Enhancement of Dielectric Properties in Epitaxial Bismuth Ferrite-Bismuth Samarium Ferrite Superlattices

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 (epsilon(33) approximate to 170 at 1 MHz), reduced loss tangent, and increased tunability (tau approximate to 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 (epsilon(33) approximate to 150), increases in dielectric loss tangent and decreased tunability (tau approximate to 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.