Microscopic origin of large negative magneto-electric coupling in   Sr$_{1/2}$Ba$_{1/2}$MnO$_{3}$
  With a combined ab initio density functional and model Hamiltonian approach we establish that in the recently discovered multiferroic phase of the manganite Sr$_{1/2}$Ba$_{1/2}$MnO$_{3}$ the polar distortion of Mn and O ions is stabilized via enhanced in-plane Mn-O hybridizations. The magnetic superexchange interaction is very sensitive to the polar bond-bending distortion, and we find that this dependence directly causes a strong magnetoelectric coupling. This novel mechanism for multiferroicity is consistent with the experimentally observed reduced ferroelectric polarization upon the onset of magnetic ordering. 
Gianluca Giovannetti, Sanjeev Kumar, Carmine Ortix, Massimo Capone,   Jeroen van den Brink
cond-mat.mtrl-sci cond-mat.str-el