The magnetic properties of an iron nitride thin film obtained by ion implantation have been investigated. N2+ ions were implanted in a pristine iron layer epitaxially grown on ZnSe/GaAs (001). X-ray diffraction measurements revealed the formation of body-centered tetragonal N-martensite whose c-axis is perpendicular to the thin film plane and c-parameter is close to that of alpha' Fe8N. Magnetic measurements disclosed a weak perpendicular magnetic anisotropy (PMA) whose energy density KPMA was assessed to about 105 J/m3. A sharp decline of the in-plane magnetocrystalline anisotropy (MCA) was also observed, in comparison with the body-centered cubic iron. The origin of the PMA is attributed to the MCA of N-martensite and/or stress-induced anisotropy. As a result of the PMA, weak magnetic stripe domains with a period of about 130nm aligned along the last saturating magnetic field direction were observed at remanence by magnetic force microscopy. The application of an increasing in-plane magnetic field transverse to the stripes Htrans highlighted a threshold value above which these magnetic domains irreversibly rotated. Interestingly, below this threshold, the stripes do not rotate, leading to a zero remanent magnetization along the direction of the applied field. The interest of this system for magnetization dynamics is discussed.