Ultrasmall iron oxide nanoparticles: Magnetic and NMR relaxometric properties

摘要

AbstractUltrasmall iron oxide (USPIO) nanoparticles, with diameter mostly less than 3?nm dispersed in an organic carrier fluid were synthesized by polyol route. The evolution of ZFC-FC magnetization curves with temperature, as well as the shift of theacsusceptibility peaks upon changing the frequency, reveal that the nanoparticles in the fluid are non-interacting and superparamagnetic with the blocking temperatureTB～10?K. The M?ssbauer spectra analysis proposed the core/shell structure of the nanoparticles consisting of stoichiometric γ-Fe2O3core and non-stoichiometric shell. The nanoparticle surface layer has a great influence on their properties which is principally manifested in significant reduction of the magnetization and in a large increase in magnetic anisotropy. Magnetic moments do not saturate in fields up to 5?T, even at the lowest measured temperature,T?=?5?K. The average magnetic particle diameter is changed from 1.3 to 1.8?nm with increasing magnetic field from 0 to 5?T which is noticeably smaller than the particle sizes measured by TEM. The estimated effective magnetic anisotropy constant value,Keff?=?2?×?105?J/m3, is two orders of magnitude higher than in the bulk maghemite. Measurements of the longitudinal and transverse NMR relaxivity parameters on water diluted nanoparticle dispersions at 1.5?T gave the valuesr1?=?0.028?mmol