Critical radius for exchange bias in naturally oxidized Fe nanoparticles

Abstract

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