Continuous, uniform, and crack/void-free CoFe2O4/polymer films with thicknesses in the range 200 nm to 1.6 μm were systematically prepared by multiple spin/cast coating followed by thermal treatment to dry the film. Figure  3 shows SEM images with a CFO weight fraction of 25%

where the white dots are the CFO nanoparticles and the dark background is the P(VDF-HFP) copolymer. The top surface view of the microstructure of the nanocomposite film demonstrates that monodisperse, ultrafine cobalt ferrite YH25448 nanoparticles are well embedded in the polymer matrix, forming typical 0–3, particulate type nanocomposites. Loose agglomeration occurs locally due to the magnetic interaction among the nanopowders. Defects, pores, or phase separation unfavorable for device fabrication was not observed. The cross-sectional image (Figure  3b) confirms the thickness of the free standing film of approximately 1.5 μm. The observation of intimate physical contact between the CFO and P(VDF-HFP) phase components is a good starting point for attempting to generate mechanical, magnetic, or electrical coupling between them. Figure 3 SEM images of CoFe 2 O 4 / P ( VDF-HFP ) thin-films deposited on Si substrate. With cobalt ferrite

fraction of 25 wt.% and film thickness of 1.5 μm. (a) Top surface view; (b) cross-sectional view. The effective permittivity (ϵ eff) and loss tangent (tan δ) of the ferrites/polymer thin films (thickness of approximately 1 μm) were measured over the frequency range from 100 Hz to 1 MHz (Figure  4). Both the effective permittivity and loss tangent of the nanostructured films PX-478 solubility dmso show a systemic increase as a function

of the loading of CFO nanocrystals. The dielectric constant of the pure P(VDF-HFP) film is measured to be 8 at 100 Hz (Figure  4a), consistent with the reported data [24, 25], and increases to 44 in the case of the 30 wt.% CFO samples due to the inclusion of the higher dielectric constant magnetic component (k(CoFe2O4) ≈ 400) [26]. The polarization in ferrites originates from the electronic exchange Fe2+ ⇔ Fe3+ and hole transfer between Co2+ ⇔ Co3+ in the spinel phase, which cannot follow the alternating external field beyond a certain frequency [27]. When until the space charge carriers fail to keep up with the field and lag behind the alternation of its direction, the composites’ permittivity and loss tangent decrease monotonically with frequency. Once the frequency is over 10 kHz, the relaxation mechanism associated with the P(VDF-HFP) phase dominates the overall dielectric behavior [20]. The decrease in loss (Figure  4b) with frequency at low frequencies (<1 kHz) is attributed to the ionic DC conduction contribution from the P(VDF-HFP) copolymer phase, which yields interfacial or spatial charge polarization [28]. The increase in loss at high frequencies (>10 kHz) results from the β relaxation associated with the glass transition of the copolymer.