Neutron powder diffraction and single crystal x-ray resonant magnetic scattering measurements suggest that Dy plays an active role in enhancing the ferroelectric polarization in multiferroic DyMnO3 above T(Dy)(N)=6.5 K. We observe the evolution of an incommensurate ordering of Dy moments with the same periodicity as the Mn spiral ordering. It closely tracks the evolution of the ferroelectric polarization. Below T(Dy)(N), where Dy spins order commensurately, the polarization decreases to values similar for those of TbMnO3. The higher P(s) found just above T(Dy)(N) arises from the contribution of Dy spins so as to effectively increase the amplitude of the Mn spin spiral.

The magnetic excitations in multiferroic TbMnO3 have been studied by inelastic neutron scattering in the spiral and sinusoidally ordered phases. At the incommensurate magnetic zone center of the spiral phase, we find three low-lying magnons whose character has been fully determined using neutron-polarization analysis. The excitation at the lowest energy is the sliding mode of the spiral, and two modes at 1.1 and 2.5 meV correspond to rotations of the spiral rotation plane. These latter modes are expected to couple to the electric polarization. The 2.5 meV mode is in perfect agreement with recent infrared-spectroscopy data giving strong support to its interpretation as a hybridized phonon-magnon excitation.

We have used in-field neutron and x-ray single-crystal diffraction to measure the incommensurability ␦ of the crystal and magnetic structures of multiferroic TbMnO 3 . We show that the flop in the electric polarization at the critical field H C , for field H along the a and b axes, coincides with a first-order transition to a commensurate phase with propagation vector = ͑ 0, 1 4 ,0 ͒ . In-field x-ray diffraction measurements show that the quadratic magnetoelastic coupling breaks down with applied field as shown by the observation of the first harmonic lattice reflections above and below H C . This indicates that magnetic field induces a linear magnetoelastic coupling. DOI: 10.1103/PhysRevB.73.020102 PACS number͑s͒: 77.80.Bh, 64.70.Kb, 75.25.ϩz, 77.22.Ej Control of the spontaneous ferroelectric polarization ͑P s ͒ with an external magnetic field ͑H͒ in a material opens the opportunity for new types of magnetoelectric devices. The realization of such devices is based on multiferroic materials in which magnetism and ferroelectricity are strongly coupled. While available multiferroics are limited, it has been shown that frustrated spin materials may offer a unique class of enhanced multiferroics. [1][2][3] In one of these materials, TbMnO 3 , we find that multiferroic behavior arises as a consequence of the release of frustration with H. Here ferroelectricity arises below the Néel temperature ͑T N ͒ from a coupling to the lattice of an incommensurate ͑IC͒ modulation of the magnetic structure ͓Fig. 1͑a͔͒ that is caused from frustration in the ordering of the Mn d orbitals. 1,3 In this communication we show that magnetic field releases this frustration, inducing a linear magnetoelastic coupling, so that ferroelectricity is no longer a secondary effect. The linear magnetoelastic coupling drives a magnetostructural transition from an IC phase, which has P s along the c axis ͑P ʈ c͒, to a commensurate ͑C͒ phase with P s along the a axis ͑P ʈ a͒.In TbMnO 3 , when a magnetic field is applied along the b axis ͑H ʈ b͒ at 2 K, parallel to the direction of the IC magnetic modulation ͓see Fig. 1͑a͔͒, the electric polarization of the lattice flops from P ʈ c to P ʈ a at the critical field H C b ϳ 4.5 T ͓Fig. 2͑d͔͒. When field is applied along the a axis ͑H ʈ a͒, perpendicular to the magnetic modulation, a similar flop is found but at a higher critical field, H C a ϳ 9 T ͓Fig. 2͑a͔͒. Recently there have been a number of examples of magnetoelastic coupling in complex multiferroic oxides such as TbMn 2 O 5 which exhibit a reversable polarization switch with applied field, 4 and hexagonal HoMnO 3 where one magnetic phase is selected over another by applying an electric field.5 However, TbMnO 3 is unique as it is the only known example of a material that exhibits a field-induced flop of its polarization.In TbMnO 3 the staggered ordering of Mn 3+ 3d 3x 2 −r 2 /3d 3y 2 −r 2 orbitals as found in LaMnO 3 is frustrated partly due to the small ionic size of Tb 3+3 . This leads to an IC spin ordering which drives a ferroelectric lattice...

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The magnetic excitations in multiferroic TbMnO 3 have been investigated by inelastic scattering of polarized and unpolarized neutrons in the ferroelectric cycloidal and in the paraelectric collinear phase. The polarization analysis of the excitations at the incommensurate magnetic zone center allows one to determine the characters of three distinct modes. In particular we may identify those modes which may directly couple to the ferroelectric polarization. We find a rather complex magnon dispersion with branches split throughout the Brillouin zone, which should be a generic characteristic of elliptical cycloidal order.