Magnetic birefringence of sound and magnetoacoustic oscillations in hematite

An experimental study of the magnetic birefringence of sound in the α-Fe₂O₃ easy-plane antiferromagnet is reported. The amplitude of transverse sound transmitted through the crystal along the C₃ trigonal axis was found to oscillate depending on the magnitude of the magnetic field H applied in the basal plane (H ⊥ C₃). The experiments provide qualitative support for the theory of this phenomenon developed earlier by Turov. Possible reasons for the substantial quantitative disagreement between theory and experiment observed in the field dependence of the sonic-amplitude oscillation period are discussed. Unannealed samples of the hematite revealed a 60° periodic dependence of the position of the intensity oscillation extrema on magnetic field orientation in the basal plane, which is associated with basal anisotropy of higher than second order. The observation of pronounced (～6×10³ Oe) effective magnetic anisotropy fields in the basal plane can be assigned to the existence of large residual strains in such samples.     

Theory of birefringence of sound in easy-plane rhombohedral antiferromagnets with inclusion of hexagonal anisotropy and mechanical stresses

The constructed thermodynamic theory of magnetic birefringence of transverse sound in easyplane weak ferromagnets with an axial basal anisotropy induced by boundary conditions has been generalized to the case of additional inclusion of the hexagonal anisotropy enhanced by isotropic mechanical stresses. The magnetoelastic contribution ΔC _a to the effective elastic modulus C ₄₄^eff = C ₄₄ + ΔC _a , which determines the phase velocity of the magnetic mode of the acoustic wave in a crystal, has been calculated. It has been shown that the induced anisotropy with the twofold and sixfold symmetry leads to the appearance of an additional anisotropic term of the fourfold symmetry in the contribution ΔC _a . The developed theory has made it possible to satisfactorily describe the existing experiments.     

Hydrostatic pressure and hexagonal magnetic anisotropy of hematite

The effect of hydrostatic pressure on the hexagonal basal magnetic anisotropy of a rhombohedral easy-plane weak ferromagnet is investigated in terms of the thermodynamic theory. It is shown that the contribution to the hexagonal anisotropy is determined by the effective constants of the uniaxial and cubic anisotropy, which include the additions proportional to the hydrostatic pressure. The hydrostatic pressure, which leads to the basal anisotropy observed in experiments for stressed samples, is determined for hematite.     

Magnetic birefringence of light in hematite

Peculiarities in the propagation of IR radiation in the easy-plane antiferromagnet α-Fe₂O₃ are investigated. In the geometry of the Cotton-Mouton effect, magnetic birefringence is measured and magnetooptical constants are estimated.     

Rotation of the plane of polarization and linear birefringence of sound in hematite below the morin point

The linear antiferromagnetic birefringence of sound in hematite (α-Fe₂O₃) residing in the collinear easy-axis phase (L ‖ C ₃) below the Morin point is experimentally studied. The plane of polarization of a linearly polarized transverse acoustic wave propagating along the trigonal axis C ₃ of a hematite crystal placed in a magnetic field H applied in the basal plane (H ⊥ C ₃, 3.5 ≤ H ≤ 15 kOe) is found to rotate after a temperature-driven orientational phase transition to the easy-axis state. The angle of rotation exhibits a 180° angular dependence on the direction of the magnetic field in the basal plane and varies from zero to ～π/2. Numerical estimates suggest that the conditions necessary for rotation of the plane of polarization through appreciable angles (～π/2) can be satisfied in the easy-axis phase at orientational phase transition temperatures close to the Morin point, which actually takes place in the fields employed. The results obtained are described sufficiently well by the theory of linear antiferromagnetic birefringence of sound (E.A. Turov) and confirm its main conclusions.     

Acoustic anisotropy and birefringence in fibre-reinforced composite materials

All nine elastic moduli of an orthotropic composite material, namely, polypropylene reinforced with glass fiber, are determined from the measured values of the bulk acoustic wave velocities along specific directions in the planes of symmetry of the material. These data are used to calculate the angular dependences of phase velocities, polarization vectors, and directions of ray velocities of bulk waves in the composite. It is demonstrated that the difference in the velocities of shear waves polarized along and across the glass fiber gives rise to an acoustic birefringence and can lead to an elliptical polarization of waves. The measurement of the phase velocities of shear waves as functions of the wave polarization is suggested as a method for the determination of the fiber orientation in a composite material.     

Effect of anisotropy on acoustoelastic birefringence in wood

The ultrasonic velocity of shear waves propagating through radial direction of a wood plate specimen, transversely to the loading direction, was measured. By rotating an ultrasonic sensor, the oscillation direction of the shear waves was varied with respect to the wood plate axis and loading direction. The relationship between shear wave velocity and oscillation direction was examined to discuss the effect of anisotropy on the acoustoelastic birefringence in wood. The results obtained were summarized as follows. When the oscillation direction of the shear wave corresponded to the tangential direction of the wood specimen regardless of the stress direction, shear wave velocity decreased markedly and the relationship between shear wave velocity and rotation angle tended to become discontinuous. That is, when the shear waves oscillated in the anisotropic axis of the wood, the shear wave velocity peaked unlike in the case of oscillation in the stress direction. In an isotropic material (acrylic, aluminum 5052), on the contrary, when the shear waves oscillated in the stress direction of the specimen, the shear wave velocity peaked regardless of the main-axis direction of the specimen. On the basis of the discussion of these results, the ultrasonic shear wave propagating in wood under stress is confirmed to be polarized in the anisotropic axis of the wood.     

磁性液体在磁场中产生光的双折射效应机理

磁性液体是一种特殊的高分子稳定胶体,在磁场中会产生光的双折射效应,对磁性液体在胶体学科方面展开研究,发现磁性液体在磁场中的弱絮凝行为表现异常明显,显示出特有的方向性,且又不至胶体系统失稳,证明了磁性液体中的磁性微粒在磁场中聚集成方向性的链状而又不失稳的临界状态存在。从而揭示了方向性弱絮凝是磁性液体在磁场中产生光的双折射效应的机理。     

New magnetic regime in hematite hollow spheres

A new magnetic regime is presented in hematite hollow spheres. The numerous dangling bonds, formed in the H⁺ etching process during fabrication, endow the surfaces of the primary particles with paramagnetic and ferrimagnetic attributes. The freezing and coupling of these surface states have profound effect on the magnetic performance of the host hematite spheres – including enhanced ferromagnetism, a peculiar variation of thermal remanent magnetization, and an exchange bias training effect, which may find their potential applications in spintronic fields. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Multimodal and birefringence effects in magnetic photonic crystals

Photon trapping in magnetic photonic crystals leads to an enhanced Faraday rotation by increasing the optical path-length of the transmitted beam. The integration of these structures into on-chip photonic circuits, while advantageous from the point of view of component connectivity in multifunctional systems, faces several challenges. Photonic waveguide structures in magnetic films may support more than one mode depending on the waveguide thickness and refractive index. Differences in effective refractive indices between TE and TM modes engender phase disparities, thus hindering the Faraday response of the material. The effects of birefringence and multimodality on the performance of waveguide magnetic photonic crystals in magnetic garnets are addressed in paper. In particular, Faraday rotation enhancement in magnetophotonic crystals in the presence of waveguide birefringence and modal multiplicity on the photonic bandgap spectral response are discussed. Multiple stopbands and significant polarization rotation are observed in multimode Bi-substituted iron garnet film waveguides with single-defect photonic crystal structures. The photonic crystals for this study are patterned on ridge waveguide films by focused-ion-beam (FIB) milling.     

Stationary states for a mechanical system with stochastic boundary conditions

We consider a system of Newtonian particles, with a long-range repulsive pair potential, moving in a cavity whose surface temperature is spatially varying. When a particle hits the surface, it is thermalized at the temperature of the collision point. We prove that this system has a unique stationary ensemble, to which any initial distribution converges for large times. We show that this stationary ensemble depends continuously on the surface temperature profile.     

A study of the interphase boundary between ilmenite and hematite

The misfit dislocation network which forms between hematite and the ilmenite matrix from which it exsolves has been studied utilizing transmission electron microscopy techniques. It has been shown that the misfit dislocation network consists of a three-fold, symmetric dislocation configuration of half partial dislocations. The measured spacings between the misfit dislocations are in good agreement with those predicted from the postulated model of the network.     

Growth-induced magnetic anisotropy in YIG

YIG film were grown by LPE and the substrate was carefully removed by grinding. From the⁵⁷Fe Moessbauer spectrum the orientation of magnetization was determined and the growth-induced magnetic anisotropy constant deduced. For (111) orientation YIG, the magnetic uniaxial anisotropy constant Ku was determined to be 2.3×10³ erg cm^–3.     

Relationship between segmental anisotropy in polarizability and stationary ultrasonically induced birefringence in polymer solutions

Stationary ultrasonically induced birefringence in various polymer solutions was measured in order to investigate its relation to the anisotropy in polarizability of a polymer chain. From the concentration dependence of stationary ultrasonically induced birefringence in polystyrene–toluene, polycarbonate–chloroform and polybutadiene–toluene solutions, the intrinsic values of the birefringence were obtained. A linear relationship between the intrinsic value of the birefringence per segment and the segmental anisotropy in polarizability was obtained. In aqueous solutions of polyelectrolytes, sodium polystyrenesulfonate and tetramethylammonium polystyrenesulfonate, the birefringence decreased with ionic strength by the addition of the salts. The stationary birefringence per ultrasonic intensity for all polymer solutions investigated here decreased with increasing frequency.     

Measurement of the magnetic birefringence in oxygen and nitrogen gases

The Cotton-Mouton constant for oxygen and nitrogen has been measured at room temperature and pressure using a laser with λ = 514.5 nm.     

Effect of boundary conditions on quantum-anti-dots in the presence of a magnetic field

Many people have studied the conductance properties through an array of anti-dots, especially since the observation of Weiss oscillations. In most cases, however, in which the recursive Greenźs functions are used on a spatial lattice, periodic boundary conditions are employed. In this paper, we analyse the effects of boundary conditions and magnetic field on the conductance behavior in a number of anti-dot-shaped, GaAs/AlGaAs 2DEG quantum systems. The effect of periodic boundary conditions causes a reduction in the overall conductance. The effect of changing the boundary conditions is more profound for lower numbers of anti-dots.     

Experimental validation of the sound transmission of rectangular baffled plates with general elastic boundary conditions

Several prediction methods have recently been developed for systematically studying the effects of general boundary conditions on the sound transmission loss (STL) of plate-like structures. But corresponding experimental validation studies remain scarce owing to the difficulty of obtaining accurate boundary conditions for practical structures. This paper presents a convincing experiment conducted on a baffled plate system to validate the STL prediction model in a previous paper by Yu et al. [Noise Control Eng. J. 58(2), 187-200, 2010]. A method is proposed to determine the boundary conditions of this system, and experimental STL compares well with the predictions based on the identified boundary condition.