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

## TL;DR

This paper studies how the structure of iron-oxide nanoflowers affects their magnetic behavior, revealing how to optimize them for better cancer hyperthermia treatments.

## Contribution

The study reveals how spin disorder and grain boundaries influence vortex reversal in iron-oxide nanoflowers, enabling tailored hyperthermia performance.

## Key findings

- Iron-oxide nanoflowers larger than 50 nm exhibit a vortex state with a secondary coercivity maximum.
- Two distinct vortex reversal modes are identified, with opposing coercivity-size dependencies.
- Tuning grain size allows for optimizing the coercivity 'sweet spot' for efficient nanoheaters.

## Abstract

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.

Large‐scale micromagnetic simulations show how disorder and grain boundaries control magnetization dynamics in iron‐oxide nanoflowers beyond the single‐domain regime. Two competing vortex‐reversal modes emerge, producing a coercivity maximum at their transition. Linking microstructure to macroscopic response, this work demonstrates how tuning intraparticle disorder via grain size manipulation enables more efficient and controllable magnetic hyperthermia nanoheaters.© 2026 WILEY‐VCH GmbH