Time-dependent hyperfine interactions in moderately diluted Ni(Hf)

The magnetic hyperfine interaction of 181Ta in Ni(Hf) has been measured as a function of temperature by means of the time differential γγ angular correlation technique. While for very low concentrations of less that 10-4 a pure static interaction is observed, which follows the bulk magnetization apart from small deviations, spectra obtained from a source with 1 x 10-3 Hf content show a more complex behaviour. From the comparison of the experimental data with theoretical spectra, which result from various dynamical models, it is concluded that for a Hf concentration of 1 x 10-3 the impurity-impurity interaction causes a local breakdown of the ferromagnetic ordering near the Curie point. The arising paramagnetic domains may change their geometrical form and position so that a probe nucleus is subject to a time-dependent hyperfine interaction. (Auth.)