  We present x-ray resonant magnetic reflectivity (XRMR) as a very sensitive
tool to detect proximity induced interface spin polarization in Pt/Fe,
Pt/Ni$_{33}$Fe$_{67}$, Pt/Ni$_{81}$Fe$_{19}$ (permalloy), and Pt/Ni bilayers.
We demonstrate that a detailed analysis of the reflected x-ray intensity gives
insight in the spatial distribution of the spin polarization of a non-magnetic
metal across the interface to a ferromagnetic layer. The evaluation of the
experimental results with simulations based on optical data from ab initio
calculations provides the induced magnetic moment per Pt atom in the spin
polarized volume adjacent to the ferromagnet. We find the largest spin
polarization in Pt/Fe and a much smaller magnetic proximity effect in Pt/Ni.
Additional XRMR experiments with varying photon energy are in good agreement
with the theoretical predictions for the energy dependence of the magnetooptic
parameters and allow identifying the optical dispersion $\delta$ and absorption
$\beta$ across the Pt L3-absorption edge.
