Spin correlation in the frustrated antiferromagnetMnS2above the Néel temperature

Spin correlations in the antiferromagnet ${\mathrm{MnS}}_2$ based on an inherently frustrated face-centered-cubic lattice have been investigated by quasielastic neutron scattering above the Néel temperature. The diffuse scattering above ${\mathit{T}}_{\mathit{N}}$ is centered at an incommensurate position k=(1,${\mathit{k}}_{\mathit{y}}$,0) and is temperature dependent. The component ${\mathit{k}}_{\mathit{y}}$ increases continuously from ${\mathit{k}}_{\mathit{y}}$=0.40 at T=115 K to ${\mathit{k}}_{\mathit{y}}$=0.44 at a temperature just above ${\mathit{T}}_{\mathit{N}}$=48.2 K. At ${\mathit{T}}_{\mathit{N}}$, a first-order phase transition takes place and ${\mathit{k}}_{\mathit{y}}$ jumps abruptly to the commensurate value ${\mathit{k}}_{\mathit{y}}$=1/2. The ratio of the correlation lengths along and perpendicular to the incommensurate modulation is very close to unity at ${\mathit{T}}_{\mathit{N}}$. The spatial correlations extend only over little more than one unit cell (ξ=8 Å) even just above ${\mathit{T}}_{\mathit{N}}$. The inverse time correlation increases continuously with temperature from 0.16 THz at ${\mathit{T}}_{\mathit{N}}$ up to 2.9 THz at 150 K, the highest temperature investigated. Mean-field calculations of the diffuse scattering in a framework of a Heisenberg Hamiltonian with exchange up to the fourth neighbors agree reasonably well with the experimental data. However, such calculations fail to explain the centering of the diffuse scattering at the incommensurate wave vector.