Effect of Disorder and Dipolar Interactions in Two-Dimensional Assemblies of Iron-Oxide Magnetic Nanoparticles

Abstract

We examine the magnetic properties of two-dimensional monolayers of ironoxide magnetic nanoparticles arranged on a hexagonal lattice and interacting through magnetic dipole forces. Monte Carlo simulations are used to reconstruct the zero-field cooled magnetization curve, as well as other relevant quantities (Zeeman and dipolar energies). The blocking temperature is found to increase with shorter interparticle distance. The effect of structural disorder is studied by randomly removing some nanoparticles from their lattice sites. Surprisingly, it is found that disorder has little impact on the global magnetic structure of the sample. The numerical results are compared to existing experimental measurements.

Structural and magnetic properties of cobalt iron disulfide (CoxFe1−xS2) nanocrystals

“…For a ferromagnetic exchange system, the mean field of ferromagnetic exchange coupling is positive, yielding T CW > 0. For a disorder (random anisotropy axes) system with dipolar interaction 30 , such as in the present case with x = 0.5, a negative mean field is expected from the first part of the dipolar energy while the average over the random field of the second part of the dipolar energy would give zero, yielding T CW < 0. In a competing interaction system, with x ≥ 0.5, T CW can increase from negative to positive as J increases for a fixed value of g .…”

Effect of Dipolar Interactions on the Assembly Process of Iron Oxide Nanoparticles Promoted by the CuAAC “Click” Chemistry Reaction

“…In a 2D assembly, the randomness of the assembly generates some geometrical frustration which results in a spin glass state (Table 2). 10,21,46 Isolated NP5 and NP10 exhibit the highest coercive fields which can be ascribed to the significant contribution of the surface anisotropy energy in the case of the absence of dipolar interactions. H C decreases significantly when the density of NP5 and NP10 increases (Table 2).…”

“…H C decreases significantly when the density of NP5 and NP10 increases (Table ). The interpretation of the variation of the coercive field and the remanent magnetization as a function of the spatial arrangement of nanoparticles is not trivial. ,− Nanoparticle assemblies present a distribution of interparticle distances and domain sizes which affect dipolar interactions. , Indeed, dipolar interactions favor the reversal of magnetic moments upon applying a magnetic field which agree with a lower H C in comparison to isolated nanoparticles which need much more energy to counterbalance the oscillation of magnetic moments induced by thermal fluctuations. Such a behavior is perfectly observed for NP5 and NP10 assemblies, as shown by the decrease of H C by 5 times for NP5 and 4 times for NP10 between assemblies prepared after 1 and 48 h (Table ).…”

“…When the nanoparticle size increases, the energy barrier E a is higher and results in the increase of T Max . Furthermore, dipolar interactions between nanoparticles in assemblies also contribute to the significant increase of T MAX . ,,,, Given the 2-dipole approximation, the dipolar energy between two nanoparticles can be calculated as where μ is the magnetic moment of the nanoparticle and a is the interparticle distance. , Although E d is favored by short interparticle distances (Figure S11), it also dramatically increases with the nanoparticle size because the magnetic moment μ of each nanoparticle is described as μ = M S × V with M S being the magnetization saturation. Considering similar interparticle distances of 3 nm resulting for the coating of oleic acid at the nanoparticle surface, regardless of their size, E d increases significantly with the nanoparticle size (Table ).…”

“…Furthermore, dipolar interactions between nanoparticles in assemblies also contribute to the significant increase of T MAX . 10,27,47,50,46 Given the 2dipole approximation, the dipolar energy between two nanoparticles can be calculated as…”