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We present an infrared transmission Pr 3ϩ crystal-field study of as-grown, reduced, and oxygenated Pr 2Ϫx Ce x CuO 4Ϯ␦ single crystals and thin films. Excitations from the ground-state multiplet 3 H 4 to the 3 H 5 , 3 H 6 , 3 F 2 , and 3 F 3 excited multiplets are observed in all samples. In addition to the Pr 3ϩ regular sites, which remain unperturbed following the cerium doping or the oxygen content modifications, Pr 3ϩ sites are detected. A precise set of crystal-field parameters, which reproduces the energy and the symmetry of the levels, is determined. The reduction process, which drives the electron-doped cuprates superconducting, is discussed in detail and scenarios for the reduction mechanism and induced vacancies are proposed. In contrast to the common belief, the apical oxygen, which is clearly detected in all samples, is not removed in Pr 1.85 Ce 0.15 CuO 4 following reduction. This observation questions the role attributed to the apical oxygen removal in triggering the superconductivity.

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LaCoO 3 displays two broad anomalies in the DC magnetic susceptibility DC , occurring, respectively, around 50 K and 500 K. We have investigated the first of them within the 10 K Ͻ T Ͻ RT temperature range using Co K␣ 1 x-ray absorption spectroscopy ͑XAS͒ in the partial fluorescence yield mode. In contrast with previous O K-edge XAS reports, our data show the existence of abrupt changes around 50 K which can be nicely correlated with the anomaly in DC. To our knowledge, this is the first time that a clear, quantitative relationship between the temperature dependence of the magnetic susceptibility and that of the XAS spectra is reported. The intensity changes in the preedge region, which are consistent with a transition from a lower to a higher spin state, have been analyzed using a minimal model including the Co 3d and O 2p hybridization in the initial state. The temperature dependence of the Co magnetic moment obtained from the estimated e g and t 2g occupations could be satisfactorily reproduced. Also, the decrease of the Co 3d and O 2p hybridization by increasing temperature obtained from this simple model compares favorably with the values estimated from thermal evolution of the crystallographic structure.

We present measurements of the in-plane (κ ab ) and out-of-plane (κc) thermal conductivity of Pr2CuO4 and Gd2CuO4 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 CuO2 planes. However, the absolute values of κmag are lower than previous results on La2CuO4. These differences probably arise from deviations from the nominal oxygen stoichiometry. This has a drastic influence on κmag, which is shown by an investigation of a La2CuO 4+δ polycrystal.

We introduce a minimal spin model for describing the magnetic properties of CuCrO 2 . Our Monte Carlo simulations of this model reveal a rich magnetic field induced phase diagram, which explains the measured field dependence of the electric polarization. The sequence of phase transitions between different mutiferroic states arises from a subtle interplay between spatial and spin anisotropy, magnetic frustration and thermal fluctuations. Our calculations are compared to new measurements up to 92 T. PACS numbers: 75.85.+t, 75.30.Kz, 75.50.Ee Triangular lattice antiferromagnets (TLA) are widely studied in the field of frustrated magnetism. Complex orderings and rich phase diagrams arise because three antiferromagnetic interactions within a triangle cannot be simultaneously satisfied. Delafossite CuCrO 2 is a particularly clean example of a TLA where quasi-classical Cr 3+ S = 3/2 spins form a triangular lattice in the ab plane. [1, 2] The spins have outof-plane anisotropy and weak interlayer coupling that is one to two orders of magnitude smaller than the in-plane interactions. [3][4][5] The three spins of each triangle form a nearly 120• structure and all three sublattices form proper-screw spirals that propagate along the same [110] axis with propagation vector q = 0.329 (the spins rotate in and out of the ab plane). [1,6,7] The spiral can propagate along any of six directions (three choices for the [110] axis and two choices for the helicity) leading to six possible domains.The proper-screw spiral induces an electric polarization P along the spiral propagation vector. [3,[6][7][8][9][10][11][12] This allows us to probe phase transitions between spiral states at high applied magnetic fields H a , while the magnetization is largely insensitive to these transitions. The non-zero P is consistent with Arima's mechanism for multiferroic behavior, [13] where the spiral magnetic structure slightly influences the hybridization between the Cr d-orbitals and the O p-orbitals via spin-orbit coupling, creating a net P q. Thus, a pattern of electromagnetic domains forms below the magnetic ordering temperature that can be influenced by small electric and magnetic fields relative to the dominant exchange interactions. [6,10] The triangular layers of CuCrO 2 stack along the c-axis such that a Cr 3+ ion from one layer lies at the center of a triangle of Cr 3+ ions in the next layer. [1, 2, 9] The triangular lattice distorts by about 0.01% as a result of the spiral magnetic ordering, leading to two different exchange interactions, J and J , along different bonds of the triangle [3,10,11,14,15] (Fig. 1). Thermodynamic measurements show two close-lying phase transitions. Elastic neutron diffraction measurements suggest that below T N = 24.2 K, the triangular plane develops collinear spin correlations. A spiral long-range order appears below T MF = 23.6 K and also induces net P, possibly via a first-order transition. [8,15,16] The H a dependence of this spiral ordering is only partially explored in experiments and theory. [8,9,17,1...