Study of magnetization and magnetoelectricity in CoFe2O4/BiFeO3 core-shell composites

CoFe₂O₄ (core)/BiFeO₃ (shell) nanoparticles are prepared by varying the relative molar concentration of core and shell materials (40%CoFe₂O₄-60%BiFeO₃, 50%CoFe₂O₄-50%BiFeO₃, and 60%CoFe₂O₄-40%BiFeO₃). The core-shell nature is confirmed from transmission electron microscopy on these samples. A plot of ΔM (=M_FC-M_ZFC) vs temperature suggests the presence of two types of spin dynamics: (a) particle size dependent spin blocking and (b) spin-disorder. These two spin dynamic processes are found to contribute independently to the generation of magnetoelectric voltage. Very clear first order and second order magnetoelectric voltages are recorded. The resemblance of the first order magnetoelectric coefficient vs temperature plot to that of building up of order parameters in the mean field theory suggests that spin disorder can act like one of the essential ingredients in building the magnetoelectric coupling. The best result is obtained for the 50-50 composition sample, which may be due to better coupling of magnetostrictive CoFe₂O₄, and piezoelectric BiFeO₃, because of the optimum thickness of shell and core.