Surface and bulk magnetic properties of pyriteNiS2: Magnetization and neutron-scattering studies

Abstract

In pyrite-structure antiferromagnetic ${\mathrm{NiS}}_2$, the magnetic susceptibility χ(T) shows downward curvature above the Néel transition (${\mathit{T}}_{\mathit{N}1}$=39.2 K), suggesting that magnetic correlations persist to T=400 K. However, neutron-scattering measurements indicate that the bulk magnetic behavior above ${\mathit{T}}_{\mathit{N}1}$ is conventional. We present evidence that the unusual temperature dependence of χ(T) arises from a surface contribution. At ${\mathit{T}}_{\mathit{N}2}$=29.75 K a transition occurs to a weakly ferromagnetic state, accompanied by a small structural distortion. We show that this distortion is not tetragonal as suggested previously, but is most likely rhombohedral. Below ${\mathit{T}}_{\mathit{N}2}$, χ(T) is anisotropic; our data suggest that in this range χ(T) is dominated by next-nearest-neighbor exchange interactions.