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We report on the resistivity (ρ), thermoelectric power (S for Seebeck) and magnetic measurements of the series Pr1-xCaxCrO3 (x=0.0–0.5). These orthochromites exhibit a T independent, large and positive S, from 100 K to 700 K which follows the Cr4+ concentration. Upon Ca2+ for Pr3+ substitution, a concomitant decrease of the resistivity and S values is found. The evolution of S as a function of hole concentration (Cr4+) can be nicely fitted by the modified Heikes formula which takes into account the orbital degeneracy associated to Cr3+/Cr4+. This is in good agreement with the model previously calculated by Marsh and Parris, in the case of weak magnetic coupling [Phys. Rev. B 54, 7720 (1996)]. The magnetic susceptibility measurements support the assumption of a weak magnetic coupling since the antiferromagnetic ordering TN values are found to be lower than 250 K. For Pr0.7Ca0.3CrO3, the power factor PF= S2/ρ measured at 700 K is equal to 1.9×10-4 W m-1K-2. The present system, chemically stable in air up to T > 1000 °C, is promising for thermoelectric application at very high temperature.

The substitution of 1%-Ce4 + for La3 + in LaCoO3 is found to change the sign of the Seebeck coefficient at room temperature. This demonstrates that not only holes but also electrons can be created in LaCoO3. The result is compatible with the Heikes formula for doping levels close to the “pure” trivalent Co3 + state. Nonetheless, the physical properties such as magnetic susceptibility, magnetization, thermal conductivity and resistivity are found to be asymmetric for hole and electron-doped LaCoO3. Such a different behaviour is ascribed to the very different spin-states of Co4 + (low-spin, t 2g 5 e g 0) and Co2 + (high-spin, t 2g 5 e g 2).

The low temperature magnetic and transport properties of the Pr0.5Ca0.5Mn1-xNixO3 manganites ( 0≤x≤0.1) have been investigated. The presence of Ni hinders the charge and orbital ordering observed in Pr0.5Ca0.5MnO3 and favors the creation of ferromagnetic regions, leading to phase separation. The ferromagnetic fractions induced by the Ni substitution have been estimated from magnetization measurements, they are large and reach 40% for 4% of Ni. Steps are observed in the M(H) and ρ(H) curves of all the samples at T < 5 K. They are similar to the steps observed in Pr0.5Ca0.5Mn1-xMxO3, where M is a non magnetic cation (Mg2+, Ga3+,...), and for which the ferromagnetic fractions are very small (less than 2%), however, their appearance is restricted to lower temperatures (T < 5 K) with Ni dopant than with non magnetic cations. This study shows that steps can be observed in a wide range of phase-separated systems, even when the ferromagnetic fraction is very large.

The doping of the manganese site by iridium (up to 15%) in the small A cation manganites Pr_{1-x}Ca_xMnO_3 (0.4 < x < 0.8), has been investigated as a new method to suppress charge-ordering and induce CMR effects. Ir doping leads to ferromagnetism and to insulator to metal transitions, with high transition temperatures reaching 180K and CMR ratio in 7T as large as 10^4. The efficiency with which iridium induces ferromagnetism and CMR is compared to previous results obtained with other substitutions (Ru, Rh, Ni, Cr, ...). The ionic radius of the foreign cations and their mixed-valencies are found to be the main parameters governing the ability to collapse the charge-ordered state., 9 pages, 5 figures, accepted for publication in Eur. Phys. J. B

To investigate the existence of a splay effect in Bi2Sr2CaCu2O8 (Bi-2212), vortex pinning has been studied in different configurations of strongly inclined columnar defects (75 ° from the c axis), installed by heavy-ion irradiation. It is shown that the symmetry of the track setting with respect to the field direction is a more influent parameter than the presence of a dispersion in the track directions. We claim that the enhanced pinning efficiency which is observed in some splayed configurations of columnar defects in Bi-2212 can be interpreted without invoking a splay effect.