¹H-NMR study of the idle-spin magnet Cu₃(OH)₄SO₄

¹H-NMR study of a spin-1/2 triple-chain system Cu₃(OH)₄SO₄, in which the realization of an `idle-spin' state has been suggested recently from neutron experiments, were carried out in the temperature range down to 1.4 K. In the experiments under about 1 T, we found a clear anomaly of the nuclear magnetic relaxation rate and drastic change of the spectrum at around T_N = 5.4 K. NMR spectrum below T_N has a characteristic shape of an antiferromagnetic ordered state. This low-temperature spectrum is consistently reproduced by the calculation of local fields at proton sites on the basis of the idel-spin state. Further, T_N decreased as the applied field is increased up to 6.9 T.