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Magnetic hyperthermia is a cancer treatment based on the exposure of magnetic nanoparticles to an alternating magnetic field in order to generate local heat. In this work, 3D cell culture models were prepared to observe the effect that a different number of internalized particles had on the mechanisms of cell death triggered upon the magnetic hyperthermia treatment. Macrophages were selected by their high capacity to uptake nanoparticles. Intracellular nanoparticle concentrations up to 7.5 pg Fe/cell were measured both by elemental analysis and magnetic characterization techniques. Cell viability after the magnetic hyperthermia treatment was decreased to <25% for intracellular iron contents above 1 pg per cell. Theoretical calculations of the intracellular thermal effects that occurred during the alternating magnetic field application indicated a very low increase in the global cell temperature. Different apoptotic routes were triggered depending on the number of internalized particles. At low intracellular magnetic nanoparticle amounts (below 1 pg Fe/cell), the intrinsic route was the main mechanism to induce apoptosis, as observed by the high Bax/Bcl-2mRNA ratio and low caspase-8 activity. In contrast, at higher concentrations of internalized magnetic nanoparticles (1–7.5 pg Fe/cell), the extrinsic route was observed through the increased activity of caspase-8. Nevertheless, both mechanisms may coexist at intermediate iron concentrations. Knowledge on the different mechanisms of cell death triggered after the magnetic hyperthermia treatment is fundamental to understand the biological events activated by this procedure and their role in its effectiveness.

Magnetic hyperthermia (MH) based on magnetic nanoparticles (MNPs) is a promising adjuvant therapy for cancer treatment. Particle clustering leading to complex magnetic interactions affects the heat generated by MNPs during MH. The heat efficiencies, theoretically predicted, are still poorly understood because of a lack of control of the fabrication of such clusters with defined geometries and thus their functionality. This study aims to correlate the heating efficiency under MH of individually coated iron oxide nanocubes (IONCs) versussoft colloidal nanoclusters made of small groupings of nanocubes arranged in different geometries. The controlled clustering of alkyl-stabilized IONCs is achieved here during the water transfer procedure by tuning the fraction of the amphiphilic copolymer, poly(styrene-co-maleic anhydride) cumene-terminated, to the nanoparticle surface. It is found that increasing the polymer-to-nanoparticle surface ratio leads to the formation of increasingly large nanoclusters with defined geometries. When compared to the individual nanocubes, we show here that controlled grouping of nanoparticles—so-called “dimers” and “trimers” composed of two and three nanocubes, respectively—increases specific absorption rate (SAR) values, while conversely, forming centrosymmetric clusters having more than four nanocubes leads to lower SAR values. Magnetization measurements and Monte Carlo-based simulations support the observed SAR trend and reveal the importance of the dipolar interaction effect and its dependence on the details of the particle arrangements within the different clusters.

Revealing the ultimate speed limit at which magnetic order can be controlled, is a fundamental challenge of modern magnetism having far reaching implications for magnetic recording industry. Exchange interaction is the strongest force in magnetism, being responsible for ferromagnetic or antiferromagnetic spin order. How do spins react after being optically perturbed on an ultrashort timescales pertinent to the characteristic time of the exchange interaction? Here we demonstrate that femtosecond measurements of X-ray magnetic circular dichroism provide revolutionary new insights into the problem of ultrafast magnetism. In particular, we show that upon femtosecond optical excitation the ultrafast spin reversal of Gd(FeCo) - a material with antiferromagnetic coupling of spins - occurs via a transient ferromagnetic state. The latter one emerges due to different dynamics of Gd and Fe magnetic moments: Gd switches within 1.5 ps while it takes only 300 fs for Fe. Thus, by using a single fs laser pulse one can force the spin system to evolve via an energetically unfavorable way and temporary switch from an antiferromagnetic to ferromagnetic type of ordering. These observations supported by atomistic simulations, present a novel concept of manipulating magnetic order on different classes of magnetic materials on timescales of the exchange interaction.

We have conducted Stokesian dynamics simulations to investigate the dynamic properties of ferromagnetic colloidal dispersions subjected to a sinusoidal shear flow. Thick chain-like cluster formation is significantly influenced by an oscillatory shear flow even if the amplitude is relatively small, since the internal structures of thick chain-like clusters are highly sensitive to the change in the direction of the shear flow. The motion of thick chain-like clusters is out of phase to a sinusoidal shear rate, and the phase difference is strongly correlated with that of the viscosity and normal stress coefficients. The viscoelastic properties become more apparent with decreasing frequency of the oscillatory shear flow, since such properties have a strong relationship with the thick chain-like cluster formation. In other words, since thick chain-like clusters are more stable for the case of a smaller frequency shear flow, such stable clusters induce significant viscoelastic properties of ferromagnetic colloidal dispersions in a strong, applied magnetic field.

We have carried out Brownian dynamics simulations to investigate the behavior of clusters of ferromagnetic particles in a colloidal dispersion subjected to a simple shear flow. The results have been compared with those of Stokesian dynamics and Monte Carlo simulations. The main results obtained here are summarized as follows. The Brownian dynamics method can capture thick chain-like clusters formed along a magnetic field, in agreement with the Stokesian dynamics and Monte Carlo methods. However, Brownian motions of the particles give a subtle influence on the internal structure of thick chain-like clusters even in the case in which the magnetostatic interactions dominate the particle Brownian motions. The Stokesian dynamics method leads to a physically unreasonable cluster formation in the case in which the particle Brownian motions play an important role compared with that of the magnetic particle–particle interactions. We conclude that the Brownian dynamics method gives rise to physically realistic cluster formations in simulation of colloidal dispersions and therefore the method is to be preferred to the Stokesian dynamics method.

The present study theoretically investigates the aggregate structures of thick chainlike clusters arising in ferromagnetic colloidal dispersions by means of the cluster-moving Monte Carlo method. Pair correlation functions are also evaluated to clarify the internal structures themselves of such clusters. The following aggregation image of thick chainlike clusters has been obtained from the Monte Carlo simulations for a two-dimensional model system. Particles aggregate to form chainlike clusters when magnetic attractions between particles become strong over a certain value. Such thin chainlike clusters prefer nearly linear formation along a magnetic field direction, if the strength of magnetic field is sufficiently large. In this situation, attractive forces come to act between these nearly linear chainlike clusters, and these forces increase with the lengths of the clusters. Such attractions between the clusters lead to the formation of thick chainlike clusters.

Three-dimensional Monte Carlo simulations have been carried out in order to investigate the thick chainlike cluster formation of ferromagnetic fine particles in an applied magnetic field. The results of the aggregate structures are summarized as follows. If a magnetic field is sufficiently strong and magnetic particle–particle interactions are relatively strong, thick chainlike clusters are formed along a field direction. The thick chainlike clusters obtained by the simulations agree well with those observed experimentally. For the increased particle–particle interactions, high potential barriers arise between thin chainlike clusters, so that it is impossible for the thin chainlike clusters to aggregate to form thick chainlike clusters. In small magnetic fields, thick chainlike clusters are not observed, since nonlinear clusters are preferred to linear clusters in this situation, whereby strong attractive forces do not act between clusters, due to flux closure.

We have investigated the behavior of clusters of ferromagnetic particles in a colloidal dispersion subjected to a simple shear flow. To do so, the Stokesian dynamics method has been used under the assumption that the effect of Brownian motion is negligible. For the case of no shear flow, the aggregate structures obtained by the Stokesian dynamics simulations agree well with Monte Carlo results qualitatively. We can, therefore, conclude that the Stokesian dynamics simulations can capture thick chainlike clusters without introducing a specific clustering algorithm, which is indispensable for Monte Carlo simulations. The behavior of the thick chainlike clusters in a simple shear flow is summarized as follows. The thick chainlike clusters decline in the shear flow direction as time advances. Since longer clusters experience larger shear forces, it is difficult for them to survive in such a situation. The thick chainlike clusters, therefore, dissociate into some short clusters. Such clusters are relatively stable in a shear flow, so that they do not decrease significantly any more. The viscosities have a strong relationship with the internal structures of the aggregates. The instantaneous viscosities, therefore, fluctuate significantly for the case of the thick chainlike clusters.