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XAFS studies of amorphous binary alloys: Local order and method of preparation
54
Physica B 158 (1989) 54-55 North-Holland, Amsterdam
XAFS Studies of Amorphous Binary Alloys: Local Order and Method of Preparation T.I. Morrison Illinois Institute of Technology Carl L. Foiles Michigan State University X-ray absorption spectroscopic techniques (EXAFS and XANES) have been used to study the effects of the method of preparation on local order in amorphous magnetic alloys. Previous work has shown that atomic correlations are not necessarily random in amorphous binary magnetic materials, and consequently it cannot be assumed that there is a unique average local configuration for a given composition. Several models exist for the presence of ferromagnetic interactions in The Heisenberg model, for example, presents the idea that amorphous systems. the hamiltonian can be written as the sum of dot-products between spin vectors Si and Sj, coupled by an exchange function Jij(r). The Ising model is similar, but presumes that all the spins are aligned. In each case, regardless of the sophistication or simplicity of the model, it is necessary to know the number of interacting spins and the distance through which they interact. This is precisely the information that X-ray absorption fine structure spectroscopy can provide. In a “perfectly” random binary alloy A, B I _x, it is in principle possible predict the local order about the average atom A by considering only composition of the material. If each atom A has Z nearest neighbors, probability of finding N near neighbors of type B can be written
P(N) = {Z! (l-x) N x~-~)/(N! The expected number of near neighbors number of type B) could then be written
of
to the the
(Z-N)!]
type
B
(i.e.
average
coordination
CNB = C Ni P(Ni) However, since amorphous alloys are not in a state of thermodynamic equilibrium, it would not be expected a priori that alloys with different thermal histories In this study, the would have the same local configurations. amorphous alloys were prepared by triode sputtering and “splat-cooling”. In triode sputtering, Ar+ ions bombard a target of appropriate composition, knocking off constituent atoms which will deposit on a substrate. In splatcooling, a spring-loaded hammer The hits a molten drop of the alloy splat-cooling technique used can achieve a cooling rate of about 10 6 K/set, while the vapor deposition technique has an effective cooling rate of about 0921-4526/89/$03.50 @ Elsevier Science Publishers (North-IIolland Physics Publishing Division)
B.V.
T.I. Morrison, CL. Foiles/Amorphous
55
binary alloys
Thus we are able to compare local order in two systems with 10 ’ ‘K/xc. and we can also compare our experimental extemely different thermal histories, results with predicitions for a “perfectly random” system.
Near-neighbor
ratios
in
Fe_SY
.7
NFe/NY about Fe based on assumption of randomness:
418
NFe/NY about Fe in splat-cooled
517
Fe.SY 7:
NFe/NY about Fe in sputtered Fe.3Ys7:
1012
We are forced to conclude that the random packing models for amorphous alloys are not adequate as bases for theories of magnetic behavior in these systems. Rather, it is necessary to experimentally determine the local environment of taking into account both the relative numbers of nearest the magnetic atoms, neighbors and the. shape of the pair distribution functions. Division of Materials Sciences, We ackowledge the support of the U.S. D.O.E., under contract Number DE-AS05-ER10742 for its role in the development and The NSLS is supported by the U.S. D.O.E. operation of Beamline X- 11 at the NSLS. Divisions of Materials Sciences and Chemical Sciences, under Contract No. DE-AC02-76CH00016.
Physica B 158 (1989) 54-55 North-Holland, Amsterdam
XAFS Studies of Amorphous Binary Alloys: Local Order and Method of Preparation T.I. Morrison Illinois Institute of Technology Carl L. Foiles Michigan State University X-ray absorption spectroscopic techniques (EXAFS and XANES) have been used to study the effects of the method of preparation on local order in amorphous magnetic alloys. Previous work has shown that atomic correlations are not necessarily random in amorphous binary magnetic materials, and consequently it cannot be assumed that there is a unique average local configuration for a given composition. Several models exist for the presence of ferromagnetic interactions in The Heisenberg model, for example, presents the idea that amorphous systems. the hamiltonian can be written as the sum of dot-products between spin vectors Si and Sj, coupled by an exchange function Jij(r). The Ising model is similar, but presumes that all the spins are aligned. In each case, regardless of the sophistication or simplicity of the model, it is necessary to know the number of interacting spins and the distance through which they interact. This is precisely the information that X-ray absorption fine structure spectroscopy can provide. In a “perfectly” random binary alloy A, B I _x, it is in principle possible predict the local order about the average atom A by considering only composition of the material. If each atom A has Z nearest neighbors, probability of finding N near neighbors of type B can be written
P(N) = {Z! (l-x) N x~-~)/(N! The expected number of near neighbors number of type B) could then be written
of
to the the
(Z-N)!]
type
B
(i.e.
average
coordination
CNB = C Ni P(Ni) However, since amorphous alloys are not in a state of thermodynamic equilibrium, it would not be expected a priori that alloys with different thermal histories In this study, the would have the same local configurations. amorphous alloys were prepared by triode sputtering and “splat-cooling”. In triode sputtering, Ar+ ions bombard a target of appropriate composition, knocking off constituent atoms which will deposit on a substrate. In splatcooling, a spring-loaded hammer The hits a molten drop of the alloy splat-cooling technique used can achieve a cooling rate of about 10 6 K/set, while the vapor deposition technique has an effective cooling rate of about 0921-4526/89/$03.50 @ Elsevier Science Publishers (North-IIolland Physics Publishing Division)
B.V.
T.I. Morrison, CL. Foiles/Amorphous
55
binary alloys
Thus we are able to compare local order in two systems with 10 ’ ‘K/xc. and we can also compare our experimental extemely different thermal histories, results with predicitions for a “perfectly random” system.
Near-neighbor
ratios
in
Fe_SY
.7
NFe/NY about Fe based on assumption of randomness:
418
NFe/NY about Fe in splat-cooled
517
Fe.SY 7:
NFe/NY about Fe in sputtered Fe.3Ys7:
1012
We are forced to conclude that the random packing models for amorphous alloys are not adequate as bases for theories of magnetic behavior in these systems. Rather, it is necessary to experimentally determine the local environment of taking into account both the relative numbers of nearest the magnetic atoms, neighbors and the. shape of the pair distribution functions. Division of Materials Sciences, We ackowledge the support of the U.S. D.O.E., under contract Number DE-AS05-ER10742 for its role in the development and The NSLS is supported by the U.S. D.O.E. operation of Beamline X- 11 at the NSLS. Divisions of Materials Sciences and Chemical Sciences, under Contract No. DE-AC02-76CH00016.