Transient excitation of nuclei in ferromagnetic metals

Free precession signals and spin echoes have been observed from Fe ⁵⁷, Co⁵⁹, and Ni⁶¹ nuclei in finely divided multidomain iron, cobalt, and nickel, respectively. Spin-lattice relaxation has been studied in all three metals from temperatures in the liquid helium range to room temperature or above. Although the relaxation is not strictly exponential, the rate of recovery of the magnetization appears to be proportional to the absolute temperature. Spin-spin relaxation has been studied in natural cobalt and in both natural and enriched samples of iron and nickel. The spin-spin coupling is very much stronger in cobalt than in iron or nickel, as expected, because of the very much larger nuclear magnetic moment of Co⁵⁹. Spin diffusion through the frequency spectrum of cobalt has been investigated at 4.2 and 77°K by the stimulated echo technique. The results are consistent with a theory of one-dimensional diffusion with exchange rate and exchange distance of the order of 1/T₂. The spin-echo technique has also been used in all three metals to study the effect of domain wall motion on the nuclear resonance. A dc pulse, which displaces the domain walls by a controllable amount, is placed between the first rf pulse and the echo. This study confirms that the induction signals arise from nuclei in domain walls and further suggests a connection between line broadening and domain wall processes. A weak free-precession signal, arising from domain rotation, has been observed from cobalt in high magnetic fields. The sign of the precession has been determined, confirming that the hyperfine field is directed opposite to the magnetization.