Charge carrier self-organization in ferroelectromagnetic semiconductors Eu₀.₈Ce₀.₂Mn₂O₅

The state with a giant permittivity (ε~10⁴) and ferromagnetism has been observed above 185 K (including room temperature) in single crystals of diluted semiconductor manganite–ferroelectromagnetic Eu₀.₈Ce₀.₂Mn₂O₅ in the investigations of x-ray diffraction, dielectric and magnetic properties, conductivity. X-ray diffraction study has revealed a layered superstructure along the c axis at room temperature. A model of the state with a giant ε including as-grown 2D layers with doping impurities, charge carriers, and double-exchange coupled Mn³⁺–Mn⁴⁺ 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 grows and hopping conductivity increases, the charge carrier self-organization in the crystal cause formation of a layered superstructure consisting of charged layers (with an excess Mn³⁺ concentration) alternating with dielectric layers of the initial crystal — the ferroelectricity state due to charge ordering. Ferromagnetism results from double exchange between Mn³⁺ and Mn⁴⁺ ions through of charge carriers in the charged layers

Magnetically Frustrated Behavior In Multiferroics R Mn2 O 5 (r=bi, Eu, And Dy): A Raman Scattering Study

A temperature dependent Raman scattering study in multiferroic single crystals R Mn2 O5 (R=Bi, Eu, and Dy) was performed. The Raman spectra were measured in the range from 150 to 450 cm-1 involving mostly Mn-O-Mn bending vibrations, complementing our previous work in a higher frequency range involving Mn-O stretching modes. A number of studied phonons present anomalous frequency behavior below a characteristic temperature, T* ∼60-65 K, such as that found for the stretching modes. The sign and magnitude of such anomalous behavior appear to be correlated with the ionic radius of R, being softening for R=Bi and hardening for R=Eu and Dy in the range between TC TN and T*. The anomalous phonon behaviors in both bending and stretching modes are consistent with an interpretation in terms of the spin-phonon coupling in a scenario of strong magnetic correlations. © 2007 American Institute of Physics.1019Wang, J., (2003) Science, 299, p. 1719Hur, N., Park, S., Sharma, P.A., Ahn, J.S., Guha, S., Cheong, S.-W., (2004) Nature (London), 429, p. 392Alonso, J.A., Casais, M.T., Martínez-Lope, M.J., Martínez, J.L., Fernández-Díaz, M.T., (1997) J. Phys.: Condens. Matter, 9, p. 8515Kagomiya, I., Kohn, K., Uchiyama, T., (2002) Ferroelectrics, 280, p. 297Hur, N., Park, S., Sharma, P.A., Guha, S., Cheong, S.-W., (2004) Phys. Rev. Lett., 93, p. 107207Muoz, A., Alonso, J.A., Casais, M.T., Martínez-Lope, M.J., Martínez, J.L., Fernández-Díaz, M.T., (2002) Phys. Rev. B, 65, p. 144423Golovenchits, E.I., Sanina, V.A., Babinskii, A.V., (1997) JETP, 85, p. 156Chapon, L.C., Blake, G.R., Gutmann, M.J., Park, S., Hur, N., Radaelli, P.G., Cheong, S.-W., (2004) Phys. Rev. Lett., 93, p. 177402Blake, G.P., Chapon, L.C., Radaelli, P.G., Park, S., Hur, N., Cheong, S.-W., Rodríguez-Carvajal, J., (2005) Phys. Rev. B, 71, p. 214402Polyakov, V., Plakhty, V., Bonnet, M., Burlet, P., Regnault, L.-P., Gavrilov, S., Zobkalo, I., Smirnov, O., (2001) Physica B, 297, p. 208Higashiyama, D., Miyasaka, S., Kida, N., Arima, T., Tokura, Y., (2004) Phys. Rev. B, 70, p. 174405Ramirez, A.P., (2001) Handbook of Magnetic Materials, 13, pp. 423-520. , Elsevier, New YorkGarcía-Flores, A.F., (2006) Phys. Rev. B, 73, p. 104411Baltensperger, W., Helman, J.S., (1968) Helv. Phys. Acta, 41, p. 668Granado, E., (2001) Phys. Rev. Lett., 86, p. 5385Sushkov, A.B., Tchernyshyov, O., Ratcliff, I.I.W., Cheong, S.-W., Drew, H.D., (2004) Phys. Rev. Lett., 94, p. 13720

Anomalous Phonon Shifts In The Paramagnetic Phase Of Multiferroic R Mn2 O5 (r=bi, Eu, Dy): Possible Manifestations Of Unconventional Magnetic Correlations

A Raman spectroscopic study of the high-frequency optical phonons in single crystals of the multiferroic system R Mn2 O5 (R=Bi, Eu, Dy) was performed. All studied materials show anomalous phonon shifts, below a new characteristic temperature for these materials, T* ∼60-65 K. The sign and magnitude of such shifts appear to be correlated with the ionic radius of R, envolving from softenings for R=Bi to hardenings for R=Dy and showing an intermediary behavior for R=Eu. Additional phonon anomalies were identified below ∼ TN ∼40-43 K, reflecting the onset of long-range ferroelectric and/or magnetic order of the Mn sublattice. Complementary dc-magnetic susceptibility [χ (T)] measurements for Bi Mn2 O5 up to 800 K yield a Curie-Weiss temperature θCW =-253 (3) K, revealing a fairly large frustration ratio (θCW TN =6.3). 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Neutron powder diffraction and single crystal X ray magnetic resonant and non resonant scattering studies of the doped multiferroic Tb Bi MnO3

The results of powder neutron diffraction, X ray resonant and non resonant scattering in Tb0.95Bi0.05MnO3 demonstrate that substitution of Bi for Tb leads to partial suppression of intrinsic long range magnetic order in the Tb sub lattice, which transforms to correlated short range order. The magnetic order in the Mn sub lattice remains similar to that of TbMnO3. In the Bi substituted sample an anomaly in the temperature dependencies of the unit cell parameters was detected at about 100