Nuclear-magnetic-resonance studies of dilute molybdenum--cobalt and tungsten--cobalt alloys

A detailed nuclear-magnetic-resonance study of the dilute magnetic alloys $M_{o}Co(T_K\approx {24}^{\circ }K)$ and $WCo(T_K\approx {11}^{\circ }K)$ has been carried out in the temperature range 0.5-300 °K and for cobalt impurity concentrations of 0.1-1.0 at.%. Two distinct types of ⁵⁹Co impurity resonances were observed in every sample. The dominant resonance is characterized by a large temperature-dependent negative frequency shift and is associated with the magnetic impurity sites. Severe inhomogeneous broadening of the magnetic-site resonances indicates an inhomogeneous impurity polarization resulting from long-range oscillatory spin polarizations of the conduction electrons. A weaker, essentially unshifted resonance whose intensity increases with increasing impurity concentration is associated with relatively nonmagnetic impurity sites in regions of high local impurity concentration. At temperatures well above $T_K$, the magnetic-site resonance shifts exhibit the same Curie-Weiss temperature dependences (with $\theta =-T_K$) as do the respective bulk susceptibilities. The slope dK/dχ yields cobalt hyperfine fields of -23 and $-3\frac{kOe}{{\mu }_B}$ in MoCo and WCo, respectively. In the temperature range 0.5-4 °K, the ⁵⁹Co resonance shifts in MoCo, and to a much lesser extent in WCo, become composition dependent unlike the bulk susceptibility. The low-temperature (1-4 °K) ⁵⁹Co spin-lattice relaxation rates in MoCo are directly proportional to T and have magnitudes which correlate well with the observed resonance shifts using a Korringa-like relationship. From an analysis of the experimental Korringa products, a positive 1-2% orbital contribution to the measured resonance shifts is inferred. The molybdenum and tungsten nuclear resonances exhibit inhomogeneous broadening and enhanced spin-lattice relaxation rates. Within the experimental uncertainties, the widths and enhancement factors scale linearly with the impurity susceptibility. Our results provide no evidence for the existence below $T_K$ of significant nonperturbative long-range spin-correlation contributions to the field-induced magnetization of isolated cobalt impurities in either MoCo or WCo. In this regard MoCo and WCo, despite their lower Kondo temperatures, resemble $AuV(T_K\approx {300}^{\circ }K)$.