Coercivity-size map of magnetic nanoflowers: spin disorder tunes the vortex reversal mechanism and tailors the hyperthermia sweet spot

Abstract: Iron-oxide nanoflowers (NFs) are one of the most efficient nanoheaters for magnetic hyperthermia therapy (MHT). However, the physics underlying the spin texture of disordered iron-oxide nanoparticles beyond the single-domain limit remains still poorly understood. Using large-scale micromagnetic simulations we completely map the magnetization of NFs over an unprecedented size range, from 10 to 400 nm in diameter, connecting their microstructure to their macroscopic magnetic response. Above the single domain (d > 50 nm), the magnetization folds into a vortex state, within which the coercivity describes a secondary maximum, not present for non-disordered nanoparticles. We have extended our understanding by resolving also the NF magnetization dynamics, capturing the physics of the magnetization reversal. Within the vortex regime, two distinct reversal modes exist: i) A core-dominated one, in which the core immediately switches along the direction of the applied field, resulting in an increasing coercivity for larger sizes; and ii) a flux-closure dominated reversal mode, going through the perpendicular alignment of the vortex core to the field, resulting in a decreasing coercivity-size dependence. The coercivity maximum is located at the transition between both reversal modes, and results from the combination of grain anisotropy and grain-boundary pinning: weak (but non-negligible) inter-grain exchange keeps the vortex profile coherent, yet allows the core to be pinned by the random anisotropy easy axes of the single grains, maximizing magnetic losses. Our results provide the first full description of spin textures in iron oxide NFs beyond the macrospin framework, and clarify the role of internal spin disorder in magnetic hyperthermia heating. By adjusting the grain size, the coercivity "sweet spot" can be tailored, offering a practical route to next-generation, high-efficiency nanoheaters.