  We revisit the problem of diffusive proximity systems involving
superconductors and normal metals (or ferromagnets) with magnetic disorder. On
the length scales much larger than its correlation length, the effect of
sufficiently weak magnetic disorder may be incorporated as a local spin-flip
term in the Usadel equations. We derive this spin-flip term in the general case
of a three-dimensional disordered Zeeman-type field with an arbitrary
correlation length. Three different regimes may be distinguished: pointlike
impurities (the correlation length is shorter than the Fermi wavelength),
medium-range disorder (the correlation length between the Fermi wavelength and
the mean free path), and long-range disorder (the correlation length longer
than the mean free path). We discuss the relations between these three regimes
by using the three overlapping approaches: the Usadel equations, the non-linear
sigma model, and the diagrammatic expansion. The expressions for the spin-flip
rate agree with the existing results obtained in less general situations.
