An ac susceptibility study in capped Ni/Ni(OH)₂ core-shell nanoassemblies: Dual peak observations

In this study, the ac susceptibility (χ′ and χ″) variation with temperature (10-100 K) for oleic acid (OA) capped Ni/Ni(OH) ₂ core-shell nanoparticle assemblies are reported at frequencies varying from 0.1 to 1000 Hz. Nanoparticle assemblies, with two average particle diameters of ∼34 nm and ∼14 nm, were synthesized using a wet chemical synthesis approach. Two peaks in the ac susceptibility versus temperature curves are clearly discernable for each of the samples. The first, occurring at ∼22 K was attributed to the paramagnetic/antiferromagnetic transition of the Ni(OH)₂ present in the shell. The second higher temperature peak was attributed to the superparamagnetic blocking of the pure Ni situated at the core of the nanoparticles. The higher temperature peaks in both the χ′ and χ″ curves were observed to increase with increasing frequency. Thus the Néel and the blocking temperatures for such core-shell nanoassemblies were clearly identified from the ac analysis, whereas they were not discernible (superimposed) even from very low dc (FC/ZFC) field measurements. Interparticle interactions within the assemblies were studied through the fitting of phenomenological laws to the experimental datasets. It is observed that even with an OA capping layer, larger Ni/Ni(OH)₂ nanoparticles experience a greater degree of sub-capping layer oxidation thus producing lower magnetic interaction strengths.