Abstract
The origin of the spiral spin order in perovskite multiferroic manganites RMnO3 R=Tb or Dy is here
investigated using a two eg-orbital double-exchange model. Our main result is that the experimentally observed
spiral phase can be stabilized by introducing a relatively weak next-nearest-neighbor superexchange coupling
10% of the nearest-neighbor superexchange. Moreover, the Jahn-Teller lattice distortion is also shown to be
essential to obtain a realistic spiral period. Supporting our conclusions, the generic phase diagram of undoped
perovskite manganites is obtained using Monte Carlo simulations, showing phase transitions from the A-type
antiferromagnet, to the spiral phase, and finally to the E-type antiferromagnet, with decreasing size of the R
ions. These results are qualitatively explained by the enhanced relative intensity of the superexchanges.
