  Magnetic reversal processes of a FePt/{\alpha}-Fe/FePt trilayer system with
in-plane easy axes have been investigated within a micromagnetic approach. It
is found that the magnetic reversal process consists of three steps: nucleation
of a prototype of domain wall in the soft phase, the evolution as well as the
motion of the domain wall from the soft to the hard phase and finally, the
magnetic reversal of the hard phase. For small soft layer thickness Ls, the
three steps are reduced to one single step, where the magnetizations in the two
phases reverse simultaneously and the hysteresis loops are square with
nucleation as the coercivity mechanism. As Ls increases, both nucleation and
pinning fields decrease. In the meantime, the single-step reversal expands to a
standard three-step one and the coercivity mechanism changes from nucleation to
pinning. The critical thickness where the coercivity mechanism alters, could be
derived analytically, which is found to be inversely proportional to the square
root of the crystalline anisotropy of the hard phase. Further increase of Ls
leads to the change of the coercivity mechanism from pinning to nucleation.
