Local Magnetic Susceptibility of the Positive Muon in the Quasi 1D S=1/2   Antiferromagnet KCuF$_3$
  We report muon spin rotation measurements of the local magnetic susceptibility around a positive muon in the paramagnetic state of the quasi one-dimensional spin 1/2 antiferromagnet KCuF$_3$. Signals from two distinct sites are resolved which have a temperature dependent frequency shift which is different than the magnetic susceptibility. This difference is attributed to a muon induced perturbation of the spin 1/2 chain. 
J. Chakhalian, R.F. Kiefl, R. Miller, S.R. Dunsiger, G. Morris, S.   Kreitzman, S. Kreitzman, J. Sonier, S. Eggert, I. Affle...