Giant Magnetoresistance Effect of the Nanoscale Layered-Perovskite La_2.5-xK_0.5+xMn₂O₇ (0 < x < 0.5)

Nanoscale layered-perovskite La_2.5-xK_0.5+xMn₂O₇ (0 < x < 0.5) have been prepared by the modified sol-gel method, and their structures, magnetic and electric properties have been studied. The experimental results show that these materials exhibit paramagnetic-to-ferromagnetic phase transitions at 183, 216, 230, 238, and 249 K for x = 0.05, 0.15, 0.25, 0.35, and 0.45, respectively. For all compositions of La_2.5-xK_0.5+xMn₂O₇ investigated here, we have not observed the metal-nonmetal transition phenomena for the whole temperature region (80 - 300 K). The nanoscale samples exhibit highly enhanced magnetoresistance (MR) effects especially at low temperatures (< T_C). The MR behaviour can be explained by taking account of the two-dimensionality with anisotropy of the Mn-O networks and nanoscale grain size effect in the layered manganese perovskites.