  Very high magnetic anisotropies have been theoretically predicted for
strained Fe-Co(-X) and indeed several experiments on epitaxial thin films
seemed to confirm strain induced anisotropy enhancement. This study presents a
critical analysis of the different contributions to perpendicular anisotropy:
volume, interface and surface anisotropies. Tracing these contributions,
thickness series of single layer films as well as multilayers with Au-Cu
buffers/interlayers of different lattice parameters have been prepared. The
analysis of their magnetic anisotropy reveals a negligible influence of the
lattice parameter of the buffer. Electronic effects, originating from both, the
Au-Cu interface and the film surface, outrange the elastic effects. Surface
anisotropy, however, exceeds the interface anisotropy by more than a factor of
three. A comparison with results from Density Functional Theory suggests, that
the experimentally observed strong perpendicular surface anisotropy originates
from a deviation from an ideal oxide-free surface. Accordingly, tailored
Fe-Co-X/oxide interfaces may open a route towards high anisotropy in rare-earth
free materials.
