  Gd6FeBi2 is reported as the only one room-temperature magnet with a Curie
temperature (Tc) of ca. 350 K among more than hundreds of compounds with its
structural type, which makes it more attractive in potential applications. To
reveal the origin of such high ordering temperature, critical behaviors,
electronic structure and crystal-field effects of Gd6FeBi2 are investigated in
this work. The short-range Gd-Fe ferrimagnetic interaction is supported by the
non-Curie-Weiss paramagnetic behavior, crystal and electronic structure
analyses, in agreement with previous DFT calculations. Unlike the strong TM-TM
exchange interactions, the Gd-Fe exchange interaction shows limited influence
on the critical exponents determined by long-range exchange interactions, which
seems a common feature in RE-TM based alloys without TM-TM exchange
interactions. However, the strong Gd-Fe hybridization reduces the influence of
vibronic couplings on the short-range exchange interaction and thus allows a
high Tc. The broadening or splitting mechanism of Gd 4f-electron bands is
addressed based on crystal-field analysis and likely another factor for
elevated Tc in Gd6FeBi2 and Gd-based compounds with non-magnetic elements.
Different magnetic behaviors among isostructural compounds, and the
relationship between the band splitting and crystal-field effects is also
discussed.
