Line Shapes, Saturation Behavior, and Temperature Studies in the Nuclear Resonance of Nickel

Abstract

The nuclear magnetic resonance of ${\mathrm{Ni}}^{61}$ has been studied in pure nickel powders. Both natural nickel (1.25% abundant in ${\mathrm{Ni}}^{61}$) and enriched nickel (99.9% abundant in ${\mathrm{Ni}}^{61}$) were used. The signal observed with a circuit sensitive to the absorption of power is a combination of absorption and dispersion modes. The amount of mode mixing is found to depend on the applied rf field, ambient temperature, annealing temperature, and external magnetic-field intensity. The mode mixing of the resonance is related to electronic losses in the sample. The saturation behavior of the individual absorption and dispersion modes is that of an inhomogeneously broadened line. From the saturation behavior an estimate of the enhancement of the applied rf level for nuclei in domain walls of about 7\ifmmode\times\else\texttimes\fi{}${10}^{3}$ is made. An additional broadening mechanism is observed in the enriched nickel, particularly at 77\ifmmode^\circ\else\textdegree\fi{}K. Application of Redfield's theory of saturation at high rf levels leads to the conclusion that the additional broadening can be attributed to strong indirect exchange between nuclei. Frequency measurements from room temperature to 4.2\ifmmode^\circ\else\textdegree\fi{}K made with a marginal oscillator in both the enriched and unenriched nickel are presented. A temperature-dependent frequency shift is found between the enriched and unenriched nickel which appears to be a maximum at about 200\ifmmode^\circ\else\textdegree\fi{}K where the enriched nickel frequency is about 30 kc/sec higher. From room temperature to 536\ifmmode^\circ\else\textdegree\fi{}K, frequency measurements made with super-regenerative circuits are reported. A comparison of the temperature dependence of the resonance frequency with the saturation magnetization is made.