Experimental Study of the Low-Temperature Spin Correlations in the Magnetic-Impurity Problem

In an attempt to verify the existence of the extended spin-polarization cloud associated with the ground state of the magnetic-impurity problem, we have presented results of a new set of NMR experiments together with a review of the Mössbauer and bulk-susceptibility data on $\mathrm{Cu}$ Fe. The results demonstrate the existence of the quasiparticle polarization cloud in this system. Analysis of the published Mössbauer and bulk-susceptibility data demonstrates that the local $d$-spin susceptibility accounts for only one-half of the total susceptibility for $T\ll T_K$. From a study of the nuclear-resonance linewidth data we see that the remainder of the susceptibility is located in a spatially extended spin-polarization cloud around the impurity sites. By a model-independent analysis of the data, we obtain a measure of the temperature and magnetic-field dependence of the amplitude of the quasiparticle. In addition, the NMR linewidth studies are extended to magnetic fields as low as 170 G. At fields below 3 kG, a strong field dependence is observed which is attributed to the presence of very small amounts of precipitated Fe in the form of superparamagnetic clusters.