  We present the characterization of Mg-Co-Zr tri-layer stacks by using x-ray
fluorescence induced by x-ray standing waves, both in the grazing incidence
(GI) and grazing exit (GE) modes. The introduction of a slit in the direction
of the detector improves the angular resolution by a factor 2 and significantly
the sensitivity of the technique for the chemical characterization of the
buried interfaces. By observing the intensity variations of the Mg Kalpha and
Co Lalpha characteristic emissions as a function of the incident (GI mode) or
detection (GE mode) angle, we show that the interfaces of the Si/[Mg/Co/Zr] x30
multilayer are abrupt, whereas in the Si/[Mg/Zr/Co] x30 multilayer a strong
intermixing occurs at the Co-on-Zr interfaces. The explanation of this opposite
behaviour of the Co-on-Zr and Zr-on-Co interfacesis given by the calculation of
the mixing enthalpies of the Co-Mg, Co-Zr and Mg-Zr systems, which shows that
the Co-Zr system presents anegative value and the two others positive values.
Together with the difference of the surface free energies of Zr and Co, this
leads us to consider the Mg/Zr/Co system as aMg/Co x Zr y bi-layer stack, with
x/y estimated around 3.5.
