  By means of new muon spin relaxation experiments, we disentangle extrinsic
and intrinsic sources of low-temperature bulk magnetism in the candidate
topological Kondo insulator (TKI) SmB6. Results on Al-flux grown SmB6 single
crystals are compared to those on a large floating-zone grown ^{154}Sm^{11}B6
single crystal in which a 14 meV bulk spin exciton has been detected by
inelastic neutron scattering. Below ~ 10 K we detect the gradual development of
quasi-static magnetism due to rare-earth impurities and Sm vacancies. Our
measurements also reveal two additional forms of intrinsic magnetism: 1)
underlying low-energy (~ 100 neV) weak magnetic moment (~ 10^{-2} mu_B)
fluctuations similar to those detected in the related candidate TKI YbB12 that
persist down to millikelvin temperatures, and 2) magnetic fluctuations
consistent with a 2.6 meV bulk magnetic excitation at zero magnetic field that
appears to hinder surface conductivity above ~ 4.5 K. We discuss potential
origins of the magnetism.
