Crafting the strain state in epitaxial thin films: A case study of CoFe2 O4 films on Pb(Mg,Nb) O3-PbTiO3

The strain dependence of electric and magnetic properties has been widely investigated, both from a fundamental science perspective and an applications point of view. Electromechanical coupling through field-induced polarization rotation (PRO) and polarization reorientation (PRE) in piezoelectric single crystals can provide an effective strain in film/substrate epitaxial heterostructures. However, the specific pathway of PRO and PRE is a complex thermodynamic process, depending on chemical composition, temperature, electric field, and mechanical load. Here, systematic studies of the temperature-dependent field-induced phase transitions in Pb(Mg,Nb)O3-PbTiO3 single crystals with different initial phase and orientation configurations have been performed. Different types of strains, volatile/nonvolatile and biaxial/uniaxial, have been measured by both macroscopic and in situ x-ray diffraction techniques. In addition, the strain state of epitaxial Mn-doped CoFe2O4 thin films was examined by magnetic anisotropy measurements, where a giant magnetoelectric coupling has been demonstrated.