Numerical simulations of collective magnetic properties and magnetoresistance in 2D ferromagnetic nanoparticle arrays

Magnetic properties and magnetoresistance (MR) in 2D magnetic nanoparticle (NP) arrays are investigated by solving the Landau--Lifshitz--Gilbert equation at T = 0 K. The interparticle interactions induce a decrease in the coercive field and in the MR amplitude compared with the non-interacting case, while in some cases, the variation of the remanent magnetization is found to be non-monotonic when increasing the dipolar strength. For different values of the anisotropy, these variations of the coercive field, the remanent magnetization and the MR ratio are reproduced and exhibit a scaling on the dipolar/anisotropy ratio. These results suggest that the magnetic properties of the assemblies can be described by an individual or collective behaviour depending on the balance between the magnetic anisotropy and the dipolar interactions. In the case of strongly interacting NPs, the corresponding configurations of the magnetic moments at the remanent state reveal the formation of a ferromagnetic order at moderate dipolar strength (increase in the remanent magnetization) while small ferromagnetic domains/chains coupled antiferromagnetically are obtained in the case of strongly interacting NPs (decrease in the remanent magnetization). Such domains lead to a reduction in the MR amplitude and to a deviation from the m2-law in the resistance-magnetic field [R(H)] characteristic of the non-interacting case.