Influence of lattice instabilities on magnetic order in uniaxially strained EuTiO3 from first principles

Magnetoelectric coupling remains a promising avenue to future nonvolatile memory technologies and low power magnetic sensors. Bulk materials with strong spin-lattice coupling are a particularly appealing medium for harnessing this effect due to linear coupling between polar and magnetic order parameters. Several challenges, including low Curie temperatures, however, prevent the adoption of these materials and mechanisms of the magnetoelectric effect are often hard to elucidate due to the presence of multiple lattice instabilities and competing exchange interactions. In this study, we present the phase diagram of uniaxially strained EuTiO3 and identify the influence that key lattice distortions have on exchange interactions up to the third-nearest neighbor. Our method relies on the use of first-principles density functional theory calculations and machine learning algorithms to predict the free energy and exchange interactions as a function of lattice distortion magnitudes. This is made possible using Landau theory to construct free energy models for EuTiO3 with different magnetic orderings and provides a generalizable approach for comparing magnetic ordering energetics for common structural distortions in perovskite oxides.