Unusual magnetism in templated NiS nanoparticles

Nanostructured NiS was prepared by inclusion into anodic alumina templates. The resultant particles were found to be stoichiometric and highly crystalline. The particles displayed small particle superparamagnetism, and a low temperature (at 48K (T_sg)) spin-freezing phenomenon (a spin-glass) and higher temperature (170K) thermal blocking of small particle magnetic moment fluctuations were both observed for the first time for a sulfide material. Very unusually, these NiS materials are quite distinct from antiferromagnetic nanoparticulate sulfide materials, as they display a high temperature ferromagnetic-like phase. The saturation magnetization, the remanent magnetization, the coercivity and the ferromagnetic mass susceptibility were measured as 0.58emug^-1 (at 100K), 0.19emug^-1, 219.5Oe (at 170K) and ∼ 900 × 10^-6emuOe^-1g^-1 respectively and these are consistent with a moderately strong ferromagnetism. The materials had an unexpectedly high Curie temperature of 390K. The decrease of the saturation magnetization value at 30K suggests that the ferromagnetic response is a surface phenomenon and the high coercivity of the paramagnetic component well above T_sg suggests that the core can be described as superparamagnetic.