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

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 T-C 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 Neel temperature T-N 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 T-C. In the AFM phase, the electrical resistivity rho shows a Fermi surface instability characterized by the upturn of rho below approximately T-N. The competition between the FM and AFM states would be ascribed to the anisotropic x dependences of Fe-Fe bond lengths.