Understanding high ordering temperature in Gd₆FeBi₂ magnet : Critical behavior, electronic structure and crystal-field analysis

Gd₆FeBi₂ is reported as the only one room-temperature magnet 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 crystal and electronic structure analyses. Unlike the strong Fe-Fe 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 rare earth – transition metal based alloys without exchange interactions between transition-metal atoms. However, the strong Gd-Fe hybridization reduces the influence of vibronic couplings on the short-range exchange interaction and thus allows a high T_c. The broadening or splitting mechanism of Gd 4f-electron bands is addressed based on crystal-field analysis and likely another factor for elevated T_c 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.