Spontaneous toroidal moment and field-induced magnetotoroidic effects in Ba2CoGe2O7

The unusual magnetoelectric effects observed in the multiferroic phase arising below T-N = 6.7 K in Ba2CoGe2O7 (BCG) are related to the spontaneous toroidal moment existing in this compound. The transition to the multiferroic state, which involves spontaneous magnetization, polarization, and toroidal moment gives rise to spontaneous toroidic effects. These effects correspond to specific contributions to the induced polarization and magnetization under applied magnetic or electric fields which can be differentiated from standard nonlinear magnetoelectric contributions. The toroidic contribution to the electric polarization in BCG is shown to result from single-ion effects.