We show that an in situ Kerr rotation measurement is a very effective technique for the study of antiferromagnetic (AF) ferromagnetic (F) film couples. Magnetic signals can be obtained even in the case where the (AF) is the top layer up to at least 200 Å of AF thickness. We have used this in situ approach combined with ion milling to study the thickness dependence of the magnetic properties of Mn50Fe50/Ni80Fe20 systems. We observe that the exchange bias field has a surprisingly sharp onset at a critical thickness of AF∼50 Å. We show that this is consistent with a simple model and that the magnetic anisotropy of MnFe can be estimated from the observed critical thickness to be ∼1.35×105 erg/cm 3. The exchange field showed the predicted proportionality to the inverse of the F thickness from ∼50 to 400 Å. Auger spectroscopy and spin polarized secondary electron emission have been used to rule out gross artifacts due to ion milling.