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Abstract:
Fe-57-Mossbauer studies of superconducting Rb0.8Fe1.6Se2.0 with T-C =
32.4 K were performed on single-crystalline and polycrystalline samples
in the temperature range 4.2-295 K. They reveal the presence of 88%
magnetic and 12% nonmagnetic Fe2+ species with the same polarization
dependence of their hyperfine spectra. The magnetic species are
attributed to the 16i sites of the root 5 x root 5 x 1 superstructure
and the nonmagnetic Fe species to a nanosized phase observed in recent
structural studies of superconducting KxFe2-ySe2 systems rather than to
the vacant 4d sites in the root 5 x root 5 x 1 superstructure. The Fe-57
spectrum of a single-crystalline sample in an external field of 50 kOe
applied parallel to the crystallographic c axis confirms the
antiferromagnetic order between the fourfold ferromagnetic Fe(16i)
supermoments and the absence of a magnetic moment at the Fe sites in the
minority phase. A discussion of all spectral information and comparison
with superconducting FeSe provides convincing evidence that the
nanoscale phase separation is monitored by Mossbauer spectroscopy in
Rb0.8Fe1.6Se2.0.
