Dependence of exchange anisotropy and coercivity on the Fe-oxide structurein oxygen-passivated Fe nanoparticles

Ultrafine Fe particles have been prepared by the inert gas condensation met hod and subsequently oxygen passivated. The as-obtained particles consist i n an Fe core surrounded by an amorphous Fe-oxide surface layer. The antifer romagnetic character of the Fe-oxide surface induces an exchange anisotropy in the ferromagnetic Fe core when the system is field cooled. Samples have been heat treated in vacuum at different temperatures. Structural changes of the Fe-O layer have been monitored by x-ray diffraction and transmission electron microscopy. Magnetic properties as coercivity, hysteresis loop sh ift, and evolution of magnetization with temperature have been analyzed for different oxide crystallization stages. A decrease of the exchange anisotr opy strength is reported as the structural disorder of the surface oxide la yer is decreased with thermal treatment. (C) 1999 American Institute of Phy sics. [S0021-8979(99)38808-3].