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.
By comprehensive neutron diffraction measurements we have studied the magnetic structure of aegirine (NaFeSi 2 O 6 ) in and above its multiferroic phase. Natural aegirine exhibits two magnetic transitions into incommensurate magnetic order with a propagation vector of k inc = (0, ∼ 0.78,0). Between 9 and 6 K, we find a transverse spin-density wave with moments pointing near the c direction. Below 6 K, magnetic order becomes helical and spins rotate in the ac plane. The same irreducible representation is involved in the two successive transitions. In addition, the ferroelectric polarization P appearing along the b direction cannot be described by the most common multiferroic mechanism but follows P ∝ S i × S j . Synthetic NaFeSi 2 O 6 does not exhibit the pure incommensurate helical order but shows coexistence of this order with a commensurate magnetic structure. By applying moderate pressure to natural aegirine, we find that the incommensurate magnetic ordering partially transforms to the commensurate one, underlining the nearly degenerate character of the two types of order in NaFeSi 2 O 6 .
Abstract. The control of multiferroic domains through external electric fields has been studied by dielectric measurements and by polarized neutron diffraction on singlecrystalline TbMnO 3 . Full hysteresis cycles were recorded by varying an external field of the order of several kV/mm and by recording the chiral magnetic scattering as well as the charge in a sample capacitor. Both methods yield comparable coercive fields that increase upon cooling.
Polarized neutron scattering experiments reveal that type-II multiferroics allow for controlling the spin chirality by external electric fields even in the absence of long-range multiferroic order. In the two prototype compounds TbMnO 3 and MnWO 4 , chiral magnetism associated with soft overdamped electromagnons can be observed above the long-range multiferroic transition temperature T MF , and it is possible to control it through an electric field. While MnWO 4 exhibits chiral correlations only in a tiny temperature interval above T MF , in TbMnO 3 chiral magnetism can be observed over several kelvin up to the lock-in transition, which is well separated from T MF . DOI: 10.1103/PhysRevLett.119.177201 Multiferroic materials with coupled magnetic and ferroelectric order bear considerable application potential [1,2]. In type-II multiferroics, magnetic order directly induces ferroelectric polarization and giant magnetoelectric coupling. External magnetic fields imply a flop of electric polarization, and electric fields can control chiral magnetic domains [1][2][3][4][5]. Various neutron experiments [6][7][8][9][10][11][12] as well as resonant and nonresonant x-ray studies [13,14] show that cooling in electric fields enforces a monodomain chiral state, and varying external electric fields at constant temperature drives the chiral magnetic order [9][10][11][12], which corresponds to the most promising application as a data storage medium. In addition, time resolved soft x-ray diffraction showed that chiral magnetism can be manipulated by THz-radiation pulses at an electromagnon energy [15].So far, studies of the multiferroic coupling and hysteresis curves were restricted to the phases with long-range magnetic order on bulk or film materials [16], while only small multiferroic blocks would be vital for applications. Also, from the fundamental point of view, one may ask whether multiferroic hysteresis and control can be achieved in short-range systems above the long-range static multiferroic transition, and how far spin chirality persists above the static and long-range multiferroic order. The mixed system Ni 0.42 Mn 0.58 TiO 3 already indicates that magnetoelectric coupling can persist in cluster systems with competing magnetic structures [17], but until now there has been no information about the control and multiferroic coupling of chiral ordering that is limited in space and time. Here, we study two prototype type-II multiferroics, TbMnO 3 [1, 3,4] and MnWO 4 [18-20], above the long-range ferroelectric transition at zero electric field T MF , where it is still possible to pole and control chiral magnetic correlations. Although the two materials exhibit a similar sequence of magnetic transitions, it turns out that only in TbMnO 3 can chiral scattering be controlled over a large temperature interval of several kelvin.TbMnO 3 (MnWO 4 ) both exhibit a first magnetic transition at T N ¼ 42 K (13.5 K), followed by a second transition at lower temperature, at which cycloid order develops at T MF ¼ 27.6 K (12.6 K). For ...
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.