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.

Impurity Nuclear-Magnetic-Resonance Shifts and Spin-Lattice Relaxation Rates in Al: V, Al: Cr, and Al: Mn

“…13 ' 14 The NMR shifts and relaxation rates are determined by the local static and dynamic susceptibilities, respectively, and thus provide a microscopic probe of the magnetic response of the impurity. The notation "magnetic" and "nonmagnetic" refers to the models (i.e., Kondo or spin fluctuation) which have been most successful to date in accounting for the bulk properties of the respective alloys.…”