NH 4 ) 2 [FeCl 5 (H 2 O)], a member of the family of antiferromagnetic A 2 [FeX 5 (H 2 O)] compounds (X = halide ion and A = alkali metal or ammonium ion) is classified as a new multiferroic material. We report the onset of ferroelectricity 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 we characterize the magnetoelectric, dielectric and magnetic properties of (NH 4 ) 2 [FeCl 5 (H 2 O)]. By combining these data with measurements of thermal expansion, magnetostriction and specific heat, we derive detailed magnetic field versus temperature phase diagrams. 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, we observe an additional phase transition at 79 K, which we suspect to be of structural origin. IntroductionMultiferroic materials with simultaneous ferroelectric and (anti-)ferromagnetic order in the same phase have attracted considerable interest during the last decade [1][2][3][4]. In particular, the discovery of spin-driven ferroelectricity in magnetically frustrated systems [5], such as, e.g., in transition metal oxides R EMnO 3 (R E = Tb, Dy) [6], Ni 3 V 2 O 8 [7], LiCu 2 O 2 [8], MnWO 4 [9-11], NaFeX 2 O 6 (X = Si, Ge) [12, 13], CuO [14] or CaMn 7 O 12 [15] (but also in non-oxide systems such as, e.g., CuCl 2 [16] or K 3 Fe 5 F 15 [17]) revived the search for new multiferroic materials. Typically, these multiferroics show complex, non-collinear spin structures and a strong coupling between magnetic and ferroelectric order exists. Consequently, the spontaneous electric polarization can be strongly modified by applying external magnetic fields. Depending on the direction and strength of the magnetic field, reversal, rotation or suppression of the electric polarization may occur. Such magnetic-field-induced changes of the electric polarization or, vice versa, electric-field-dependent magnetization changes, are not only interesting from the fundamental physical point of view but also are interesting for potential new devices in the fields of data memory or sensor systems.Here, we report the discovery and the basic characterization of the new multiferroic material ammonium pentachloroaquaferrate(III), (NH 4 ) 2 [FeCl 5 (H 2 O)]. It belongs to the family of erythrosiderite-type compounds A 2 [FeX 5 (H 2 O)], where A stands for an alkali metal or ammonium ion and X for a halide ion. As for most members of this family, the roomtemperature crystal structure of (NH 4 ) 2 [FeCl 5 (H 2 O)] is orthorhombic with space group Pnma [18] and lattice constants (at room temperature) a = 13.706(2) Å, b = 9.924(1) Å and c = 7.024 (1) Å [19]. The structure consists of isolated (NH 4 ) + units and isolated complex groups [Fe...

The compounds A(2)[FeCl(5)(H(2)O)] with A = K, Rb, Cs are identified as new linear magnetoelectric (non-multiferroic) materials. We present a detailed investigation of their linear magnetoelectric properties through measurements of pyroelectric currents, dielectric constants and magnetization. The anisotropy of the linear magnetoelectric effect of the K-based and Rb-based compound is consistent with the magnetic point group m'm'm', already reported in literature. A symmetry analysis of the magnetoelectric effect of the Cs-based compound allows us to determine the magnetic point group mmm' and to develop a model for its magnetic structure. In addition, magnetic-field versus temperature phase diagrams are derived and compared to the closely related multiferroic (NH(4))(2)[FeCl(5)(H(2)O)].

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We present a study of the anisotropy of the dielectric, magnetic and magnetoelastic properties of the multiferroic clinopyroxene NaFeGe 2 O 6 . Pyroelectric currents, dielectric constants and magnetic susceptibilities as well as the thermal expansion and the magnetostriction were examined on large synthetic single crystals of NaFeGe 2 O 6 . The spontaneous electric polarization detected below ≃ T 11.6 K C in an antiferromagnetically ordered state ( ≃ T 13 K N ) is mainly lying within the ac plane with a small component along b, indicating a triclinic symmetry of the multiferroic phase of NaFeGe 2 O 6 . The electric polarization can be strongly modified by applying magnetic fields along different directions. We derive detailed magnetic field versus temperature phase diagrams and identify three multiferroic lowtemperature phases, which are separated by a non-ferroelectric, antiferromagnetically ordered state from the paramagnetic high-temperature phase.

The crystal structure of the double tungstate NaFe(WO4)2 arises from that of the spin-driven multiferroic MnWO4 by inserting non-magnetic Na layers. NaFe(WO4)2 exhibits a three-dimensional incommensurate spin-spiral structure at low temperature and zero magnetic field, which, however, competes with commensurate order induced by magnetic field. The incommensurate zero-field phase corresponds to the condensation of a single irreducible representation but it does not imply ferroelectric polarization because spirals with opposite chirality coexist. Sizable anharmonic modulations emerge in this incommensurate structure, which are accompanied by large magneto-elastic anomalies, while the onset of the harmonic order is invisible in the thermal expansion coefficient. In magnetic fields applied along the monoclinic axis, we observe a first-order transition to a commensurate structure that again is accompanied by large magneto-elastic effects. The large magnetoelastic coupling, a reduction of the b lattice parameter, is thus associated only with the commensurate order. Upon releasing the field at low temperature, the magnetic order transforms to another commensurate structure that considerably differs from the incommensurate low-temperature phase emerging upon zero-field cooling. The latter phase, which exhibits a reduced ordered moment, seems to be metastable.