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 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.