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Nanoparticles of the ferrimagnetic epsilon-Fe2O3 oxide have been synthesized by sol-gel method. Here, we report on the measurements of the dielectric permittivity as a function of temperature, frequency and magnetic field. It is found that, coinciding with the transition from collinear ferrimagnetic ordering to an incommensurate magnetic state occurring at about 100 K, there is an abrupt change (about 30 %) of permittivity suggesting the existence of a magnetoelectric coupling in this material. Indeed, magnetic field dependent measurements at 100 K have revealed an increase of the permittivity by about 0.3 % in 6 T. Prospective advantages of epsilon-Fe2O3 as multiferroic material are discussed.

The magnetic measurements of the Josephson critical field and intergranular critical currents are presented for the Bi2223 compound. The investigations prove that the Josephson critical field value is mainly dependent on the intergranular critical current density. The other factors like the grain dimensions and shapes, sample porosity and density are less important. The relation between the Josephson critical field and critical current HcJ(Jc) is well described within the single junction model, Hcj∝JcλJ. The temperature dependence of HcJ can also be well fitted according to the single junction model with a critical current dependence, Jc∝(1-T/TcJ)α. The critical current density versus temperature is almost linear indicating the presence of superconductor-insulator-superconductor (SIS) type junctions.

Dc magnetic relaxation measurements were performed on pristine and irradiated Hg-1201 single crystals. In relation to the doping level, we analyse the measurements on the pristine single crystals near the fishtail line. The fishtail effect originates from a competition between a critical current that increases against the magnetic field and the pinning energy which decreases against the magnetic field. Depending on the doping level of the single crystals, the fishtail effect may be observed or not, without any correlation with a transition of the vortex relaxation creep (plastic to elastic). The pinning properties have been modified by the introduction of columnar defects along the c-axis by irradiation with 6 GeV Pb ions. The columnar nature of the induced latent tracks is probed by high-resolution electron microscopy. It is shown that, in the presence of correlated disorder, Hg-1201 exhibits features similar to those previously reported in Y-123, despite a larger electronic anisotropy.

An `exact' numerical solution of Laplace's equation is performed in order to extract the anisotropic resistivity in a single crystal of a superconducting oxide from a multi-terminal transport measurement. A phenomenological fit of the results is proposed to provide a very simple method of extraction of the anisotropic resistivity. This extraction is successfully applied in a crystal of at room temperature.

Current - voltage characteristics are reported for a thin film with a granular structure. Our data provide support for a Berezinskii - Kosterlitz - Thouless transition type with an unusual vortex - antivortex interaction . Dissipation is discussed in terms of large-scale pair excitations through the sample inhomogeneities. Consistency is found with the theory worked out by Lobb, Abraham and Tinkham to explain resistive transition data in arrays of superconducting junctions arranged two-dimensionally. On this basis, the temperature behaviour of A(T) accounts for a weakening of the pair potential at interfaces due to the short coherence length of high- oxides, as was pointed out by Deutscher and Muller. We investigate the spatial renormalization of the vortex - antivortex interaction over the length scale probed by the current in the same manner as was first demonstrated by Kadin, Epstein and Goldman. Then good agreement with the renormalized Kosterlitz - Thouless theory is found.

The magnetic behaviour of the 58 K superconductor oxycarbonate thin film, which consists of an intergrowth of several members of a family with agreement composition (CaCuO2)m(Ba2CuCO3O2)n has been studied. Its pinning properties in terms of critical current densities and irreversibility line have been established using their hysteresis cycles. Typical Jc was calculated to approximately 105 A cm-2 at 5 K in zero field. Finally, the irreversibility line is compared to those obtained by Kumakura et al. (1994) on bulk oxycarbonate superconductors synthesized at high pressure.

We reported a resistive study of a Tl2Ba2Ca2Cu3O10- delta (Tl-2223) single crystal with magnetic fields up to 7 T applied in the two orientations H//c and H//ab. We obtained the anisotropic upper critical lines Hc2(T) and the irreversibility lines Hirr(T). The derivation of the coherence lengths leads to a rather small anisotropic ratio and exhibits a correlation between the coherence length along the c-axis and the thickness of the Cu-O layers. Different models of dissipation proposed in the literature have been unsuccessfully tested and, in particular, the occurrence of a 2D fluctuation mechanism was not observed in the studied part of the phase diagram.

International audience; Two kinds of epitaxial structures were grown by standard pulsed laser deposition on (001) Si, namely La0.7Sr0.3MnO3/Bi4Ti3O12/CeO2/YSZ/Si (BTO-based), and La0.7Sr0.3MnO3/SrTiO3/ CeO2 /YSZ/Si (STO-based) multilayers. The samples were investigated by means of x-ray diffraction, transmission electron microscopy, magnetic and transport measurements. The Curie temperature TC of the BTO-based samples was found to be higher (360 K) than for the typical reference epitaxial LSMO film grown on (001) SrTiO3 single crystal (345 K), due to high compressive in-plane strain. The STO-based samples show high structural quality, low roughness and high TC (350 K), making them interesting candidates for use in innovative LSMO-based bolometers or spintronic devices operating at room temperature. (Some figures in this article are in colour only in the electronic version)

We report on structural and magnetic properties of 0.5 and 1 at.% Al-doped Zn1?xCoxO polycrystalline samples with x=0.03 and 0.15 prepared by the co-precipitation method. X-ray diffraction patterns reveal that all samples crystallize in the hexagonal wurtzite structure without any parasitic phase. This space group is confirmed by the electron diffraction image to be P63mc. Paramagnetic behaviour was observed for samples that contain up to 3% Co whatever the Al content was and no ferromagnetic coupling was observed. Antiferromagnetic interaction started to appear for samples containing 15% Co.

The effect of calcium doping on the magnetic and transport properties of Eu1−xCaxBaCo2O5.50 ± δ has been investigated for x≤0.2. The substitution of calcium for europium does not change the cobalt valency, which remains +3, due to a small oxygen deficiency with respect to the ideal 'O5.5' composition. This substitution induces a large expansion of the ferromagnetic state in the whole temperature range from 10 to 320 K, TC increasing with the calcium content. Moreover, we observe a substantial decrease of the effective moment in the paramagnetic phase as x increases. In contrast, the Ca-doping does not significantly affect the metal–insulator transition. However, the resistivity and magnetoresistance decrease drastically for a small doping rate. These results are explained by a new scenario where we consider the presence of canted antiferromagnetic domains and a disproportionation mechanism of the cobalt Co3+ into Co2+ and Co4+.

We have studied the behaviour of the diffusion constant by incoherent quasi-elastic neutron scattering at the disorder transition of the second-stage α-HNO3 intercalated in graphite. The transition is continuous. This disordered phase is a lattice liquid phase. It shows simultaneously strong orientational correlations as previously observed by X-ray diffraction and translational diffusion. The diffusion constant follows an exp [- 2b/(T - Tc)1/2] law, as predicted for 2D defect-mediated phase transitions.