Conduction-Electron Contributions to the Hyperfine Fields in Metallic Eu and Gd

The Mössbauer effect of the 21.7-keV $\gamma$ line of ${\mathrm{Eu}}^{151}$ has been employed to study the isomer shifts and the magnetic fields in Eu-Yb alloys. From these measurements the conduction-electron contribution to the hyperfine field at the Eu atom in Eu metal due to its own $4f$ electrons and due to its neighboring atoms can be deduced. In addition an explanation is given for the apparent lack of conduction-electron contribution to the observed hyperfine field in metallic Gd.