The magnetoresistances (MR’s) and magnetic properties of layered manganese oxides, ${\mathrm{Sr}}_{1.6}{R}_{1.4}{\mathrm{Mn}}_2{\mathrm{O}}_7$ ($R=\mathrm{L}\mathrm{a},\mathrm{P}\mathrm{r},\mathrm{N}\mathrm{d},\mathrm{G}\mathrm{d}$; $R$-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 $A{\mathrm{MnO}}_3$ 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 $T_c.$

• Received 4 September 1997

DOI:https://doi.org/10.1103/PhysRevB.57.10740