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The linear and non-linear AC magnetic susceptibilities of the perovskite La1−xSrxCoO3, with 0.15 ⩽ x ⩽ 0.30 , are presented. It is observed that, for x 0.20 , a freezing temperature is defined by AC magnetic susceptibility, pointing to a glassy behaviour whose origin is the phase separation of ferromagnetic metallic-like clusters embedded in an insulating matrix. At this temperature, the non-linear magnetic susceptibility shows critical divergence, similar to spin-glasses, although the width of the diverging peak does not compare well with canonical spin-glasses.

The dependence of the ac-magnetic susceptibility of La0.85Sr0.15CoO3 on the annealing temperature used during synthesis is addressed. Such dependence has been previously attributed to compositional inhomogeneities. Nevertheless, the presence of distinct phases with different chemical compositions is excluded after explorations by several techniques. Instead it is proposed that an electronic phase separation takes place in the material, whose state is changed after charge-carrier redistributions due to the thermal history of the samples, analogously to parent manganese-based perovskites.

We report a rather high dependence of the dielectric permittivity on the magnetic field in La2/3Ca1/3MnO3. The variation is maximum at around 270 K, little above the Curie temperature, TC, and it reaches a 30% under only 0.5 T. We attribute this phenomenon to the space-charge or interfacial polarization produced between the insulator and the metallic regions segregated intrinsically in the material above TC., Comment: 4 figures

Magnetic anisotropy has been measured for the ferromagnetic La0.7Sr0.3CoO3 perovskite from an analysis of the high-field part of the magnetization vs. field curves, i.e., the magnetic saturation regime. These measurements give a magnetic anistropy one order of magnitude higher than that of reference manganites. Surprisingly, the values of the magnetic anisotropy calculated in this way do not coincide with those estimated from measurements of coercive fields which are one order of magnitude smaller. It is proposed that the reason of this anomalous behaviour is a transition of the trivalent Co ions under the external magnetic field from a low-spin to an intermediate-spin state. Such a transition converts the Co3+ ions into Jahn-Teller ions having an only partially quenched orbital angular momentum, which enhances the intra-atomic spin-orbit coupling and magnetic anisotropy., Accepted of publication in Europhysics Letters, 11 pages, 5 figures

A study of the reliability of the title compounds as a basis for magnetoresistive sensors is presented. For this purpose we have carried out long-term systematic studies of the evolution of the resistive and magnetic properties of samples synthesized by both ceramic and wet chemical routes, allowing them to age during several months. It is observed that, although the magnetic properties remain unaltered, their resistivity does change appreciably. With time, samples that are initially metallic over all the measured temperature range (T < 300 K), change to semiconductor-like behaviour at low temperatures and above their ferromagnetic to paramagnetic phase transition temperature, giving rise to a 'reentrant semiconducting behaviour' with associated high electrical noise, relaxation effects of the resistivity and even diodic behaviour. Fil: Mira, J.. Universidad de Santiago de Compostela. Facultad de Física; España Fil: Rivas, J.. Universidad de Santiago de Compostela. Facultad de Física; España Fil: Breijo, M. P.. Universidad da Coruña; España Fil: Señarís Rodríguez, M. A.. Universidad da Coruña; España Fil: Sanchez, Rodolfo Daniel. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina