  We have discovered strong magnetoelectric (ME) effects in the single
chiral-helical magnetic state of single-crystalline langasite Ba3NbFe3Si2O14
that is crystallographically chiral. The ferroelectric polarization,
predominantly aligned along the a axis below the Neel temperature of ~27 K,
changes in a highly non-linear fashion in the presence of in-plane magnetic
fields (H) perpendicular to the a axis (H//b*). This ME effect as well as
smaller ME effects in other directions exhibit no poling dependence, suggesting
the presence of a self-formed single ME domain. In addition, these ME effects
accompany no-measurable hysteresis, which is crucial for many technological
applications.
