Effect of periodicity on order parameters of multiferroic superlattices*

Superlattice (SL) structures with alternating perovskite oxide layers have attracted enormous attention due to involved fascinating physics and technology. The half-metallic oxide La$_{0.67}$Sr$_{0.33}$MnO$_{3}$(LSMO) and multiferroic Pb(Zr$_{0.53}$Ti$_{0.47})_{0.60}$(Fe$_{0.5}$Ta$_{0.5})_{0.40}$O$_{3}$(PZTFT) materials have been chosen to fabricate SLs by pulsed laser deposition technique on cubic LSAT substrates with LSMO or LaNiO$_{3}$ as bottom electrodes. X-ray diffraction studies revealed superlattice structure with satellite peaks modulated around main peaks. Atomic force microscopy studies disclosed a systematic decrease in grain size with decrease of modulation periodicity ( $\Lambda )$ in SLs. Piezo force microscopy studies of SL films confirmed ferroelectricity at a nanoscale level. XPS studies of SLs with $\Lambda =$5 nm confirmed the existence of all elements in the films. A relatively small reduction in saturation magnetization from 28 to 20 emu/cm$^{3\, }$at H$=$5 kOe, remanant polarization from 21 to 10 $\mu $C/cm$^{2}$ and increase in dielectric constant from 530 to 743 were observed with decrease of $\Lambda $.The observed features will be explained in context of finite size, interfaces, stress, lattice distortion, and grain sizes effects.