Short note on magnetic impurities in SmFeAsO$_{1-x}$F$_x$ (x=0, 0.07)   compounds revealed by zero-field $^{75}$As NMR
  We have performed zero-field $^{75}$As nuclear magnetic resonance study of SmFeAsO$_{1-x}$F$_x$ (x=0, 0.07) polycrystals in a wide frequency range at various temperatures. $^{75}$As resonance line was found at around 265 MHz revealing the formation of the intermetallic FeAs clusters in the new layered superconductors. We have also demonstrated that NMR is a sensitive tool for probing the quality of these materials. 
A. A. Sidorenko, R. De Renzi, A. Martinelli, A. Palenzona