Tunable Magnetic Exchange between Rare-Earth Metal 5d and Iron 3d States: A Case Study of the Multiple Magnetic Transitions in Gd₆FeBi₂ and the Solid Solutions Dy_6–xGd_xFeBi₂ (1 ≤ x ≤ 5) with Curie Temperatures in the Range 120–350 K

In this paper, the room-temperature magnet Gd₆FeBi₂ was comprehensively characterized by means of temperature-dependent single-crystal X-ray diffraction, magnetization measurements, and experimental electron density and electronic structure computations. This work explores the electron-spin effects of Fe on this structure and suggests that Gd₆FeBi₂ shows structural features and exchange interactions, which are clearly distinguishable from other analogues, including the solid solutions Dy_6–xGd_xFeBi₂ (0 ≤ x ≤ 5). The unique traits of Gd₆FeBi₂ and its derivatives encompass abnormal variations of lattice parameters with the temperature, as well as the presence of multiple magnetic transitions of complex nature. Based on the comprehensive analyses, it can be suggested that the unique magnetic response in this material is the result from tunable Fe spin states, which are coupled with strong Gd 5d–Fe 3d interactions, which differ from other members of this large, structural family. In the ground state, according to density functional theory calculations, the Fe atom carries two net electrons, whose spins are coupled in an antiparallel fashion to the spins of the Gd electrons. Near the Curie temperature, the Fe net moment is reduced intermittently, coupled with a multiple of magnetic transitions. The magnetic correlations are manifested in unexpected variations of the lattice parameters as a function of temperature, suggesting spin–lattice interactions. This study emphasizes the important role of Gd 5d electrons in tuning Fe 3d-based magnetic contribution, which enables better understanding on the spin coupling in other related compounds and sheds light on the development of new magnetic and spintronic compounds.