Magnetic Phase Transitions and Electric Polarization in RMn2O5 Oxides

Resumo

Possible phase transitions from paramagnetic to the magnetic phases in the RMn₂O₅ (R = Bi, Pr, La) compounds have been studied in the framework of a phenomenological approach based on a symmetry analysis. The basis functions of the irreducible representations of the space group for these compounds are determined, which are expressed in terms of the magnetic vector components. It was found from renormalization group analysis of two magnetic transitions that polarization appears in BiMn₂O₅ oxide when at least one of these transitions becomes a first-order transition as a result of spin fluctuations. It was shown that the commensurate antiferromagnetic phase resulting from magnetic phase transition of the second order in PrMn₂O₅ has no electric polarization because this polarization is forbidden by the symmetry of the system. The conditions under which the parameters of the thermodynamic potential must satisfy the realization of a second-order transition in LaMn₂O₅ to have been found. The line of the first-order phase transitions in this system and the critical point of the second-order phase transition line have been determined. It was shown that a dynamic electric polarization can form in the antiferromagnetic phase for this compound.