We have performed detailed 57Fe M\"ossbauer spectroscopy measurements on Ba0.78K0.22Fe2As2 and BaFe2−xNixAs2 single crystal mosaics showing antiferromagnetic ordering below TN with superconductivity below TC. Analysis of the M\"ossbauer spectra shows a decrease in the magnetic hyperfine (hf) field but no change in the magnetic volume fraction below TC. This clearly indicates the coexistence of magnetism and superconductivity in these compounds. The decrease in the magnetic hf field below TC depends on the difference between TN and TC, being the largest for TN close to TC. Two different explanations for this observation are given. We also find that the non-magnetic volume fraction below TN correlates with the Ni doping x, being large for high TC and small for high TN.
@article{arxiv.1111.5853,
title = {Interplay between superconductivity and antiferromagnetism in some iron-pnictide single crystals studied by $^{57}$Fe M\"ossbauer spectroscopy},
author = {J. Munevar and H. Micklitz and J. Agüero and C. L. Zhang and H. Q. Luo and C. Argüello and Y. J. Uemura and Pengcheng Dai and E. Baggio-Saitovitch},
journal= {arXiv preprint arXiv:1111.5853},
year = {2011}
}