The generic magnetic phase diagram of multiferroic RMn2O5 (with R=Y, Ho, Tb, Er, Tm), which allows different sequences of ordered magnetic structures for different R's and different control parameters, is described using order parameters which explicitly incorporate the magnetic symmetry. A phenomenological magneto-electric coupling is used to explain why some of these magnetic phases are also ferroelectric. Several new experiments, which can test this theory, are proposed.
@article{arxiv.0802.0604,
title = {Order Parameters and Phase Diagram of Multiferroic RMn$_2$O$_5$},
author = {A. B. Harris and Amnon Aharony and Ora Entin-Wohlman},
journal= {arXiv preprint arXiv:0802.0604},
year = {2009}
}