Classical and quantum spin dynamics in the fcc antiferromagnet NiS2 with frustration -: art. no. 094409

The unusual coexistence of two antiferromagnetic (AF) long-range orderings (LRO) in single-crystal NiS2 is investigated through measurements of inelastic neutron scattering, specific heat, uniform magnetic susceptibility, and resistivity. Neutron scattering intensity reveals a honeycomb pattern of the intensity distribution in reciprocal lattice space (continuous-line structure along the Brillouin zone boundaries) in the extended critical temperature region (T-N1=39.3 K<150 K) providing direct evidence for nearly frustrated antiferromagnetism on the face centered cubic (fcc) lattice. The observed geometrical pattern of the critical scattering makes it possible to determine the exchange coupling constants between the first, second, and third nearest neighbors, which are found to be consistent with LDA+U band calculation. The system is found to be located very close to the phase boundary between the type-I AF state with Q(AF)=(1,0,0), and the type-II AF state with Q(AF)=(1/2,1/2,1/2). Theoretical analysis based on the Onsager reaction field reproduces the various features in experiments at a semiquantitative level. The type-I AF order appears the highest Neel temperature T-N1=39.3 K, while at T-N2=30.6 K, the type-I and type-II AF LRO coexist via a first-order phase transition accompanied by a small but finite lattice distortion. This coexistence is explained in terms of coupling between the spin and lattice in the nearly degenerate case, although the type-I and type-II AF LRO become mutually incompatible in the fcc symmetry at higher temperatures. The AF LRO and lattice distortion lift the degeneracy and the accumulation of spectral weight at low energy, recovering the quantum nature (spin-wave excitation) of the spin fluctuation.