New Determination of the Nuclear Gyromagnetic RatioγofCo59

The nuclear gyromagnetic ratio $\gamma$ of ${\mathrm{Co}}^{59}$ has been redetermined from NMR studies of the intermetallic compounds CoSi and Co${\mathrm{Si}}_2$, yielding the new value $\frac{\gamma }{2\pi }=1.0054\mathrm{}\pm 0.002$ kHz/G. These materials possess weak dia- and paramagnetism, respectively. The analysis consists of placing upper limits on the NMR shifts in these compounds by using measured values of susceptibilities and spin-lattice relaxation times $T_1$ and the well-known relations between shift and susceptibility contributions from orbital and spin-paramagnetic sources. Our findings are further supported by shifts observed for dilute cobalt in a number of transition-metal alloys. The new value of $\gamma$ given above is larger by ∼0.9% than the commonly accepted one of Freeman, Murray, and Richards that was obtained by extrapolation in their study of NMR shifts and optical splittings in cobaltic complexes. In view of this discrepancy, the theory of Van Vleck shifts in these complexes is reexamined in terms of a molecular orbital model of covalent mixing.