Role of Frozen Spins in the Exchange Anisotropy of Core-Shell Fe@Fe3O4 Nanoparticles

Core-shell Fe@Fe3O4 nanoparticles exhibit substantial exchange bias at low temperatures, mediated by unidirectionally aligned moments at the core shell interface. These spins are frozen into magnetic alignment with field cooling and are depinned in a temperature-dependent manner. The population of such frozen spins has a direct impact on both coercivity (H-C) and the exchange-bias field (H-E), which are modulated by external physical parameters such as the strength of the applied cooling field and the cycling history of magnetic field sweeps (training effect). Aging of the core-shell nanoparticles under ambient conditions results in a gradual decrease in magnetization but overall retention of H-C and H-E, as well as a large increase in the population of frozen spins. These changes are accompanied by a structural evolution from well-defined core-shell structures to particles containing multiple voids, attributable to the Kirkendall effect. Energy-filtered and high-resolution transmission electron microscopy both indicate further oxidation of the shell layer, but the Fe core is remarkably well preserved. The increase in frozen spin population with age is responsible for the overall retention of exchange bias, despite void formation and other oxidation-dependent changes. The exchange-bias field becomes negligible upon deliberate oxidation of Fe@Fe3O4 nanoparticles into nanoparticles into yolk-shell particles, with a nearly complete physical separation of core and shell.