Magnetoelectric coupling in lead-free piezoelectric Lix(K0.5Na0.5)1 − xNb1 − yTayO3 and magnetostrictive CoFe2O4 laminated composites

To replace lead zirconium titanate in magnetoelectric (ME) composites owing to concerns regarding its toxicity, we investigate the ME coupling in bilayer composites comprising lead-free Lix(K0.5Na0.5)1 − xNb1 − yTayO3 (LKNNT) (piezoelectric) and CoFe2O4 (magnetostrictive) phases. We prepare the LKNNT ceramics and measure its piezoelectric coefficient d31, a crucial ingredient determining ME couplings, for several Li (x=0.03,0.035,0.04) and Ta (y=0.15,0.2,0.25) concentrations, and find that the highest d31 occurs at y=0.2 for all the values of x studied here. We then evaluate both the transverse (αE,31) and the longitudinal (αE,33) low-frequency ME coupling coefficients of our composites, for each the above composition of (x,y). At x=0.03, we find the usual scenario of αE,31 and αE,33, i.e., the strongest ME coupling occurs when d31 is maximal, namely at y=0.2. On the other hand, interestingly, we also obtain the strongest ME coupling when the LKNNT layer has a relatively weaker d31, e.g., at y=0.25 for x=0.035 and y=0.15 for x=0.04, following from the interplay of d31 and other ingredients (e.g., dielectric constant). Our calculated ME couplings, with αE,31 in magnitude around twice of αE,33, are comparable to those in lead-based composites. The effect of the volume fraction and interface parameter on the ME coupling is also discussed.