NMR and Magnetic Susceptibility of Co Magnetic Impurities in Transition Metal Hosts

Abstract

The 59Co NMR has been observed in Mo-y at. % Co and W-x at. % Co alloys where: y=0.54 and 1.08; x=0.53 and 1.05. The NMR frequency shifts exhibit a strong temperature dependence which can be correlated with the temperature dependent part of the bulk susceptibility. The resonance lineshapes are symmetrical and the linewidths show a temperature and field dependence which is approximately linear in H/T. The magnetic susceptibilities can be fitted to an equation of the Curie-Weiss form: χ−χ0=C/(T−θ) with θ∼−11°K and −24°K for the W and Mo hosts, respectively. In each alloy the temperature-independent susceptibility is approximately equal to that of the host metal, and the values of the Curie constant correspond to ``effective'' moments per Co atom (gS) of 1.79±0.10 in W and 1.52±0.15 in Mo. Using temperature as an implicit parameter over the range of 4.2°–300°K, a plot of the Knight shift vs susceptibility for each alloy yields a linear relationship from which the hyperfine field per Bohr magneton is found to be much smaller than expected for Co. Assuming pure d states this suggests that the ``effective moment'' in these alloys does not reside entirely on the Co atom, and the host matrix is polarized in the vicinity of the impurity; however an alternate explanation is that positive contributions to the internal field such as an admixture of s character into the d states of the impurity tend to cancel the large negative hyperfine field due to core polarization by the unpaired d spins.