Fyzikální ústav Akademie věd ČR

The extensive study of the magnetic systems appropriate for use in medicine, in particular for the magnetic hyperthermia, has been devoted especially to nanoparticles of CoFe₂O₄. Annealing of the nanoparticle powder at temperatures of 400-800 °C resulted in the average particle size of 18 to 40 nm. By Mössbauer spectroscopy of the ⁵⁷Fe isotope enabled to determine the distribution of Fe ions between tetrahedral and octahedral positions. Based on these data the inversion degree of the spinel lattice was determined. We investigated behavior of diluted nanoparticle suspensions and modeled their presence in tissues, in ac fields, and compared it with the measured ac losses. We conclude that these losses and the corresponding heating power of the particles depends on the relative amplitude of the ac field by far more strongly than on koercivity value.
In the project Self-organized magnetic structures of the GAAV program "Nanotechnology for society" we investigated nanogranular thin films of Fe₄₀Co₄₀Al₁₀N₁₀ prepared by plasma jet. Their structure consisted of ferromagnetic particles separated by a poorly electrically conductive matrix, which implies their potential use at significantly higher frequencies than with metallic alloys. The thin films were characterized by XRD and scanning electron microprobe, and their volume and surface magnetic properties were determined. The results of Mössbauer spectroscopy of conversion electrons (CEMS) contributed to the information about orientation of local magnetic moments of Fe atoms and together with NMR results it gave information on the effect of the neighborhood of Co atoms on this orientation.