  The spin configuration in the ferromagnetic part during the magnetization
reversal plays a crucial role in the exchange bias effect. Through Monte Carlo
simulation, the exchange bias effect in ferromagnetic-antiferromagnetic
core-shell nanoparticles is investigated. Magnetization reversals in the
ferromagnetic core were controlled between the coherent rotation and the domain
wall motion by modulating ferromagnetic domain wall width with parameters of
uniaxial anisotropy constant and exchange coupling strength. An anomalous
monotonic dependence of exchange bias on the uniaxial anisotropy constant is
found in systems with small exchange coupling, showing an obvious violation of
classic Meiklejohn-Bean model, while domain walls are found to form close to
the interface and propagate in the ferromagnetic core with larger uniaxial
anisotropy in both branches of the hysteresis. The asymmetric magnetization
reversal with the formation of a spherical domain wall dramatically reduces the
coercive field in the ascending branch, leading to the enhancement of the
exchange bias. The results provide another degree of freedom to optimize the
magnetic properties of magnetic nanoparticles for applications.
