The state with a giant permittivity ͑Ј ϳ 10 4 ͒ and ferromagnetism have been observed above 185 K ͑including room temperature͒ in single crystals of diluted semiconductor manganite-multiferroic Eu 0.8 Ce 0.2 Mn 2 O 5 in the investigations of x-ray diffraction, heat capacity, dielectric and magnetic properties, conductivity, and Raman light-scattering spectra of this material. X-ray diffraction study has revealed a layered superstructure along the c axis at room temperature. A model of the state with a giant Ј including as-grown two-dimensional layers with doping impurities, charge carriers, and double-exchange-coupled Mn 3+-Mn 4+ ion pairs is suggested. At low temperatures these layers form isolated electrically neutral small-size one-dimensional superlattices, in which de Haas-van Alphen oscillations were observed. As temperature grows and hopping conductivity increases, the charge carrier self-organization in the crystal causes formation of a layered superstructure consisting of charged layers ͑with an excess Mn 3+ concentration͒ alternating with dielectric layers of the initial crystal-the ferroelectricity due to charge-ordering state. Ferromagnetism results from double exchange between Mn 3+ and Mn 4+ ions by means of charge carriers in the charged layers. Temperature evolution of frequency shifts of A g modes and quasielastic scattering in Raman-scattering spectra agree with the pattern of phase transitions in ECMO suggested.
Studies of magnetization, magnetoresistance, and magnetic oscillations in semiconductor-multiferroics Eu(1-x)Ce(x)Mn2O5 (x = 0.2-0.25) (ECMO) at temperatures ranging from 5 to 350 K in magnetic fields up to 6 T are presented. It is shown that phase separation and charge carrier self-organization in the crystals give rise to a layered superstructure perpendicular to the c axis. An effect of magnetic field cycling on the superstructure, magnetization, and magnetoresistance is demonstrated. X-ray diffraction studies of ECMO demonstrating the effect of magnetic field on the superstructure are presented. The de Haas-van Alphen magnetization oscillations in high magnetic fields and the temperature-induced magnetic oscillations in a fixed magnetic field are observed at low temperatures. Below 10 K the quantum corrections to magnetization due to the weak charge carrier localization in 2D superlattice layers occur. It is shown that at all the temperatures the Eu(1-x)Ce(x)Mn2O5 magnetic state is dictated by superparamagnetism of isolated ferromagnetic domains.
Spin-wave excitations revealed in the dynamically equilibrated one-dimensional superlattices formed due to phase separation and charge carrier self-organization in doped single crystals of Eu(0.8)Ce(0.2)Mn(2)O(5) and Tb(0.95)Bi(0.05)MnO(3) multiferroics are discussed. Similar excitations, but having lower intensities, were also observed in undoped RMn(2)O(5) (R=Eu, Er, Tb, Bi). This suggests that a charge transfer between manganese ions with different valences, which give rise to the superlattice formation, occurs in undoped multiferroics as well. The spin excitations observed in the native superlattices represent a set of homogeneous spin-wave resonances excited in individual superlattice layers. The positions of these resonances depend on the relation between the numbers of Mn(3+) and Mn(4+) ions, charge carrier concentrations, and barrier depths in the superlattice layers. It has been found that the spin-wave excitations observed in the frequency interval studied (30-50 GHz) form two spin-wave minibands with a gap between them.
Spin wave resonances have been observed in superlattices arising due to the phase separation and self orga nization of charge carriers in Eu 0.8 Ce 0.2 Mn 2 O 5 single crystals. The resonances are found within the 5-80 K temperature range at frequencies close to 30 GHz. Similar resonances with intensities about an order of mag nitude lower are also observed in EuMn 2 O 5 . The latter suggests the existence of charge transfer processes between the manganese ions of different valences in EuMn 2 O 5 .
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