ESR in Situ with a Josephson Tunnel Junction

The in situ electron-spin resonance of a voltage-biased NbAuNb Josephson junction is reported. The Au barrier is doped with Gd or ${}_{}{}^{167}{}_{}{}^{}\mathrm{Er}$ ions. Sharp resonances appear in the $I-V$ curves at frequencies equivalent to the crystal-field splitting of $\mathrm{Au}\mathrm{Gd}$ (1.0 and 1.7 GHz) and to the hyperfine splitting of $\mathrm{Au}{}_{}{}^{167}{}_{}{}^{}\mathrm{Er}$ (2.87 GHz). The principle of this new type of ESR-Josephson-junction spectrometer, as well as its application, is discussed.