Tuning dipolar effects on magnetic hyperthermia of Zn_0.3Fe_2.7O₄/SiO₂ nanoparticles by silica shell

Effects of magnetic dipole interactions on specific loss power (SLP) for Zn_0.3Fe_2.7O₄/SiO₂ core/shell nanoparticles (NPs) with identical magnetic core size and varying non-magnetic shell thicknesses were systematically investigated. In the case of thin silica shell, NPs form chain-like structures due to strong dipole interactions between NPs. The chain formation induces additional shape anisotropy. With increasing thickness of the silica shell, the dipole interaction and therefore the anisotropy induced by the dipole interactions decrease. Meanwhile, the hydrodynamic size of the NPs shows a counterintuitive reduction with increasing shell thickness, also due to the decrease in interactions. The magnetic anisotropy and hydrodynamic size affect the SLP through changes in the Néel and Brownian relaxation rates, leading to different field-dependent SLP values for NPs with thin and thick shells.