We report a magnetic characterization of iron thin films of 6 nm prepared on Si (001) substrates by sputtering at different substrate temperatures. Magnetization loops and zero field cooling-field cooling measurements were performed to unveil the magnetic features of such systems. Magnetic parameters show that films prepared at very low temperature (T(S) = 200 K) present features of a core-shell granular systems, formed by ferromagnetic iron grains surrounded by an antiferromagnetic iron oxide phase.