Double-exchange model study of multiferroic $R$MnO${}_3$ perovskites

In this proceeding, recent theoretical investigations by the authors on the multiferroic $R$MnO${}_3$ perovskites are briefly reviewed at first. Using the double-exchange model, the realistic spiral spin order in undoped manganites such as TbMnO${}_3$ and DyMnO${}_3$ is well reproduced by incorporating a weak next-nearest neighbor superexchange ($\sim 10$ of nearest neighbor superexchange) and moderate Jahn-Teller distortion. The phase transitions from the A-type antiferromagnet (as in LaMnO${}_3$), to the spiral phase (as in TbMnO${}_3$), and finally to the E-type antiferromagnet (as in HoMnO${}_3$), 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 LaMnO${}_3$ and TbMnO${}_3$ thin films is explained by considering the substrate stress. Finally, the relationship between our double-exchange model and a previously used $J_1$-$J_2$-$J_3$ model is further discussed from the perspective of spin wave excitations.