Exploring magnetic disorder in inverted core-shell nanoparticles: the role of surface anisotropy and core/shell coupling

In this work, we have studied the effect of internal coupling in magnetic nanoparticles with inverted core-shell structure (antiferromagnet-ferrimagnet) and also magnetic surface anisotropy, performing Monte Carlo simulations based on a micromagnetic model applied in the limit of lattice size equal to the crystalline unit cell. In the treatment, different internal regions of the particle were labeled in order to analyze the magnetic order and the degree of coupling between them. The results obtained are in agreement with experimental observations in CoO/CoFe2O4 and ZnO/CoFe2O systems, which we have taken as reference. It is observed that the surface anisotropy decreases the coercive field and the blocking temperature of the system. However, the core/shell coupling improves these properties and magnetically hardens the system. Our study shows that a significant magnetic stress is generated in the system, leading to magnetic disorder in the spins of the particle interface. On the other hand, in cases of high surface anisotropy, within a range of interfacial exchange values, a clear magnetic disorder is observed in the shell, which leads to anomalous behavior because the magnetization reversal process is no longer coherent.