Symmetry analysis of the magnetic structures of bilayered manganites La2-2xSr1-2xMn2O7 near x=0.3: Phase separations and percolation

We analyze the symmetry of the magnetic structures of tetragonal bilayered manganites La2-2xSr1+2xMn2O7 with doping near x=0.3 and formulate a corresp onding 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. Accor dingly, a schematic magnetic phase diagram near x=0.3 is constructed which may consistently account for the controversial experimental observations. P ossible phase separations and a percolation mechanism of the colossal magne toresistance no discussed.