  Models of exchange-bias in thin films have been able to describe various
aspects of this technologically relevant effect. Through appropriate choices of
free parameters the modelled hysteresis loops adequately match experiment, and
typical domain structures can be simulated. However, the use of these
parameters, notably the coupling strength between the systems' ferromagnetic
(F) and antiferromagnetic (AF) layers, obscures conclusions about their
influence on the magnetization reversal processes. Here we develop a 2D
phase-field model of the magnetization process in exchange-biased CoO/(Co/Pt)xn
that incorporates the 10 nm-resolved measured local biasing characteristics of
the antiferromagnet. Just three interrelated parameters set to measured
physical quantities of the ferromagnet and the measured density of
uncompensated spins thus suffice to match the experiment in microscopic and
macroscopic detail. We use the model to study changes in bias and coercivity
caused by different distributions of pinned uncompensated spins of the
antiferromagnet, in application-relevant situations where domain wall motion
dominates the ferromagnetic reversal. We show the excess coercivity can arise
solely from inhomogeneity in the density of biasing- and anti-biasing pinned
uncompensated spins in the antiferromagnet. Counter to conventional wisdom,
irreversible processes in the latter are not essential.
