Unraveling the origin of training in granular Co-CoO exchange bias systems with buried antiferromagnetic constituents

•A model to simulate non-layered exchange bias systems was developed.•Low-anisotropic buried uncompensated spins generate exchange bias.•Perpendicular anisotropy is responsible for training effect.•Asymmetric magnetization reversals are linked to out-of-plane anisotropy. Training is a common effect in exchange bias systems and accounts for the decrease of the exchange bias loop shift and coercivity with consecutively measured hysteresis loops until steady values. This is an ageing-like phenomenon that is related to the metastable state of the ferromagnetic/antiferromagnetic interface after field cooling. However, its origin still remains intriguing and not univoquely established. Here, by micromagnetic simulations considering discrete non-interacting antiferromagnetic grains embedded in a ferromagnetic matrix, it is demonstrated that the origin of training in granular Co-CoO exchange bias systems prepared by O ion implantation into Co thin films is linked to the perpendicular anisotropy of rotatable interface uncompensated spins. The simulations are compared to experimental data as reported in Physical Review B 89 (2014) 144407. The out-of-plane nature of the rotatable anisotropy of the system is also responsible for the magnetic reversal asymmetry between the first and the second magnetic reversals, evidencing the interconnection between training and magnetization reversal, suggesting that training effect and magnetization reversal asymmetry are ultimately interconnected through perpendicular anisotropy.