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

We have studied the growth and magnetic properties of thin Fe-Ge films synthesized (codeposited at room temperature and postannealed at 250 deg. 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 ∼48 at. % (∼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 [1120]Fe-Ge||110]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(2x2) and c continuously decreases with Fe content, whereas from Fe1.5Ge to Fe1.1Ge the surface periodicity is (√(3)x√(3))R30 deg. 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 ∼450 K 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.