Enhanced local magnetoelectric effect in core-shell structured CoFe₂O₄–BaTiO₃ nanocomposites

Magnetoelectric nanocomposites capable of converting remotely applied magnetic fields into local electrical responses are promising materials for biomedical and bioelectronic applications. However, quantitative probing of magnetoelectric-related electrical output at the level of nanoparticles or nanoparticle agglomerates remains challenging. In this work, we investigate core–shell-like CoFe₂O₄@BaTiO₃ nanocomposites consisting of magnetostrictive CoFe₂O₄ cores embedded in or surrounded by a piezoelectric BaTiO₃ shell/matrix. The central idea of this study is to use Kelvin probe force microscopy as a complementary local electrical readout method for detecting magnetic-field-induced surface-potential modulation associated with local magnetoelectric coupling. TEM analysis showed that the primary CoFe₂O₄ cores had an average diameter of 58 ± 13 nm, while the average distance between neighboring CFO cores within agglomerated nanocomposite regions was approximately 130 ± 70 nm. XRD confirmed the presence of the main CoFe₂O₄ and BaTiO₃ phases, with weak additional reflections attributed to minor secondary phases. VSM measurements revealed soft ferrimagnetic behavior with weak hysteresis at room temperature, with Ms = 6.06 emu/g, Mr = 0.4 emu/g, and Hc = 38.2 Oe. Magnetic-field-assisted PFM showed a change in phase contrast under a 100 Oe DC magnetic field, providing qualitative evidence of field-induced modification of the local electromechanical response. KPFM measurements under an AC magnetic field of 40 Oe at 100 Hz revealed relative surface-potential increases from 70.5 to 87.7 mV in one analyzed region and from 73.2 to 81.2 mV in another region. These changes correspond to absolute modulations of 17.2 and 8.0 mV, or relative increases of 24.3% and 10.9%, respectively. The results demonstrate that KPFM can be used as a complementary approach for probing relative magnetic-field-induced surface-potential changes in CoFe₂O₄@BaTiO₃ nanocomposite agglomerates. The method should be regarded as a proof-of-concept local characterization approach rather than a direct measurement of the full magnetoelectric coefficient.