Asymmetric hysteresis loops and horizontal loop shifts in purely ferromagnetic nanoparticles

Horizontally shifted hysteresis loops and asymmetric hysteresis loops are commonly related with exchange-biased samples, consisting of a ferromagnet exchange-coupled to an antiferromagnet or a ferrimagnet. In pure ferromagnetic samples, such effects may experimentally occur erroneously due to undetected minor loops or additional anisotropies, while in simulations they may occur due to thermal effects. However, performing simulations of ferromagnetic nanostructures at zero temperature with large enough saturation fields should not result in such asymmetries. Here we report on micromagnetic simulations using the Object Oriented MicroMagnetic Framework (OOMMF) at zero temperature, performed on symmetric sputtered nanoparticles with different shapes and holes or slits inside. Our results show that not only the small deviations of the systems under investigation due to random anisotropy orientations in the different grains may result in strong deviations of the magnetization reversal processes and hysteresis loops, but that there may also occur clearly asymmetric, horizontally shifted hysteresis loops in a purely ferromagnetic nanoparticle.