Effect of grain size on exchange-spring mechanism of [BaFe12O19 (BaM)-Na0.5Bi0.5TiO3 (NBT)] magnetoelectric composite system

•Synthesis of four ME composites with variation of grain size.•Ferromagnetic (FM) nature study.•Exchange-spring mechanism (ESM) observed in ME composites.•For exchange-spring mechanism (ESM) both phases need not be nano grain size. Four magnetoelectric (ME) composite systems: having composition [80 wt% BaFe12O19 (BaM) – 20 wt% Na0.5Bi0.5TiO3 (NBT)] considering the variation of grain size of both the phases are synthesized by solid state method. X-ray diffraction, FESEM and TEM studies are employed to confirm the pure phase formation, grains size and connectivity of both the phases in composite systems. The exchange-spring mechanism (ESM) has been examined with the help of MH loops and magneto transport studies. From analysis it has been found that spin polarized electron tunnelling at ferromagnetic (FM)-insulator (I) interfaces has significant effect on ESM.