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2021-04-30 - Colloque/Présentation - poster - Anglais - page(s)

Martin Eléonore , Gossuin Yves , Vuong Quoc Lam , "Monte Carlo and Experimental Study of the Magnetic Relaxation of Superparamagnetic Nanoparticle Ensembles" in Intermag 2021, Lyon, France, 2021

  • Codes CREF : Physique (DI1200), Biophysique (DI3113), Sciences exactes et naturelles (DI1000), Electromagnétisme - analyse du signal (DI1232)
  • Unités de recherche UMONS : Physique biomédicale (M104)
  • Instituts UMONS : Institut de Recherche en Science et Ingénierie des Matériaux (Matériaux), Institut des Sciences et Technologies de la Santé (Santé)
Texte intégral :

Abstract(s) :

(Anglais) Superparamagnetic Iron Oxide Nanoparticles (SPION) are nanosize beads of iron oxides. Their peculiar magnetic behaviour makes them particularly suited for a variety of biomedical applications, ranging from cellular imaging to cancer treatment by hyperthermia. The usual theory used to describe their magnetic behaviour is that developed by Paul Langevin [1], which only applies to idealized (isotropic, monodisperse in size and non-interacting) nanoparticles at high temperatures. Reality however usually deviates from that theoretical framework: real samples are polydisperse in size, particles usually have at least one easy magnetization axis, and, particulary in biological media, they tend to aggregate, leading to locally high particle volumic fractions and therefore interaction between their magnetic moments [2]. Moreover, as demonstrated by Néel and Brown [3], at low temperatures the particles’ magnetic moments and anisotropy axises appear effectively blocked at their initial directions. All those phenomena impact the magnetization of particle ensembles in a non-trivial way and are impossible to study simultaneously theoretically. In this work, these deviations from the Langevin law are studied numerically, at thermodynamic equilibrium, using a Metropolis algorithm, and compared with experimental data obtained on a Vibrating Sample Magnetometer. The algorithm is adapted to take into account the Néel and Brown blocking of the particles as those time-dependant effects are not explained by free energy minimization. Thorough tests are led on the simulation program to ensure correct convergence of the magnetization, as demonstrated in figure 1, which shows that the temperature step used in the ZFC curve does not impact the simulation results, as expected. The advantage of using numerical simulation is that it allows to discriminate the effects of each tweak to the theory, leading to a better understanding of their various impacts, the ultimate goal being to replicate by numerical simulation the experimental results. Two types of magnetization curves in particular are obtained: so called MH curves, where the mag- netization is plotted versus the external magnetic field that is applied, and magnetization vs temperature curves. These can be obtained following different protocols, leading to different type of curves, the most common being the zero field cooled (ZFC) curve and the field cooled (FC) curve. Their shape is naturally impacted by the different effects studied, and by the precise experimental protocol, as can be seen in figure 2, where a ZFC curve is compared with two FC curves: one obtained after freezing the sample under no magnetic field and one after freezing it under a 5 mT magnetic field. References [1] P. Langevin, J. Phys. Theor. Appl., 1905, 4(1), 678 [2] M. Lévy, C. Wilhelm, et al., Nanoscale, 2011, 3, 4402 [3] W.F. Brown, Jr, Physical Review, 1963, 130(5), 1677