Composition- and Phase-Controlled High-Magnetic-Moment Fe $_{1 - {\rm x}}$ Co $_{\rm x}$

Fe-Co alloy nanoparticles are studied in this work of different composition condition. Fe_1-xCo_x nanoparticles were synthesized by a physical method with well-developed crystal structure, uniform size, and high saturation magnetization. Their magnetic property is found to be a function of composition, which indicates successful control of the synthesis process. Exchange bias was found for Fe_1-xCo_x core-oxide shell particles after natural oxidization in air. It was a result of interfacial coupling between Fe/Co and their oxides. The property of the surface oxide varies with composition and so does the coupling strength. Oxidation and interparticle interactions lead to much different magnetic response of nanoparticles compared to un-oxidized, non-interacting ones.