oscillations in the A-type primary of the eccentric binary HD 209295

We have discovered both intermediate-order gravity mode and low-order pressure mode pulsation in the same star, HD 209295. It is therefore both a γ Doradus and a δ Scuti star, which makes it the first pulsating star to be a member of two classes. The analysis of our 128 h of multisite spectroscopic observations carried out over two seasons reveals that the star is a single-lined spectroscopic binary with an orbital period of 3.10575±0.00010 d and an eccentricity of 0.352±0.011. Only weak pulsational signals are found in both the radial velocity and line-profile variations, but we have succeeded in showing that the two highest-amplitude γ Doradus pulsation modes are consistent with ℓ=1 and ∣m∣=1. These two modes dominated our 280 h of BVIC multisite photometry, also obtained over two seasons. We detected altogether ten frequencies in the light variations, one in the δ Scuti regime and nine in the γ Doradus domain. Five of the γ Doradus frequencies are exact integer multiples of the orbital frequency. This observation leads us to suspect they are tidally excited. Attempts to identify modes from the multicolour photometry failed. We performed model calculations and a stability analysis of the pulsations. The frequency range in which δ Scuti modes are excited agrees well with observations. However, our models do not show excitation of γ Doradus pulsations, although the damping is smaller in the observed range. We also investigated tidal excitation of γ Doradus modes. Some of the observed harmonics of the orbital period were found to be unstable. The observed orbital harmonics which are stable in the models can be understood as linear combinations of the unstable modes. We could not detect the secondary component of the system in infrared photometry, suggesting that it may not be a main-sequence star. Archival data of this star show that it has a strong ultraviolet (UV) excess, the origin of which is not known. The orbit of the primary is consistent with a secondary mass of M>1.04 M⊙, which is indicative of a neutron star, although a white dwarf companion is not ruled ou