PRESSURE-INDUCED CHANGES IN TRANSPORT-PROPERTIES OF LAYERED LA1.2CA1.8MN2O7

The transport properties of a single crystal of La1.2Ca1.8Mn2O7 with t he layered Sr3Ti2O7-type perovskite structure have been studied under hydrostatic pressures of up to similar to 9 kbar. It is found that La1 .2Ca1.8Mn2O7 undergoes a first-order phase transition from a paramagne tic insulating to a low-temperature ferromagnetic metallic state which is accompanied by a large decrease in the electrical resistivity. At ambient pressure the magnetic phase transition takes place at T-c=242 K and as the external pressure is applied the critical temperature inc reases Linearly at a rate of 1.1 K/kbar. The present results reflect t he competition between double and superexchange resulting from two-dim ensional Mn-O-Mn networks and can be interpreted in terms of the excha nge striction model.