Role of dipolar interaction in magnetic hyperthermia

The dynamic properties of magnetic nanoparticles are known to be substantially influenced by the dipole-dipole interaction. In this paper we study how this affects the efficiency of magnetic hyperthermia experiments. In particular we ask whether it is possible to use the dipolar interaction as a mechanism to increase the heat released by the nanoparticles, thus improving the application. The investigation is carried out via numerical simulations based on a mean-field model developed to include the dipolar interaction in the Fokker-Planck equation describing the time evolution of the system. Both the linear and nonlinear regimes (related to the amplitude of the external magnetic field) are studied in detail. It is shown that even moderate changes in the particle concentration may have substantial effects on the magnetization dynamics of the system, being capable of increasing or decreasing the heat released by orders of magnitude, depending on the values of other system parameters. It is found that the dipolar interaction can be used to increase the dissipation of magnetically soft particles, but should be avoided in the case of hard particles.

Figure 1

Hysteresis loops in the linear regime, $h_0=0.005$. From (a) to (f), different values of $\sigma$, as denoted. Black solid, $\gamma =0$; blue dashed, $\gamma =0.01$; red dotted, $\gamma =0.05$.

Figure 2

Hysteresis loops in the nonlinear regime, $h_0=0.5$. All other details are similar to Fig. 1.

Figure 3

The SAR as a function of $\sigma$ for different values of $\gamma$ in the linear regime [$h_0=0.005$; panel (a)] and nonlinear regime [$h_0=0.5$; panel (b)]. Black solid, $\gamma =0$; blue dashed, $\gamma =0.01$; red dotted, $\gamma =0.05$.

Figure 4

The SAR as a function of $\gamma$ for different values of $\sigma$ in the linear [$h_0=0.005$; panel (a)] and nonlinear regime [$h_0=0.5$; panel (b)]. Filled circles, $\sigma =6$; filled squares, $\sigma =8$; stars, $\sigma =10$; open circles, $\sigma =12$; open squares, $\sigma =14$.