Unconventional spin freezing and fluctuations in the frustrated antiferromagnet NiGa2S4
Abstract
Muon spin rotation (muSR) experiments reveal unconventional spin freezing and dynamics in the two-dimensional (2D) triangular lattice antiferromagnet NiGa2S4. Long-lived disordered Ni-spin freezing (correlation time > 10-6 s at 2 K) sets in below Tf = 8.5 +- 0.5 K with a mean-field-like temperature dependence. The observed exponential temperature dependence of the muon spin relaxation above Tf is strong evidence for 2D critical spin fluctuations. Slow Ni spin fluctuations coexist with quasistatic magnetism at low temperatures but are rapidly suppressed for fields > 10 mT, in marked contrast with the field-independent specific heat. The muSR and bulk susceptibility data indicate a well-defined 2D phase transition at Tf, below which NiGa2S4 is neither a conventional magnet nor a singlet spin liquid.
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