Giant improper ferroelectricity in the ferroaxial magnet CaMn7O12
  In rhombohedral CaMn7O12, an improper ferroelectric polarization of magnitude 2870 {\mu}C m-2 is induced by an incommensurate helical magnetic structure that evolves below TN1 = 90 K. The electric polarization was found to be constrained to the high symmetry three-fold rotation axis of the crystal structure, perpendicular to the in-plane rotation of the magnetic moments. The multiferroicity is explained by the ferroaxial coupling mechanism, which in CaMn7O12 gives rise to the largest magnetically induced, electric polarization measured to date. 
R. D. Johnson, L. C. Chapon, D. D. Khalyavin, P. Manuel, P. G.   Radaelli and C. Martin
cond-mat.str-el