FIG. 6.

The total signal from all measured harmonics (2–9) for a $400\mathrm{\mu }\mathrm{g}$ sample of iron oxide nanoparticles and for a $150\mathrm{\mu }\mathrm{g}$ sample of iron core composite nanoparticles as functions of the bias field, $H_{\mathrm{bias}}$. The total signal increases significantly with the addition of a bias field and it shows that the signal is difficult to saturate. The total signal can be estimated numerically from an estimate of $H_{\mathrm{TE}}$ obtained from the ratio of the signal at the 3rd and 5th harmonic frequencies. The predicted peaks are marked with longer vertical lines for the iron oxide nanoparticles, $H_{\mathrm{TE}}=1.98\mathrm{mT}$, and with shorter vertical lines for the iron core composite nanoparticles, $H_{\mathrm{TE}}=1.71\mathrm{mT}$.