Modeling the Magnetic-Hyperthermia Response of Linear Chains of Nanoparticles with Low Anisotropy: A Key to Improving Specific Power Absorption

Abstract

The effect of magnetic interactions is a key issue for the performance of nanoparticles in magnetic fluid hyperthermia. There are reports informing on beneficial or detrimental effects in terms of the specific power absorption depending on the intrinsic magnetic properties and the spatial arrangement of the nanoparticles. To understand this effect, our model treats a simple system: an ensemble of identical nanoparticles arranged in an ideal chain with the easy axis of the effective uniaxial anisotropy of each particle aligned parallel to the chain. We study the magnetic relaxation of linear chains with low anisotropy in magnetic-fluid-hyperthermia experiments, a system that yields a larger hysteresis area than the noninteracting case (i.e., improved specific power absorption) for all orientations of the chain (even in the perpendicular configuration and the randomly oriented case). The most-favorable case is the chain parallel to the external field; however, we show that the incorporation of a dipolar-field component perpendicular to the external field is necessary for the correct modeling of chains nearly in the perpendicular configuration, which is not always done. The mechanism involved in the hysteresis-area increase can be interpreted as a shift between the local field and the applied field.

Time-dependent AC magnetometry and chain formation in magnetite: the influence of particle size, initial temperature and the shortening of the relaxation time by the applied field

“…However, as can be seen in Figure 3, MAG-30 is the only one having remanence close to 60% for 0H > 25 mT. This suggests the formation of an anisotropic assembly under the influence of the magnetic field, as already reported 7,11,40 .…”

Probing dynamics of nanoparticle chains formation during magnetic hyperthermia using time-dependent high-frequency hysteresis loops

“…The first one is that the changes in hysteresis loops would be due to the alignment of the MNP easy axis along the magnetic field: the particles would at the beginning of the experiment be randomly oriented and would then rotate to get aligned, without chain formation. This hypothesis seems unlikely for two reasons: i) if the particles were randomly oriented, the remnant magnetization would be half the saturation one; it is experimentally observed that it is much lower; ii) the typical time for a rotation of a nanoparticle under the influence of a magnetic field is of the order of the microsecond, much shorter than the timescale of the phenomena detected here [20]. The second hypothesis is that the observed features would be due to a strong increase of the local temperature of the MNPs; the latter would, at the beginning of the experiment, display a coercive field much larger than the applied field, explaining the flat hysteresis loops.…”

Nanoparticles for Magnetic Heating: When Two (or More) Is Better Than One

“…Hence, it was also shown that the heating efficiency of these systems could be modified by changing the viscosity of the agarose matrix and the relative orientation between the aggregate long axis and the direction of the AMF [ 207 , 208 , 209 ]. This last item was also theoretically addressed by Valdes et al, who also analyzed the case of randomly oriented NP chains and predicted significantly better heating performance with respect to a system of dispersed non-interacting NPs [ 210 , 211 ].…”