Controlled magnetic properties of Ni nanoparticles embedded in polyimide films

We have studied the magnetic properties of Ni nanoparticles incorporated in polyimide films using dc- and ac-magnetization measurements. Transmission electron microscopy images reveal that both the mean particle diameter $\left(d_{\mathit{ave}}\right)$ and the volume fraction of particles $\left({\eta }_{\mathit{ave}}\right)$ in the composite layer can be controlled independently. In a series of samples with fixed $d_{\mathit{ave}}$, the system with the lowest ${\eta }_{\mathit{ave}}$ behaves magnetically as an ensemble of noninteracting superparamagnets. With increasing ${\eta }_{\mathit{ave}}$, typical signatures of the dipole-dipole interparticle interaction, such as an enhanced blocking temperature, are observed. In dense systems, with a particle concentration above the threshold for geometrical percolation, the magnetic response is ferromagnetic due to the exchange interaction between particles in direct contact.

Figure 1

Cross-sectional TEM images of the films in series II annealed at $300°\mathrm{C}$ for (a) $30\min$ with ${\eta }_{\mathit{ave}}=5.5\%$ and (c) $130\min$ with ${\eta }_{\mathit{ave}}=18\%$. [(b) and (d)] Size distribution of the Ni particles observed in (a) and (c), respectively.

Figure 2

(Color online) Temperature dependence of the in- and out-of-phase components of the ac susceptibility (${\chi }^{\prime }$ and ${\chi }^{″}$) of a film with (a) ${\eta }_{\mathit{ave}}=5.5\%$ and (b) ${\eta }_{\mathit{ave}}=18\%$ in series II.

Figure 3

(Color online) ${\chi }^{″}$ vs $E_b=-T\ln \left(2\pi f{\tau }_0\right)$ for the two samples with ${\eta }_{\mathit{ave}}=5.5\%$ (solid symbols) and ${\eta }_{\mathit{ave}}=18\%$ (open symbols) in series II. ${\tau }_0=1\times {10}^{-14}\mathrm{s}$ was used both for the 5.5% and 18% samples. The inset illustrates the tail of large particles in the 18% sample by plotting ${\chi }^{″}$ vs $E_b∕E_b^{\mathit{max}}$.

Figure 4

(Color online) Blocking temperatures $\left(T_B\right)$ as a function of particle volume fraction in the composite layer $\left({\eta }_{\mathit{ave}}\right)$. Squares and circles correspond to series I $(d_{\mathit{ave}}\sim 5\mathrm{nm})$ and II $(d_{\mathit{ave}}\sim 8\mathrm{nm})$, respectively.

Figure 5

(Color online) $M\text{−}H$ hysteresis curves at $300\mathrm{K}$ for a sample with ${\eta }_{\mathit{ave}}=5.5\%$ and the percolated sample in series II. The magnetic field is applied along the film. Demagnetization effects should therefore be small.