59CoNMR studies of metallicNaCo2O4

Abstract

${}^{59}\mathrm{Co}$ NMR studies have been performed in metallic ${\mathrm{NaCo}}_{2}{\mathrm{O}}_{4}$ in the temperature range 350--100 K. The powder patterns predominantly correspond to typical second-order quadrupolar splitted central transition of two inequivalent cobalt sites. Features of magnetic interaction are also present. The analysis of the NMR line shape shows that the electric field gradient (EFG) experienced by one of the two sites (site 1) is independent of temperature, whereas around 47% increment in EFG has been observed for site 2 as the temperature is lowered from 350 to 100 K. ${}^{59}\mathrm{Co}$ NMR shift for each site is almost isotropic in character which varies linearly with susceptibility and basically consists of two parts. Both the sites suffer a large temperature independent shift due to the contribution from orbital paramagnetism $({\ensuremath{\chi}}_{\mathrm{vv}})$ which is analogous in metals to the Van Vleck paramagnetic susceptibility. The temperature-dependent Knight shift for site 1 reveals a negative hyperfine coupling constant ${(A}_{\mathrm{hf}}^{d})$ arising from inner s core polarization by the d electrons which are itinerant in character. A positive ${A}_{\mathrm{hf}}^{d}$ observed for site 2 cannot be interpreted directly in a metallic system such as ${\mathrm{NaCo}}_{2}{\mathrm{O}}_{4}.$ A comparison of the magnitude of ${A}_{\mathrm{hf}}^{d}$ with those of other cobalt oxides suggests that the nucleus which is being probed by NMR, belongs to a diamagnetic cobalt state. Thus the present NMR results together with the susceptibility indicate the presence of ${\mathrm{Co}}^{3+}$ ion with S = 0 and the magnetic ${\mathrm{Co}}^{4+}$ ion.