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

Abstract

In this work, we have studied the effect of internal coupling in magnetic nanoparticles withinverted core-shell structure (antiferromagnet-ferrimagnet) and also magnetic surfaceanisotropy, performing Monte Carlo simulations based on a micromagnetic model applied in thelimit of lattice size equal to the crystalline unit cell. In the treatment, different internal regions ofthe particle were labeled in order to analyze the magnetic order and the degree of couplingbetween them. The results obtained are in agreement with experimental observations inCoO/CoFe2O4 and ZnO/CoFe2 O systems, which we have taken as reference. It is observed thatthe surface anisotropy decreases the coercive field and the blocking temperature of the system.However, the core/shell coupling improves these properties and magnetically hardens thesystem. Our study shows that a significant magnetic stress is generated in the system, leading tomagnetic disorder in the spins of the particle interface. On the other hand, in cases of highsurface anisotropy, within a range of interfacial exchange values, a clear magnetic disorder isobserved in the shell, which leads to anomalous behavior because the magnetization reversalprocess is no longer coherent.