Abstract. We report the multiferroic behaviour of MnWO 4 , a magnetic oxide with monoclinic crystal structure and spiral long-range magnetic order. Based upon recent theoretical predictions MnWO 4 should exhibit ferroelectric polarization coexisting with the proper magnetic structure. We have confirmed the multiferroic state below 13 K by observing a finite electrical polarization in the magnetically ordered state via pyroelectric current measurements.Multiferroic materials which combine magnetism and ferroelectricity currently attract considerable attention [1][2][3][4]. There are already several multiferroic materials recently discovered among transition metal oxides:. Nevertheless, the search for novel systems with multiferroic properties presents a definite interest. In this letter we report that yet another transition metal oxide, MnWO 4 , belongs to the same class of materials and develops spontaneous electric polarization in a spiral magnetically ordered state [8].There exist several different microscopic mechanisms which may cause multiferroic behavior [3]. One of the most interesting cases is when a spontaneous polarization exists only in a magnetically ordered phase with a particular type of ordering. This is e.g. the case in TbMnO 3 and TbMn 2 O 5 . Microscopic [9] and phenomenological [10] treatments have shown that this happens particularly in spiral magnetic structures with the spin rotation axis − → e not coinciding with the magnetic propagation vector − → Q : theoretical treatment shows that in this case a finite spontaneous polarization perpendicular to the plane spanned by − → e and − → Q may appearThis is not the only source for a magnetically driven ferroelectricity [11,12], but perhaps the most common one. Accordingly, one strategy to search for new multiferroic materials is to look for magnetic systems with proper magnetic structures. MnWO 4 (also known as the mineral hübnerite) appears to be just such a system. Detailed studies of the magnetic ordering in this material have shown [13,14] that below 12.3 K a spiral magnetic ordering develops which seems to satisfy the criterion of Eq. (1). In order to test this we carried out measurements of the dielectric response and of spontaneous polarization of MnWO 4 using single-crystalline samples. The crystals of MnWO4 were grown from melt solution. On the basis of earlier work [15] we applied a modified flux technique, using a melt solvent from the system Na 2 WO 4 -WO 3 . The resulting crystals are of dimensions up to 15 x 5 x 3 mm 3 and of 3 Author to whom correspondence should be addressed (khomskii@ph2.uni-koeln.de).
Pyroxenes with the general formula AM Si 2 O 6 (A = mono-or divalent metal, M = di-or trivalent metal) are shown to be a new class of multiferroic materials. In particular, we have found so far that NaFeSi 2 O 6 becomes ferroelectric in a magnetically ordered state below 6 K. Similarly, magnetically driven ferroelectricity is also detected in the Li homologues, LiFeSi 2 O 6 (T C 18 K) and LiCrSi 2 O 6 (T C 11 K). In all these monoclinic systems the electric polarization can be strongly modified by magnetic fields. Measurements of magnetic susceptibility, pyroelectric current and dielectric constants (and their dependence on magnetic field) are performed using a natural crystal of aegirine (NaFeSi 2 O 6 ) and synthetic crystals of LiFeSi 2 O 6 and LiCrSi 2 O 6 grown from melt solution. For NaFeSi 2 O 6 a temperature versus magnetic field phase diagram for NaFeSi 2 O 6 is proposed. Exchange constants are computed on the basis of ab initio band structure calculations. The possibility of a spiral magnetic structure caused by frustration as origin of the multiferroic behaviour is discussed. We propose that other pyroxenes may also be multiferroic, and that the versatility of this family offers an exceptional opportunity to study general conditions for and mechanisms of magnetically driven ferroelectricity.
New materials for nonlinear optical (NLO) applications are still of great interest. The monoclinic BiB3O6 (BIBO) shows exceptionally large NLO coefficients. In this article we will present the linear optical properties in the wavelength range between 350 and 2400 nm, the phase matching conditions calculated for second harmonic generation, and optical parametric oscillation. Angular bandwidth data are also given. The wide tuning range of phase matched directions together with the monoclinic symmetry allow a broad variety of applications. The laser damage threshold is comparable to high quality lithium triborate.
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...
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