YCr1-xFexO3 materials with x = 0; 0.125; 0.25; 0.33; 0.5; 0.67; 0.75; 0.875 and 1 have been prepared by solution combustion method followed by thermal treatment at 800 degrees C in air. X-ray powder diffraction spectra of samples confirmed the formation of solid solutions within the whole concentration interval. All spectra were successfully indexed in orthorhombic Pnma space group. Neutron diffraction measurements at room temperature have confirmed that some of the samples studied are antiferromagnetically ordered. This is consistent with the asymptotic Curie temperatures determined from the dependence of the reverse magnetic susceptibility on temperature The effective magnetic moments were determined from the Rietveld refinement of the neutron powder diffraction data.

The influence of the electric field E on the magnetic properties in multiferroic thin films is studied by a combination of modified Heisenberg and transverse Ising models (TIMs) using a Green's function technique. It is shown that the magnetization M, the Neel temperature TN and the spin-wave energies Em increase, whereas their damping gm decreases with increase in electric field E. This is an evidence for the strong intrinsic magnetoelectric (ME) coupling. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

The phonon properties of multiferroic hexagonal RMnO3 thin films are studied based on a microscopic model including anharmonic spin–phonon interactions. We obtain an anomaly near the magnetic phase transition temperature TN which can be attributed to the magnetoelectric nature of RMnO3 and a strong spin–phonon interaction. The size and surface effects on the phonon properties are discussed. It is demonstrated that the phonon energies can be larger or smaller for different substrates or doping ions, whereas their damping is always enhanced. The influence of an external magnetic field on the phonon properties is observed.

The influence of an external electric field E is studied on the phonon properties of multiferroic BiFeO3 and hexagonal R MnO3 materials using a microscopic model and Green’s function technique. The phonon energy decreases whereas the damping increases with enhancing of E.

The influence of the surface, film thickness, temperature and ion doping effects on the magnetic and electric properties in multiferroic thin films is studied by a combination of modified Heisenberg and transverse Ising models using a Green’s function technique. It is demonstrated that the magnetization M, the polarization P, the critical temperatures TN and TC, the spin-wave energies Em and Ee, and their damping are very sensitive to the exchange interaction constants on the surface (A1s and Js) and in the defect layers (A1d and Jd). It has been found that the damping is enhanced compared to the case with no defect layers. We have obtained that M, P, TN, TC and the spin-wave energies could be increased or decreased by using different kinds of doping ions. The concurrent interaction mechanisms of the magnetic and electric subsystems are shown. The results are in qualitative accordance with the experimental data.

Size, substrate, doping and magnetic field effects on the phonon properties in multiferroic BiFeO3 thin films are studied based on a microscopic model. We obtain an anomaly near the magnetic phase transition temperature TN which can be attributed to the magnetoelectric nature of BiFeO3 and strong anharmonic spin–phonon interaction. It is shown that due to crystal lattice distortion for dopants with ionic radius smaller than that of the host ions the phonon energy decreases (for example Tb or Ti), whereas for the opposite case (larger radius of the doping ions, for example Co or Ni) it increases. The phonon damping is always enhanced compared to the undoped thin film.

The renormalized phonon spectrum beyond the random phase approximation in ferroelectric thin films is obtained using a Green's function technique and the transverse Ising model including anharmonic spin-phonon and phonon-phonon interactions. The temperature and film thickness dependence of the soft phonon mode energy omega is calculated for the case of positive and negative anharmonic spin-phonon interaction constants R and for different surface constants R-s. For R > 0 the phonon mode energy increases with increasing temperature, T -> T-C, whereas for R < 0 the phonon mode decreases with increasing temperature. For the case of smaller surface constant compared to the bulk, R-s < R-b, we obtain that omega of the thin film is larger in comparison to that of the bulk, i.e., we obtain hardening of the soft mode. In the other case, R-s > R-b, the soft phonon mode energy of the thin films is smaller compared to the bulk, i.e., we obtain softening of the phonon mode. The first case could explain the behavior of the phonon mode, soft mode hardening in SrTiO3 and (Ba,Sr)TiO3 thin films obtained experimentally, whereas the second case is responsible for PbTiO3 thin films. The dependence of the phase transition temperature on film thickness and different surface spin-spin J(s) and spin-phonon interaction constants R-s is discussed, too. The spin-phonon interaction enhances the phonon damping of the thin film. The damping increases near T-c for the two cases R > 0 and R < 0. It is larger for thin films in comparison to the bulk. Taking into account the influence of an external electric field we obtain hardening of the soft phonon modes for R > 0 and R < 0.

The magnetic and electric static and dynamic properties of hexagonal multiferroic RMnO3 are studied based on the transverse Ising and Heisenberg models using a Green’s function technique. The influence of the magnetoelectric coupling and the exchange interaction constants is discussed. The coexistence of ferroelectricity and magnetism leads to interplay between the electric and magnetic properties which was found as an anomaly, as a kink in the temperature dependence of different static and dynamic properties in the vicinity of the magnetic phase transition temperature TN. The spin excitations show a novel softening around TN due to the magnetoelectric coupling. The magnetic-field-induced polarization is investigated. The obtained results are in qualitative agreement with the experimental data.

The phonon properties of hexagonal multiferroic RMnO3 materials are studied using a Green’s function technique. The calculations are performed on the basis of the Heisenberg and the transverse Ising model taking into account anharmonic spin–phonon and phonon–phonon interaction terms. The strong spin–phonon interaction leads to an anomaly in the phonon energy and the damping around the magnetic and ferroelectric phase transitions. The phonon spectrum is discussed for different exchange, magnetoelectric and spin–phonon interaction constants. It is shown that the phonon energy depends on the radius of the rare earth ion rR. The influence of an applied magnetic field on the phonon spectrum is studied, too. The predictions are consistent with experimental results.

The phonon properties of orthorhombic manganites RMnO(3) are studied using a Green's function technique taking into account anharmonic spin-phonon and phonon-phonon interaction terms. The strong spin-phonon interaction leads to anomalies in the phonon energy and the damping around the magnetic phase transition. The phonon spectrum is discussed for different exchange interaction J(1) and spin-phonon interaction R(sp) constants. In dependence on the sign of R(sp) we obtain softening or hardening of the phonon modes with decreasing temperature below the phase transition temperature T(N). This is associated with phonon modulation of the exchange interaction. It is shown that the phonon energy and the phonon damping depend on the radius of the rare earth ion r(R) and on the ion doping. The influence of the external magnetic field is discussed, too.