A. B. Harris

Magnetically driven ferroelectric order in Ni3V2O8

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.

Direct Transition From a Disordered to a Multiferroic Phase on a Triangular Lattice

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.

Competing magnetic phases on a kagomé staircase

We present thermodynamic and neutron data on Ni3V2O8, a spin-1 system on a kagomé staircase. The extreme degeneracy of the kagomé antiferromagnet is lifted to produce two incommensurate phases at finite T--one amplitude modulated, the other helical--plus a commensurate canted antiferromagnet for T-->0. The H-T phase diagram is described by a model of competing first and second neighbor interactions with smaller anisotropic terms. Ni3V2O8 thus provides an elegant example of order from subleading interactions in a highly frustrated system.

Landau Analysis of the Symmetry of the Magnetic Structure and Magnetoelectric Interaction in Multiferroics

This paper presents a detailed instruction manual for constructing the Landau expansion for magnetoelectric coupling in incommensurate ferroelectric magnets, including

Ordering by quantum fluctuations in a strongly frustrated Heisenberg antiferromagnet

{\mathrm{Ni}}_{3}{\mathrm{V}}_{2}{\mathrm{O}}_{8}

Towards a Microscopic Model of Magnetoelectric Interactions in Ni 3 V 2 O 8

,

Rotational and Vibrational Dynamics of Interstitial Molecular Hydrogen

\mathrm{Tb}\mathrm{Mn}{\mathrm{O}}_{3}

Effect of Inversion Symmetry on the Incommensurate Order in Multiferroic R Mn 2 O 5 ( R =rare earth)

,

Magnetic Structure and Magnetoelectric Coupling in Bulk and Thin Film FeVO 4

\mathrm{Mn}\mathrm{W}{\mathrm{O}}_{4}

Order Parameters and Phase Diagram of Multiferroic RMn2O5

,