Nuclear Magnetic Relaxation of the Impurity Nucleus in Dilute Ferromagnetic Alloys

Abstract

We have measured the nuclear relaxation times ${T}_{1}$ and ${T}_{2}$ for 1% ${\mathrm{Ni}}^{61}$ in 99% iron, 1% ${\mathrm{Ni}}^{61}$ in 99% cobalt, and 0.5% ${\mathrm{Co}}^{59}$ 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 ${T}_{1}$ 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 ${T}_{1}$ are lower limits for relaxation times for nuclei in domains. The longest ${T}_{1}'\mathrm{s}$ are found to be inversely proportional to temperature with ${T}_{1}T=1.0$ sec \ifmmode^\circ\else\textdegree\fi{}K for 1% ${\mathrm{Ni}}^{61}$ in cobalt, ${T}_{1}T=1.2$ sec \ifmmode^\circ\else\textdegree\fi{}K for 1% ${\mathrm{Ni}}^{61}$ in iron, and ${T}_{1}T=0.3$ sec \ifmmode^\circ\else\textdegree\fi{}K for 0.5% ${\mathrm{Co}}^{59}$ in nickel, and are believed to be due to conduction electron relaxation. The quantity $\frac{1}{{{\ensuremath{\gamma}}_{n}}^{2}{T}_{1}T}$ (where ${\ensuremath{\gamma}}_{n}$ 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. ${T}_{2}$ for ${\mathrm{Ni}}^{61}$ in cobalt is 0.15 msec at room temperature and is believed to be due to a coupling of ${\mathrm{Ni}}^{61}$ and ${\mathrm{Co}}^{59}$ nuclei by the Ruderman-Kittel interaction.