Effects of strain on ferroelectric polarization and magnetism in orthorhombic HoMnO3

We explore how the ferroelectric polarization of antiferromagnetic E-type orthorhombic HoMnO3 can be increased, by investigating the effects of in-plane strain on both the magnetic properties and the ferroelectric polarization, using combined density functional theory calculations and a model Hamiltonian technique. Our results show that the net polarization is strongly enhanced under compressive strain, due to an increase of the electronic contribution to the polarization. In contrast, the ionic contribution is found to decrease. We identify the electron-lattice coupling, due to Jahn-Teller (JT) distortions, and its response to strain, to be responsible for the observed behavior. The JT-induced orbital ordering of occupied Mn-e(g)(1) electrons in alternating 3x(2)-r(2)/3y(2)-r(2) orbital states in the unstrained structure, changes under in-plane compressive strain to a mixture with x(2)-z(2)/y(2)-Z(2) states. The asymmetric hopping of e(g) electrons between Mn ions along zigzag spin chains (typical of the AFM-E spin configuration) is therefore enhanced under strain, explaining the large value of the polarization. Using a degenerate double-exchange model including electron-phonon interaction, we reproduce the change in the orbital ordering pattern. In this picture, the orbital ordering change is related to a change of the Berry phase of the e(g) electrons. This causes an increase of the electronic contribution to the polarization. DOI: 10.1103/PhysRevB.87.014403