Enhanced magnetism in amorphous Co-Y alloys: An ab initio approach

The magnetic properties of Co-Y crystalline intermetallic compounds and amorphous alloys have been investigated using molecular-dynamics simulations of the amorphous structure (based on effective tight-binding-bond forces) and self-consistent spin-polarized electronic-structure calculations (using the supercell approximation for the amorphous phases). We find that the amorphous structure is characterized by a rather strong chemical short-range order (stronger than in amorphous Fe-Y, but weaker than in Ni-Y allows). As a consequence, the total electronic density of states (DOS) is also similar in the crystalline and amorphous phases, apart from a smearing of the fine-structure characteristic for the long-range order in the intermetallic compounds. All crystalline Co_xY_100-x alloys with x≥75 and all amorphous alloys with x≥45 are ferrimagnetic. The Laves phase Co₂Y shows metamagnetism. The disorder-induced smearing of the electronic DOS eliminates the metamagnetic instability and is responsible for the increase of the paramagnetic DOS at the Fermi level and for the enhancement of magnetism. We find that the ferrimagnetic coupling, together with the strong tendency to heterocoordination is important for the persistence of magnetic ordering in the Y-rich regime.