Effect of particle size on magnetodielectric and magnetoelectric coupling effect of CoFe2O4@BaTiO3 composite fluids

CoFe2O4 particles with two different sizes were prepared using hydrothermal method by controlling the technological parameters. On that basis, core–shell structure CoFe2O4@BaTiO3 (CFO@BTO) particles were prepared by sol–gel process. The surfactant treated composite particles were then distributed into a highly-insulating heptane to form CFO@BTO composite fluids. Influences of CFO particle size on the magnetodielectric and magnetoelectric coupling effect were comparatively investigated. XRD, TEM and SAED results indicate that the size of CFO particles are ~ 12 and ~ 18 nm, respectively, while the prepared CFO@BTO particles show bi-phase and core–shell composite structure, and the size of CFO@BTO particles are ~ 28 and ~ 35 nm, respectively. The composite fluids show quasi super paramagnetic behavior at room temperature due to the size effect and Brovnian motion. When the size of CFO is 18 nm, the CFO@BTO composite fluids shows larger remnant magnetization and coercive field. Strong dependence of dielectric properties on external magnetic field has been observed, of which the composite fluids show negative magnetodielectric effect, and the value of permittivity was decreased by 8% at 500 Oe. When size of CFO is 12 nm, the corresponding specimen with shows more obvious magnetic response, and it has larger remnant polarization. The values of both coercive electric field and remnant polarization of the two kinds of CFO@BTO composite fluids have been enhanced distinctly under the action of external magnetic field. When the particle size of CFO is smaller, the magnetolectric coupling coefficient αE of CFO@BTO composite fluid is larger, and this value decreases with increasing external magnetic field. When the particle sizes of CFO are 12 and 18 nm, the maximal values of αE are about 7 and 10 V/cm·Oe, respectively. These values are several orders of magnitude larger than that of CFO/BTO and other magnetoelectric composite ceramics. The strong coupling effect can be explained based on the formation, movement and variation of chain-like structure in the presence of external field. These results may supply useful information for enhancing coupling effect and might play important roles in practical applications in novel magnetoelectric devices.