Microstructure was analyzed with transmission Tanaproget microscopy (TEM) using bright field and dark field imaging. Fig. 5 presents the TEM cross-section micrographs of the coatings deposited for ton = 20 s using either dark field ( Fig. 5) or bright field ( Fig. 6a). A multilayer microstructure is clearly evidenced that may result from the pulsing of the oxygen flow rate introduced into the deposition chamber. Furthermore, the sample starts to develop a typical columnar microstructure ( Fig. 5a). Such microstructure with multilayer morphology of the deposited coating has been already observed in several coatings obtained by magnetron sputtering such as TiN/Ti(C,N)  or TiAlN/CrN films . The bilayer period thickness in the iron oxynitride films is approx. 14 ± 1 nm. Remarkable is the continuous columnar structure in spite of the multilayer morphology as illustrated in Fig. 5b. The occurrence of a continuous columnar microstructure in the multilayer can find an explanation in the successive deposition of layers exhibiting the same structure with relatively close lattice parameters, as previously encountered in local epitaxial growth  and . Parreira et al. , studying W–O coatings deposited by magnetron sputtering with reactive gas pulsing, have pointed out the formation of multilayer structure oxide/metal. In their study, two different layers are visible during one oxygen pulse period: a tungsten-rich and an oxygen-rich layers corresponding to the sputtering during toff (metallic mode) and ton (transition and compound modes). This process has also been used to deposit TiO/TiO2 periodic multilayer thin films . Similarly to what has been observed in the W–O coatings, one can also distinguish in our study two different layers, a nitrogen-rich and an oxygen-rich corresponding to the sputtering time (toff) and (ton). This periodicity of 14 nm confirmed by secondary ion mass spectrometry with the profiles of oxygen and nitrogen is in phase opposition (not shown here). The polycrystalline feature of the film is easily evidenced by the presence of the discontinuous diffraction rings (insert Fig. 5a). Therefore, the layer presents a texture effect. We also have observed that the average grain size is smaller than 5 nm. In addition, the HRTEM micrograph ( Fig. 7) shows that the grains are stretched on the growth direction with different orientations.