Mössbauer study of superconducting NdFeAsO0.88F0.12 and its parent compound NdFeAsO

57Fe Mossbauer spectroscopy has been used to investigate the magnetic order of non-superconducting NdFeAsO (TN = 140 K) and superconducting NdFeAsO0.88F0.12 (Tc = 45 K). A magnetic hyperfine field Bhf was observed at the 57Fe nucleus below TN~140 K for NdFeAsO. Below ~2 K an increase of Bhf relative to the saturation value was attributed to the transferred Bhf at the Fe site resulting from the collinear antiferromagnetic (AF) spin structure of the Nd moments. The analysis of the spectra is consistent with a commensurate AF order of Fe spins. No Bhf is observed in superconducting NdFeAsO0.88F0.12 down to 1.5 K.