Ferromagnetic resonance of NiCoFe2O4 nanoparticles and microwave absorption properties of flexible NiCoFe2O4–carbon black/poly(vinyl alcohol) composites

Abstract

The effect of cationic disorder and particle morphology on the ferromagnetic resonance (FMR) of NiCoFe₂O₄ nanoparticles (NPs) and the electromagnetic shielding effectiveness of flexible composites (wherein the nanoparticles are used as fillers) has been presented. Upon annealing at 1000 °C, spherical, ∼25 nm, single crystalline (as-prepared) NPs are transformed into octahedral, ∼200 nm, polycrystalline (annealed) NPs and change the cationic distribution significantly. The effect of shape, size and cationic distribution on the resonance properties has been discussed using the randomly-oriented anisotropic-axis model. The temperature dependent evolution of FMR spectra has been found to be consistent with a Bloch spin-relaxation model. Analysis of the FMR spectra reveals that NiCoFe₂O₄ nanoparticles have a large internal magnetic field along with broad FMR linewidths of ∼2–3 kOe, signifying high magnetic losses that are essential for the absorption of electromagnetic (EM) waves. Next, NiCoFe₂O₄–carbon black (NCF–CB) hybrids grafted in a PVA matrix, as flexible composite films with a thickness of ∼1.5 mm, are assessed for EM wave absorption properties in the range of 8–18 GHz. As compared to annealed-NCF–CB/PVA (21 dB, ∼99.5%), the as-prepared-NCF–CB/PVA composite film exhibits significantly large SE of 27 dB (∼99.9% attenuation of the EM wave), with a dominant contribution from absorption (SE_A ∼ 21 dB). The electrical conductivity, the electric modulus, and Cole–Cole plots reveal that the dielectric losses in the as-prepared-NCF–CB/PVA have significant contributions from cationic disorder and particle size, as compared to the annealed-NCF–CB/PVA composites. Cationic disorder increases the d–d electron transition probability between adjacent ionic pairs such as Co²⁺/Fe³⁺ and a reduced particle size creates large interfacial polarization in the as-prepared NCF/CB hybrids. Considerably large values of the Landes g-factor, magnetic anisotropy and better impedance matching indicate a dominant magnetic loss contribution in ap-NCF (g = 4.5) as compared to an-NCF (g = 2.5) at 300 K.