Multiferroic behavior in EuTiO3 films constrained by symmetry

We have elucidated the spin, lattice, charge, and orbital coupling mechanism underlying the multiferroic character in tensile-strained EuTiO3 films. Symmetry determined by oxygen octahedral tilting shapes the hybridization between the Eu 4f and the Ti 3d orbitals, and this inhibits predicted Ti displacement proper ferroelectricity. Instead, phonon softening emerges at low temperatures within the pseudocube (110) plane, orthogonal to the anticipated ferroelectric polarization symmetry. Additionally, the magnetic anisotropy is determined by orbital distortion through hybridization between the Ti 3d and the typically isotropic Eu2+ 4f states. This unique scenario demonstrates the critical role symmetry plays in the coupling of order parameters defining multiferroic behavior.