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We report thermal-expansion and specific-heat data of the series La 1 - x Ca x CoO 3 for 0 ⩽ x ⩽ 0.3 . For x = 0 the thermal-expansion coefficient α ( T ) features a pronounced maximum around T = 50 K caused by a temperature-dependent spin-state transition from a low-spin state ( S = 0 ) at low temperatures towards a higher spin state of the Co 3 + ions. The partial substitution of the La 3 + ions by divalent Ca 2 + ions causes drastic changes in the macroscopic properties of LaCoO 3 . Around x ≈ 0.125 the large maximum in α ( T ) has completely vanished. With further increasing x three different anomalies develop.

We present diffraction data on a single-layered manganite La(0.42)Sr(1.58)MnO4 with hole doping (x>0.5). Overdoped La(0.42)Sr(1.58)MnO4 exhibits a complex ordering of charges, orbitals and spins. Single crystal neutron diffraction experiments reveal three incommensurate and one commensurate order parameters to be tightly coupled. The position and the shape of the distinct superstructure scattering points to a stripe arrangement in which ferromagnetic zigzag chains are disrupted by additional Mn4+ stripes.

Neutron diffraction with spherical polarization analysis is a powerful tool for studying the multiferroic materials where the ferroelectric polarization arises from a complex magnetic structure. Analyzing the off-diagonal terms in the polarization matrix one may directly detect the chiral contributions even in a multidomain arrangement. In MnWO4 one can control the chiral magnetism by varying an electric field at constant temperature. The analysis of multiferroic hysteresis cycles at four equivalent magnetic Bragg peaks fully agrees with a nearly monodomain chiral arrangement controlled by the electric field. A pronounced asymmetry of the hysteresis cycles and memory effects point to strong pinning of the chiral magnetism in MnWO4. We find a second-order harmonic modulation which exhibits both magnetic and structural character and which may be related with the domain pinning. The observed interference between the nuclear and the magnetic modulation is another manifestation of the coupling between the crystal structure and the magnetism in the multiferroic oxides.

We report thermal-expansion, lattice-constant, and specific-heat data of the series ${\text{La}}_{1\ensuremath{-}x}{A}_{x}{\text{CoO}}_{3}$ for $0\ensuremath{\le}x\ensuremath{\le}0.30$ with $A=\text{Ca}$, Sr, and Ba. For the undoped compound ${\text{LaCoO}}_{3}$, the thermal-expansion coefficient $\ensuremath{\alpha}(T)$ exhibits a pronounced maximum around $T=50\text{ }\text{K}$ caused by a temperature-driven spin-state transition from a low-spin state of the ${\text{Co}}^{3+}$ ions at low temperatures toward a higher spin state at higher temperatures. The partial substitution of the ${\text{La}}^{3+}$ ions by divalent ${\text{Ca}}^{2+}$, ${\text{Sr}}^{2+}$, or ${\text{Ba}}^{2+}$ ions causes drastic changes in the macroscopic properties of ${\text{LaCoO}}_{3}$. The large maximum in $\ensuremath{\alpha}(T)$ is suppressed and completely vanishes for $x\ensuremath{\gtrsim}0.125$. For $A=\text{Ca}$ three different anomalies develop in $\ensuremath{\alpha}(T)$ with further increasing $x$, which are visible in specific-heat data as well. Together with temperature-dependent x-ray data, we identify several phase transitions as a function of the doping concentration $x$ and temperature. From these data we propose an extended phase diagram for ${\text{La}}_{1\ensuremath{-}x}{\text{Ca}}_{x}{\text{CoO}}_{3}$.

Using in-field single-crystal neutron diffraction, we have determined the magnetic structure of TbMnO(3) in the high field P parallel a phase. We unambiguously establish that the ferroelectric polarization arises from a cycloidal Mn spin ordering, with spins rotating in the ab plane. Our results demonstrate directly that the flop of the ferroelectric polarization in TbMnO(3) with applied magnetic field is caused from the flop of the Mn cycloidal plane.

Spin correlations in La2-xSrxCoO4 (0.3 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 magnetic correlations in the charge and orbital ordered manganite ${\text{La}}_{1/2}{\text{Sr}}_{3/2}{\text{MnO}}_{4}$ have been studied by elastic and inelastic neutron scattering techniques. Out of the well-defined charge exchange (CE)-type magnetic structure with the corresponding magnons, a competition between CE-type and ferromagnetic fluctuations develops. Whereas ferromagnetic correlations are fully suppressed by the static CE-type order at low temperature, elastic and inelastic CE-type correlations disappear with the melting of the charge-orbital order at high temperature. In its charge-orbital disordered phase, ${\text{La}}_{1/2}{\text{Sr}}_{3/2}{\text{MnO}}_{4}$ exhibits a dispersion of ferromagnetic correlations, which remarkably resembles the magnon dispersion in ferromagnetically ordered metallic perovskite manganites.

The field-induced multiferroic transition in TbMnO3 has been studied by neutron scattering. Apart strong hysteresis, the magnetic transition associated with the flop of electronic polarization exhibits a memory effect: after a field sweep, TbMnO3 does not exhibit the same phase as that obtained by zero-field cooling. The strong changes in the magnetic excitations across the transition perfectly agree with a rotation of the cycloidal spiral plane indicating that the inverse Dzyaloshinski-Moriya coupling causes the giant magnetoelectric effect at the field-induced transition. The analysis of the zone-center magnetic excitations identifies the electromagnon of the multiferroic high-field phase.

figuration 10Dq= 1.2 eV, eg= 1.4 eV, and t2g = 0.2 eV. The spectral weight of the lowest absorption feature at 1 – 2 eV 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 Neel temperature TN = 133 K. Interpreting LaSrMnO4 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 = 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.

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 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.

We have studied the thermal conductivity $\ensuremath{\kappa}$ on single crystalline samples of the antiferromagnetic monolayer cuprates ${R}_{2}\mathrm{Cu}{\mathrm{O}}_{4}$ with $R=\mathrm{La}$, Pr, Nd, Sm, Eu, and Gd. For a heat current within the $\mathrm{Cu}{\mathrm{O}}_{2}$ planes, i.e., for ${\ensuremath{\kappa}}_{ab}$ we find high-temperature anomalies around $250\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ in all samples. In contrast, the thermal conductivity ${\ensuremath{\kappa}}_{c}$ perpendicular to the $\mathrm{Cu}{\mathrm{O}}_{2}$ planes, which we measured for $R=\mathrm{La}$, Pr, and Gd, shows a conventional temperature dependence as expected for a purely phononic thermal conductivity. This qualitative anisotropy of ${\ensuremath{\kappa}}_{i}$ and the anomalous temperature dependence of ${\ensuremath{\kappa}}_{ab}$ give evidence for a significant magnetic contribution ${\ensuremath{\kappa}}_{\mathrm{mag}}$ to the heat transport within the $\mathrm{Cu}{\mathrm{O}}_{2}$ planes. Our results suggest, that a large magnetic contribution to the heat current is a common feature of single-layer cuprates. We find that ${\ensuremath{\kappa}}_{\mathrm{mag}}$ is hardly affected by structural instabilities, whereas already weak charge carrier doping causes a strong suppression of ${\ensuremath{\kappa}}_{\mathrm{mag}}$.

The crystal and magnetic structure of ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{1+x}\mathrm{Mn}{\mathrm{O}}_{4}$ $(0\ensuremath{\leqslant}x\ensuremath{\leqslant}0.6)$ has been studied by diffraction techniques and high resolution capacitance dilatometry. There is no evidence for a structural phase transition related to octahedron tilting 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.

We present measurements of the in-plane ( κ ab ) and out-of-plane ( κ c ) thermal conductivity of Pr 2 CuO 4 and Gd 2 CuO 4 single crystals. The anisotropy gives strong evidence for a large contribution of magnetic excitations to κ ab , i.e. for a heat current within the CuO 2 planes. However, the absolute values of κ mag are lower than previous results on La 2 CuO 4 . These differences probably arise from deviations from the nominal oxygen stoichiometry. This has a drastic influence on κ mag , which is shown by an investigation of a La 2 CuO 4 + δ polycrystal.