Nanocrystallization Studies in Co and Fe-Rich Amorphous Alloys
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S. Aburto*, M. Jim•nez* , R. G6mez* V. Marquina%, M.L. Marquina*, R. Ridaura*, P. Santiago**, L. Rendon**, R. Valenzuela***. *Facultad de Ciencias. **Instituto de Fisica. ***Instituto de Investigaciones en Materiales.. Universidad Aut6noma de Mdxico. Circuito Exterior, CU, 04510 Mdxico.
ABSTRACT Nanocrystallization from Metglas® 2605 SC and Vitrovac® amorphous ribbons were studied by M6ssbauer spectroscopy, transmission electron microscopy and differential scanning calorimetry. The spectra showed spin texture. The relative intensities change to a random distribution of spin orientation. New lines appear indicating new magnetic phases. TEM and DSC observations confirmed the crystallization of nanostructured grains. INTRODUCTION The nanocrystallization of amorphous ferromagnetic alloys such as FeBSiNbCu (also know as "Finemet" (1) has raised a strong interest since it leads to a substantial softening in its magnetic properties, presumably by a randomization of magnetic anisotropy, and a decrease in magnetostriction constant (2). A similar decrease in coercive field and increase in permeability has recently been observed (3,4) in Co-based, commercially available ribbons of the Vitrovac type. However, the exact nature of the changes involved in nanocrystallization process are not fully understood. In this paper, we present an investigation of the effects of crystallization process on the microstructure and magnetic properties of Vitrovac 6025, and also of Metglas 2605 SC amorphous ribbons. EXPERIMENTAL Metglas ® 2605 SC and Vitrovac ® 6025 amorphous ribbons of nominal compositions Fe8sBl 3 .5Si3 .5C2 and Co 66Fe 4Mo2 B16 Si12, respectively, were annealed at 550 TC (metglas) and at 450 'C (vitrovac) in nitrogen gas atmosphere, for various times in the 0-60 mmin range. The DSC curves were recorded in a DuPont 2100 system, with a standard DSC cell. The same samples were analyzed in a M6ssbauer spectrometer with constant acceleration with 57Co in Rh source kept at room temperature. All the M6ssbauer spectra were obtained at 300 K. Also the same samples were treated in order to be observed with TEM, with a ME Jeol 4000EX of 1.7 A point resolution. The treatment consists in thinning mechanically the ribbon with different grain size alumina, continuing with ion milling. The samples were cooled with liquid nitrogen to avoid heating. RESULTS AND DISCUSSION M6ssbauer hyperfine spectra showed a distribution in the hyperfine field (Hf) for the as quenched samples as well as for the annealed ones. A reasonable fit to a single six-line pattern was 507
Mat. Res. Soc. Symp. Proc. Vol. 405 01996 Materials Research Society
obtained by assuming a continuous distribution of HW leading to average values. M6ssbauer spectra of as-quenched samples from both amorphous ribbons are presented in Fig. 1.
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Fig. 2. Relative line intensities
Fig. 1.As- quenched samples
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