Paramagnetic Resonance Width in Iron and Nickel

A theory is presented to account for the unusually large linewidths found by Rodbell in the microwave resonance absorption of iron and nickel single crystals above the Curie points. The perturbing spin coupling is assumed to contain pseudodipolar ($D$) plus quadrupolar ($Q$) components. By the method of moments it is shown that the paramagnetic linewidth is proportional to $\frac{(D^2+b{Q}^2)}{J}$, where $J$ is the exchange integral and $b=0\mathrm{}$ for $S=\frac{1}{2}$, and $b=\frac{1}{27}$ for $S=1\mathrm{}$. The ferromagnetic anisotropy constant at 0°K is proportional to $(-\frac{D^2}{J})+eQ$, where $e=0\mathrm{}$ for $S=\frac{1}{2}$, $e\sim 2 \mathrm{to} 5$ for $S=1\mathrm{}$. Since $D$, $Q$, and $J$ are probably not very temperature sensitive, measurements of paramagnetic linewidth and ferromagnetic anisotropy, together with well-known techniques for estimating $J$, may be used to deduce values of both $D$ and $Q$. For iron and nickel it is shown that the contribution of $Q$ to the linewidth is negligible, whereas the contribution from $D$ is enormous, thus accounting for Rodbell's results.