⁵⁷Fe Mossbauer Study Of Iron-based Superconductors

The discovery of iron-based superconductors, a new member of high temperature superconductor, has triggered enormous studies of superconducting and other proper-ties of the related materials. There is no argument that iron is playing an important role both in the magnetism and superconductivity. At the same time, iron is the most common Mossbauer nucleus, so it’s naturally containing iron that provides a rare oppor-tunity to employ Mossbauer spectroscopy to study the magnetic and superconductivity in those materials. Based on this motivation, we performed systematic Mossbauer stud-ies on Ba(Fe1-xMnx)2As2 (x= 0.016,0.064), CaFe2As2, Lu2Fe3Si5, Ba(Fe0.94Co0.06)2As2 and Sr2VO3FeAs under different temperatures. An enumeration of importantly obtained results are followed.1. Ba(Fe1-xMnx)2As2 single crystal compounds with x=0.016 and 0.064 have been prepared and studied by 57Fe Mossbauer spectroscopy in the temperature range from 30 K to 300 K. The unusual magnetic splitting spectra at lower temperatures have been analyzed using the distribution of hyperfine field. It is found that the influence of Mn dopant spreads beyond the nearest Fe magnetic moments, and the Fe-3d electrons behave more localized compared with those in the electron-doped compounds. This reduces the hyperfine interactions between the iron nuclear and the surrounding electrons. The shape of the spin density wave is near rectangular at 6.4%-Mn doping, indicating that a quite different interband interaction compared with electron-doped compounds. The distinct broadening of the spectral linewidth around the spin density wave transition temperature has been observed and the spin correlation time is deduced according to the linewidth. The correlation time is further related to the spin-lattice relaxation rate by a simple model and the magnetic fluctuations can be explored effectively. It is found that the magnetic fluctuations of iron spins in Mn-doped compounds can be well described using a phenomenological two-component model and the yielded Curie-Weiss temperature is far from the quantum critical point at the present doping levels.2. CaFe2As2 is known as the most pressure sensitive of the AFe2As2 (A= Ca, Ba, Sr), so it is an ideal compound to investigate the delicate effect on magnetism of the crystal structure. Therefore, we performed Mossbauer measurements on CaFe2As2 single crystal, the ground single crystal powder, and the annealed ground single crystal powder. Our Mossbauer result suggests that the complicated un-uniform strain, induced by grinding, has two kinds of affection on the compounds. A second paramagnetic phase emerges below the Neel temperature, and can even survive at our base temperature 4.6 K both in the ground and annealed ground single crystal powder, indicating a suppressed effect caused by the strain. Out of our expectation, the strain can also induce a slight increase of the magnetic phase transition without influencing the absolute value of the magnetic field. Once the long range spin density wave magnetic order forms, the amplitude will maintain at a robust-fixed value, around 10 T at 4.6 K.3. With the advent of FeAs based superconductivity, it has been becoming im-portant to study how superconductivity manifests itself in details of 57Fe Mossbauer spectroscopy of Fe-bearing superconductors. To this end, we performed detailed tem-perature dependent Mossbauer measurement on conventional, albeit multigap supercon-ductors, Lu2Fe3Si5, new discovered iron-based superconductors Ba(Fe0.94Co0.06)2As2 and Sr2VO3FeAs. For Lu2Fe3Si5, there are two crystallographic sites for Fe in this com-pound, so the observed spectra appear to have a pattern consisting of two doublets over the whole temperature range. However, neither abnormal behavior of the hyperfine pa-rameters at or near critical temperature, nor phonon softening were observed. While, for Ba(Fe0.94Co0.06)2As2, a sudden increasing of Linewidth and normalized spectral area above critical temperature, can be observed, which is attributed to an existence of strong mag-netic fluctuation at low temperature range in the optimum doping superconductor. And for freshly prepared Sr2VO3FeAs compound, below superconducting transition tempera-ture the distinct enhancement of spectral area and simultaneous increasing of linewidth also suggest the emergence of magnetic fluctuations in FeAs layer. Our experimental results strongly indicate the coexistence of magnetism and superconductivity in freshly prepared Sr2VO3FeAs compound and support the view that magnetism is playing an important role in the presence of superconductivity.