Nuclear magnetic resonance (NMR) [#!ChibaPRB91!#,#!GaveauJAP91!#,#!FujiwaraPRB92!#,#!FujiwaraPRB93!#]

The temperature dependence of the proton T₁ relaxation rate was also well described by excitations over an energy gap, as $T_1^{-1}\propto\exp(-E_{\rm g}/kT)$[85,86,87]. Fujiwara et al. [87] reported that the field dependence of the gap ($E_{\rm g}$) agreed well with that of the gap observed in the neutron scattering measurements [82]. Below 4 K, the T₁ relaxation rate took a maximum at around the critical field ($H_c\sim9.5$ T). This is because the external field brought the first excited state to the zero-energy, maximizing the spectral density responsible for the T₁ relaxation of the proton nuclear spins [87]. In magnetic fields below $\sim 3$ T, the field suppressed the T₁ relaxation rate. In this regime, it has been found that Zn doping to NENP enhances the T₁ relaxation rate, suggesting a contribution of doping induced unpaired spins [88].