  Magnetoelectric coupling in ferromagnet/multiferroic systems is often
manifested in the exchange bias effect, which may have combined contributions
from multiple sources, such as domain walls, chemical defects or strain. In
this study we magnetically "fingerprint" the coupling behavior of CoFe grown on
epitaxial BiFeO3 (BFO) thin films by magnetometry and
first-order-reversal-curves (FORC). The contribution to exchange bias from
71{\deg}, 109{\deg} and charged ferroelectric domain walls (DWs) was elucidated
by the FORC distribution. CoFe samples grown on BFO with 71{\deg} DWs only
exhibit an enhancement of the coercivity, but little exchange bias. Samples
grown on BFO with 109{\deg} DWs and mosaic DWs exhibit a much larger exchange
bias, with the main enhancement attributed to 109{\deg} and charged DWs. Based
on the Malozemoff random field model, a varying-anisotropy model is proposed to
account for the exchange bias enhancement. This work sheds light on the
relationship between the exchange bias effect of the CoFe/BFO heterointerface
and the ferroelectric DWs, and provides a path for multiferroic device analysis
and design.
