Direct evidence of spin frustration in the fcc antiferromagnet NiS2

NiS2 is a well-known Mott insulator with anomalous antiferromagnetic long-range order of coexistent type I (Q(M) = (1, 0, 0), T-N1 = 40 K) and type 11 (Q(M) = (1, 1 1) 2 21 2 1 T-N2 = 30 K). Extensive neutron-scattering measurements reveal that magnetism in NiS2 is governed by geometrical spin frustration, resulting in magnetic diffuse scattering extending along the fcc zone boundary. Although the diffuse scattering exists at temperatures as high as 250 K (6T(N1)), it disappears rapidly below T-N2, associated with minor crystal distortion. We observed a clear energy gap in addition to the low-energy spin-wave excitation at significantly below 30 K, and obtain evidence that degeneracy due to the coexistence of the two types of antiferromagnetism is relieved in the ground state via the reduction in symmetry due to distortion.