Dual Nature of Improper Ferroelectricity in a Magnetoelectric Multiferroic...

Using first-principles calculations, we study the microscopic origin of ferroelectricity (FE) induced by
magnetic order in the orthorhombic HoMnO3. We obtain the largest ferroelectric polarization observed in
the whole class of improper magnetic ferroelectrics to date. We find that the two proposed mechanisms for
FE in multiferroics, lattice and electronic based, are simultaneously active in this compound: a large
portion of the ferroelectric polarization arises due to quantum-mechanical effects of electron orbital
polarization, in addition to the conventional polar atomic displacements. An interesting mechanism for
switching the magnetoelectric domains by an electric field via a 180
coherent rotation of Mn spins is also
proposed.