A major concern about the biomedical application of magnetic nanoparticles is their biocompatibility. A possible solution is coating them with hydroxyapatite (HA) [Ca5(PO4)3OH], which is the inorganic component of biological hard tissues, e.g. bone and teeth. This approach appears especially appealing for uses in the field of bone tissue engineering. \ud We have synthetized a novel nanogranular system, consisting of magnetite nanoparticles embedded in biomimetic carbonate HA, through an original two-step method: i) magnetite nanoparticles are prepared by refluxing an aqueous solution of Fe(SO4) and Fe2(SO4)3 in an excess of Tetrabutilammonium hydroxide acting as surfactant; ii) the nanoparticles are coated with a Ca(OH)2 layer, to induce the growth of HA directly on their surface, by reaction of Ca(OH)2 with HPO42-. \ud Two samples have been collected with magnetite content ~ 0.8 wt. % and ~ 4 wt.%. The magnetite nanoparticles and the nanogranular material have been investigated by X-ray Diffraction, Fourier Transform Infrared Spectroscopy and Transmission Electron Microscopy. These analyses have provided structural information on the as-prepared nanoparticles (mean size ~ 6 nm) and revealed the presence of surface hydroxyl groups, which promote the growth of the HA phase featuring a nanocrystalline lamellar structure. \ud Hysteresis loops (temperature range 5-300 K), thermal and time dependence of the magnetization under different magnetic fields and field dependence of the remanence have been measured by SQUID magnetometer. Moreover, a Mössbauer spectroscopy investigation on the as-prepared nanoparticles has been carried out between T = 4 K and room temperature; to better distinguish the contribution coming from Fe different lattice sites, low temperature measurements were performed with and without an applied magnetic field of 8 T; the field was applied parallel to the direction of the incoming 14.4 keV -radiation direction. Room temperature Mössbauer spectra show just the presence of a doublet contribution; below 200 K, a sextet contribution appears and it gets narrower as T is decreased. If the sextet is described in terms of a hyperfine fields distribution, that shows three main contributions that shift to higher hyperfine fields values as T approaches 4 K. At 4 K, with the external magnetic field, two sextets can be clearly observed corresponding to Fe in tetrahedral and octahedral sites, showing a slight canting with respect to the field direction. Both the as-prepared and the HA-coated magnetite nanoparticles show a superparamagnetic behaviour at T=300 K. However, the magnetization relaxation process is affected by dipolar magnetic interactions of comparable strength in the three samples, also inducing the onset of a collective frozen magnetic regime below T~20 K. The results indicate that the magnetite nanoparticles tend to form agglomerates in the as-prepared state, which are not substantially altered by the HA growth, coherently with the creation of electrostatic hydrogen bonds among the surface hydroxyl groups
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