Irina A. Gudim

Optical Spectra of Gd3Ga5O12:Mn Crystals

The absorption and luminescence spectra of gadolinium gallium garnet crystals activated with Mn during high-temperature solution growth are analyzed in relation to the valence state of Mn. The results indicate that, in contrast to melt growth, the flux growth process stabilizes manganese in the valence state 3+.

Mode Splitting in 37–42 GHz Barium Hexaferrite Resonator: Theory and Device Applications

Dielectric resonances in the 37-42 GHz frequency band in the single crystal barium hexaferrite that occur well below the ferromagnetic resonance (FMR) have been investigated. Sample dimensions have been chosen so that the mode frequency is lower than the spin-wave excitation (in this case, the natural domain resonance) frequency. Such dielectric mode frequencies, being a function of both permittivity and permeability, are magnetic field H tunable. Here, we report on below-FMR magnetodielectric resonance (MDR) axially magnetized barium hexaferrites and their H-tuning characteristics. Our studies reveal H-tuning by up to 2.5 GHz and the tuning is the largest when the ferrite is in a magnetically unsaturated state. A theory for the MDR is presented and a fairly good agreement between experimental data and theory has been obtained. Tunable millimeter wave phase shifters and isolators utilizing the below-FMR dielectric resonances have been demonstrated.

Magnetoelectric Polarization of Paramagnetic HoAl3-XGaX(BO3)4 Single Crystals

HoAl3-xGax(BO3)4 (0 ≤ х ≤ 3) single crystals were grown from Bi2Mo3O12Li2MoO4-based fluxes. Structural inversion twinning depending on crystal composition and heat treatment conditions is revealed. It is established that upon substitution of Ga3+ ions for Al3+ the magnetic characteristics of the crystals do not significantly change, while the magnetoelectric polarization decreases with increasing gallium content. The measured value and sign of polarization charges depend on a ratio between the left and right isomers.

Crystal structure, phase transition and structural deformations in iron borate (Y0.95Bi0.05)Fe3(BO3)4 in the temperature range 90–500 K

An accurate X-ray diffraction study of (Y0.95Bi0.05)Fe3(BO3)4 single crystals in the temperature range 90-500u2005K was performed on a laboratory diffractometer and used synchrotron radiation. It was established that the crystal undergoes a diffuse structural phase transition in the temperature range 350-380u2005K. The complexity of localization of such a transition over temperature was overcome by means of special analysis of systematic extinction reflections by symmetry. The transition temperature can be considered to be Tstr ≃ 370u2005K. The crystal has a trigonal structure in the space group P3121 at temperatures of 90-370u2005K, and it has a trigonal structure in the space group R32 at 375-500u2005K. There is one type of chain formed by the FeO6 octahedra along the c axis in the R32 phase. When going into the P3121 phase, two types of nonequivalent chains arise, in which Fe atoms are separated from the Y atoms by a different distance. Upon lowering the temperature from 500 to 90u2005K, a distortion of the Y(Bi)O6, FeO6, B(2,3)O3 coordination polyhedra is observed. The distances between atoms in helical Fe chains and Fe-O-Fe angles change non-uniformly. A sharp jump in the equivalent isotropic displacement parameters of O1 and O2 atoms within the Fe-Fe chains and fluctuations of the equivalent isotropic displacement parameters of B2 and B3 atoms were observed in the region of structural transition as well as noticeable elongation of O1, O2, B2, B3, Fe1, Fe2 atomic displacement ellipsoids. It was established that the helices of electron density formed by Fe, O1 and O2 atoms may be structural elements determining chirality, optical activity and multiferroicity of rare-earth iron borates. Compression and stretching of these helices account for the symmetry change and for the manifestation of a number of properties, whose geometry is controlled by an indirect exchange interaction between iron cations that compete with the thermal motion of atoms in the structure. Structural analysis detected these changes as variations of a number of structural characteristics in the c unit-cell direction, that is, the direction of the helices. Structural results for the local surrounding of the atoms in (Y0.95Bi0.05)Fe3(BO3)4 were confirmed by EXAFS and Mössbauer spectroscopies.

Magnetic and Magnetoelectric Properties of Sm1-xLaxFe3(BO3)4 Single Crystals

Sm1-xLaxFe3(BO3)4 (х = 0, 0.5, and 0.75) single crystals are grown by the flux method and their characteristics are investigated in the temperature range 5−300 K and magnetic fields of up to 9 T. It is established that substitution of nonmagnetic La3+ ions for magnetic Sm3+ ions increases the magnetic moment and weakens the magnetoelectric effect in Sm1-xLaxFe3(BO3)4.