Volume 36, Issue 6, June 2010
Index of content:
36(2010); http://dx.doi.org/10.1063/1.3462535View Description Hide Description
A brief review of research performed on ferroelectromagnetic crystals over the following the discovery of these materials by G. A. Smolenskiĭ and his colleagues is given. During the first decades single crystals and solid solutions of ferrloelectromagnetic substances were synthesized and the first evidence of an interaction between the ferroelectric and magnetic subsystems was obtained. The first two colossal magnetoelectric (ME) effects in nickel-iodine boracite were discovered. The theory explained qualitatively the observed and predicted new ME phenomena. Subsequently, the class of ferroelectromagnets grew, and compounds in which the electric polarization was induced by a spiral magnetic structure appeared. Measuring techniques improved. The discovery at the end of the 20th century of a new optical method of second-harmonic generation set the conditions for a new renaissance in ME studies. An entire series of new colossal ME effects attesting to the realization of cross ME control of the electric (magnetic) properties of a crystal by a magnetic (electric) field has now been discovered. The ME effects of greatest significance which have been discovered in the last few years are described. It is noted that ferroelectromagnets with electric polarization of an electronic nature (compounds with mixed valence, semiconductors) are promising materials.
Optical spectroscopy of charge transfer transitions in multiferroic manganites, ferrites, and related insulators36(2010); http://dx.doi.org/10.1063/1.3455721View Description Hide Description
The results of theoretical and experimental studies of charge transfer(CT) transitions in multiferroic manganites, ferrites, and related insulators are reviewed. Starting with a simple cluster model approach one-center and two-center CT transitions, their polarization properties, the role played by structural parameters, orbital mixing, and spin degree of freedom are all addressed. Optical ellipsometry data in the spectral range of in perovskite and hexagonal rare-earth manganites and in orthorhombic manganites are analyzed. Two groups of iron oxides, where ions occupy either only octahedral positions (, orhoferrites et al.) and materials with ions both in octahedral and tetrahedral positions (hematite , garnets , lithium ferrite, et al.), are discussed.
36(2010); http://dx.doi.org/10.1063/1.3457390View Description Hide Description
The magnetic, electric, magnetoelectric, and magnetoelastic properties of rare-earth ferroborates (, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er) as well as yttrium ferroborate have been studied comprehensively. A strong dependence not only of the magnetic but also magnetoelectric properties on the type of rare-earth ion, specifically, on its anisotropy, which determines the magnetic structure and the large contribution to the electric polarization, has been found. This is manifested in the strong temperature dependence of the polarization below the Néel point and its specific field dependence, which is determined by the competition between the external and exchange fields. A close correlation has been found between the magnetoelastic properties of ferroborates and the magnetoelastic and magnetic anomalies at magnetic-field induced phase transitions. It is found that in easy-plane ferroborates, together with magnetic-field induced electric polarization spontaneous polarization also arises below the Néel point. The ferroelectric ordering in ferroborates is of extrinsic character, giving rise to strong magnetoelectric coupling below . Aside from the antiferromagnetic phase transition, the particulars of the structural phase transition accompanied by anomalies of the dielectric and magnetoelectric properties are examined for the first time. The character of the dielectric anomalies at a structural transition is analyzed for the first time on the basis of Landau’s approach.
36(2010); http://dx.doi.org/10.1063/1.3455792View Description Hide Description
Giant permittivity and ferromagnetism have been observed above (including room temperature) in single crystals of the dilute semiconductor manganite-ferroelectromagnetic in the investigations of x-ray diffraction, dielectric and magnetic properties, and conductivity. X-ray diffraction study has revealed a layered superstructure along the axis at room temperature. A model of the state with giant including as-grown 2D layers with doping impurities, charge carriers, and double-exchange coupled ion pairs is suggested. At low temperatures these layers form isolated electrically neutral small-size 1D superlattices, in which de Haas van Alphen oscillations were observed. As temperature and hopping conductivity increase, charge-carrier self-organization in the crystal cause the formation of a layered superstructure consisting of charged layers (with an excess ) alternating with dielectric layers of the initial crystal—the ferroelectricity state due to charge ordering. Ferromagnetism results from double exchange between and ions through charge carriers in the charged layers.
36(2010); http://dx.doi.org/10.1063/1.3455798View Description Hide Description
Symmetry analysis is used to study magnetic anisotropy and magnetoelectric interactions in the ferroelectricantiferromagnet theoretically. It is shown that symmetry admits the existence in this material of a nonuniform magnetoelectric interaction (flexomagnetoelectric type) with a specific dependence on the magnitude and orientation of the external magnetic field. With increasing magnetic field this interaction can cause a phase transition into a magnetic incommensurate phase with a characteristic jump of the electric polarization. The linear and quadratic magnetoelectric effects and the question of the relativistic canting of the sublattices, which is due to the magnetoelectric interaction, are examined. The proposed approach is a natural generalization of the method of constructing invariants from the magnetic modes of the crystal to the case of phase transitions with a doubled unit cell of the crystal and can be used for symmetry analysis of other ferroelectric magnetic compounds.
Light reflection from nonlinear optical dielectric film on a bigyrotropic magnetoelectric substrate at angles close to Brewster angles36(2010); http://dx.doi.org/10.1063/1.3455805View Description Hide Description
Light reflection from the interface of a dielectric film, characterized by a cubic optical nonlinearity, on a bigyrotropic substrate magnetoelectric substrate is investigated theoretically. Relations are obtained for the reflection coefficient as a function of the incidence angle of the light for the main magneto-optic configurations: polar, longitudinal, and transverse. The effect of the magneto-electric and magneto-optical contributions to the electric polarization of such a biaxial structure on the polarization state of the light reflected at angles close to the Brewster angles is studied.
36(2010); http://dx.doi.org/10.1063/1.3456999View Description Hide Description
The present stage in the study of magnetoelectric (ME) compositemultiferroics is analyzed. The ME effect in materials of this kind is due to the magnetostriction and piezoelectric properties of the components. The elastic mechanical interaction between the magnetostriction and piezoelectric phases engenders a giant magnetoelectric response in magnetoelectric composite materials. The ME effect is more than 100 times stronger near an electromechanical resonance. The recently obtained nanostructural composites made from ferroelectric and magnetic oxides with dimensions of the order of nanometers made in the form of films on a substrate engender interest in the possibility of constructing integrated devices. The ME interaction between ferroelectrics and magnetic oxides with nanometer dimensions is the same as in ordinary composite materials. Just as for bulk ME composites, sensors, transducers and diverse reading/writing devices are some of the possible applications of the ME effect in nanocomposites
36(2010); http://dx.doi.org/10.1063/1.3455808View Description Hide Description
sublattices ferrimagnet with a cubic symmetry and a linear magnetoelectric effect. There is no spectroscopic evidence for structural lattice distortions below , which are expected due to magnetoelectric coupling. Using symmetry arguments we explain this observation by considering a special type of ferrimagnetic ground state which does not generate a spontaneous electric polarization. Interestingly, Raman scattering shows a strong increase of electric polarization of media through a dynamic magnetoelectric effect as a remarkable enhancement of the scattering intensity below . New lines of purely magnetic origin have been detected in the magnetically ordered state. A part of them are attributed as scattering on exchange magnons. Using this observation and further symmetry considerations we argue for strong Dzyaloshinskii–Moriya interaction existing in the .
36(2010); http://dx.doi.org/10.1063/1.3457377View Description Hide Description
It has been found experimentally that in the strongly anisotropic orthorhombic antiferromagnetic crystal the destruction of antiferromagnetic order by a magnetic field directed along the antiferromagnetism axis occurs in steps by means of three phase transitions: two first-order transitions in the fields , and one second-order phase transition in the field . A section of magnetic fields (, ) where the magnetization of the crystal varies almost linearly with the field was found. Possible magnetic structures formed in a magnetic field are discussed. The values of the exchange parameters are estimated.