Suppression of magnetic excitations near the surface of the topological Kondo insulator SmB6

We present a detailed investigation of the temperature and depth dependence of the magnetic properties of the three-dimensional topological Kondo insulator ${\mathrm{SmB}}_{6}$, in particular, near its surface. We find that local magnetic field fluctuations detected in the bulk are suppressed rapidly with decreasing depths, disappearing almost completely at the surface. We attribute the magnetic excitations to spin excitons in bulk ${\mathrm{SmB}}_{6}$, which produce local magnetic fields of about $\ensuremath{\sim}1.8\phantom{\rule{0.28em}{0ex}}\mathrm{mT}$ fluctuating on a time scale of $\ensuremath{\sim}60\phantom{\rule{0.28em}{0ex}}\mathrm{ns}$. We find that the excitonic fluctuations are suppressed when approaching the surface on a length scale of $\ensuremath{\sim}40\char21{}90\phantom{\rule{0.28em}{0ex}}\mathrm{nm}$, accompanied by a small enhancement in static magnetic fields. We associate this length scale to the size of the excitonic state.