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[en] The charge order-like transition observed at 260 K in La_0.4Ca_0.6MnO₃ is suppressed by 20% Cr or Fe substitution at Mn site. Whereas no magnetoresistance (MR) was observed in the parent compound, even in a field of 14 T, a sizeable MR is observed in the substituted samples. However, at 50 K in a field of 5 T, the MR of the Cr-doped sample was larger by a factor of 10 compared with that of Fe-doped one

[en] We have studied the structural and superconducting properties of the superconductor SmSrBaCu₃O_6+z. This compound when annealed in oxygen at 450 deg. C showed a tetragonal structure and a T_c of 79 K. When the same sample was heated in argon followed by oxygen annealing; we observed an orthorhombic structure and an increase of T_c by 6 K. Further, there was an enhancement in the shielding and the intergranular critical current. A combination of several factors such as decrease in the Cu(1)-apical oxygen distance, increase in phase purity, in hole density and chain oxygen ordering for the [AO] sample may qualitatively account for the observed data

[en] Measurements of magnetisation, magnetostriction and specific heat were performed on Nd_0.04Sm_0.16Ca_0.8MnO₃ in order to investigate the influence of isovalent substitution in the rare-earth subsystem on spontaneous and field-induced magnetostructural transitions found earlier in electron-doped manganites Sm_1-xCa_xMnO₃ (x=0.8, 0.85). It is shown that the temperature of the magnetostructural transition from the orthorhombic to the monoclinic phase, as well as the parameters of the magnetic subsystem in the orthorhombic phase, change non-significantly at partial substitution of Nd for Sm. However, this substitution strongly reduces the volume effect at the transition. Therefore, the volume anomaly at this magnetostructural transition appears to be quite sensitive to the ionic radius and/or the f-electronic state of the rare-earth metal

[en] The magnetic properties of electron-doped manganite La_0.3Ca_0.7MnO₃ and La_0.3Ca_0.7Mn_0.8Cr_0.2O₃ polycrystalline samples prepared by sol-gel technique have been investigated between 5 and 300 K in magnetic fields ranging from 0 to 5 T. The transition at 260 K, attributed to charge ordering in La_0.3Ca_0.7MnO₃, is completely suppressed in the Cr-substituted sample while the onset of a magnetic remanence followed by the appearance of a magnetic irreversibility at lower temperatures is observed in both samples. These features indicate that ferromagnetic clusters coexist with either an antiferromagnetic phase for La_0.3Ca_0.7MnO₃ or a spin-cluster glass phase for La_0.3Ca_0.7Mn_0.8Cr_0.2O₃ at the lowest temperatures. The exponential temperature dependence of the resistivity for the Cr-substituted sample is consistent with the small polaron hopping model for 120 K< T<300 K, while the data are better described by Mott's hopping mechanism for T<120 K. Whereas the parent compound La_0.3Ca_0.7MnO₃ is known to show no magnetoresistance, a large negative magnetoresistance is observed in the La_0.3Ca_0.7Mn_0.8Cr_0.2O₃ sample below 120 K. The appearance of the CMR is attributed to spin dependent hopping between spin clusters and/or between ferromagnetic domains

[en] Nernst coefficient measurements of La_0.88MnO₃ (T_c∼265 K) and La_0.7Ca_0.3MnO₃ (T_c∼265 K) are reported as a function of temperature (100-320 K) in moderate applied magnetic fields (0.38< H<1.8 T). For T>T_c, our data can be accounted for by the transport behaviour of small polarons. We find the semiconductor-metal transition is dominated by an increase in the hole density. For T≤T_c, an anomalous suppression of the Nernst coefficient was observed when the field increased from 0.38 to 1.8 T. The colossal magnetoresistance effect is attributed to a field-induced increase in the mobility of the charge carriers

[en] Studies aimed at the nature of magnetic interactions in Fe and Cr substituted lanthanum deficient manganites, La_0.88Mn_1-xFe_xO₃ (x=0-0.15) and La_0.88Mn_1-yCr_yO₃ (y=0-0.175) are reported. The oxides order ferromagnetically for the entire range of x- and y-values. A decrease in the Curie temperature T_c with increasing substitution is observed for both compounds, but the decrease in T_c is large for Fe substitution. A large decrease was also seen in the low-temperature magnetic moment of Fe substituted samples. The data indicate possible ferromagnetic ordering of Cr and antiferromagnetic ordering of Fe moments in the oxides. Ferromagnetic resonance studies indicate (i) strong spin-lattice effects in the paramagnetic state, (ii) valence-exchange effects in the ferromagnetic state, and (iii) possible microscopic inhomogeneities in Cr-substituted samples

[en] Nearly three years have elapsed since Bednorz and Muller opened the gateway of high temperature superconductivity (HTSC) by discovering superconductivity in LaBaCuO. However, a satisfactory model has not evolved yet to quantitatively account for the different properties observed. Among the interesting properties observed in the cuprate superconductors, mention should be made of the normal state transport properties. Whereas it is important to explain the occurrence of high superconducting transition temperatures (T_c), it turns out that a quantitative model of the temperature dependence of the resistivity e and the Hall constant R_H for T > T_c and the influence of carrier density n(p) = 1/eR_H on T_c is yet to be successfully proposed and which might contain eventually the key to the understanding of the underlying mechanism of superconductivity in these materials. In this paper the authors review experimental data on Hall measurements on several CuO based superconductors and briefly comment on some of the theoretical models

[en] Films of thickness ranging from 50 to 400 microns were deposited on alumina and copper sustrates in air by means of a plasma torch starting from prefabricated UBaCuO powder. The samples became superconducting after a heat treatment in oxygen at 500⁰C. The lattice parameters, resistivity and magnetic properties are presented

[en] We have carried out thermoelectric power (S), magnetization and magnetoresistance measurements on LaSr₂Mn_2-yCr_yO₇ compounds (y=0.1, 0.2 and 0.4). In the bilayer manganite LaSr₂Mn₂O₇ (hole doping level x=1/2), it is well known that the coexistence of metallic A-type antiferromagnetic (AF) phase and CE-type charge/orbital order (CO/OO) phase is essential to understand physical properties. The Cr-doping strongly suppressed the AF transition temperature and its associated peak in S down to lower temperatures. The Cr impurities on the Mn site in the bilayer manganite system did not cause such a dramatic influence as the insulator to metal transition in cubic manganites. This finding is discussed on the basis of phase separation model concerning the A-type majority and CE-type minority phases