New amorphous magnetic materials of Fe-B-Be and Fe-B-Au

Severin, C.
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Materials Science and Engineering

Substitution of Be for B in the amorphous binary alloy Fe(,82)B(,18) caused an initial increase in saturation magnetization (M(,s)) to a maximum of 200 emu/g at 4.2K, followed by a decrease for alloys with more than 4 at.% Be. Concurrently, the Curie temperature (T(,C)) of the Fe(,82)B(,18-x)Be(,x) alloy decreased progressively with Be content. These changes in M(,s) and T(,C) differ from those observed in Fe-B-M' metallic glasses, where M' is another metalloid (P, C, Si or Ge). Results from Auger electron and Mossbauer spectroscopies also detected the reversal trend established by the magnetization measurements. The Auger results indicated that there is a charge transfer from Be in the alloys with x (LESSTHEQ) 4, but no such transfer for x > 4. ('57)Fe Mossbauer spectra obtained at 77K and 300K on this series of alloys indicated an initial increase in effective hyperfine field for x (LESSTHEQ) 4, but a decrease for x > 4. The isomer shift was -0.032 mm/sec for x (LESSTHEQ) 4, but changed to -0.050 mm/sec for x > 4. The annealing behavior of Fe(,82)B(,18-x)Be(,x) was also studied by X-rays and Mossbauer spectroscopy and a two-step crystallization process was observed. For x > 0, a solid solution of (alpha)-Fe-Be was formed in the first stage and then Fe(,2)B was precipitated at higher temperatures;Additions of Au to Fe-B tended to increase the average Fe^moment, (mu)(,Fe), resulting in values for (mu)(,Fe) at 2.20 (mu)(,B) in(' )^Fe(,82)B(,16.5)Au(,1.5) and 2.46 (mu)(,B) in Fe(,87)B(,11)Au(,2). The ternary alloys^containing Au up to 1.0 at.% displayed two crystallization stages^(with products of an (alpha)-Fe-Au solid solution followed by Fe(,2)B) while^those with higher Au content displayed a third stage with an Au-^rich solid solution as the crystallization product. Annealing of^Fe(,87)B(,11)Au(,2) resulted in lower M(,s) values, unlike the annealing effect usually observed in Fe-base metallic glasses;Radial distribution function (RDF) analyses were conducted on Fe(,87)B(,13), Fe(,82)B(,12)C(,6), Fe(,82)B(,12)Si(,6), Fe(,82)B(,14)Be(,4), and Fe(,82)B(,13)Be(,5). When compared to Fe(,87)B(,13), the results for the alloys containing C and Si indicated a reduction in the intensity on the lower r-side of the first peak in the RDF. The results were explained in terms of an increase in the spin-wave stiffness constant. An important result of the RDF analyses is the determination of the Fe-Fe interatomic distance (r(,Fe-Fe)) at 2.51(ANGSTROM) and 2.47(ANGSTROM) for Fe(,82)B(,14)Be(,4) and Fe(,82)B(,13)Be(,5), respectively. This decrease in r(,Fe-Fe) corresponds to a change in the number of nearest neighbors from 10.5 to 9.1. The RDF results for these two alloys containing Be were correlated with the changes observed in T(,C) and the average Fe moment;('1)This work was done at the Ames Laboratory, Iowa State University, Ames, IA, operated for the U.S. Department of Energy by I.S.U. under contract No. W-7405-eng-82. The research was supported by the Director of Energy Research, Office of Basic Energy Sciences, WPAS-KC-02-01.