Enhanced nanocomposite susceptibility by field-alignment of superparamagnetic particles

Abstract

Nanocomposites comprised of insulated magnetic single-domain particles are promising candidates for high-frequency, eddy current free, soft magnetic materials, but tend to suffer from low magnetic susceptibility ($<20$). Particle alignment has been proposed to increase nanocomposite susceptibility and reduce magnetic losses but experimental verification has been lacking. Here, magnetic nanocomposites containing 3-57 vol\% field-aligned 11$\pm$3 nm maghemite particles in a poly-vinyl matrix were investigated for potential use as high-frequency inductor core materials. The particles were aligned by a homogenous static alignment field during nanocomposite drying, fixating the particle orientation. Particle aggregation was disproved by small-angle scattering. The dependence of the alignment field strength and particle concentration on the nanocomposite's susceptibility and hysteresis losses were investigated from DC up to 922 kHz by vibrating sample magnetometry, AC-susceptibility and high-frequency hysteresis measurements. Nanocomposite susceptibility increased super-linearly with particle fraction due to weak particle interactions. Alignment of the particles increased the nanocomposite susceptibility from 21 to 50 for samples with a particle content of 57 vol\%. Hence, the synergy between particle alignment and interaction allows for a higher than expected susceptibility of nanocomposites. The results show that magnetically aligning particles in a nanocomposite reduces magnetic losses when using well-dispersed single-domain superparamagnetic nanoparticles. Measured nanocomposite susceptibility could be modelled by a combination of directional dependent Debye-models including mean-field interaction effects and partial particle alignment. Measured susceptibility of 50 is among the highest obtained for nanocomposites, making it a relevant candidate for applications in power electronics.

Figures (8)

• Figure 1: Illustration of nanocomposite synthesis and in-plane measurement directions on nanocomposite disk samples. Nanoparticle solution was mixed with PVA and cast into disk shape (field-series) or printed in layers, yielding flat disks (concentration-series). White markings indicate alignment field direction on sample.
• Figure 2: Small-angle neutron scattering (SANS) data for field-series samples containing 3 vol% 11$\pm$3 nm maghemite particles together with two-model-fit. Scattering intensity was scaled with $10^{x}$, $x\in[0,1,..]$ for better visibility.
• Figure 3: Small-angle x-ray scattering (SAXS) data for concentration-series samples with (500 mT) and without alignment field (0 mT) together with two-model-fit. Scattering intensity was scaled with $10^{x}$, $x\in[0,1,..]$ for better visibility.
• Figure 4: Normalised VSM hysteresis curves for the 56 vol% sample with and without alignment field together with Langevin fit. Measurements were obtained at room temperature. For the samples synthesised with 500 mT alignment field, hysteresis for the direction measuring parallel ($||$) and perpendicular ($\perp$) to the alignment field direction are shown. VSM results and fitting parameters are listed in table \ref{['tab:VSMResults']}
• Figure 5: Nanocomposite susceptibility found from VSM as function of volume fraction for samples synthesised with 500 mT alignment field and without alignment field. Predicted susceptibility for randomly oriented, non-interacting linear model (eq. \ref{['eq:ChiLin']}) and for randomly oriented, weakly interacting particles (eqs. \ref{['eq:ChiLin']} and \ref{['eq:ChiInt']}) for 11$\pm$3 nm maghemite particles are shown as full / dotted lines.