Thermodynamic properties of the new multiferroic material (NH₄)₂[FeCl₅(H₂O)]

Abstract

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...

Magnetic-field-induced change of magnetoelectric coupling in the hybrid multiferroic (ND4)2[FeCl5·<

“…The series of Q-scans performed with the crystal oriented with the c-axis parallel to B shows qualitatively the same results -not surprisingly, given the close similarity of the B-T phase diagrams for both orientations of the magnetic field [24] . The resolution is however markedly poorer with B//c, as the experimental geometry ('normal beam') implies measurements with a departure from the horizontal scattering plane ( Fig.…”

“…Moreover, in order to include the effect of the magnetic field, a weak ferromagnetic component along the a-axis (direction of application of the external magnetic field) has been included in the model. The value of the field-induced ferromagnetic moment was derived from the magnetization curves as function of the external field [24] , with an estimated value of 0.4  B along the a-axis. shows two remarkable differences with respect to the previously obtained at zero field [26] .…”

Spin-lattice coupling mediated multiferroicity in (ND4)2FeCl5·D2O

“…This offers a unique opportunity to test theoretical models that will shed light on the underlying multiferroic mechanism. In this paper, we report a neutron diffraction study of (NH 4 ) 2 FeCl 5 ·H 2 O, a molecular compound that was recently discovered to exhibit magnetically induced multiferroicity [19]. Using both polarized and unpolarized neutrons, we show that the onset of the electric polarization is accompanied by a lattice modulation indicating spin-lattice coupling plays a critical role in the ferroelectric mechanism…”

“…Combined with the large magnetostriction and negative thermal expansion [19] Note: While working on the revision of this paper, we become aware of a recent work by Rodríguez-Velamazán et al which reports that no higher-order harmonics and coexistence of the IC and C phases were observed in their experiment on the same material [34].…”

Anisotropy study of multiferroicity in the pyroxene NaFeGe2O6

“…Compared to conventional ferroelectrics, the spontaneous electric polarizations are often rather small, but can be strongly modified and controlled by external magnetic fields. As a result of the intense research activities, a series of new spin-driven multiferroic materials has been found in the last few years; see for example [6][7][8][9][10][11][12][13][14][15][16][17]. In particular the pyroxenes (general formula AMX 2 O 6 with A = mono-or divalent metal, M = di-or trivalent metal and X = tri-or tetravalent cation) form a huge family of potentially multiferroic and magnetoelectric materials [18].…”