In this paper we provide an initial, qualitative description of the basic m
agnetization reversal processes that occur when a soft ferromagnetic layer
is coupled to an antiferromagnet. We find that the magnetization reversal i
n the ferromagnetic layer adjacent to the antiferromagnet, i.e., the pinned
layer, is dominated by thermal activation processes, We have developed, an
d report here, a model that accounts for seven distinct features of the mag
netization curve of the pinned layer. The model is based upon the formation
of domains in the antiferromagnetic layer whose growth is dominated by the
rmal activation processes. The thermal activation of these domain processes
can result in shifts of the hysteresis loop in either direction, depending
on the magnetic history of the sample and the rate of field sweep. In cons
equence, we call into question the current definition of the exchange field
H-ex typically used to characterize such systems.