Crystallization Behavior and Recoilless Fraction Determination of Amorphous and Nanocrystalline Fe 56 Co 24 Nb 4 B 13 Si
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Crystallization Behavior and Recoilless Fraction Determination of Amorphous and Nanocrystalline Fe56Co24Nb4B13Si2Cu1 System Monica Sorescu1, Julia Limongelli1, Christopher Stroh1 and Kevin Byerly2 1 2
Duquesne University, Department of Physics, Fisher Hall, Pittsburgh, PA 15282 SPANG, Magnetic Technology Center, 110 Delta Drive, Pittsburgh, PA 15238
ABSTRACT Amorphous ferromagnetic alloy with the composition Fe56Co24Nb4B13Si2Cu1 was obtained by rapid quenching from the melt. Samples cut from the ribbons were annealed at 450, 550, 650 and 750 oC in a vacuum furnace. 57Fe Mössbauer spectroscopy was used to identify the phases formed based on the refined values of the hyperfine parameters. The as-quenched specimen was analyzed with a hyperfine magnetic field distribution and corresponded to an inplane orientation of the magnetic moment directions. The sample annealed at 450 oC was found to be in a nanocrystalline state due to observation of the (FeCo)-Si alloy with the DO3 structure. The balance of the composition was represented by a metalloid-enriched amorphous grain boundary phase. In contradistinction to this, the samples annealed at 550-750 oC were totally crystallized and the new phases formed were D-(FeCo), (FeCo)2(BSi) and (FeCo)3(BSi). These findings suggest that nanocrystallization is obtained only at select processing temperatures. A new set of Mössbauer spectra was obtained by recording simultaneously the intensity transmitted by a sandwich of the sample with the stainless steel etalon, based on the dual absorber method recently introduced by us. The values of the recoilless fraction can be derived from the relative spectral areas. The f factor value dropped from 0.6 to 0.37 for the sample annealed at 450 oC, consistent with the onset of nanocrystallization in the system. For the completely crystallized specimens, the f factor maintained values close to 0.5. This indicates that the presence of quenched-in stresses may play a role in the ability of samples to undergo recoilless emission and absorption of gamma rays. INTRODUCTION The study of structure, magnetic properties and phase composition of amorphous ferromagnetic alloys has been undertaken as part of developments seeking to elucidate the nature of the microscopic structure-property relationships relevant to rational construction of new magnetic materials [1-12]. Finemet Fe73.5Cu1Nb3Si13.5B9 alloy is a FeSiB-based alloy with small amount of additives of Cu and Nb. The early transition metal Nb atoms are stabilizers of the amorphous phase and are preferentially located at the periphery of crystal grains, which limits the grain growth and delays the B precipitation when the amorphous phase is under heat treatment. Cu atoms are not soluble in Fe and serve as nucleating agent for the ferromagnetic nanocrystalline Fe–Si phase, favoring the nanocrystallization process [7, 12]. On the other hand, amorphous iron-cobalt based alloys (of the type FeCoSiB) have been the subject of intensive theoretical and experimental investigations [11], due to their interest
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