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 manga nese oxides, Sr1.6R.4Mn2O7 (R = La, Pr, Nd, Gd; R-327), have been inve stigated. The La-, Pr-, and Nd-substituted compounds show metal insula tor transitions below 150 K, whereas isostructural Gd-327 compound is semiconducting and paramagnetic. For La-327, the transition temperatur e corresponds well to the onset of ferromagnetic ordering and the MR r atio is fairly large compared with perovskite-type AMnO(3) oxides. The Pr and Nd analogs show negative MR's, but both compounds display othe r anomalous transitions to semiconductors below 50 K. Neither material shows a transition to a ferromagnetic state below 350 K. The observat ion of colossal magnetoresistance in the absence of bulk ferromagnetis m suggests that the double-exchange interaction alone is not sufficien t to explain the resistivity of R-327 near T-c. [S0163-1829(98)04217-9 ].