Narrowing effects in the ESR spectra of Gd in metals: Application to LaSb:Gd

The ESR spectra of Gd in metallic single crystals of LaSb exhibit appreciable variation in the ESR line shape upon changing the temperature from 1.7 to 300 K. At low temperatures, a completely resolved fine-structure splitting appropriate to Gd in a cubic crystalline field is observed. The fourth-order cubic-crystalline-field parameter is $b_4=+32\mathrm{}\pm 2$ Oe. The increase of the temperature leads to significant narrowing effects with the result of a single anisotropic line at room temperature. A Korringa relaxation rate of $\frac{\Delta H_K}{T}=0.19\mathrm{}\pm 0.03$ Oe/K was extracted from the thermal broadening of the collapsed spectrum. The thermal broadening of the $\frac{-1\mathrm{}}{2}\leftrightarrow \frac{+1\mathrm{}}{2}$ fine-structure transition is considerably enhanced, however, with respect to this Korringa thermal broadening. All these features are in complete agreement with the predictions of the theory for exchange narrowing of the fine structure in metals. The ESR line shape was calculated using this theory. Comparison of the theoretical line shape with the one observed experimentally indicates the presence of an additional narrowing mechanism. This narrowing is due to Gd spin-spin interaction in LaSb. The line shape analysis yields an exchange-interaction parameter of 0.03 eV between the spin of Gd and that of the conduction electron. The same value for the exchange parameter is derived, independently, from the experimental Korringa thermal broadening.