In this work, a straightforward aqueous synthesis for mass production (up to 20 g) of uniform and crystalline magnetite nanoparticles with a core sizes between 20 and 30 nm, which are the optimum nanoparticle core sizes for hyperthermia application, is proposed. Magnetic and heating properties have been analyzed showing very high saturation magnetization and magnetic heating values. To stabilize the naked magnetite nanocrystals at physiological pH and increase their circulation time in blood, they have been covalently coated with carboxy-methyl-dextran, a biocompatible polymer.The influence of this superficial modification on magnetic and heating properties has been studied showing that these biocompatible magnetic nanocrystals maintain high magnetization saturation values, good colloidal stability and hyperthermia properties in the presence of the polymeric external layer. These particles, properly functionalized, could be used to selectively kill cancer cells under a moderate alternating magnetic field (44 mT and 70 kHz).

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We present experimental intrinsic loss power (ILP) values, measured at an excitation frequency of 1 MHz and at relatively low field amplitudes of 3.4 to 9.9 kA/m, as a function of the mean core diameter, for selected magnetic nanoparticle (MNP) samples synthesized in the recent EU-funded NanoMag project. The mean core sizes ranged from ca. 8 nm to 31 nm. Transmission electron microscopy indicated that those with smaller core sizes (less than ca. 22 nm) were single-core MNPs, while those with larger core sizes (ca. 29 nm to 31 nm) were multi-core MNPs. The ILP data showed a peak at ca. 20 nm. We show here that this behaviour correlates well with the predicted ILP values obtained using either a non-interacting Debye model, or via dynamic Monte-Carlo simulations, the latter including core-core magnetic interactions for the multi-core particles. We show that this alignment of the models is a consequence of the low field amplitudes used.

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Spherical magnetite nanoparticles having average particle size = 5 nm have been synthesized by coprecipitation of Fe(II) and Fe(III) salts in KOH with Polyvinylalcohol (PVA). The resulting dry powder displayed superparamgnetic (SPM) behaviour at room temperature, with a transition to a blocked state at T B ~ 45 K for applied field H app = 500 Oe. The effect of dipolar interactions was investigated by measuring the dependence of T B on the applied field H ap and driven ac field in susceptibility data. A thermally activated model has been used to fit the dynamic data to obtain the single-particle energy barriers E a = K eff V, allowing us to estimate the contributions of dipolar interactions to the single-particle effective magnetic anisotropy K eff . We have measured the dependence of T B with H ap in order to draw the transition contours of a H-T diagram. Two different regimes are found for the (T B -T 0 ) ~H λ dependence at low and high fields, that can be understood within a pure SPM relaxation-time (Néel-Brown) landscape. The T B (H) data shows a crossover from λ = 2/3 to λ ~2 for applied magnetic fields of ≈ 550 Oe.

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␥-Fe 2 O 3 /SiO 2 and Fe/SiO 2 nanocomposites, with a Fe/Si molar ratio of 0.25, were prepared by the sol-gel method starting from ethanolic solutions of tetraethoxysilane and iron (III) nitrate. After gelation the xerogels were oxidated or reduced. Samples were investigated by transmission electron microscopy, x-ray diffraction, differential scanning calorimetry, and thermogravimetry. Magnetic properties of the samples were investigated at room temperature (RT) and at 77 K. Nanometric particles supported in the silica matrix were obtained in all cases. Bigger particles (10 nm) were obtained in the case of Fe/SiO 2 nanocomposites with respect to the ␥-Fe 2 O 3 /SiO 2 samples (5-8 nm). A slight effect of sol dilution on particle size was observed only in the case of ␥-Fe 2 O 3 /SiO 2 nanocomposites. A superparamagnetic behavior was shown at RT only by ␥-Fe 2 O 3 /SiO 2 nanocomposites. Iron-based composites exhibited coercivity values higher than 700 Oe at RT.

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