Ferromagnetoelectric phases of hexaferrites: a group-theoretical analysis

A symmetry-guided approach for designing and tailoring magnetoelectric and multiferroic hexaferrites is proposed. A group-theoretical study of the magnetoferroelectric structures of a hexagonal ferrite was carried out. The results were applied to M-, W- and Z-type hexaferrites. It is shown that the magnetic structure cannot be collinear in the main magnetic phase, with the direction of the magnetic moment along the hexagonal axis. Magnetic sublattices are shown in which cation substitution should lead to the formation of a multiferroic state. A thermodynamic model of a hexagonal ferroelectric ferrimagnet was constructed, in which isostructural phase transitions were observed.

Phonons in Short-Period GaN/AlN Superlattices: Group-Theoretical Analysis, Ab initio Calculations, and Raman Spectra

We report the results of experimental and theoretical studies of phonon modes in GaN/AlN superlattices (SLs) with a period of several atomic layers, grown by submonolayer digital plasma-assisted molecular-beam epitaxy, which have a great potential for use in quantum and stress engineering. Using detailed group-theoretical analysis, the genesis of the SL vibrational modes from the modes of bulk AlN and GaN crystals is established. Ab initio calculations in the framework of the density functional theory, aimed at studying the phonon states, are performed for SLs with both equal and unequal layer thicknesses. The frequencies of the vibrational modes are calculated, and atomic displacement patterns are obtained. Raman spectra are calculated and compared with the experimental ones. The results of the ab initio calculations are in good agreement with the experimental Raman spectra and the results of the group-theoretical analysis. As a result of comprehensive studies, the correlations between the parameters of acoustic and optical phonons and the structure of SLs are obtained. This opens up new possibilities for the analysis of the structural characteristics of short-period GaN/AlN SLs using Raman spectroscopy. The results obtained can be used to optimize the growth technologies aimed to form structurally perfect short-period GaN/AlN SLs.

Термодинамическая стабильность твердых растворов In-=SUB=-x-=/SUB=-Ga-=SUB=-1-x-=/SUB=-N

A group-theoretical analysis of solid solutions of indium and gallium nitrides InxGa1-xN was carried out, and all the main symmetry groups were found for these solutions with the initial hexagonal structure. The thermodynamic potentials of the main phases with different compositions x are calculated using the density functional theory. It is shown that for small and large x, i.e. at 0 <x <0.2 and 0.8 <x <1, there is a large number of monoclinic phases Pm and P2_1, which are stable with respect to decomposition into InN and GaN at room temperature. In the range 0.2 <x <0.8, there are only two stable orthorhombic phases Cmc2_1 with compositions x = 1/3 and x = 2/3. All basic geometric and thermodynamic properties of various InxGa1-xN phases have been calculated. It was found that the stability of InxGa1-xN epitaxial films increases with growth on InN and decreases with growth on GaN.

Антиферродисторсионная мягкая мода в кристалле PbZr-=SUB=-0.024-=/SUB=-Ti-=SUB=-0.976-=/SUB=-O-=SUB=-3-=/SUB=-

The antiferrodistortive soft mode was detected at the M point of the Brillouin zone in a crystal PbZr0.024Ti0.976O3 using the method of inelastic scattering of synchrotron radiation. The performed group-theoretical analysis and calculations of inelastic structural factors made it possible to relate the detected critical excitation to oxygen octahedra rotations. The traced temperature evolution of the soft mode frequency obeys the Curie-Weiss law with Curie temperature T AFD = 438 ± 5 K, which is close to the ferroelectric Curie temperature T FE = 479.5 K.

ИК-активные фононы ионного кристалла LiNiPO-=SUB=-4-=/SUB=-

The results of an experimental study of the phonon spectrum of a LiNiPO4 single crystal by IR reflection spectroscopy are presented. The spectroscopic parameters of IR phonons are obtained. The observed number of phonons is mainly in agreement with the performed group-theoretical analysis, and the reasons for small discrepancies are discussed. With decreasing temperature, a hardening of phonon frequencies is observed, associated with temperature changes in the parameters of the crystal lattice.