One presented some lattice models, while the theoretic derivation has not been found, and the importance of correlation effects has to be emphasized. On the basis of the non-relativistic Hamiltonian from the Dirac equation, we derive in detail a tight binding model. We find that both ferromagnetism and ferroelectricity are from the correlation effect between electrons, and magnetic and electric orders are strongly coupled due to the spin-orbit interaction.
@article{arxiv.0908.1232,
title = {Multiferroics seen from theoretic derivation of a tight binding model},
author = {Tian De Cao},
journal= {arXiv preprint arXiv:0908.1232},
year = {2009}
}