General Theory for the Ferroelectric Polarization Induced by Spin-Spiral   Order
  The ferroelectric polarization of triangular-lattice antiferromagnets induced by helical spin-spiral order is not explained by any existing model of magnetic-order-driven ferroelectricity. We resolve this problem by developing a general theory for the ferroelectric polarization induced by spin-spiral order and then by evaluating the coefficients needed to specify the general theory on the basis of density functional calculations. Our theory correctly describes the ferroelectricity of triangular-lattice antiferromagnets driven by helical spin-spiral order, and incorporates known models of magnetic-order-driven ferroelectricity as special cases. 
H. J. Xiang, E. J. Kan, Y. Zhang, M.-H. Whangbo, and X. G. Gong
cond-mat.str-el cond-mat.mtrl-sci