Exploring the intrinsic dielectric behavior of SrTiO ₃ -based heterostructures: the impact of layer architecture on soft mode dynamics

The design of heterostructures offers opportunities for developing new materials with emergent functionalities and tunable physical properties through layer engineering. However, the underlying mechanisms of interlayer interactions that influence the intrinsic dielectric response in promising multiferroic heterostructures remain largely unexplored. In this study, we investigate the well-defined ferroelectric soft mode in SrTiO₃ (STO), which serves as a benchmark for comparing the intrinsic dielectric properties of STO layers embedded in the potential multiferroic heterostructure of 5 × (50 + 50) nm STO/AlFeO₃ (AFO) with single-layer STO films of similar individual (50 nm) and total (250 nm) thickness. Our findings reveal that increasing the thickness of the STO thin films from 50 nm to 250 nm leads to significant changes in the film microstructure and the degradation of dielectric properties. Notably, the average dielectric response and structural characteristics of the 5 × 50 nm STO layers in the heterostructure remain comparable to those of the single-layer 50 nm STO film, suggesting that the incorporation of intermediate AFO layers effectively mitigates the deterioration of the intrinsic dielectric characteristics of the STO layers. The pronounced temperature-dependent variations in soft mode frequency allow us to propose two potential mechanisms for this effect, which are based on the thermal strain build-up and the formation of defect-induced polar nanoregions. Our study demonstrates that intermediate AFO layers contribute not only magnetic properties to potential multiferroic heterostructures but also enhance the dielectric response, thereby expanding the possibilities of layer engineering.