Ti-substituted perovskites, La 0.7 Sr 0.3 Mn 1−x Ti x O 3 , with 0≤ x ≤ 0.20, were investigated by neutron diffraction, magnetization, electric resistivity, and magnetoresistance(MR) measurements. All samples show a rhombohedral structure (space group R3c) from 10 K to room temperature. At room temperature, the cell parameters a, c and the unit cell volume increase with increasing Ti content. However, at 10 K, the cell parameter a has a maximum value for x = 0.10, and decreases for x > 0.10, while the unit cell volume remains nearly constant for x > 0.10. The average (Mn,Ti)-O bond length increases up to x=0.15, and the (Mn,Ti)-O-(Mn,Ti) bond angle decreases with increasing Ti content to its minimum value at x=0.15 at room temperature. Below the Curie temperature T C , the resistance exhibits metallic behavior for the x ≤ 0.05 samples. A metal(semiconductor) to insulator transition is observed for the x ≥ 0.10 samples. A peak in resistivity appears below T C for all samples, and shifts to a lower temperature as x increases. The substitution of Mn by Ti decreases the 2p − 3d hybridization between O and Mn ions, reduces the bandwidth W , and increases the electron-phonon coupling. Therefore, the T C shifts to a lower temperature and the resistivity increases with increasing Ti content. A field-induced shift of the resistivity maximum occurs at x ≤ 0.10. The maximum MR effect is about 70% for La 0.7 Sr 0.3 Mn 0.8 Ti 0.2 O 3 . The separation of T C and the resistivity maximum temperature T ρ,max enhances the MR effect in these compounds due to the weak coupling between the magnetic ordering and the resistivity as compared with La 0.7 Sr 0.3 MnO 3 .

The effects of Cu-doping on the structural, magnetic, and transport properties of La 0.7 Sr 0.3 Mn 1−x Cu x O 3 (0 ≤ x ≤ 0.20) have been studied using neutron diffraction, magnetization and magnetoresistance(MR) measurements. All samples show the rhombohedral structure with the R3c space-group from 10K to room temperature(RT). Neutron diffraction data suggest that some of the Cu ions have a Cu 3+ state in these compounds. The substitution of Mn by Cu affects the Mn-O bond length and Mn-O-Mn bond angle resulting from the minimization of the distortion of the MnO 6 octahedron. Resistivity measurements show that a metal to insulator transition occurs for the x ≥ 0.15 samples. The x = 0.15 sample shows the highest MR(≈80%), which might result from the co-existence of Cu 3+ /Cu 2+ and the dilution effect of Cu-doping on the double exchange interaction.

No abstract

The RuSr 2 Gd 1.5 Ce 0.5 Cu 2 O 10-δ (Ru-1222) compounds, with varying oxygen content, crystallize in a tetragonal crystal structure (space group I4/mmm).Resistance (R) versus temperature (T) measurements show that the air -annealed samples exhibit superconductivity with superconducting transition temperature (T c ) onset at around 32 K and R=0 at 3.5 K. On the other hand , the N 2 -annealed sample is semiconducting down to 2 K. Magneto-transport measurements on airannealed sample in applied magnetic fields of 3 and 6 Tesla show a decrease in both T c onset and T R=0 . Magnetoresistance of up to 20% is observed in N 2annealed sample at 2 K and 3 T applied field. The DC magnetization data (M vs. T ) reveal magnetic transitions (T mag. ) at 100 K and 106 K, respectively, for both air-and N 2 -annealed samples. Ferromagnetic components in the magnetization 2 are observed for both samples at 5K and 20 K. The superconducting transition temperature (T c ) seems to compete with the magnetic transition temperature (T mag. ). Our results suggest that the magnetic ordering temperature (T mag. ) of Ru moments in RuO 6 octahedra may have direct influence/connection with the appearance of superconductivity in Cu-O 2 planes of Ru-1222 compounds.

No abstract