Thermodynamic properties of new multiferroic and linear magnetoelectric crystals

The present work is concerned with the search for new multiferroic or magnetoelectric crystals, with strong magnetoelectric couplings and their basic characterization by investigating the dielectric and other thermodynamic properties. The focus of the experimental work lies on two different classes of compounds, the pyroxenes AMX2O6 (A= mono- or divalent metal, M= di- or trivalent metal and X= tri- or tetravalent cation) and the erythrosiderite-type family A2[FeX5(H2O)] (A= alkali metal or ammonium ion, X= halide ion). An analysis of the anisotropy of the linear magnetoelectric effect of LiFeSi2O6 by means of dielectric measurements and investigations with a polarized-light microscope reveal that the magnetic space group P2_1/c' proposed in literature for its antiferromagnetic phase is actually wrong. The correct space group has to be instead P\bar{1}'. The multiferroic properties of NaFeGe2O6, the first prototype multiferroic within the pyroxene family, are characterized by dielectric investigations, magnetic-susceptibility, thermal-expansion and magnetostriction measurements on large single crystals. Ferroelectricity arises below ~11.6 K within an antiferromagnetically ordered state (T_N~13 K). The corresponding electric polarization can be strongly modified by applying magnetic fields. Detailed magnetic-field versus temperature phase diagrams are derived. (NH4)2[FeCl5(H2O)] is classified as a new multiferroic material. The onset of ferroelectricity is found below ~6.9 K within an antiferromagnetically ordered state (T_N~7.25 K). The corresponding electric polarization can drastically be influenced by applying magnetic fields. Based on measurements of pyroelectric currents, dielectric constants and magnetization the magnetoelectric, dielectric and magnetic properties of (NH4)2[FeCl5(H2O)] are characterized. Combining these data with measurements of thermal expansion, magnetostriction and specific heat detailed magnetic-field versus temperature phase diagrams are derived. Depending on the direction of the magnetic field up to three different multiferroic phases are identified, which are separated by a magnetically ordered, but non-ferroelectric phase from the paramagnetic phase. Besides these low-temperature transitions, a ferroelastic phase transition at ~79 K is observed and investigated. Three other members of the erythrosiderite-type family (K2[FeCl5(H2O)], Rb2[FeCl5(H2O)], Cs2[FeCl5(H2O)]) are classified as linear magnetoelectric materials by means of dielectric investigations and measurements of the magnetic susceptibility. From the K-based to the Cs-based compound the transition temperature to the respective magnetoelectric phase decreases from T_N^K=14.3 K and T_N^Rb=10.2 K to T_N^Cs=6.8 K. For all three compounds the anisotropy and the temperature dependence of the linear magnetoelectric effect is analysed. Based on the anisotropy study of the magnetoelectric effect of Cs2[FeCl5(H2O)]) a model for its unknown magnetic structure, described by the magnetic space group $C2'/m 2'/c 2_1/m', is developed.