Recoilless Fraction in Amorphous and Nanocrystalline FeCuNbSiB System
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Recoilless Fraction in Amorphous and Nanocrystalline FeCuNbSiB System Monica Sorescu, Tianhong Xu and Steven Herchko
Duquesne University, Department of Physics, Fisher Hall, Pittsburgh, PA 15282-0321, USA
ABSTRACT Differential scanning calorimetry, X-ray diffraction, and room temperature Mössbauer spectrum measurements of Fe73.5Cu1Nb3Si13.5B9 (Finemet) alloy have been carried out in order to study its structural and magnetic properties as a function of annealing temperature. The Mössbauer spectra of annealed Finemet alloy could be fitted with 4 or 5 sextets and one doublet at higher annealing temperatures, revealing the appearance of different crystalline phases corresponding to the different Fe sites above the crystallization temperature. The appearance of the nanocrystalline phases at different annealing temperatures was further confirmed by the recoilless fraction measurements. These made use of our recently-developed dual absorber method, which made it possible to determine precisely the recoilless fractions of the amorphous, nanocrystalline and grain boundary phases separately. INTRODUCTION The recoilless fraction f of the Mössbauer effect is a measure of the average square of the amplitude of vibrations of the nuclear probe in the direction of the wavevector of the gammaray photon. There were a few methods developed and applied for the measurement of the recoilless fraction [1-7] and the main one involved the determination of the Debye temperature from complicated equation plots. The errors were significant, especially due to the determination of the background. To avoid these difficulties, we recently proposed a new method for the direct determination of the recoilless fraction using a single room temperature transmission Mössbauer measurement [8]. The method relies on recording simultaneously the spectra of two absorbers, the sample under investigation and an etalon, such as stainless steel (SS) or the natural iron foil (fe=0.7). Most recently, our approach was favorably cited in [9], as a way around the subtle background problem, which offers a very attractive, accurate and extremely simple method to determine the recoilless fraction. Moreover, in [8] we demonstrated, using interchanged and split-area absorbers, that the thickness effects do not play a measurable role in this dual-absorber method. The method proposed turned out to be extremely reliable in the study of a variety of systems, from iron chlorides to cobalt-doped magnetite and nanoparticles [10-19]. In this study we investigate the thermal, structural and magnetic properties of the FeCuNbSiB alloy system by complementary techniques and we determine the recoilless fraction of the amorphous, nanocrystalline and grain boundary phases using the dual absorber method.
EXPERIMENTAL Finemet alloy with the nominal composition of Fe73.5Cu1Nb3Si13.5B9 was prepared by single roller method with a thickness of ~ 20 μm. To obtain different crystallization stages, the as-quenched amorphous Finemet ribbons were annealed for 1 hour at 450, 550, 650 and 750 ºC
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