The co-deposition of two metals using a molecular beam epitaxy (MBE) technique at various growth temperatures (TG) yields single-crystal alloy thin films with a columnar microstructure whose structural and magnetic properties can be different from those of the corresponding equilibrium bulk alloys. A general overview of results obtained in hcp Co-Ru and hcp or fcc Co-Pt thin films grown on a hcp (0002) substrate will be presented. Around the A 3B composition (A = Co, B = Ru or Pt) the films display a composition modulation along the growth direction whose amplitude is strongly dependent of TG passing through a maximum at respectively 600 and 650 K. This long range order (LRO) that does not exist in equilibrium phases is explained as resulting from the competition between two phenomena occurring simultaneously during the growth process: a surface effect driven by surface interactions and surface diffusion that tends to enrich the surface layer in one element (Co, Ru or Pt segregation) and a bulk effect driven by bulk interactions and bulk diffusion that tends to restore the bulk equilibrium phase when the bulk diffusion becomes efficient during the growth time. A thermally activated model that takes into account both surface and bulk diffusion during the growth time reproduces quite satisfactorily the TG dependencies of the LRO in Co3Ru and Co3Pt as well as that of the uniaxial magnetic anisotropy in CoPt3.
At high TG ( > 800 K) or after ex situ anneals, the diffusion of the Ru buffer through the grain boundaries of the columnar microstructure, that occurs before the inside grain diffusion, isolates the columns magnetically and is thought to be responsible of the large observed coercive field and of a substantial modification of the magnetic domain shapes.