It is well known that magnetic amorphous materials, in particular the commercial metallic glass ribbons, have good soft magnetic properties, such as: low coercivity, high magnetic saturation field, and high magnetic permeability. However, these properties change significantly when these materials undergo thermal treatments. For these reasons, the study and controlling of thermal stability of such alloys are key questions for sensor applications. In this work we have investigated the thermal and magnetic properties of the Fe78B13Si9 metallic glass.Facultad de Ciencias Exacta

Fernandes, António A.

Freitas, Jair C. C.

Larica, C.

Nunes, E

Passamani, Edson C.

Pereira, R. D.

Sánchez, Francisco Homero

Journal of Materials Science

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J M A T E R S C I 4 1 (2 0 0 6 ) 1 6 4 9 –1 6 5 1 L E T T E R SThermal stability and magnetic properties of FeSiB amorphous alloyE. NUNES, R. D. PEREIRA, J. C. C. FREITAS, E. C. PASSAMANI, C. LARICA, A. A. R. FERNANDES ∗Departamento de Fı́sica, Universidade Federal do Espı́rito Santo, 29060-900 Vitória, ES, BrasilE-mail: alberto@cce.ufes.br.F. H. SANCHEZDepartamento de Fı́sica, Universidad Nacional de La Plata, 1900 La Plata, ArgentinaPublished online: 1 November 2005It is well known that magnetic amorphous materials, inparticular the commercial metallic glass ribbons, havegood soft magnetic properties, such as: low coercivity,high magnetic saturation field, and high magnetic per-meability. However, these properties change significantlywhen these materials undergo thermal treatments. Forthese reasons, the study and controlling of thermal sta-bility of such alloys are key questions for sensor applica-tions. In this work we have investigated the thermal andmagnetic properties of the Fe78B13Si9 metallic glass.All experiments were carried out on pieces of approxi-mately 20 mm2 of the Fe78B13Si9 metallic glass (suppliedby Honeywell International Inc.) prepared by melt spin-ning technique. Differential scanning calorimetry (DSC)analyses were performed with a Shimadzu DSC-50equipment between room temperature and 998 K, underan argon flux of 30 ml/min. The signals were alwaysrecorded in the heating ramps, with heating-rates (β) of10, 15 and 20 K/min. Kinetic analysis was carried outaccording to the Ozawa method [1]. Structural changes inas-received and isothermally annealed samples were ex-plored by X-ray diffraction (XRD) with Cu-Kα radiation.57Fe transmission Mössbauer spectra (MS) were recordedat room temperature (RT), with a 25mCi 57Co radioactivesource in Pd matrix. The γ -rays propagation directionwas perpendicular to the ribbon surface. Heat treatmentswere performed in air with temperature accuracy of±2 K. The Mössbauer spectra were fitted by a hyperfinefield distribution using a histogram method describedby Brand and Le Caer [2]. To reproduce the smallasymmetry of the spectra, a linear correlation betweenhyperfine magnetic field (Bhf) and isomer shift (IS) wasassumed.Fig. 1 shows the DSC curves recorded with threedifferent heating rates. The occurrence of two exothermicpeaks is clearly recognized and associated with distinctcrystallization processes occurring upon heating. Thesepeaks are attributed to the crystallization of the Fe2B(at lower temperature) and Fe3Si (at higher temperature)∗ Author to whom all correspondence should be addressed.phases. The activation energies of these two processes,obtained from the DSC curves (inset), were found to be370 kJ/mol and 351 kJ/mol, respectively. These valuesare in agreement with previous results in other amorphousalloys [3, 4].The XRD patterns of the as-received and annealed sam-ples are displayed in Fig. 2 (right side). For annealingtimes shorter than 8 min, no Bragg peaks associated withcrystalline phases are clearly detected, within our exper-imental resolution. For longer times (8 and 24 min), itis observed the presence of the Fe3Si and Fe2B crys-talline peaks superimposed on the residual amorphousphase (broad maximum). Finally, the XRD pattern of thesample annealed for 63 hr exhibits only the presence ofthe above mentioned crystalline phases. Fig. 2 (left side)also displays the MS of the as-received and annealed sam-ples. The broad MS absorption resonance lines of the as-received samples are due to a topological and chemicaldisorder on the 57Fe atoms neighborhood. For annealingtime of 8 min and above, it is observed a change in the2 and 5 line intensities (counting from left to right). Thisresult is explained considering that the orientation of theFe magnetic moments relative to the ribbons plane de-termined directly from the relative line intensities. Theseintensities have a 3:x:1:1:x:3 ratio, where x is a functionof the angle between the γ -rays propagation directionand magnetic moments orientation in the ribbons plane.For in-plane magnetization, x assumes value of 4, andzero for out-of-plane, considering the γ -rays propagationdirection perpendicular to the ribbon plane. It was calcu-lated that, for as-received sample, x has a value close to3.2, which indicates an average magnetization near theribbon plane.The hyperfine magnetic field distributions obtainedfrom the MS of the samples annealed up to 24 min re-main nearly unchanged (Fig. 2, inset), indicating that thesamples are chemically and structurally similar, in con-trast to the clear magnetic reorientation observed for thesame samples.0022-2461 C° 2006 Springer Science + Business Media, Inc.DOI: 10.1007/s10853-005-4229-0 1649Figure 1 DSC curve for Fe78B13Si9 recorded with three values of heating-rates (β). Inset: Ozawa plot [1].Figure 2 Mössbauer spectra (left) and X-ray diffractograms (right) of as-received and annealed at 773 K samples. (x) Fe2B and (∗) Fe3Si.1650In order to get total crystallization in the material, asample was annealed for 63 hr at 773 K. Based on XRDresults and on magnetic hyperfine parameters found inthe literature, the spectrum for this sample was fully fittedwith a convolution of five magnetic subspectra, two fromthe Fe2B ordered phase [5] and three from the DO3 phaseof Fe3Si [6].Looking at the results obtained for annealing times up to24 min, it is apparent that crystallized fractions observedby the two techniques are quite different. For instance,while there is a considerable crystalline contribution to thediffractogram for the sample annealed during 24 min, thecorresponding Mössbauer spectrum shows no crystallinephase formation.The crystallization begins with the formation of verysmall crystals and/or atomic clusters with crystalline-likeshort-range order (embryos), embedded in the amorphousmatrix. Mössbauer spectroscopy is more sensitive to thepresence of these small associations of atoms with aparticular short-range order than X-ray diffractometry.Therefore, the above stated difference between the twotechniques results indicates that it may originate fromtheir different sampling ability. The gamma rays emittedby the 57Co source have sufficient energy (14.4 keV)to explore sample bulk as well as the surface regions,whereas the less energetic X-rays can effectively searchonly few microns deep the ribbons surface. Then,transmission Mössbauer results reflect mostly the state ofthe sample bulk while XRD seems to be more sensitiveto the ribbon regions close to the surface. It may be con-cluded that the crystalline phases detected by XRD existessentially at the ribbon surfaces. This conclusion is sup-ported by the well known results which show that smallcrystalline regions are present at the so called wheel-sideof the melt-spun ribbons, even for as-prepared samples.Now the question to be answered is how the two pro-cesses, spin reorientation and crystallization onset, arecorrelated.During the annealing at 773 K frozen-in stresses aregradually removed from the ribbons [7]. These stressesare of tensile type in the as-prepared sample and fa-vor the in-plane magnetization state due to the positivemagnetostriction of the amorphous alloy. Therefore in-plane domains are large and dominate the magnetizationstate of the sample as shown by the Mössbauer spec-trum of the as-prepared ribbon. As annealing progresses,magnetic domains with different magnetization orienta-tions grow at the expenses of in-plane ones, leading toa more isotropic state, in order to minimize the mag-netostatic energy of the material. The fact that for longannealing times (i.e. 24 min) out-of-plane magnetizationbegins to dominate might be associated with magneticanisotropy effects at the interfaces amorphous/air or amor-phous/crystal. From XRD peak intensities, it seems thatthe Fe3Si phase grows with preferential orientation, a factthat would support the previous interpretation. Recently,a structure-magnetization reorientation was also found inFeCuNbSiB films [8].The frozen-in stress also induces a preferential tex-ture for crystalline phase formation at the ribbons surfaceduring fabrication and annealing processes. This prefer-ential texture would in turn provide a definite magneticanisotropy at the grain boundaries, with the amorphousphase favoring an out of plane spin distribution after24 min annealing. A similar behavior, but using tempera-ture instead of time as annealing parameter, was describedby Ok and Morrish and also by Herzer and Hilzinger[9, 10]. As shown in Fig. 1, the annealing treatmentsshown are carried out at temperature well below the firstcrystallization peak. Furthermore, the present results mayalso indicate that the thermally activated process had pro-vided a simple way to prepare amorphous ribbons withdefined magnetic orientations.In summary, a clear re-orientation spin effect has beenobserved in Metglas Fe78B13Si9 amorphous ribbon, fromin-plane to out-of-plane, which may be correlated withthe beginning of the crystallization process at the surface.We also found that the two processes are related to theexistence of a frozen-in stress in the melt-spun alloy.This stress induces an in-plane preferential orientation ofthe magnetization and its gradual release with annealingmay favor an out-of-plane magnetization.References1. J . O Z AWA , J. Therm. Anal. Cal. 64 (2001) 109.2. R . A . B R A N D and G. L E C A E R , Nucl. Instrum. Methods B34(1988) 272.3. R . S . D E B I A S I and A. A. R . F E R NA N D E S , J. Magn. Magn.Mater. 54–57 (1986) 253.4. Idem., J. Mater. Sci. 24 (1989) 41.5. I . V I N E Z E , S . D . B O U R D E AU X and M. T E G Z E , Phys. Rev. B19 (1979) 4898.6. V. A . P. RO D R I G U E Z, E . BAG G I O-S A I TOV I T C H, E . C .PA S S A M A N I , F. G A R C I A, A. Y. TA K E U C H I , J . M. B O R-R E G O and A. C O N D E , Hyp. Int. 122 (1999) 1.7. C . L . C H I E N , Phys. Rev. B 18 (1978) 1003.8. X . D . L I , W. Z . Y UA N, Z . J . Z H AO, X. Z . WA N G, J . Z .RUA N and X. L . YA N G , J. Magn. Magn. Mater. 279 (2004) 429.9. H . N . O K and A. H. M O R R I S H , Phys. Rev. B 23 (1981) 2257.10. G . H E R Z E R and H. R . H I L Z I N G E R , J. Magn. Magn. Mater. 62(1986) 143.Received 13 Marchand accepted 11 July 20051651

Thermal stability and magnetic properties of FeSiB amorphous alloy

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Thermal stability and magnetic properties of FeSiB amorphous alloy

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