The relationships among magnetism, lattice modulation, and dielectric properties have been investigated for RMnO3 (R=Eu, Gd, Tb, and Dy). These compounds show a transition to an incommensurate ...lattice structure below their Néel temperature, and subsequently undergo an incommensurate-commensurate (IC-C) phase transition. For TbMnO3 and DyMnO3 it was found that the IC-C transition is accompanied by a ferroelectric transition, associated with a lattice modulation in the C phase. DyMnO3 shows a gigantic magnetocapacitance with a change of dielectric constant up to Deltaepsilon/epsilon approximately 500%.
We report on the control of electric polarization (P) by using magnetic fields (B) in a hexaferrite having magnetic order above room temperature (RT). The material investigated is hexagonal ...Ba0.5Sr1.5Zn2Fe12O22, which is a nonferroelectric helimagnetic insulator in the zero-field ground state. By applying B, the system undergoes successive metamagnetic transitions, and shows concomitant ferroelectric order in some of the B-induced phases with long-wavelength magnetic structures. The magnetoelectrically induced P can be rotated 360 degrees by external B. This opens up the potential for not only RT magnetoelectric devices but also devices based on the magnetically controlled electro-optical response.
We report the first direct transition from a paramagnetic and paraelectric phase to an incommensurate multiferroic in the triangular lattice antiferromagnet RbFe(MoO4)(2). Ferroelectricity is ...observed only when the magnetic structure has chirality and breaks inversion symmetry. A Landau expansion of symmetry-allowed terms in the free energy demonstrates that chiral magnetic order can give rise to a pseudoelectric field, whose temperature dependence agrees with experiment.
We show that long-range ferroelectric and incommensurate magnetic order appear simultaneously in a single phase transition in Ni3V2O8. The temperature and magnetic-field dependence of the spontaneous ...polarization show a strong coupling between magnetic and ferroelectric orders. We determine the magnetic symmetry using Landau theory for continuous phase transitions, which shows that the spin structure alone can break spatial inversion symmetry leading to ferroelectric order. This phenomenological theory explains our experimental observation that the spontaneous polarization is restricted to lie along the crystal b axis and predicts that the magnitude should be proportional to a magnetic order parameter.
The development of methods to economically synthesize single wire structured multiferroic systems with room temperature spin–charge coupling is expected to be important for building next-generation ...multifunctional devices with ultralow power consumption. We demonstrate the fabrication of a single nanowire multiferroic system, a new geometry, exhibiting room temperature magnetodielectric coupling. A coaxial nanotube/nanowire heterostructure of barium titanate (BaTiO3, BTO) and cobalt (Co) has been synthesized using a template-assisted method. Room temperature ferromagnetism and ferroelectricity were exhibited by this coaxial system, indicating the coexistence of more than one ferroic interaction in this composite system.
We investigated magnetic and thermal transport properties of bulk FeVO 4 multiferroic systems using temperature-dependent magnetic and thermal conductivity measurements. These studies exhibit ...long-range magnetic ordering with two successive ferroic orderings at 22 and 15 K. The temperature dependence of thermal conductivity shows a phonon-assisted peak at ~30 K with an additional anomaly near ~22 K. The Debye-Einstein phonon model fits suggest that the main contribution to thermal conductivity is coming from the acoustic phonons at low temperatures, whereas optical phonon contribution is coming at high temperatures. The onset of thermal conductivity at 22 K is associated with antiferromagnetic ferroic ordering and suggests strong coupling between magnetic ordering and phonons in FeVO 4 multiferroic.
We report on the effects of spin fluctuations, magnetic ordering, and external magnetic field on the dielectric constant of the ferromagnet SeCuO3, and the antiferromagnet TeCuO3. A model based on ...the coupling between uniform polarization and the q-dependent spin-spin correlation function is presented to explain the different behaviors for these isostructural compounds. The large magnetocapacitance near the transition temperature in the ferromagnet SeCuO3 suggests routes to enhancing the magnetodielectric response for practical applications.
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