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

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

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)

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

Magnetization Dynamics of Iron Garnet Crystals in Oscillating Magnetic Field

AbstractUsing direct observations via stroboscopic technique it is shown that in iron garnet crystals placed in an alternating magnetic field aligned perpendicular to the plane of the sample the dynamic magnetization reversal is carried out by the oscillations of the domain walls with their subsequent drift. For the first time the dependences of the maximum speed of domain walls motion during oscillations Vosc and of the domain walls oscillations amplitude Aosc in external oscillating magnetic field on amplitude of the external magnetic field H0 are obtained. It is shown that these dependences can be approximated by linear functions. Numerical simulations of domain walls motion in an alternating magnetic field were performed with parameters of the real sample. It is established that the experimental dependences Vosc(H0) and Aosc(H0) at different frequencies are in a qualitative agreement with the results of numerical simulations

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

A multisite photometric study of two unusual Beta Cep stars: the magnetic V2052 Oph and the massive rapid rotator V986 Oph

We report a multisite photometric campaign for the Beta Cep stars V2052 Oph and V986 Oph. 670 hours of high-quality differential photoelectric Stromgren, Johnson and Geneva time-series photometry were obtained with eight telescopes on five continents during 182 nights. Frequency analyses of the V2052 Oph data enabled the detection of three pulsation frequencies, the first harmonic of the strongest signal, and the rotation frequency with its first harmonic. Pulsational mode identification from analysing the colour amplitude ratios confirms the dominant mode as being radial, whereas the other two oscillations are most likely l=4. Combining seismic constraints on the inclination of the rotation axis with published magnetic field analyses we conclude that the radial mode must be the fundamental. The rotational light modulation is in phase with published spectroscopic variability, and consistent with an oblique rotator for which both magnetic poles pass through the line of sight. The inclination of the rotation axis is 54o <i< 58o and the magnetic obliquity 58o <beta< 66o. The possibility that V2052 Oph has a magnetically confined wind is discussed. The photometric amplitudes of the single oscillation of V986 Oph are most consistent with an l=3 mode, but this identification is uncertain. Additional intrinsic, apparently temporally incoherent, light variations of V986 Oph are reported. Different interpretations thereof cannot be distinguished at this point, but this kind of variability appears to be present in many OB stars. The prospects of obtaining asteroseismic information for more rapidly rotating Beta Cep stars, which appear to prefer modes of higher l, are briefly discussed.Comment: 12 pages, 8 figures, MNRAS, in pres

Probable nonradial g-mode pulsation in early A-type stars

A survey for line profile variability in early A-type stars has been performed in order to detect nonradial pulsation signatures. The star HR 6139, with spectral type A2V and estimated T_eff=8800 K, shows evident line profile variations that can be explained by oscillations in prograde g-modes. This feature and the known photometric variability are similar to those observed in the Slowly Pulsating B-type stars. However HR 6139 is much cooler than the cool border of the instability strip of such variables, and it is hotter than the blue edge of the delta Scuti instability strip. There are indications of a tiny variability also in other four objects, whose nature is not yet clear.Comment: 4 pages, 5 figures; accepted for publication in A&A (letter