M agnetodielectric materials are characterized by a strong coupling of the magnetic and dielectric properties and, in rare cases, simultaneously show both magnetic and polar order. Among other multiferroics, TbMnO 3 and GdMnO 3 reveal a strong magneto-dielectric coupling and as a consequence fundamentally different spin excitations exist: electro-active magnons (or electromagnons), spin waves that can be excited by a.c. electric fields. Here we provide evidence that these excitations appear in the phase with an incommensurate magnetic structure of the manganese spins. In external magnetic fields this incommensurate structure can be suppressed and the electromagnons wiped out, thereby inducing considerable changes in the index of refraction from d.c. up to terahertz frequencies. Hence, besides adding a creature to the zoo of fundamental excitations, the refractive index can be tuned by moderate magnetic fields, which enables the design of the next generation of optical switches and optoelectronic devices.Enormous progress has been made in the field of multiferroics and the discovery of new classes of ferroelectromagnets (FEMs) with the simultaneous occurrence of magnetic and polar order [1][2][3][4][5] has triggered a revival 6,7 of this old field of magneto-dielectric effects and the electrodynamics of multiferroic media 8 . As well as promising applications of FEMs in the field of modern electronics, for example, as multiple-state memory devices with mutual magnetic or electric control or as magnetically switchable optical devices, fascinating new problems can be tackled in basic research, such as the search for magneto-dielectric excitations.

We report on structural, magnetic, dielectric, and thermodynamic properties of Eu 1−x Y x MnO 3 for Y doping levels 0 ഛ x Ͻ 1. This system resembles the multiferroic perovskite manganites RMnO 3 ͑with R = Gd, Dy, Tb͒ but without the interference of magnetic contributions of the 4f ions. In addition, it offers the possibility to continuously tune the influence of the A-site ionic radii. For small concentrations x ഛ 0.1 we find a canted antiferromagnetic and paraelectric ground state. For higher concentrations x ജ 0.3 ferroelectric polarization coexists with the features of a presumably spiral magnetic phase analogous to the observations in TbMnO 3 . In the intermediate concentration range around x Ϸ 0.2 a multiferroic scenario is realized combining weak ferroelectricity and weak ferromagnetism, presumably due to a conelike magnetic structure.

We report on structural, magnetic and electrical properties of Sr-doped LaMnO3 single crystals for doping levels 0.4 ≤ x ≤ 0.85. The complex structural and magnetic phase diagram can only be explained assuming significant contributions from the orbital degrees of freedom. Close to x = 0.6 a ferromagnetic metal is followed by an antiferromagnetic metallic phase below 200 K. This antiferromagnetic metallic phase exists in a monoclinic crystallographic structure. Following theoretical predictions this metallic antiferromagnet is expected to reveal an (x 2 -y 2 )-type orbital order. For higher Sr concentrations an antiferromagnetic insulator is established below room temperature.

The magnetic, electric, magnetoelectric, and magnetoelastic properties of rare-earth ferroborates RFe3(BO3)4 (R=Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er) as well as yttrium ferroborate YFe3(BO3)4 have been studied comprehensively. A strong dependence not only of the magnetic but also magnetoelectric properties on the type of rare-earth ion, specifically, on its anisotropy, which determines the magnetic structure and the large contribution to the electric polarization, has been found. This is manifested in the strong temperature dependence of the polarization below the Néel point TN and its specific field dependence, which is determined by the competition between the external and exchange f-d fields. A close correlation has been found between the magnetoelastic properties of ferroborates and the magnetoelastic and magnetic anomalies at magnetic-field induced phase transitions. It is found that in easy-plane ferroborates, together with magnetic-field induced electric polarization spontaneous polarization also arises below the Néel point. The ferroelectric ordering in ferroborates is of extrinsic character, giving rise to strong magnetoelectric coupling below TN. Aside from the antiferromagnetic phase transition, the particulars of the structural phase transition accompanied by anomalies of the dielectric and magnetoelectric properties are examined for the first time. The character of the dielectric anomalies at a structural transition is analyzed for the first time on the basis of Landau’s approach.

We have investigated the complex ac conductivity of for , temperatures and frequencies . In addition, results from dc measurements are presented. From the frequency dependence of the complex conductivity we find hopping of Anderson-localized charge carriers as the dominant transport process in certain temperature and composition ranges. We deduce that, while Anderson localization is not the driving mechanism for the metal-insulator transition observed in this compound, it is responsible for the high-resistivity regions observed at low doping levels and low temperatures. The results indicate a polaronic nature of the charge carriers. From the temperature dependence of the ac conductivity and the magnetic permeability, deduced in the skin-effect-dominated regime, various phase transition temperatures have been determined.