Magnetoresistance in La-Sr-Cr-Mn-O ceramic compounds

Manganates with nominal compositions of La0.7Sr0..3CrzMnl-zO3-δ with z = 0.05, 0.10, 0.15, 0.20, 0.25 and 0.30 were prepared by solid state reactions in air. The effects of Cr substitution for La, Sr and Mn on magnetic and electrical properties were investigated. In all cases, the electrical resistivity in zero field increases, with increasing Cr. The pure lanthanum manganese oxides, LaMnO3, is an antiferromagnetic insulator with perovskite crystal structure. Substituting divalent Sr2+ ion into the La3+ ions sites changes some of the Mn3+ ions to Mn 4+ ions. Electrical conduction is then possible through electron hopping between the two manganese ions. When La3+ ions are replaced with smaller ions, the overlap between the orbitals and then the width of the energy band decrease. Materials with narrow energy bands also show weaker ferromagnetic coupling, leading to lower Curie temperature. The resistivity of the materials is metal-like at TTp. The resistivity of all samples decreases while Tp increases on applying a magnetic field in the range of 0 - 1 T. Magnetoresistances (MR) at room temperature (TR= 302 K) are highest for z 0.05, which is 24.6% in a magnetic field of 1 T. This is due to MR is highest at Tp, which is about equal to room temperature for those compositions. The content of Mn3+ and Mn4+ ions in the compound of La0.7Sr0..3CrzMnl-zO3-δ were evaluated by X-ray photoelectron spectroscopy (XPS). It was found that Mn4+ ion content decreases with increasing Cr. Cr substitution is beneficial in preventing the oxidation on Mn3+ ions to Mn4+ ions in the sample at high sintering temperature. The ratio of the amount of Mn4+ ions to Mn3+ ions for z 0.12 was found to be the same as the theoretical value of 30% in the pure La0.7Sr0..3CrzMnl-zO3-δ. For z > 0.15, Mn4+ ion content was 100% and magnetoresistive effect was not shown within the temperature of 150-673 K. The occurrence of MR is directly linked to the presence of Mn4+ ions with respect to Mn3+ ions in the materials. The Mn4+ ion content of 62%, for z = 0.05, which corresponds to the highest MR, is concluded as its optimum value in the compound. (author)