Heat Generation in Magnetic Hyperthermia by Manganese Ferrite-Based Nanoparticles Arises from Neel Collective Magnetic Relaxation

Collective magnetic relaxation of coupled nanoparticle's magnetic moments and its influence in magnetic nanoparticle hyperthermia (MNH) therapy are studied by combining experimental data, numerical simulations, and theoretical approaches. Frequency-dependent MNH of Mn-ferrite nanoparticles with different particle sizes and different nanoparticle arrangements, controlled by medium pH and surface coating, revealed that the hyperthermia efficiency could increase or decrease depending on the nanoparticle's organization within the aggregate. Effective relaxation times of similar to 10(-7) s were obtained for heat generation that are not explained by Brownian or single-particle Neel relaxation. In particular, we propose a theoretical approach that is a generalization of the Allia-Knobel phenomenological model that allows us to build magnetic regime diagrams and find the conditions for single-particle relaxation (superparamagnetic, interacting superparamagnetic, and single-particle blocked regimes) and collective magnetic relaxation. The regimes depend on dipolar strength, temperature, particle size, aggregate shape and length, magnetization, and magnetic anisotropy (together with axes arrangement). We demonstrate through a detailed nanoparticle characterization (including the temperature dependence of magnetization and anisotropy) that the collective relaxation is responsible for the heat generation of magnetic nanostructures. We believe that our findings and our approach to study the collective magnetic relaxation open new perspectives for designing more efficient magnetic nanocarriers for hyperthermia and explain superferromagnetism as a collective blocked regime.