Possible undercompensation effect in the Kondo insulator (Yb,Tm)B${}_{12}$J

The effects of Tm substitution on the dynamical magnetic response of Yb${}_{1\ensuremath{-}x}$Tm${}_{x}$B${}_{12}$ $(x=0$, 0.08, 0.15, and 0.75) and Lu${}_{0.92}$Tm${}_{0.08}$B${}_{12}$ compounds have been studied using time-of-flight inelastic neutron scattering. Major changes were observed in the spectral structure and temperature evolution of the Yb contribution to the inelastic response for a rather low content of magnetic Tm ions. A sizable influence of the $R$B${}_{12}$ host (YbB${}_{12}$, as compared to LuB${}_{12}$ or pure TmB${}_{12}$) on the crystal-field splitting of the Tm${}^{3+}$ ion is also reported. The results point to a specific effect of impurities carrying a magnetic moment (Tm, as compared to Lu or Zr) in a Kondo insulator, which is thought to reflect the ``undercompensation'' of Yb magnetic moments, originally Kondo screened in pure YbB${}_{12}$. A parallel is made with the strong effect of Tm substitution on the temperature dependence of the Seebeck coefficient in Yb${}_{1\ensuremath{-}x}$Tm${}_{x}$B${}_{12}$, which was reported previously.