Nuclear-Magnetic-Resonance Studies of Dilute Molybdenum-Cobalt and Tungsten-Cobalt Alloys

A detailed nuclear-magnetic-resonance study of the dilute magnetic alloys $M_o\mathrm{Co} \left(T_K\approx 24 {}_{}{}^{\circ }{}_{}{}^{}\mathrm{K}\right)$ and $W\mathrm{Co} \left(T_K\approx 11 {}_{}{}^{\circ }{}_{}{}^{}\mathrm{K}\right)$ 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 ${}_{}{}^{59}{}_{}{}^{}\mathrm{Co}$ 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 $\frac{\mathrm{dK}}{d\chi }$ yields cobalt hyperfine fields of -23 and $-3\mathrm{} \frac{\mathrm{kOe}}{{\mu }_B}$ in $\mathrm{Mo}\mathrm{Co}$ and $W\mathrm{Co}$, respectively. In the temperature range 0.5-4 °K, the ${}_{}{}^{59}{}_{}{}^{}\mathrm{Co}$ resonance shifts in $\mathrm{Mo}\mathrm{Co}$, and to a much lesser extent in $W\mathrm{Co}$, become composition dependent unlike the bulk susceptibility. The low-temperature (1-4 °K) ${}_{}{}^{59}{}_{}{}^{}\mathrm{Co}$ spin-lattice relaxation rates in $\mathrm{Mo}\mathrm{Co}$ 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 $M\mathrm{oCo}$ or $W\mathrm{Co}$. In this regard $\mathrm{Mo}\mathrm{Co}$ and $W\mathrm{Co}$, despite their lower Kondo temperatures, resemble $\mathrm{Au}\mathrm{V} \left(T_K\approx 300 {}_{}{}^{\circ }{}_{}{}^{}\mathrm{K}\right)$.