The structure of short- and medium-range order of Fe-rich amorphous Fe-La alloys

Abstract

The ferromagnetic state of iron-rich amorphous FexLa1-x alloys (a-FexLa1-x) is known to become unstable with increasing iron concentration and at x=0.9 a cusp characteristic of a spin glass appears in the AC susceptibility. In order to elucidate the correlations between the magnetic properties and the structure, the authors measured both large-angle and small-angle X-ray scatterings for a-FexLa1-x(0.65<or=x<or=0.9) alloys. From large-angle scattering measurements, interatomic distances and partial coordination numbers between Fe-Fe, Fe-La and La-La atoms were determined. The Fe-Fe interatomic distances for these alloys are all very close to the critical value (=2.54 AA) at which an antiferromagnetic exchange competes with a ferromagnetic one. The Fe-Fe coordination number of 8.6 for a-Fe0.9La0.1was found to be close to 8.8 for crystalline (Fe0.86Al0.14)13La alloy where the ferromagnetic ordering collapses and the antiferromagnetic one dominates. The small-angle scattering for a-Fe0.9La0.1 shows pronounced high intensity in contrast to those for others (x=0.7 and 0.8). This fact suggests that inhomogeneous regions exist in a-Fe0.9La0.1. From the correlation function gamma (r) and Q-invariant 2 pi 2 gamma (0), the authors conjecture that the inhomogenous regions consist of iron clusters whose size is widely distributed and their volume fraction is estimated to be about 2%. The correlations of iron clusters with the spin glass state of a-Fe0.9La0.1 are discussed.