Unconventional spin freezing and fluctuations in the frustrated antiferromagnet NiGa₂S₄

Longitudinal-field muon spin rotation (LF-μSR) experiments in the two- dimensional geometrically-frustrated antiferromagnet NiGa2S4 reveal a number of features of spin freezing and dynamics in this system. Long-lived (correlation time 10-6 s at 2 K) disordered Ni spin freezing sets in abruptly below Tf = 8:5±0:5 K. At low temperatures slow Ni spin fluctuations are found in low applied field HL that are rapidly suppressed for HL and 0:04 T. Activated muon spin relaxation above Tf is strong evidence for 2D critical behaviour. The LF-μSR data indicate that NiGa₂S₄ is neither a conventional magnet nor a singlet spin liquid, and raise the question of how to reconcile the strongly field-dependent muon relaxation with the field-independent specific heat.