Magnetoelectricity in CoFe ₂O ₄ nanocrystal-P(VDF-HFP) thin films

Abstract

Transition metal ferrites such as CoFe ₂O ₄, possessing a large magnetostriction coefficient and high Curie temperature ( T _c > 600 K), are excellent candidates for creating magnetic order at the nanoscale and provide a pathway to the fabrication of uniform particle-matrix films with optimized potential for magnetoelectric coupling. Here, a series of 0–3 type nanocomposite thin films composed of ferrimagnetic cobalt ferrite nanocrystals (8 to 18 nm) and a ferroelectric/piezoelectric polymer poly(vinylidene fluoride-co-hexafluoropropene), P(VDF-HFP), were prepared by multiple spin coating and cast coating over a thickness range of 200 nm to 1.6 μm. We describe the synthesis and structural characterization of the nanocrystals and composite films by XRD, TEM, HRTEM, STEM, and SEM, as well as dielectric and magnetic properties, in order to identify evidence of cooperative interactions between the two phases. The CoFe ₂O ₄ polymer nanocomposite thin films exhibit composition-dependent effective permittivity, loss tangent, and specific saturation magnetization ( M _s). An enhancement of the effective permittivity and saturation magnetization of the CoFe ₂O ₄-P(VDF-HFP) films was observed and directly compared with CoFe ₂O ₄-polyvinylpyrrolidone, a non-ferroelectric polymer-based nanocomposite prepared by the same method. The comparison provided evidence for the observation of a magnetoelectric effect in the case of CoFe ₂O ₄-P(VDF-HFP), attributed to a magnetostrictive/piezoelectric interaction. An enhancement of M _s up to +20.7% was observed at room temperature in the case of the 10 wt.% CoFe ₂O ₄-P(VDF-HFP) sample.