Measurement of Atomic Fractions in Multi-Phased Materials of Limited Mass via an Empirical Approach to EXAFS Modeling

Abstract

A least-square fitting analysis of EXAFS data collected from partially-crystallized Fe8uB2u thin films (t = 15 um)~ using data collected from pure phase standards of the crystalliza- tion products, was found effective in determining the relative atomic fraction of each crystalline phase present This fitting scheme provides a means for the quantitative treatment of crystall- lization and precipitation kinetics in thin films and multilayered structures A long standing limitation of extended X-ray absorption fine structure (EXAFS) as a tool for quantitative materials science has been its inability to measure the relative fraction of phases in multiphased materials. This is a prerequisite~ for example, in the study of crystallization and precipitation kinetics. Although there exist techniques which are able to perform these tasks on bulk materials, e.g. X-ray diffraction and digital scanning calorimetry~ recent trends toward the design and fabrication of low dimensional devices has made the study of thin films~ which cannot be readily measured by these techniques because of their small masses~ of particular importance. Of the popular local probes~ EXAFS is largely insensitive to small masses (lj, for example~ the signal-to-noise ratio of the EXAFS collected in total electron yield mode does not deteriorate appreciably for thin films until the sampled mass approaches Ge 10~~ grams. In an attempt to illustrate the usefulness of EXAFS in performing quantitative materials science we have applied EXAFS to study the crystallization of a model transition metal metalloid amorphous system in Fe-B. A least-square fitting analysis of the EXAFS data collected from partially-crystallized Fe-B thin films~ using data collected from pure phase standards of the crystallization products~ was found effective in determining the relative atomic fraction of each crystalline phase present. The samples used in this study are from a single 15 nm film (50 mm x 50 mm area) which was ion beam sputter-deposited from a pressed-powder target having the stoichiometry of Fe80B20. Individual pieces (7 mm x 7 mm) were encapsulated in evacuated glass ampoules and annealed at temperatures ranging from 473 K to 823 K for a period of 60 minutes. The X-ray absorption