Origin of large magnetocapacitance in K0.5Na0.5NbO3/La0.67Sr0.33MnO3 superlattices

Abstract

We have investigated magnetocapacitance (MC) performance in superlattices (SLs) of pure ferroelectric and ferromagnetic sublayers for multiferroic applications. ${\mathrm{K}}_{0.5}{\mathrm{Na}}_{0.5}\mathrm{Nb}{\mathrm{O}}_{3}$ (KNN)/${\mathrm{La}}_{0.67}{\mathrm{Sr}}_{0.33}\mathrm{Mn}{\mathrm{O}}_{3}$ (LSMO) SLs with LSMO sublayer thicknesses of 5, 7, 9, and 11 unit cell (u.c.) keeping KNN thickness fixed at 6 u.c. were deposited on (001)-oriented $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_{3}$ substrates alternately for 15 times using pulsed laser deposition (PLD) technique. The crystallinity and layer by layer growth of both constituents were confirmed from X-ray diffraction as well as transmission electron microscopy (TEM). SLs with LSMO layers of 5 and 7 u.c. experience high amount of strain while the in-plane lattice parameter almost matches with the substrate for 9 and 11 u.c. The spin-glass behavior in the SLs below 50 K and the maximum Curie temperature (${T}_{C}$) of $\ensuremath{\sim}206$ K in the thicker film imply the presence of defects as evident by TEM. The defects induced large magnetoresistance (MR) of $\ensuremath{\sim}\ensuremath{-}98%$ was realized at 100 K in the SLs with 9 and 11 u.c. However, the insulating nature in the SLs with 7 and 9 u.c. and insulator to metal transition for 11 u.c. were observed in the transport study. The capacitance as well as magnetocapacitance showed high value at low frequency and reduced monotonically upon increase in the frequency which were well explained by the MR coupled Maxwell-Wagner relaxation model. Finally, the large MC of $\ensuremath{\sim}766$% at standard frequency of 1 kHz with magnetic field of 9 T at 100 K in the SL with optimum LSMO sublayer thickness of 9 u.c. opens a path to design new artificial multiferroics with significant magnetoelectric coupling.