Structural and magnetic study of zinc-doped magnetite nanoparticles and ferrofluids for hyperthermia applications

Cubic-like shaped ZnxFe3−xO4 particles with crystallite mean sizes D between 15 and 117 nm were obtained by co-precipitation. Particle size effects and preferential occupation of spinel tetrahedral site by Zn2+ ions led to noticeable changes of physical properties. D ≥ 30 nm particles displayed nearly bulk properties, which were dominated by Zn concentration. For D ≤ 30 nm, dominant magnetic relaxation effects were observed by Mössbauer spectroscopy, with the mean blocking size DB ~ 13 to 15 nm. Saturation magnetization increased with x up to x ~ 0.1–0.3 and decreased for larger x. Power absorbed by water and chitosan-based ferrofluids from a 260 kHz radio frequency field was measured as a function of x, field amplitude H0 and ferrofluid concentration. For H0 = 41 kA m−1 the maximum specific absorption rate was 367 W g−1 for D = 16 nm and x = 0.1. Absorption results are interpreted within the framework of the linear response theory for H0 ≤ 41 kA m−1. A departure towards a saturation regime was observed for higher fields. Simulations based on a two-level description of nanoparticle magnetic moment relaxation qualitatively agree with these observations. The frequency factor of the susceptibility dissipative component, derived from experimental results, showed a sharp maximum at D ~ 16 nm. This behaviour was satisfactorily described by simulations based on moment relaxation processes, which furthermore indicated a crossover from Néel to Brown mechanisms at D ~ 18 nm. Hints for further improvement of magnetite particles as nanocalefactors for magnetic hyperthermia are discussed.