Local spin-density theory of itinerant magnetism in crystalline and amorphous transition metal alloys

Abstract

The authors present self-consistent spin-polarized electronic structure calculations for realistic models of amorphous transition metal alloys. The atomic structure is prepared by a simulated molecular-dynamic quench, based on interatomic forces calculated using hybridized nearly-free-electron tight-binding-bond theory. The electronic structure is calculated in the local spin-density approximation, using a linear muffin-tin orbital (LMTO) supercell approach. Detailed results for crystalline and amorphous alloys of Ni, Co and Fe with Zr are presented. NixZr1-x alloys are predicted to be paramagnetic for x<or=0.85, both in the crystalline and in the amorphous state. In CoxZr1-x the onset of magnetic ordering occurs at x approximately=0.67 for crystalline and at x approximately=0.50 for amorphous alloys. The Co-rich alloys are predicted to be strong ferrimagnets. The formation of the negative Zr moments is related to a covalent coupling of Co- and Zr-d states, which is strongest for the Co minority-spin states. The enhancement of magnetism in the glassy alloys is related to an increase of the density of states at the Fermi level induced by structural disorder. In both crystalline and amorphous FexZr1-x alloys, the onset of ferrimagnetic ordering occurs at x approximately=0.33. In contrast to the Co-based alloys, FexZr1-x alloys are weak magnets. For x>or=0.75 the competition between ferromagnetic and antiferromagnetic exchange interactions leads to the formation of negative moments on isolated Fe sites. The number of negative Fe moments increases strongly for x>or=0.90, leading to a decrease of the average moment in the Fe-rich limit. The authors show that the formation of negative local Fe moments is related to a large number of contracted Fe-Fe pairs. The predictions of local spin-density theory are found to be in excellent agreement with experiment. For all crystalline and amorphous alloys the local exchange splitting of the 3d states is shown to be correlated linearly with the local magnetic moment, with a slope of approximately=1 eV mu B-1. This relation holds for all types of magnetic order and also for the crystalline and amorphous pure metals.