arXiv subjects

Iago López-Vázquez

Publications and source records attributed to Iago López-Vázquez.

5 recordsLinked to original sources

arXiv2026

The Hidden Cost of Two-Dimensional TEM Reconstruction of Magnetic Nanoparticle Morphology

Conventional transmission electron microscopy (TEM) records two-dimensional (2D) projections of three-dimensional (3D) objects, complicating the inference of nanoparticle morphology. This limitation is investigated for quasi-spherical iron oxide nanoparticles (IONPs) imaged after deposition on a TEM grid and after cellular internalization and immobilization in resin. A statistical inverse-reconstruction method is developed in which synthetic 3D particle populations are randomly oriented, projected onto a TEM-like plane, and compared with the experimental equivalent-diameter and axial-ratio distributions. Models of increasing geometrical complexity are considered, from monodisperse prolate spheroids to size- and shape-polydisperse prolate populations and triaxial ellipsoids. Size polydispersity is required to reproduce the projected equivalent-diameter distribution, whereas relaxing axial symmetry through a triaxial description enables simultaneous reproduction of the size and axial-ratio distributions. The inferred population represents a statistically compatible 3D morphology rather than a unique particle-by-particle reconstruction. Its physical relevance is assessed through a macrospin model incorporating the corresponding shape-anisotropy contributions. Application to magnetic fluid hyperthermia reveals a pronounced sensitivity of the predicted heating response to particle geometry and polydispersity. The approach provides a general framework for connecting conventional TEM measurements with morphology-dependent properties of nanoparticle ensembles.

cond-mat.mtrl-sci↗

arXiv2026

AC Field-driven orientational crossover and energy dissipation in suspended magnetic nanoparticles

By combining the Landau--Lifshitz--Gilbert equation with Brownian rotational dynamics of magnetic nanoparticles (MNPs), we theoretically investigate the role of particle rotation through easy-axis reorientation in magnetic fluid hyperthermia (MFH). Our results reveal a field-driven crossover in the stationary orientation of the easy axes, from predominantly perpendicular to predominantly parallel or antiparallel to the applied field as the field amplitude increases. Although the precise crossover field depends on particle size and excitation frequency, it occurs at approximately $0.5H_k$, where $H_k$ is the uniaxial anisotropy field. These orientational regimes are directly linked to the underlying microscopic dynamics and the associated MFH performance through the occurrence of switching and non-switching hysteresis cycles, predominantly associated with Néel magnetization reversal and Brownian particle rotation, respectively. The relative importance of these dissipation mechanisms also depends on frequency: at $f=1$ MHz, Brownian heating dominates at low field amplitudes, whereas Néel heating dominates at high fields. By contrast, at $f=100$ kHz, both contributions remain comparable over most of the investigated field range.

cond-mat.mtrl-sci↗

arXiv2026

Including nanoparticle shape into macrospin models

We investigate the feasibility of the macrospin approximation to account for the actual shape of soft magnetic nanoparticles (MNPs) with realistic geometries. Specifically focusing on magnetite, we use the superellipsoidal parametrisation to account for a variety of shapes, with a continuous interpolation from spherical to cubic morphologies, as well as different elongations. Our procedure consists of the direct comparison between angular-dependent hysteresis loops obtained by full micromagnetic simulations, with those produced by an extended Stoner-Wohlfarth (SW) model that incorporates both the intrinsic cubic magnetocrystalline anisotropy, and an effective uniaxial contribution arising from the particle elongation. The limits of validity of the macrospin description are approximately 10-60 nm for axial ratios r>1.5, and 20-60 nm for 1.0<r<1.5. These results establish a direct connection between nanoparticle morphology and effective macrospin parameters, demonstrating the suitability of the generalized SW model for describing the magnetic response of realistically shaped MNPs.

cond-mat.mtrl-sci↗

arXiv2026

Tuning field amplitude to minimise heat-loss variability in magnetic hyperthermia

In this work, we theoretically investigate how shape-induced anisotropy dispersion and magnetic field amplitude jointly control both the magnitude and heterogeneity of heating in magnetite nanoparticle assemblies under AC magnetic fields. Using real time Landau-Lifshitz-Gilbert simulations with thermal fluctuations, and a macrospin model that includes both the intrinsic cubic magnetocrystalline anisotropy and a shape-induced uniaxial contribution, we analyze shape-polydisperse systems under clinically and technologically relevant field conditions. We show that for relatively large particles, around 25 to 30 nm, the relative dispersion of local (single-particle) losses exhibits a well-defined minimum at moderate field amplitudes (between 4 to 12 mT), hence identifying an optimal operating regime that minimizes heating heterogeneity while maintaining substantial power dissipation. The position of this critical field depends mainly on particle size and excitation frequency, and only weakly on shape dispersion, offering practical guidelines for improving heating uniformity in realistic MFH systems.

cond-mat.mtrl-sci↗

arXiv2025

Role of nanoparticle shape on the critical size for quasi-uniform ordering: from spheres to cubes through superballs

The equilibrium states of single-domain magnetite nanoparticles (NPs) result from a subtle interplay between size, geometry, and magnetocrystalline anisotropy. In this work, we present a micromagnetic study of shape-controlled magnetite NPs using the superball geometry, which provides a continuous interpolation between spheres and cubes. By isolating the influence of shape, we analyze the transition from quasi-uniform (single-domain) to vortex-like states as particle size increases, revealing critical sizes that depend on the superball exponent p. Our simulations show that faceted geometries promote the stabilization of vortex states at larger sizes, with marked distortions in the vortex core structure. The inclusion of cubic magnetocrystalline anisotropy, representative of magnetite, further lowers the critical size and introduces preferential alignment along the [111] easy axes. For isotropic shapes, the critical size for this transition increases with p, ranging from ~49 nm for spheres to ~56 nm for cubes, in agreement with experimental trends. In contrast, the presence of slight particle elongation increases the critical size and induces another preferential alignment direction. These results demonstrate that even small deviations from sphericity or aspect ratio significantly alter the magnetic ordering and stability of equilibrium magnetic states.

cond-mat.mtrl-sci↗