Superspin glass state and exchange bias in amorphous Fe/Fe-O core/shell nanoparticles

Abstract

Nanoparticles of iron and iron oxide are widely explored in several biomedical and technological applications. We report on the magnetic properties of amorphous Fe/Fe–O core/shell nanoparticles compared to those of a reference system with crystalline Fe–O nanoparticles. These nanoparticles are prepared by thermal decomposition of iron precursor, where the amorphous and crystalline nature of core and shell is determined by the choice and concentration of the ligand. The crystalline system exhibits a blocking temperature higher than 300 K and negligible exchange bias effect. In contrast, the amorphous systems display large exchange bias, and collective magnetic behavior at low temperatures, with features of magnetic frustration and disorder reminiscent of those observed in spin glass and superspin glass systems. We discuss the origin of the dynamical magnetic behavior of the amorphous particles and study the dependence of the exchange bias field on the cooling field.