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.
We have studied the charge disproportionation phenomenon in CaFeO3 using the local-spin density approximation with the on-site Coulomb interaction parameter U and exchange parameter J. The calculation reveals that the total number of the 3d electrons is about 5.1 for both Fe(1)(Fe5+) and Fe(2)(Fe3+) atoms, and that there are about 0.25 electron holes in the O-2p band. Therefore, the charge disproportionation can be more accurately described as 2d5L(Fe4+)=d5L2(Fe5+)+d5(Fe3+), where L denotes a hole in the oxygen 2p band, instead of 2d4(Fe4+)=d3(Fe5+)+d5(Fe3+). The hybridization between the Fe-3d and O-2p orbitals is stronger for Fe(1) than for Fe(2) due to the shorter Fe(1)–O bond. The hyperfine magnetic field contributed from conduction electron polarization is larger for Fe(2), resulting from a stronger s-d hybridization between the s orbital of Fe(2) and the d orbitals of its neighboring Fe(1) atoms. The on-site Coulomb repulsion and the exchange interaction increase the splitting between the occupied spin up and unoccupied spin down bands of Fe atoms. Fe-3d electrons become localized and the occupied d-band shifts to a lower energy range, even below the O-2p level. The calculated magnetic moments, hyperfine fields, and electron charge density agree well with the experimental data.
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