Symmetry origin of the phase transitions and phase separation in manganites at low doping

We analyze the symmetry changes of paramagnetic to A-type antiferromagnetic and to ferromagnetic phase transitions in undoped and moderately doped LaM nO3, respectively. We show that in orthorhombic-distorted perovskite mangan ites the phase separation at low doping is associated with the noncollinear nature of the magnetic orders permitted by symmetry. A simple model for th e competition between the two phase transitions is put forward within the f ramework of the Landau theory of phase transitions. [S0163-1829(99)03038-6] .