Thermal Treatment Effects on PMN-0.4PT/Fe Multiferroic Heterostructures

Multiferroic heterostructures have gained in recent years a renewed role in spintronic applications due to their possibility in controlling the magnetic properties via interfacial coupling by exploiting the ferroelectric response to various external stimuli. Whereas the main mechanisms ruling the converse magnetoelectric coupling are considered as established, the optimization of the ferroelectric properties, from both structural and ferroelectric domain points of view, is still under investigated. In particular Pb(Mg1/3Nb2/3)O3-xPbTiO3 (PMN-xPT) single crystals, with their combination of relaxor ferroelectric and photovoltaic properties, together with their complex phase diagram, still leave open questions on their interconnection and optimization. For instance, crystalline quality and ferroelectric domain thermal stability are almost neglected once coupled in heterostructures. Here we show how, by annealing PMN-0.4PT/Fe heterostructures in inert atmosphere over ferroelectric Curie temperature, the domain population significantly modifies once cooled back to room temperature, passing from a highly disordered, mostly out-of-plane domain population to a more defined crystallinity with majoritarian in-plane domains. If further annealed, the domain population returns to be mostly out-of-plane, sign that intermediate annealing steps can freeze PMN-0.4PT domain population in a sort of metastable configuration. This structural information was obtained by combining micro-Raman and x-ray diffraction measurements. In correspondence to these three states, the magnetic properties of interfacial Fe thin film are affected by the ferroelectric modifications via interfacial strain, passing from an isotropic to an anisotropic behavior, then back to an isotropic one. These results stimulate for further investigations, on both micro and macroscopic scales, on the domain population and thermal stability in ferroelectric crystals, on how structural optimization affects the global and local ferroelectric polarization, and finally on their interfacial coupling with magnetic layers.