Enhancement of Dielectric Properties in Epitaxial Bismuth FerriteBismuth Samarium Ferrite Superlattices

DOI: 10.1002/aelm.201600170 character.[1–4] The imposed 2D constraint between the interfaces has been exploited in perovskites to induce ferroelectricity,[5–7] enhance electromechanical and dielectric performance,[8–11] and even influence ferroic phase transitions and domain structures.[12–17] In the same vein, we recently demonstrated successful interface control of a morphotropic phase boundary in 200 nm thick (001) oriented BiFeO3/(Bi1–xSmx) FeO3 (BFO/BSFO) SLs.[18] It was found that the field induced ferroelectric (rhombohedral (R)) → paraelectric (orthorhombic (O)) phase transition driven by Sm3+ substitution into the Bi3+ site could be controlled by tuning the SL period, although the average composition for all SLs were kept constant. Short period SLs (5 and 10 nm thick individual layers) acted as a single coupled system with strong interlayer polar coupling. Consequently, an incommensurately modulated phase (a nanoscale mixture bridging the Bi-rich rhombohedral and Sm-rich orthorhombic phases) superstructure was found to be stabilized over an increased Sm3+ concentration range, as high as 17% Sm3+ for the short period SLs. Note that in comparison, in single layer BSFO, 17% Sm3+ is already a nonpolar paraelectric. These short period SL films demonstrate saturated ferroelectric hysteresis loops and butterfly-shaped C33–V curves even for the highest Sm3+ concentrations in the SLs. This interface coupling is found to weaken with increasing thickness of the multilayer components, in agreement with several other Artificially layered bismuth ferrite (BiFeO3)/bismuth samarium ferrite (Bi1–xSmx)FeO3 superlattices (SLs) are investigated for their dielectric properties. In short-period (5–10 nm) SLs, the stabilization of an incommensurately modulated nanoscale mixture due to a strong interlayer coupling mechanism results in a large dielectric permittivity (ε33 ≈ 170 at 1 MHz), reduced loss tangent, and increased tunability (τ ≈ 37%) for a samarium concentration range much larger than that for single-layer (Bi1–xSmx)FeO3 thin-films. The enhanced dielectric tunability is observed across a large frequency and temperature range. Increasing the thickness of the SL layers reduces the strength of the interlayer coupling, which results in reductions in dielectric permittivity (ε33 ≈ 150), increases in dielectric loss tangent and decreased tunability (τ ≈ 14%). A phenomenological model confirms that the enhanced dielectric properties, tunability and stabilization of the polar phase to higher Sm3+ concentrations over a wide range of temperatures and frequencies in the short period SLs is due to electrostatic coupling. Thus, the epitaxial short-period SLs have significant potential as a highly tunable lead (Pb)-free materials system in low-to-medium frequency applications. Electrostatic coupling effect between polar/non-polar layers in SL structures could thus be a universal method to achieve enhanced dielectric properties.