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.

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.

The crystal and magnetic structure of La1−xSr1+xMnO4 (0 ≤ x ≤ 0.7) has been studied by diffraction techniques and high resolution capacitance dilatometry. There is no evidence for a structural phase transition like those found in isostructural cuprates or nickelates, but there are significant structural changes induced by the variation of temperature and doping which we attribute to a rearrangement of the orbital occupation.

Spin correlations in La2-xSrxCoO4 (0.3 < or = x < or = 0.6) have been studied by neutron scattering. The commensurate antiferromagnetic order of La2CoO4 persists in a very short range up to a Sr content of x = 0.3, whereas small amounts of Sr suppress commensurate antiferromagnetism in cuprates and in nickelates. La2-xSrxCoO4 with x > 0.3 exhibits incommensurate spin ordering with the modulation closely following the amount of doping. These incommensurate phases strongly resemble the stripe phases observed in cuprates and nickelates, but incommensurate magnetic ordering appears only at larger Sr content in the cobaltates due to a reduced charge mobility.

The magnon dispersion in the charge, orbital, and spin ordered phase in La1/2Sr3/2MnO4 has been studied by means of inelastic neutron scattering. We find excellent agreement with a magnetic interaction model based on the CE-type superstructure. The magnetic excitations are dominated by ferromagnetic exchange parameters revealing a nearly one-dimensional character at high energies. The strong ferromagnetic interaction in the charge or orbital ordered phase appears to be essential for the capability of manganites to switch between metallic and insulating phases.

The chiral components in the magnetic order in multiferroic MnWO 4 have been studied by neutron diffraction using spherical polarization analysis as a function of temperature and of external electric field. We show that sufficiently close to the ferroelectric transition at T = 12.3 K it is possible to switch the chiral component by applying moderate electric fields at constant temperature. Full hysteresis cycles can be observed which indicate strong pinning of the magnetic order. MnWO 4 , furthermore, exhibits a magnetoelectric memory effect across heating into the paramagnetic and paraelectric phase.

Using spectroscopic ellipsometry, we study the optical conductivity ͑͒ of insulating LaSrMnO 4 in the energy range of 0.75-5.8 eV from 15 to 330 K. The layered structure gives rise to a pronounced anisotropy. A multipeak structure is observed in 1 a ͑͒ ͑ϳ2, 3.5, 4.5, 4.9, and 5.5 eV͒, while only one peak is present at 5.6 eV in 1 c ͑͒. We employ a local multiplet calculation and obtain ͑i͒ an excellent description of the optical data, ͑ii͒ a detailed peak assignment in terms of the multiplet splitting of Mott-Hubbard and charge-transfer absorption bands, and ͑iii͒ effective parameters of the electronic structure, e.g., the on-site Coulomb repulsion U eff = 2.2 eV, the in-plane charge-transfer energy ⌬ a = 4.5 eV, and the crystal-field parameters for the d 4 configuration ͑10Dq = 1.2 eV, ⌬ eg = 1.4 eV, and ⌬ t2g = 0.2 eV͒. The spectral weight of the lowest absorption feature ͑at 1 -2 eV͒ changes by a factor of 2 as a function of temperature, which can be attributed to the change of the nearest-neighbor spin-spin correlation function across the Néel temperature T N = 133 K. Interpreting LaSrMnO 4 effectively as a Mott-Hubbard insulator naturally explains this strong temperature dependence, the relative weight of the different absorption peaks, and the pronounced anisotropy. By means of transmittance measurements, we determine the onset of the optical gap ⌬ opt a = 0.4-0.45 eV at 15 K and 0.1-0.2 eV at 300 K.Our data show that the crystal-field splitting is too large to explain the anomalous temperature dependence of the c-axis lattice parameter by thermal occupation of excited crystal-field levels. Alternatively, we propose that a thermal population of the upper Hubbard band gives rise to the shrinkage of the c-axis lattice parameter.

Using in-field single crystal neutron diffraction we have determined the magnetic structure of TbMnO3 in the high field P a phase. We unambiguously establish that the ferroelectric polarization arises from a cycloidal Mn spins ordering, with spins rotating in the ab plane. Our results demonstrate directly that the flop of the ferroelectric polarization in TbMnO3 with applied magnetic field is caused from the flop of the Mn cycloidal plane.PACS numbers: 61.12. Ld, 75.30.Kz, 75.47.Lx, 75.80.+q The antisymmetric Dzyaloshinski-Moriya (DM) interaction[1, 2] between two spins, S i , S i+1 separated by r i,i+1 , provides for a natural coupling between magnetism and ferroelectricity with the spontaneous ferroelectric polarization given by P s ∼ r i,i+1 × (S i × S i+1 ) [3,4,5]. This mechanism generates ferroelectricity in a wide variety of magnets such as RMnO 3 perovskites with R=Gd, Dy, and Tb, [6,7] [10,11]. In the RMnO 3 manganites the DM interaction results in cycloidal order of Mn-spins giving a spontaneous ferroelectric polarization along the c-axis (P c) ( Fig. 1(a)). The application of magnetic field results in the flop of the polarization from the c-to the a-axis and highlights a novel control of one ferroic property by an other [6]. It has been assumed that this change in the direction of the polarization reflects the flop of the Mn spin cycloid, implying that the antisymmetric DM interaction continues to be responsible for the polarization ( Fig. 1(b)) [3,12]. However, in this high field P a-phase, the commensurate magnetic wave vector for Mn is also compatible with other magneto-electric mechanisms such as exchange striction [13,14,15]. In this letter we present a determination of the magnetic structure of TbMnO 3 in a high magnetic field in the commensurate P a phase. We find that the magnetic structure of Mn-spins is characterized by an ab-cycloid that accurately describes the direction of the observed ferroelectric polarization via the DM interaction. Our finding validates the model that the polarization flops found in the perovskite manganites result from the flop of the Mn-spin cycloid.The manganite TbMnO 3 crystallizes in the orthorhombic perovskites structure Pbnm. On cooling, below T N =41K Mn spins order incommensurately point along the magnetic wave vector τ ∼ 0.275b * [5,6]. On further cooling below T S =28K, a c-axis component of the Mn moment orders with a phase shift of π/2 with respect to the b-component so as to form a cycloidal structure where Mn-spins rotate within the bc plane and around the aaxis as shown in Fig. 1(a)[5]. The axis of spin rotation defines the DM interaction, S i × S i+1 , while the distance r i,i+1 is parallel to the modulation vector τ . For this type of spin order, inversion symmetry is broken yielding for R=Tb and Dy, a ferroelectric polarization along the c-axis as indeed is observed (P s c ∼ a × b) [5,6].It is tempting to assume that the flop in the ferroelectric polarization arises from a flop in the Mn-spin spiral, but so far there is no experimental prove for ...