  First Order Reversal Curve (FORC) diagrams are a popular tool in geophysics
and materials science for the characterization of magnetic particles of natural
and synthetic origin. However, there is still a lot of controversy about the
rigorous interpretation of the origin of certain features in a FORC diagram. In
this study, we analyze FORCs computed by micromagnetic simulations of Co cubes
with dimensions of 50, 100 and 150 nm and uniaxial magnetocrystalline
anisotropy. For the larger cubes we observe the formation of a stable
two-domain state. The nucleation of a reversed domain and its subsequent
annihilation are clearly visible as separate peaks in the FORC diagram. They
spread out along the coordinate axis in the FORC diagram, which is associated
with the bias field $H_U$ of a Preisach hysteron. Based on our findings, we
state that a FORC diagram peak spreading along the $H_U$ axis can have its
origin in the step-wise magnetization reversal driven by nucleation of domains
in a single particle. This means that we have identified another mechanism
apart from the well-known magnetostatic interaction between a set of particles
that leads to features in the FORC diagram extending along the $H_U$-axis. Our
study demonstrates that if FORCs shall be used as a quantitative tool to assess
the microstructure of samples containing magnetic material, more information
from other methods will be required to identify the correct physical mechanism
by which a certain "fingerprint" in a FORC diagram is produced.
