Muon-spin relaxation measurements on the dimerized spin-1/2 chains   NaTiSi2O6 and TiOCl
  We report muon spin relaxation (muSR) and magnetic susceptibility investigations of two Ti3+ chain compounds which each exhibit a spin gap at low temperature, NaTiSi2O6 and TiOCl. From these we conclude that the spin gap in NaTiSi2O6 is temperature independent, with a value of 2*Delta=660(50)K, arising from orbital ordering at Too = 210K; the associated structural fluctuations activate the muon spin relaxation rate up to temperatures above 270K. In TiOCl we find thermally activated spin fluctuations corresponding to a spin gap 2*Delta=420(40)K below Tc1=67K. We also compare the methods used to extract the spin gap and the concentration of free spins within the samples from muSR and magnetic susceptibility data. 
P.J. Baker, S.J. Blundell, F.L. Pratt, T. Lancaster, M.L. Brooks, W.   Hayes, M. Isobe, Y. Ueda, M. Hoinkis, M. Sing, M. Kle...