Coercivity‐Size Map of Magnetic Nanoflowers: Spin Disorder Tunes the Vortex Reversal Mechanism and Tailors the Hyperthermia Sweet Spot

Iron‐oxide nanoflowers (NFs) are one of the most efficient nanoheaters for magnetic hyperthermia therapy. However, the physics underlying the dynamic response of realistic nanoparticles, containing disorder, beyond the single‐domain limit remains poorly understood. Using large‐scale micromagnetic simulations, the magnetization of biocompatible iron‐oxide NFs ( d = 10–400 nm) has been mapped, connecting their microstructure to their macroscopic magnetic response. Above the single‐domain regime ( d > 50 nm), the magnetization folds into a vortex state, within which the coercivity reaches a secondary maximum, not present for nondisordered nanoparticles. The dynamics of the vortex shows two distinct reversal modes: 1) a core‐dominated one, with an increasing coercivity with d ; 2) a flux‐closure‐domains dominated reversal mode, with 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. The results provide the first description of spin textures in iron oxide NFs beyond the macrospin framework, revealing how particles with identical static magnetization exhibit fundamentally distinct dynamics, which result in different macroscopic behavior. By adjusting the grain size, the coercivity “sweet spot” can be tailored, offering a practical route to next‐generation, high‐efficiency nanoheaters.