A single-site theory of magnetism in amorphous Fe-Ni alloys

Abstract

Magnetic properties of amorphous FecNi1−c alloys are investigated on the basis of a single-site theory which combines the geometrical-mean model for the electronic structure calculations of amorphous alloys with the finite-temperature theory of metallic magnetism. Numerical results for magnetization versus concentration curves show strong ferromagnetism in the region 0.1≲c≲0.8. This explains the recent experimental finding of large hyperfine fields for amorphous FecNi1−c alloys in the Invar concentration region. The calculated Curie temperatures for amorphous structure show a shift of the maximum Curie temperature toward higher Fe concentration, which is in qualitative agreement with the experimental data of amorphous (Fe-Ni)90Zr10 alloys. It is demonstrated that such behavior is caused by a change of electronic structure due to structural disorder rather than by simple volume expansion; at around 60 at. % Fe, the main peak of the nonmagnetic density of states of the amorphous alloy shifts to an energy lower than that of the crystalline fcc counterpart so that the Fermi level is at the main peak, leading to a large magnetic energy gain. The agreement between calculated concentration dependence of effective Bohr magneton numbers and the experimental data for liquid alloys also supports this theory.