Magnetisation and Mossbauer studies of (Fe1-xMex)(2)SiS4, x <= 0.25 and Me = Co or Ni

• The olivine related compound(Fe1-xMex)2SiS4, Me = Co or Ni, has been studied by magnetisation (SQUID) measurements and Mössbauer spectroscopy. There are two crystallographic metal positions in the structure, M1 having an inversion center and M2 possessing a mirror plane. Fe2SiS4 becomes antiferromagnetic (AFM) at 126 K and additionally experiences a ferrimagnetic (FiM) transition at 32 K that gives rise to a net spontaneous magnetic moment of about 0.15 µB/iron atom. The Néel temperature increases slightly for Co and substantially for Ni substitution whereas the FiM transition temperature is strongly suppressed in both cases. The hyperfine field on M2 is up to 8 times smaller than on M1 in the AFM phase, while in the FiM state the hyperfine fields of the two sites become of a more comparable magnitude. A similar anomalous behaviour of the AFM hyperfine fields remains in the samples with Co or Ni substitution. We could also definitely make the assignment that Fe-ions in M2 sites give higher centroid shift values and a lower magnitude of the quadrupole splitting than Fe in M1 sites in the Mössbauer spectra. This result also ascertains that it is the M1 sites that are preferentially occupied by Co and Ni.

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