On the Strain-Dependent Multiferroic Phase Diagram of SrMnO₃

Computational [1] and experimental [2] work suggested that epitaxial strain can produce ferroelectricity (FE), and ferromagnetism (FM), in the otherwise paraelectric antiferromagnet SrMnO₃. Strain engineering thus allows for controlling the ferroic phases and their critical temperatures (T_C) in SrMnO₃. Here, computational methods combining density functional theory with Monte Carlo simulations of the Heisenberg Hamiltonian, as well as molecular dynamics with an effective Hamiltonian for FE, are used to investigate the strain-temperature ferroic phase diagram of SrMnO₃. Analysis of the strain dependent exchange interactions provides insight into the sequence of magnetic ordering. While the FM state is energetically unfavoured in the non-polar structure at all strains, a strong dependence of the out-of-plane exchange interaction on the Mn-O-Mn bond angle causes FM to be favoured by FE at high (~4-5%) strain.
While the FE T_C increases with strain after the FE onset (around 3% strain) the magnetic T_C is more insensitive to strain. This enables a situation with coincident FE and magnetic T_C’s and potentially enhanced magnetoelectric coupling. This might be of interest, e.g. in exploring multicaloric effects. [1] Phys. Rev. Lett. 104, 207204 (2010). [2] Nat. Nanotech. 10, 661 (2015).