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

Abstract

We have studied the growth and magnetic properties of thin Fe-Ge films synthesized (codeposited at room temperature and postannealed at $250\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$) 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 $\ensuremath{\sim}48\text{ }\text{at}\text{.}\text{ }\mathrm{%}$ ($\ensuremath{\sim}{\text{Fe}}_{1.1}\text{Ge}$ composition). In particular, the film is homogeneous and flat and adopts a crystalline structure of hexagonal symmetry derived from the $\text{B}{8}_{2}$ $({\text{Ni}}_{2}\text{In})$ structure over a wide stoichiometry range extending from ${\text{Fe}}_{2}\text{Ge}$ to ${\text{Fe}}_{1.1}\text{Ge}$. The epitaxial orientation between the Ge substrate and the germanide layer is $(0001)\text{Fe-Ge}\ensuremath{\parallel}(111)\text{Ge}$ with $[11\overline{2}0]\text{Fe-Ge}\ensuremath{\parallel}[\overline{1}10]\text{Ge}$. 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 $(\ensuremath{\sim}{\text{Fe}}_{1.5}\text{Ge})$. Indeed, from ${\text{Fe}}_{2}\text{Ge}$ to ${\text{Fe}}_{1.5}\text{Ge}$ the surface periodicity is $p(2\ifmmode\times\else\texttimes\fi{}2)$ and $c$ continuously decreases with Fe content, whereas from ${\text{Fe}}_{1.5}\text{Ge}$ to ${\text{Fe}}_{1.1}\text{Ge}$ the surface periodicity is $(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})R30\ifmmode^\circ\else\textdegree\fi{}$ 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 $[{\text{Fe}}_{2}\text{Ge},{\text{Fe}}_{1.1}\text{Ge}]$ composition range the layer is no more homogeneous. More precisely, for higher Fe content the film contains both the above mentioned ${\text{Ni}}_{2}\text{In}$-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 ${\text{Ni}}_{2}\text{In}$-derived epilayers are ferromagnetic with a Curie temperature that varies drastically with the stoichiometry, rising up to a high ${T}_{\text{C}}$ value of $\ensuremath{\sim}450\text{ }\text{K}$ for the Fe-rich ${\text{Fe}}_{1.9}\text{Ge}$ 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.