Epitaxial Fe-Ge thin films on Ge(111): Morphology, structure, and magnetic properties versus stoichiometry

International audienceWe have studied the growth and magnetic properties of thin Fe-Ge films synthesized (codeposited at room temperature and postannealed at 250°C ) on Ge(111) wafers versus stoichiometry. Morphology and crystal structure have been investigated in situ by means of scanning tunneling microscopy, low-energy electron diffraction, and x-ray photoelectron diffraction and ex situ with x-ray diffraction. The magnetic properties were characterized ex situ by conventional polar and longitudinal magneto-optical Kerr effect and transverse biased initial inverse susceptibility and torque measurements. It is found that the growth is epitaxial for Ge content up to ˜48at.% ( ˜Fe1.1Ge composition). In particular, the film is homogeneous and flat and adopts a crystalline structure of hexagonal symmetry derived from the B82 (Ni2In) structure over a wide stoichiometry range extending from Fe2Ge to Fe1.1Ge . The epitaxial orientation between the Ge substrate and the germanide layer is (0001)Fe-Ge∥(111)Ge with [112¯0]Fe-Ge∥[1¯10]Ge . We found however that the surface periodicity and the out-of-plane lattice parameter c evolve within this stoichiometry range and two distinct stoichiometry regimes appear on both sides of a critical stoichiometry (˜Fe1.5Ge) . Indeed, from Fe2Ge to Fe1.5Ge the surface periodicity is p(2×2) and c continuously decreases with Fe content, whereas from Fe1.5Ge to Fe1.1Ge the surface periodicity is (3×3)R30° and c remains constant. These features have been interpreted as a clear fingerprint of a minor transformation of the crystalline structure but without any change in symmetry. This structural order transformation is discussed in relation to previous results reported in the case of macroscopic single-crystal Fe-Ge ingots. On both sides of the wide [Fe2Ge,Fe1.1Ge] composition range the layer is no more homogeneous. More precisely, for higher Fe content the film contains both the above mentioned Ni2In -derived phase and a Fe-richer phase (probably bcc Fe) whereas for higher Ge content the layer is amorphous. Magnetic characterization showed in particular that the homogeneous Ni2In -derived epilayers are ferromagnetic with a Curie temperature that varies drastically with the stoichiometry, rising up to a high TC value of ˜450K for the Fe-rich Fe1.9Ge composition. Finally, whatever the stoichiometry, the magnetic easy axis of the homogeneous phase lies in the film plane and a small uniaxial anisotropy is superimposed on a sixfold order one that results from the hexagonal symmetry of the crystallographic structure