  Recently, we predicted based on symmetry considerations that KMnFeF$_{6}$ and
Ba$_{2}$Ni$_{7}$F$_{18}$ are likely magnetoelectric multiferroic materials. In
this contribution, we investigate with Landau theory and crystal structure
considerations the polarization and the linear magnetoelectric effect in these
materials. Based on these two examples, we show that any magnetoferroelectric
will display additional electrical polarization below its magnetic ordering
temperature. This additional electrical polarization is not related to the
linear magnetoelectric effect. Its magnitude depends on the dielectric
susceptibility.
