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

TL;DR

This study investigates how varying composition in Al-Fe-Ge compounds influences the magnetic states, revealing a transition from ferromagnetic to antiferromagnetic order driven by structural changes.

Contribution

It provides detailed experimental analysis of magnetic phase transitions in Al-Fe-Ge compounds as a function of composition, highlighting the competition between ferromagnetic and antiferromagnetic states.

Findings

Ferromagnetic transition at 255 K in x=0 sample.

Transition to antiferromagnetic ground state at x=2.5.

Electrical resistivity shows Fermi surface instability in the AFM phase.

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 Neel 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 rho shows a Fermi surface instability characterized by the upturn of rho below approximately TN. The competition between the FM and AFM states would be ascribed to the anisotropic x dependences of Fe-Fe bond lengths.