ORIGIN OF THE UNIAXIAL MAGNETIC-ANISOTROPY IN FE FILMS GROWN BY MOLECULAR-BEAM EPITAXY

We report the observation of a weak in-plane uniaxial magnetic anisotropy in Fe thin films grown by molecular beam epitaxy onto (001)-oriented MgO substrates. This anisotropy has been observed both in (Fe/Pd) superlattices and in single epitaxial Fe layers within a large range of thicknesses (15-800 Angstrom). We find that the uniaxial anisotropy results from the geometry of the deposition system, and in particular from the angle of incidence of the Fe atomic flux during deposition. Detailed magnetic and structural studies of a 800 Angstrom thick single Fe layer show that this anisotropy can be explained as a volume effect resulting from the magneto-elastic energy associated with an in-plane tetragonal distortion of the cubic Fe structure, with (a-b)/a=+(1.2+/-0.3)x10(-3).