Nanoscale mapping of internal magnetization dynamics reveals how disorder shapes heat generation in magnetic particle hyperthermia

Abstract

Magnetic particle hyperthermia relies on the efficient conversion of magnetic field energy into heat in biomedical applications, yet the microscopic mechanisms governing heat generation within individual particles remain poorly understood. In this study, AC magnetometry experiments are combined with dynamic micromagnetic simulations to connect microstructural features, magnetization dynamics, and macroscopic heat dissipation. Beyond macroscopic heating metrics, the heat generation is resolved at the intra-particle level, uncovering a heterogeneous landscape of localized ''hot spots'' with nanometer spatial and nanosecond temporal resolution. The results demonstrate that grain size acts as a key experimentally tunable parameter, balancing anisotropy disorder and pinning strength, thereby controlling both the magnitude and spatio-temporal distribution of heat release within the particle. In particular, nanoflower architectures composed by larger grains deliver larger heat generation, while the smaller grains offer a deeper intra-particle pinning landscape, which effectively redistributes the heat generation over extended time windows. Together, our results provide a mechanistic framework linking nanoparticle microstructure to magnetic heating and establish design principles for optimizing nanoflowers as magnetic hyperthermia transducers.

Figures (4)

• Figure 1: Iron oxide nanoflowers are multicore aggregates of nanocrystals (commonly referred to as grains) of tailorable size with promising prospects in MHT. Our model assigns each grain a random uniaxial anisotropy axis, $\vec{K_u}$, represented by a different colour in the figure. The boundaries in between those grains, highlighted in yellow, are a source of structural and magnetic disorder. We account for them through a reduction of the exchange-coupling at inter-grain boundaries, which together with the grain anisotropy, define the disorder landscape of the NFs. For maghemite NFs above the single-domain threshold ($d \gtrsim 70~\mathrm{nm}$jefremovas2026coercivitydi2012generalization), the magnetic texture folds into a vortex configuration, formed by a vortex core (represented in white) and a perpendicular flux-closure texture (each magnetization direction represented in a different color). In this work, we resolve the heat release under MHT conditions at local scale, with nanometer spatial and nanosecond temporal resolution.
• Figure 2: Specific absorption rate, $SAR$, vs. field intensity $h_{\mathrm{AC}}$, for the 4 NF ensembles measured at a)$f =$ 10, b)$f =$ 25, and c)$f =$ 100 kHz. In all cases, the NF with the smaller grain sizes $g_{s}$ display almost negligible $SAR$ throughout the explored field range. d) Numerical results of the corresponding $(d, gs)$ realizations at $f = 300~\mathrm{kHz}$. The simulations are consistent with the experiments: NFs with larger grains generate more heat than their smaller grain counterparts.
• Figure 3: Magnetization as a function of applied field (top row) and released energy $\mathcal{E}(t)$ (bottom row) over one AC-field period $T = 1/300~\mathrm{ms}$ for NFs with diameters of $d=170~\mathrm{nm}$ and $g_{s}=21~\mathrm{nm}$ (A) and $7~\mathrm{nm}$ (B), and $d=100~\mathrm{nm}$ and $g_{s}=4~\mathrm{nm}$ (C) and $15~\mathrm{nm}$ (D). The magnetization traces are colour-coded by the instantaneous heat generation, with red highlighting the points of maximum dissipation, coinciding with the jumps observed in the magnetization consequence of the reversal, i.e. moments of maximized torque. The step-like increases in $\mathcal{E}(t)$ coincide with these dissipation bursts. Sketches represent in top the grain anisotropy, where each colour represents a $\vec{K_{u}}$ direction, and the vortex configuration (bottom), where each colour represents a direction of the magnetization.
• Figure 4: 3D mappings of heat generation together with the corresponding 3D magnetization configurations and instantaneous heat release $d\mathcal{E}/dt$ as a function of time for $(d, g_{s}) = (170, 7), (170, 21), (100, 4)$ and $(100, 15)~\mathrm{nm}$ ((A)–(D), respectively). In all cases the intergrain exchange coupling parameter is fixed to $k = 1$, and the external alternating magnetic field is applied at an amplitude of $\mu_{0}h = 62~\mathrm{mT}$ and frequency $f = 300~\mathrm{kHz}$. The colour bar indicates the intensity of the heat release, ranging from black (minimum) to red (maximum). Magnetic moments within the vortex core are shown as red arrows, while flux-closure moments are represented in grey.