Electron spin resonance of the Yb 4fmoment in Yb(Rh1−xCox)2Si2

Abstract

The evolution of spin dynamics from the quantum critical system YbRh${}_{2}$Si${}_{2}$ to the stable trivalent Yb system YbCo${}_{2}$Si${}_{2}$ was investigated by electron spin resonance (ESR) spectroscopy. While the Kondo temperature changes by one order of magnitude, all compositions of the single-crystalline series Yb(Rh${}_{1\ensuremath{-}x}$Co${}_{x}$)${}_{2}$Si${}_{2}$ show well-defined ESR spectra with a clear Yb${}^{3+}$ character for temperatures below $\ensuremath{\approx}20$ K. With increasing Co content, the ESR $g$ factor along the $c$ direction strongly increases, indicating a continuous change of the ground-state wave function and, thus, a continuous change of the crystal electric field. The linewidth presents a complex dependence on the Co content and is discussed in terms of the Co-doping dependence of the Kondo interaction, the magnetic anisotropy, and the influence of ferromagnetic correlations between the 4$f$ states. The results provide evidence that, for low Co doping, the Kondo interaction allows narrow ESR spectra despite the presence of a large magnetic anisotropy, whereas at high Co concentrations, the linewidth is controlled by ferromagnetic correlations. A pronounced broadening due to critical correlations at low temperatures is only observed at the highest Co content. This might be related to the presence of incommensurate magnetic fluctuations.