Improper Magnetic Ferroelectricity Of Nearly Pure Electronic Nature In Helicoidal Spiral Camn7o12

Helicoidal magnetic order breaks inversion symmetry in quadruple perovskite CaMn7O12, generating one of the largest spin-induced ferroelectric polarizations measured to date. Here, the microscopic origin of the polarization, including exchange interactions, coupling to the spin helicity, and charge density redistribution, is explored via first-principles calculations. The B-site Mn4+ (Mn3) spin adopts a noncentrosymmetric configuration, stabilized not only by spin-orbit coupling (SOC), but also by the fully anisotropic Hubbard J parameter in the absence of SOC, to break inversion symmetry and generate polarization. Berry phase computed polarization (P-elec = 2169 mu C/m(2)) exhibits nearly pure electronic behavior, with negligible Mn displacements (approximate to 0.7 m angstrom). Orbital-resolved density of states shows that p-d orbital mixing is microscopically driven by nonrelativistic exchange striction within the commensurate ionic structure. Persistent electronic polarization induced by helical spin order in the nearly inversion-symmetric ionic crystal lattice suggests opportunities for ultrafast magnetoelectric response.