Quantum spin fluctuations in the bulk insulating state of pure and Fe-doped SmB6

The intermediate-valence compound SmB6 is a well-known Kondo insulator, in which hybridization of itinerant 5d electrons with localized 4f electrons leads to a transition from metallic to insulating behavior at low temperatures. Recent studies suggest that SmB6 is a topological insulator, with topological metallic surface states emerging from a fully insulating hybridized bulk band structure. Here we locally probe the bulk magnetic properties of pure and 0.5 % Fe-doped SmB6 by muon spin rotation/relaxation methods. Below 6 K the Fe impurity induces simultaneous sudden changes in the bulk local magnetism and the electrical conductivity. In the low-temperature insulating bulk state we observe a temperature-independent dynamic relaxation rate indicative of low-lying magnetic excitations driven primarily by quantum fluctuations. Measurements in an applied magnetic field connect the dynamic fluctuations to the sub-lattice of partially Kondo-compensated Sm moments. Our observations support the notion that SmB6 is close to a magnetic quantum critical point.