Abstract
In this proceeding, recent theoretical investigations by the authors on the multiferroic RMnO3
perovskites are briefly reviewed at first. Using the double-exchange model, the realistic spiral spin order
in undoped manganites such as TbMnO3 and DyMnO3 is well reproduced by incorporating a weak nextnearest
neighbor superexchange (∼10% of nearest neighbor superexchange) and moderate Jahn-Teller
distortion. The phase transitions from the A-type antiferromagnet (as in LaMnO3), to the spiral phase
(as in TbMnO3), and finally to the E-type antiferromagnet (as in HoMnO3), with decreasing size of the
R ions, were also explained. Moreover, new results of phase diagram of the three-dimensional lattice are
also included. The ferromagnetic tendency recently discovered in the LaMnO3 and TbMnO3 thin films is
explained by considering the substrate stress. Finally, the relationship between our double-exchange model
and a previously used J1-J2-J3 model is further discussed from the perspective of spin wave excitations.
