Magnetic couplings and exchange bias in Fe/NiO epitaxial layers

We report an investigation of the magnetic properties of Fe films epitaxially grown on NiO layers with (001) orientation. The results are interpreted with the help of a morphological characterization of the system. The onset of ferromagnetic behavior for the Fe layer is observed at 8 ML and it is ascribed to the coalescence of the islands, formed by Fe at the first stages of the growth, into larger interconnected islands. At larger Fe layer thickness the magnetization reversal is explained by taking into account the interplay between a unidirectional anisotropy, due to coupling with NiO, and the uniaxial growth anisotropy of Fe, which develops around 50 ML Fe layer thickness. No significant chemical modification of the interface is observed for thermal treatments up to 540 K. Exchange bias can be induced only for NiO layer thickness above 25 ML and its value can be tuned by changing the ferromagnetic to antiferromagnetic layer thickness ratio, reaching 220 Oe. No training effect is observed for this system.