Critical radius for exchange bias in naturally oxidized Fe nanoparticles

摘要: Details of synthesis and structural characterization highly crystalline iron oxide nanoparticles are presented together with results on the magnetic investigation as a function temperature applied field. Monodisperse Fe were prepared by thermal decomposition pentacarbonyl in presence oleic acid. These readily oxidized exposure to air. The resulting nanocrystals have been identified inverse spinels, being $\ensuremath{\gamma}\text{\ensuremath{-}}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$ dominant phase small $5\text{\ensuremath{-}}\mathrm{nm}$ nanocrystals, whereas proportion ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ component gradually increases increasing particle size. particles volume resulted finite-size effects, for instance, magnetization deviates from ${T}^{3∕2}$ Bloch's law. At same time, high field irreversibility shifted hysteresis loops after field-cooled processes detected, attributed low-temperature surface spin-glass layer. Moreover, there is critical diameter below which behavior exchange bias effect abruptly disappear.