Unusual magnetism in templated NiS nanoparticles

Abstract

Nanostructured NiS was prepared by inclusion into anodic alumina templates. Theresultant particles were found to be stoichiometric and highly crystalline. The particlesdisplayed small particle superparamagnetism, and a low temperature (at 48 K(Tsg)) spin-freezing phenomenon (a spin-glass) and higher temperature (170 K) thermal blockingof small particle magnetic moment fluctuations were both observed for the firsttime for a sulfide material. Very unusually, these NiS materials are quite distinctfrom antiferromagnetic nanoparticulate sulfide materials, as they display a hightemperature ferromagnetic-like phase. The saturation magnetization, the remanentmagnetization, the coercivity and the ferromagnetic mass susceptibility were measured as0.58 emu g − 1 (at 100 K),0.19 emu g − 1, 219.5 Oe (at170 K) and ∼ 900 × 10 − 6 emu Oe − 1 g − 1 respectively and these are consistent with a moderately strong ferromagnetism. Thematerials had an unexpectedly high Curie temperature of 390 K. The decrease of thesaturation magnetization value at 30 K suggests that the ferromagnetic response is a surfacephenomenon and the high coercivity of the paramagnetic component well aboveTsg suggests that the core can be described as superparamagnetic.