The Role of Anisotropy in Distinguishing Domination of Néel or Brownian Relaxation Contribution to Magnetic Inductive Heating: Orientations for Biomedical Applications

Magnetic inductive heating (MIH) has been a topic of great interest because of its potential applications, especially in biomedicine. In this paper, the parameters characteristic for magnetic inductive heating power including maximum specific loss power (SLP_max), optimal nanoparticle diameter (D_c) and its width (ΔD_c) are considered as being dependent on magnetic nanoparticle anisotropy (K). The calculated results suggest 3 different Néel-domination (N), overlapped Néel/Brownian (NB), and Brownian-domination (B) regions. The transition from NB- to B-region changes abruptly around critical anisotropy K_c. For magnetic nanoparticles with low K (K < K_c), the feature of SLP peaks is determined by a high value of D_c and small ΔD_c while those of the high K (K > K_c) are opposite. The decreases of the SLP_max when increasing polydispersity and viscosity are characterized by different rates of d(SLP_max)/dσ and d(SLP_max)/dη depending on each domination region. The critical anisotropy K_c varies with the frequency of an alternating magnetic field. A possibility to improve heating power via increasing anisotropy is analyzed and deduced for Fe₃O₄ magnetic nanoparticles. For MIH application, the monodispersity requirement for magnetic nanoparticles in the B-region is less stringent, while materials in the N- and/or NB-regions are much more favorable in high viscous media. Experimental results on viscosity dependence of SLP for CoFe₂O₄ and MnFe₂O₄ ferrofluids are in good agreement with the calculations. These results indicated that magnetic nanoparticles in the N- and/or NB-regions are in general better for application in elevated viscosity media. View Full-Text