The magnetic properties of partially oxidized nanocrystalline iron thin films prepared by DC-magnetron sputtering at low substrate temperatures in the 175-300 K range are studied. The preparation method is presented as a simple method for fabricating granular structures. Films prepared at intermediate temperatures exhibit granular magnetic behaviour, in which nanocrystalline grains act as almost decoupled particles, surrounded by an oxide shell, forming exchange bias core-shell systems. The magnetic features of granular systems obtained by this new method are described and the mechanisms of interaction between metallic grains and their oxide shells are explained, as are their effects in the magnetization reversal process.