Effect of lanthanide ions on the magnetotransport properties in layered Sr1.6R1.4Mn2O7 (R = La, Pr, Nd, Gd)

The magnetoresistances (MR's) and magnetic properties of layered manganese oxides, Sr1.6R.4Mn2O7 (R = La, Pr, Nd, Gd; R-327), have been investigated. The La-, Pr-, and Nd-substituted compounds show metal insulator transitions below 150 K, whereas isostructural Gd-327 compound is semiconducting and paramagnetic. For La-327, the transition temperature corresponds well to the onset of ferromagnetic ordering and the MR ratio is fairly large compared with perovskite-type AMnO(3) oxides. The Pr and Nd analogs show negative MR's, but both compounds display other anomalous transitions to semiconductors below 50 K. Neither material shows a transition to a ferromagnetic state below 350 K. The observation of colossal magnetoresistance in the absence of bulk ferromagnetism suggests that the double-exchange interaction alone is not sufficient to explain the resistivity of R-327 near T-c. [S0163-1829(98)04217-9].