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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 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 ${\text{La}}_{1\ensuremath{-}x}{\text{Ca}}_{x}{\text{CoO}}_{3}$.

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.

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.