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The heating power of magnetic nanoparticles (MNPs) submitted to high-frequency magnetic fields is generally probed using calorimetric methods, which suppose that the heating power does not evolve with time. Among the several parameters governing MNP heating properties, their organization into chains under the influence of the applied magnetic field is of key importance, though the dynamic of this phenomenon has been rarely studied experimentally. In the present article, time-resolved high-frequency hysteresis loops are used to probe the dynamics of chain formation on a sample of 17.3 ± 2.2 nm FeNi3 MNPs. Chains are formed on a timescale, which strongly depends on the magnetic field amplitude, ranging from several tens of seconds to less than 100 ms, but does not depend on frequency in the range studied here (from 9 to 78 kHz). Both the heating power and hysteresis loop squareness increase with time as chains progressively form. These findings have important methodological consequences when defining protocols or analyzing data issued from calorimetric measurements since, in samples where chains form, the heating power varies on a time scale that can be comparable to typical measurement times. [ABSTRACT FROM AUTHOR]

We report on the magnetic and hyperthermia properties of 5.5 nm in diameter iron nanoparticles synthesized by organometallic chemistry, which display the bulk magnetization. Quantitative analysis of alternative susceptibility measurements allows the determination of the effective anisotropy Keff=1.3×105 J m-3. Hyperthermia measurements are performed at a magnetic field up to 66 mT and at frequencies in the range of 5-300 kHz. Maximum measured specific absorption rate (SAR) is 280 W/g. SAR displays a square dependence with the magnetic field below 30 mT but deviates from this power law at higher value. SAR is linear with the applied frequency for μ0H =19 mT. These results are discussed in the light of linear response theory. [ABSTRACT FROM AUTHOR]

We show theoretically that, in an inhomogeneous alternating magnetic field of frequency f, due to the alternating gradient, magnetic nanoparticles oscillate mechanically and generate ultrasound waves. This effect is maximized and better controlled if a static magnetic field is superimposed to an alternating gradient. It makes possible the generation of ultrasounds intracellularly and might also explain recent experiments in magnetic hyperthermia in which cells have been killed without any global temperature increase. Combined to an efficient targeting, it could permit ultrasound therapy with an unprecedented spatial resolution. [ABSTRACT FROM AUTHOR]

Biomedical applications of superparamagnetic iron oxide particles have been of interest for quite a number of years. Recent developments show that multifunctionality can be efficiently achieved using polymers to coat the particles and to provide anchoring elements to their surface. This leads to the formation of nanobeads with a reduced number of particles trapped by the polymeric structure. While the magnetothermic behavior of isolated nanoparticles has been a subject of interest over the past several years, multicore magnetic nanobeads have thus far not received the same attention. The influence of structural and magnetic properties in the hyperthermia performance of a series of magnetic fluids designed for biomedical purposes is studied here. The fluids are made of maghemite multicore polymeric beads, with variable nanoparticle size and hydrodynamic size, dispersed in a buffer solution. The specific loss power (SLP) was measured from 5 to 100 kHz with a field intensity of 21.8 kA/m. SLP increases with increasing magnetic core size, reaching 32 W/g Fe2O3 at 100 kHz for 16.2 nm. Within the framework of the linear response theory, a graphical construction is proposed to describe the interplay of both size distributions and magnetic properties in the heating performance of such fluids in a given frequency range. Furthermore, a numerical model is developed to calculate the spare contribution of Néel and Brown relaxation mechanisms to SLP, which gives a fair reproduction of the experimental data. [ABSTRACT FROM AUTHOR]

When magnetic nanoparticles (MNPs) are single domain and magnetically independent, their magnetic properties and the conditions to optimize their efficiency in magnetic hyperthermia applications are now well understood. However, the influence of magnetic interactions on magnetic hyperthermia properties is still unclear. Here, we report hyperthermia and high-frequency hysteresis loop measurements on a model system consisting of MNPs with the same size but a varying anisotropy, which is an interesting way to tune the relative strength of magnetic interactions. A clear correlation between the MNP anisotropy and the squareness of their hysteresis loop in colloidal solution is observed: the larger the anisotropy, the smaller the squareness. Since low anisotropy MNPs display a squareness higher than the one of magnetically independent nanoparticles, magnetic interactions enhance their heating power in this case. Hysteresis loop calculations of independent and coupled MNPs are compared to experimental results. It is shown that the observed features are a natural consequence of the formation of chains and columns of MNPs during hyperthermia experiments: in these structures, when the MNP magnetocristalline anisotropy is small enough to be dominated by magnetic interactions, the hysteresis loop shape tends to be rectangular, which enhances their efficiency. On the contrary, when MNPs do not form chains and columns, magnetic interactions reduce the hysteresis loop squareness and the efficiency of MNPs compared to independent ones. Our finding can thus explain contradictory results in the literature on the influence of magnetic interactions on magnetic hyperthermia. It also provides an alternate explanation to some experiments where an enhanced specific absorption rate for MNPs in liquids has been found compared to the one of MNPs in gels, usually interpreted with some contribution of the brownian motion. The present work should improve the understanding and interpretation of magnetic hyperthermia experiments. [ABSTRACT FROM AUTHOR]