[en] Layered LiFeO₂ is an antiferromagnet with a Neel temperature T_N=20 K. Substituting Fe with diamagnetic Co³⁺ surprisingly leads first (up to x=0.4), to an increase of the Weiss temperature θ and then above x=0.4 to a decrease due to a dilution effect. Magnetic properties are qualitatively interpreted within a mean field model, with ferromagnetic layers coupled antiferromagnetically. Estimation is given of the ferro- and antiferromagnetic interactions

[en] We will present here results on the magnetic properties of two compounds, GeCo₂O₄ and GeNi₂O₄, obtained by magnetic susceptibility and high magnetic field magnetization measurements. GeCo₂O₄ shows an antiferromagnetic behaviour with an ordering temperature T_N=23 K and a Curie-Weiss temperature θ=+81 K. There is no apparent magnetic frustration. From the value of the Curie constant, we deduce that Co²⁺ ions are in high spin S=((3)/(2)) state and that they are bounded in pairs. We will discuss the possible origin of these pairs. In GeNi₂O₄, we observe a negative Curie-Weiss temperature (θ=-15 K). In this compound the ferromagnetic Ni-Ni direct interaction present in GeCo₂O₄ cannot exist. Moreover, the nickel cations are not bounded in pairs as in the cobalt compound. We show that GeNi₂O₄ is a good candidate for magnetic frustration

[en] The diversified intercalation chemistry of fullerenes has been the starting point of a still growing number of new solid phases. The intercalated species can be non-metallic (P, S, halogen, for instance) or can be an alkali or alkaline-earth metal, an organic or organometallic cation. Due to their superconducting properties, some of these phases have raised considerable interest, the magnitude of their critical temperature being just behind the one of superconducting ceramics. The new family of rare-earths fullerides, though still restricted in number, exhibit superconducting properties (found in Yb, Sm and more recently in La-based compounds) or magnetic properties related with the intrinsic characteristics of the cations of the 4f series.

[en] The effect of strain on the magnetic phase diagram of the charge ordered (CO) Pr_0.5Ca_0.5MnO₃ thin films has been investigated using magneto transport and Electron Spin Resonance measurements. The critical magnetic field required to destroy the CO phase, always smaller than in the bulk compound, correlates with the strength of the lattice mismatch between the substrate and the CO phase. The 250 nm thick film grown on LaAlO₃ presents a separated ferromagnetic phase imbedded in the CO phase, in form of thin layers

[en] The magnetization of CeAl₃ was measured at 4.2⁰K. Also plotted was the specific heat between 1.3⁰K and 12⁰K of CeAl₂ and CeAl₃ at 0 and 7 kbars. The accuracy of these measurements is about 0.5%. The specific heat anomaly of CeAl₂ is apparently related to the Kondo effect. Pressure reinforces the Kondo character of cerium thus the Kondo transition appears at higher temperatures. The magnetization behavior characterizes an induced ferromagnetic transition. When the ferromagnetism occurs, the moment of cerium takes a value between 0 and the full ionic value depending on the competition between the Kondo temperature and the exchange interactions. 3 figures

[en] In Nd_0.5Ca_0.5MnO₃, Mn³⁺ and Mn⁴⁺ ions become ordered below T_co=250 K. At 160 K, a long-range antiferromagnetic order is established. The ESR spectra have revealed that, for T_N< T< T_CO, this phase is non magnetic but presents strong antiferromagnetic fluctuations. No evidence of a separated ferromagnetic phase was detected in this temperature range. Below T_N, macroscopic antiferromagnetic modes are observed which depend on whether the magnetic field is swept upward or downward. Besides, both charge-ordered phases are unstable against magnetic field: a first-order, strongly hysteretic transition towards a ferromagnetic state occurs. This high-field phase orders ferromagnetically below T_C=270 K. For a given temperature, mixing of the charge ordered and the ferromagnetic phases occur in a restricted magnetic field range

[en] In nuclear orientation experiments performed on 3d radioactive ions of ⁵⁴Mn, ⁵¹Cr and ⁶⁰Co dissolved in FCC ytterbium, the occurrence of the 3d magnetism is exactly that found for divalent matrices. At 4.2 K the magnetisation of pure ytterbium varies linearly up to 130 kG. These measurements are in agreement with the existence, in a divalent matrix, of localised Yb³⁺ condensed at low temperature in a Kondo state. The possible origin of these Yb³⁺ ions is discussed. (author)

[en] In the Ce₃-Al₁₁ compound, two magnetic phases exist at low temperatures which are the result of the competition between the Kondo energy and the Ce-Ce interactions. Under pressure, the high temperature phase is stabilized. (orig.)

No abstract available

[en] We present ultrasonic measurements on magnetoresistance manganites Pr_1-xCa_xMnO₃ (x=0.35,0.5) in magnetic fields up to 27 T. Charge ordering occurs in Pr_1-xCa_xMnO₃ in the 0.35< x<0.5 composition regime. The associated lattice distortions also exhibit long-range ordering. A large decrease (1%) of the sound velocity was observed at the insulator-to-metal transition induced by magnetic field destroying the charge ordering. The insulator-metal transition for Pr_0.5Ca_0.5MnO₃ is accompanied with a large hysteresis which extends up to 20 T at the lowest temperatures (2 K). The field dependence of the longitudinal sound velocity measured on polycrystalline samples are reported and the magnetic diagram is determined