Classical and quantum spin dynamics in the fcc antiferromagnetNiS2with frustration

Abstract

The unusual coexistence of two antiferromagnetic ͑AF͒ long-range orderings ͑LRO͒ in single-crystal NiS 2 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 boundaries͒ in the extended critical temperature region (T N1 ϭ39.3 KϽTϽ150 K) providing direct evidence for nearly frustrated antiferromagnetism on the face centered cubic ͑fcc͒ 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 LDAϩU band calculation. The system is found to be located very close to the phase boundary between the type-I AF state with Q AF ϭ(1,0,0), and the type-II AF state with Q AF ϭ͑1/2,1/2,1/2͒. 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 Néel temperature T N1 ϭ39.3 K, while at T N2 ϭ30.6 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 excitation͒ of the spin fluctuation.

Magnetization Anomaly due to the Non-Coplanar Spin Structure in NiS2

“…2(a) inset]. As has been discussed above, various measurements such as the neutron scattering 37) and specific heat 23) measurements have proved that the phase below T N1 is not spin glass but has the AF long-range order. We have clarified for the first time the isotropic character of the hysteresis in the NAF-phase.…”

“…Previously, the surface effect was discussed to explain the sample dependence of the susceptibility observed in the paramagnetic phase (PM-phase) above T N1 ¼ 38 K. 34) It has been reported that the susceptibility χ in the PM-phase increases with increasing surface area 21) and strongly depends on the magnetic field 0 H. 23) To make a detailed comparison, we summarized the reported values of the magnetic susceptibility at 50 K, 21,23,33) χ(50 K), for NiS 2 in addition to our results (S-1) in Table I. Indeed, the samples used in the literature that claims the surface contribution show relatively large susceptibility values as well as strong field dependence.…”

Molecular Spin Resonance in the Geometrically Frustrated MagnetMgCr2O4by Inelastic Neutron Scattering

“…Therefore, magnetic quasielastic scattering in the paramagnetic phases was intensively studied by neutron scattering in spinel, pyrochlore, kagome, triangular systems, and so on. As a result, characteristic spatial correlations of spins were found, such as a small six-spin cluster [3][4][5][6][7][8], a large six-spin cluster [9], a seven-spin cluster [10], spin ice [11,12], and short-range order with propagation vector(s) [13,14].…”

Chiral Spin Density Wave Order on the Frustrated Honeycomb and Bilayer Triangle Lattice Hubbard Model at Half-Filling

“…In this paper, we focus on the antiferromagnetic (AF) χ-SDW metals and insulators with ℓ S ℓ = 0 in materials and models with strong electron correlation and magnetic frustration. They have been discovered in charge transfer insulators NiS 2 [8][9][10][11], metallic γ-FeMn alloys [12][13][14][15] and related materials where the magnetic moments reside on the frustrated face-centered-cubic lattice. Neutron scattering observed noncoplanar AF order with 4-sublattices and 3-ordering wavevectors.…”

“…On the theoretical side, it has been shown that frustrated Heisenberg two-spin exchange interactions are insufficient to produce the AF χ-SDW order; additional 4-spin exchange interactions are necessary for such a noncoplanar SDW to emerge from the many degenerate magnetic states [11,[16][17][18]. In addition, weak-coupling approaches such as nesting based models [19] and band structure (LDA) and LDA+U calculations [11,20,21] have been performed to study the complex magnetic order in these materials. While a microscopic theory for the χ-SDW order is currently lacking, it is believed that both strong correlation and geometric frustration play vital roles in its origin.…”