Nuclear Magnetic Relaxation of the Impurity Nucleus in Dilute Ferromagnetic Alloys

We have measured the nuclear relaxation times T₁ and T₂ for 1% Ni⁶¹ in 99% iron, 1% Ni⁶¹ in 99% cobalt, and 0.5% Co⁵⁹ in 99.5% nickel at several temperatures using the pulsed-free precession method. The relaxation curves are generally nonexponential and power-dependent. The signals at low-power levels are from nuclei in domain walls and T₁ is due to thermal fluctuations of the domain walls. At high-power levels, where the signal is mainly from nuclei in domains, the longest measured relaxation times T₁ are lower limits for relaxation times for nuclei in domains. The longest T₁'s are found to be inversely proportional to temperature with T₁T=1.0 sec ^∘{}K for 1% Ni⁶¹ in cobalt, T₁T=1.2 sec ^∘{}K for 1% Ni⁶¹ in iron, and T₁T=0.3 sec ^∘{}K for 0.5% Co⁵⁹ in nickel, and are believed to be due to conduction electron relaxation. The quantity 1γₙ²T₁T (where γₙ is the nuclear gyromagnetic ratio) is a normalized measure of the strength of the conduction electron relaxation mechanism and is found to be smaller for the impurity nuclei than for the nuclei of the solvent atoms in the pure solvent metal. This is believed to show that the relaxation of the nuclei by the conduction electrons (which is enhanced in the pure metals by spin waves) is slower for the nuclei on the solute atoms in the alloys due to a reduction of the spin wave enhancement. T₂ for Ni⁶¹ in cobalt is 0.15 msec at room temperature and is believed to be due to a coupling of Ni⁶¹ and Co⁵⁹ nuclei by the Ruderman-Kittel interaction.