Temperature evolution of the internal magnetic hyperfine field of Metglas: amorphous and crystallized phases

Abstract

We present differential scanning calorimetry (DSC), X-ray diffractometry, in situ Mossbauer spectroscopy (MS, and transmission electron microscopy (TEM) studies in Metglas ribbons subjected to different heat treatments. The temperature evolution of the hyperfine field H-hf(T) and the Curie temperature (T,) of the amorphous phase are determined. The magnetic field originally present in the amorphous phase has a 'normal' behavior, in the sense that it can be described by the Weiss molecular field theory. The total angular momentum of the iron atoms turns out to be 512 and this implies Fe3+ in which the electronic spins are uncoupled. When the samples are maintained near T-c ( approximate to 673 K), three new magnetic phases are detected in the Mossbauer spectra. indicating an onset of a crystallization process well below the first crystallization temperature (T-X1), as determined by DSC ( approximate to 820 K). The magnetic behavior of these phases is also described by universal Brillouin curves with J(1) = 3/2, J(2) = 5/2 and J(3) = 5/2. A pre-identification of the crystalline phases is made through the Mossbauer parameters of the three observed magnetic phases, and the final characterization by X-ray and electron diffraction. Two of the three phases were identified as alpha -FeSi and Fe2B. The third phase could be the cubic phase of Fe3Si or a solid solution of FeSi. The size of the nanocrystals formed in a sample heated at 673 K during different time intervals were determined by TEM, Even though the number of the nanocrystals increases with time, their average size remains the same. (C) 2000 Elsevier Science B.V. All rights reserved.