Synthesis of Magnetite Particles by Pulsed Alexandrite Laser Processing of Metallic Glass Precursors
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Room-temperature transmission M6ssbauer spectra were recorded with the 7 rays perpendicular to the ribbon plane using a constant acceleration spectrometer (Ranger Scientific). SEM investigations were carried out without further surface preparation using a JEOL electron microscope at 25 keV, operating in the secondary electron emission mode. RESULTS AND DISCUSSION Low-fluence effects in pulsed alexandrite laser processing of iron-based metallic glasses Room-temperature transmission M6ssbauer spectra of the FeTB 3Si9 samples, after pulsed alexandrite laser irradiation at laser fluences of 2.8 and 4.1 J/cm• are shown in Figures l(a)-l(b). The hyperfine magnetic field distributions extracted from these spectra are given in Figures 1(A)-l(B). Figures l(c)-l(d) and 1(C)-1(D) show the room-temperature transmission Mossbauer spectra and corresponding hyperfine magnetic field distributions of the laserirradiated Fe81B13.5Si. 5C 2 specimens. The refined values of the hyperfine parameters corresponding to the as-quenched and pulsed laser treated samples are listed in Table I. For the 14.4 keV y rays of 57Fe, the relative intensity of the second (fifth) to the first (sixth) lines is given, in the thin absorber approximation, by R 2,=4 sin 2ct/[3(l+cos 2cc)], where (X is the angle between the y-ray propagation direction and the direction of the net magnetic moment.`4 The ratio R2, varies from 0 to 4/3 as cc changes from 00 to 900 and for a completely random distribution of magnetic-moment directions, takes the value 0.67. Consequently, a fluence-dependent rotation of the average magnetic-moment direction from the in-plane to an out-of-plane orientation has taken place in the laser-irradiated metallic glass samples. In order to explain a similar development of transverse magnetic anisotropy in thermal annealing treatments, the model of closure domain structure has been proposed.6 It relies on the development of tensile stresses in the surface and compressive stresses in the bulk, due to the formation of surface crystalline phases of higher density. In specimens with positive magnetostriction, these stresses cause atomic spins to rotate in a direction out of the ribbon plane. This model can also explain the magnetic anisotropy induced in the present irradiation study, due to onset of crystallization in the laser-treated systems. Indeed, the fluencedependent increase in the average magnetic hyperfine field and width of the hyperfine field distribution A-I6hf (Table I) indicates the occurrence of the first stage of the crystallization process, characterized by atomic rearrangements in the laser-exposed specimens.6 Moreover, the out-of-plane magnetic anisotropy induced by low-fluence pulsed alexandrite laser processing is more pronounced in the FeB,,3 5Si3.5C2 than in the Fe 78B 13 Si 9 metallic glass. As in the case of pulsed excimer laser irradiation, this result is correlated to relative differences in the magnetostriction constants. 12 TABLE I. Relative intensity of lines R2P, average hyperfine magnetic field , mean hyperfine f
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