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We have synthesized two divanadates M2V2O7 (M2+ = Co2+ and Cu2+), that are known to show different and complex magnetic arrangements at low temperatures and have studied their dielectric behavior. We have observed a change in the slope of the dielectric constant at the magnetic transition temperature, result which confirms the existence of magnetodielectric coupling in these two divanadates. The origin of the magnetodielectric coupling seems to be different in the Cu- and Co-compounds, although in both cases it is related to their frustrated magnetic arrangement. In Cu2V2O7, the dielectric anomaly arises from a spin canting due to the antisymmetric exchange which is allowed by structural symmetry. In the Co-divanadate, the structural symmetry does not allow antisymmetric exchange but the compound shows correlation between the behavior of the dielectric constant and inverse magnetization, which points to a dependence of its electrical polarization with the pair correlation function of neighboring magnetic spins.

We present a detailed investigation on the magnetodielectric properties of Fe2OBO3, where we have found a magnetodielectric effect around 150% at room temperature and ν=2 kHz, that cannot solely arise from magnetoresistive extrinsic contributions. In addition, we find a crossover of the magnetodielectric response around TN, that we attribute to changes in the electronic microstructure of this compound that take place at that temperature. For T

We report the optimization of the dielectric properties of La2/3Ca1/3MnO3. This is achieved by synthesizing this material with small particle size, strategy that reduces the conductivity of the sample and its dielectric losses, while retaining a reasonably high dielectric permittivity and a good magnetocapacitive response. In order to further improve the properties of these nanoparticles, we have also prepared core-shell composites La2/3Ca1/3MnO3@SiO2 but in this case the magnetocapacitive effect is strongly reduced.

Chemical lithiation of High Temperature Superconductors has been used to produce materials that are both lithium ionic conductors and superconductors (Tc⋍90 K) [1]. Although their Tc is not affected by the reaction, the volume fraction of the superconducting phase seems to depend on the degree of insertion, diminishing with increasing lithium content. A detailed microstructural study performed by transmission electron microscopy correlates this behaviour with the appearance of defects in the “123” matrix upon lithiation.

We have prepared polycrystalline La 1- x Sr x CoO 3 (0 x ⩽ 0.50) particles using the ceramic method and other alternative soft-chemistry techniques that yield materials with smaller particle size. The study of their magnetic and electrical properties shows differences between these series of samples, among them: changes in the compositional range for ferromagnetic behaviour, magnetic hardening as the particles get smaller; different composition for the metal-insulator transition, and changes in the conduction regime in the semiconducting interval. We try to interpret these differences taking into account the inhomogeneous electronic structure of these doped cobaltates.

The magnetic and electrical properties of R1−xSrxCoO3 materials (R=La, Pr, Nd, Gd) (0⩽x⩽0.50) are studied. Rare-earths with angular momentum L≠0 enhance coercive field values. They are maximum for R=Nd (the highest L) and much smaller for Gd and La (L=0). The result is interpreted as the contribution of the L–S coupling in the rare-earth to the magnetic anisotropy of the system. Electrical conductivity is improved with Sr doping, but increases of rare-earth atomic number lead to decreases of conductivity. This is attributed to the increase of lattice distortions, due to the smaller R3+ ionic size, and to the effect of the rare-earth on the spin state of cobalt ions.

We have prepared a series of samples with nominal composition Bi 2−x Pb x Sr 2 Ca 2 Cu 3 O y , 0 ≤ x ≤ 2. By means of X-ray diffraction and scanning electron microscopy we have observed that for low Pb/Bi ratios lead incorporates into the structure of the so called 80 K and 110 K Bi-superconductors, which normally coexist. However the 110 K superconducting phase becomes predominant as the sintering time increases. From x=0.5 upwards an almost Bi-free second phase appears. It is non-superconducting and its structure appears to be hexagonal. Some Ca 2 PbO 4 is also present for x > 0.5.