Phase separation and enhanced charge-spin coupling near magnetic transitions

The generic changes of the electronic compressibility in systems which show magnetic instabilities are studied within a mean-field approach. It is shown that, when going into the ordered phase, the inverse compressibility is reduced by an amount comparable to its original value, making charge instabilities also possible. We discuss, within this framework, the tendency towards phase separation of the double-exchange systems, the pyrochlores, and the magnetic transitions of the Hubbard model.