[en] The doping of Mn sites with Co and Ni was studied for Ca_0.85Sm_0.15MnO₃ and Ca_0.80Sm_0.20MnO₃ and compared with Cr doping in the same oxides. It is shown that the three dopants induce strong ferromagnetic interactions and metallicity in both manganites, and that they destroy CMR effect in Ca_0.85Sm_0.15MnO₃ and induce CMR effect in Ca_0.80Sm_0.20MnO₃. The optimal doping level x for reaching the maximum resistivity ratio is larger for Cr (x = 0.08) than for Co and Ni (x 0.06)

[en] The exploration of the system Tl-Hg-Sr-Ca-Cu-O has allowed a 2212 superconducting thallium strontium based cuprate, Tl_1.5Hg_0.4Sr_2.2Ca_0.8Cu₂O_8-δ, to be isolated for the first time, with a T_c of 25 K. In the same way, the 1212 superconducting cuprate, Tl_0.88Hg_0.2-Sr_2.2Ca_0.7Cu₂O_7-δ, with a T_c of 65K has been prepared as a nearly pure phase. The combined XRD, ED, HREM, and EDS studies of these phases show the presence of an excess of strontium in the calcium sites and allows a cationic distribution to be proposed. Moreover, for intermediate compositions, one observes complex intergrowths, with T_c's ranging from 30 to 60 K. 22 refs., 11 figs., 2 tabs

[en] Single crystals of the TlSr₂CaCu₂O₇ superconductor have been grown for the first time. The structures of an as-grown crystal (T_c = 55 K) and an oxygen pressure annealed one (T_c = 15 K) have been refined. The results are compared to those reported for the polycrystalline phase. The magnetic study (H||c) of a T_c = 55 K as-grown crystal allows the comparison of the superconducting properties of TlSr₂CaCu₂O₇ with those of other thallium 1212 single crystals. 22 refs., 7 figs., 3 tabs

[en] Eighteen crystals of superconducting cuprates with T_c > 50 K have been investigated in order to establish relations between their pinning properties and their anisotropies. Both irreversibility and fishtail lines are found to be very sensitive to the thickness of the insulating layers (d_i): the lines become steeper when d_i is decreased. Moreover anisotropic resistive measurements have recently confirmed that the electronic anisotropy is decreased when d_i decreases. The role of the CuO₂ planes thickness d_s is more difficult to interpret. The authors measurements suggest that the Tl-2201 and Hg-1201 anisotropies are intermediate between those of the corresponding n=2 and n=3 members for each series. Finally, the study of Y doped Bi-2212 crystals evidences the role of doping on the pinning properties which can be optimized in those series without depressing T_c

[en] We studied the hydrostatic pressure effect on the temperature dependence of resistivity of Pr_0.7-xLa_xCa_0.3MnO₃, Pr_0.7Ca_0.3-ySr_yMnO₃ and Pr_0.66Ca_0.34-zSr_zMnO₃ compounds, with different metal-insulator transition temperatures T_MI. The pressure derivative dT_MI/dP depends linearly on T_MI^-1. For the compounds Pr_0.66Ca_0.34-zSr_zMnO₃ it achieves a record value of 30 K/kbar. In contrast, the pressure effect on the charge ordering temperature is very weak. The analysis of changes in T_MI and structural parameters at chemical and at external pressure shows that to achieve a qualitative agreement, a contribution of e-ph interactions and/or 'Jahn-Teller' effects have to be taken into account

[en] The nuclear and magnetic structures of the colossal magnetoresistive compounds Nd_0.7Ba_0.3-ySr_yMnO₃ (0≤y≤0.2) have been investigated by means of neutron powder diffraction. For all the Ba/Sr substitutions, the nuclear structure remains orthorhombic within temperature range 1.5-293 K. However, the compound Nd_0.7Ba_0.3MnO₃ is characterized by the crystallographic space group Imma whereas the Sr-doped compounds crystallize in the Pnma space group. As expected from the bigger size of Ba-ion radii, the buckling as well as the distortion of MnO₆ octahedra become less pronounced when increasing the amount of Ba in Nd_0.7Ba_0.3-ySr_yMnO₃. In order to better understand the physical properties of these samples, these results have been included in the more general diagram Nd_0.7A'_0.3-y Sr_yMnO₃ (A=Ca, Ba). In this series, the observed maximum in T_C is not easily explained with standard parameters. Whereas the Mn magnetic moments display the same characteristics as observed in similar magnetoresistive compounds, the behavior of Nd magnetic moments shows unusual features when changing the Sr/Ba ratio in Nd_0.7Ba_0.3MnO₃. In all compounds, we observe a ferromagnetic ordering of both the Nd and Mn magnetic moments along the b-axis. However, the Nd and Mn magnetic moments point in the same direction for the compounds with y≥0.05 but in opposite directions in Nd_0.7Ba_0.3MnO₃. An important evolution of the absolute value of Nd magnetic moment at 1.5 K is also observed when changing the Sr/Ba ratio

[en] The bismuth doping of the (Sm, Sr)₁Mn0₃ perovskites has been studies starting from the nominal composition Sm_0.54Bi_0.16Sr_0.3Mn0₃ sintered at various temperatures from ranging from 1,100 to 1,400 deg.C. The best results are obtained at 1,350 deg. C, leading to the final composition Sm_0.58Bi_0.04Sr_0.36Mn0₃ due to bismuth oxide volatility. The phase so obtained exhibits colossal magnetoresistance properties with a resistance ratio, more than an order of magnitude larger that that obtained for all other (Sm, Sr)₁MnO₃ perovskites without bismuth: R_o/R_B = 2 x 10⁴ at T_max = 80 K in a magnetic field of 5 T. (authors)

[en] Several different transition metal oxides with structures derived from the perovskite show magnetoresistance (MR) properties. Both Ln_1-xA_xMnO₃ manganites and Sr₂FeMoO₆ double-perovskites exhibit two kinds of MR: intrinsic (intragrain) in the vicinity of their Curie temperature T_C and extrinsic (intergrain) below T_C. The former one, colossal in the case of manganites, requires large magnetic field values (tesla or above) whereas the second one, which originates in the tunneling across a barrier (grain-boundary) of the spin-polarized carriers, is shown even in low magnetic fields. The origin of MR properties of Ln_1-xA_xCoO_3-δ cobaltites differs from that of the Mn and Fe/Mo-based oxides. In particular, no low-field MR is shown

[en] This paper reports on peculiar metamagnetic transitions which take place in antiferromagnetic, charge and orbitally ordered manganites. At very low temperatures, the virgin magnetization curves of some of these compounds exhibit several, sharp steps giving rise to a staircase-like shape. This staircase effect is shown to be sensitive to various experimental parameters. Several potential interpretations of the staircase effect are discussed in relation to this set of results. A martensitic-like scenario, involving a leading role of the structural distortions associated to the collapse of the orbital ordering, is found to be the most plausible interpretation

[en] New perovskites containing low concentration of Mn³⁺ (e_g¹) Jahn--Teller cations in a Mn⁴⁺ (e_g⁰) cation matrix are synthesized. Theses manganites are metallic at room temperature (ρ∼10^-2 Ωcm) for small concentration of dopants (y ≤ 8 %). Remarkably the samples exhibit large negative thermopower values at room temperature (-S > 100 μV· K^-1), which increases with increasing temperature. Therefore, these materials are good candidates for n-leg parts of thermoelectric devices for the conversion of heat to electricity. In order to explain this interesting behavior, a new theoretical framework taking correlation effects and realistic electronic configurations into account, is introduced. It allows for determining the temperature and charge-carrier density dependences of the thermopower. The results are compared to the observed experimental values of CaMn_1-xMxO₃, with M=Ru, Mo, which exhibit two different background states. (author)