The local susceptibilities and the 3d spin dynamics of isolated nickel ions in various alkali-metal hosts have been measured, utilizing the method of perturbed /-ray distribution after recoil implantation. In the heavy alkali-metal hosts Cs, Rb, and K, the local susceptibilities of the isolated Ni ions show the temperature behavior of a fully localized spin-orbit-coupled 3d 9 configuration. Clear host dependence of the magnetic behavior is seen in the spin-fluctuation rates which increase with decreasing host volume. In Na, the magnetic behavior drastically deviates from that of the purely ionic configuration, and Ni ions in Li as well as in Ca and Ba hosts are nonmagnetic.PACS numbers: 75.20.Hr, 75.30.Hx, 76.30.Fc, 76.80.+y In recent years the question of localization of electrons in the partially filled shells of transition metals has again become a subject of considerable interest. Following the table of transition elements by Smith and Kmetko, ! the degree of localization is the discriminating parameter for many bulk properties. The magnetism of most 4/ metals is known to be determined by localized electrons, whereas many d elements are strongly influenced by band structure and hybridization effects. Marking the change between localized and itinerant d and / electrons, the intermediate region, comprising, e.g., Ce and Fe, and Co and Ni, is rendered prominent by an extreme complexity of all-important bulk quantities.In the field of dilute magnetic alloys, the search for local moments is guided by the idea that magnetic moments will develop if the interaction of electrons "inside" one impurity atom survives hybridization or crystal fields. As nickel is estimated to have the strongest spinorbit coupling and exchange interaction in the 3d series, 2,3 a well-defined local magnetic moment might be expected in some metallic hosts, yet no such observation has been published.So the question arises whether nickel impurities will develop a fully localized configuration when incorporated in extreme environments. One such extreme system, which is characterized by sharp contrasts in atomic volume, electronegativity, and density of states between the constituents, has already been studied, Fe impurities in the alkali metals Cs, Rb, and K. 4 Here the isolated iron impurities are divalent and exhibit an unperturbed spin-orbit-coupled 3d 6 configuration. Another question of interest is the actual ionic configuration of the Ni ions in the host. Estimates on the basis of a Born-Haber cycle 5 suggest that Ni might be monovalent in the alkali metals and, even more interesting, might display charge fluctuations.In many metallic hosts Ni impurities show nonmagnetic behavior; in NiPd alloys, however, a positive magnetic hyperfine field has been found. 6 " 9 . This has been explained by negative core and positive orbital contributions and, in addition, transferred fields from neighbor-ing Pd atoms, but it is not an indication of an unperturbed well-defined moment. For Fe impurities, local orbital moments are thought to be typical f...
