The relaxation of strain in epitaxial FePd films, grown by MBE, has been studied thanks to RHEED, TEM and STM. The preferential relaxation by microtwins in well L10- ordered films grown on Pd layers has been described trough a quantitative use of STM images. This modification of the relaxation mechanism in ordered films is explained by the energy cost of ½ perfect dislocations in the L10 structure. This study outlined the role of the surface concerning the L10 ordering during growth: the ordering takes place at 300°C by a 'step-up' mechanism of Pd atoms at the surface, forming bi-atomic steps, and thus selecting one of the three possible variants of the ordered structure. The growth at temperatures lower than 300°C leads to the formation of well-ordered domains within a disordered film. This structure is explained by the high density of antiphase boundaries due to perfect dislocations taking on the relaxation of the epitaxial strain. Post growth annealing of disordered layers yielded ordered films with the 3 possible variants of the L10 structure. The proportion of the volume of each variant is set by the residual strain in the layer before annealing. The effect of irradiation by He+ ions proved to be dependant on the temperature during irradiation and on the initial structure of ordered particles in the alloy. Irradiation at 300°C of layer-by-layer-grown films improves the magnetic anisotropy: initialy in-plane magnetisation of FePd layers becomes perpendicular to the film, and FePt films become 100% remnant.
