Symmetry analysis of the magnetic structures of bilayered manganites La 2-2xSr 1+2xMn 2O 7 near x = 0.3: Phase separations and percolation

Abstract

We analyze the symmetry of the magnetic structures of tetragonal bilayered manganites La 2-2xSr 1+2xMn 2O 7 with doping near x=0.3 and formulate a corresponding Landau theory of the phase transitions involved. It is shown that a phase with a single magnetic order cannot be canting though with a mixture of different magnetic orders can, as is possibly the case near x=0.4. Accordingly, a schematic magnetic phase diagram near x=0.3 is constructed which may consistently account for the controversial experimental observations. Possible phase separations and a percolation mechanism of the colossal magnetoresistance are discussed.