Competition between ferromagnetic and antiferromagnetic states in Al8.5−xFe23Ge12.5+x (0≤x≤3)

Abstract

Polycrystalline Al7+xFe23Ge14−x-type orthorhombic Al8.5−xFe23Ge12.5+x (0 ​≤ ​x≤3) compounds were studied by X-ray diffraction measurement, metallographic examination, and magnetization and electrical resistivity measurements. The lattice parameters show anisotropic x dependences, reflecting a lower symmetry of rather complex crystal structure. In the x ​= ​0 sample, a ferromagnetic (FM) transition occurs at the Curie temperature TC of 255 ​K, which is systematically reduced with increasing x and disappears at x ​= ​2.5 where an antiferromagnetic (AFM) ground state is realized. The Néel temperature TN seems to appear at x≥1, and grows to 143 ​K ​at x ​= ​3 with the increment of x. In the intermediate range of x (1≤x≤2), a FM to AFM phase transition occurs below TC. In the AFM phase, the electrical resistivity ρ shows a Fermi surface instability characterized by the upturn of ρ below approximately TN. The competition between the FM and AFM states would be ascribed to the anisotropic x dependences of Fe–Fe bond lengths.