Study of Magnetic Materials by Mössbauer Thermal Scans. Application to Nanocrystalline Systems

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1 Journal of Metastable and Nanocrystalline Materials Vol. 22 (2004) pp online at Citation 2004 Trans & Tech Publications, Switzerland Copyright (to be inserted by the publisher ) Study of Magnetic Materials by Mössbauer Thermal Scans. Application to Nanocrystalline Systems F.H. Sánchez 1, G.A. Pasquevich 1, P.Mendoza Zélis 1, A.F. Cabrera 1, L. Ying feng 2 and M. Vázquez 2 1 Departamento de Física, Universidad Nacional de La Plata, La Plata, Argentina 2 Instituto de Ciencia de Materiales de Madrid, CSIC, Madrid, España Keywords: Mössbauer effect spectroscopy Mössbauer thermal and isothermal scans finemet type alloys magnetic transitions nanocrystallization processes. Abstract. Mössbauer thermal scans (2 K/min) have been applied to the classical amorphous finemet-type precursor Fe 73.5 Si 13.5 Nb 6 Cu 1 B 9 alloy. They allowed the determination of the ordering temperatures of the amorphous phase before (T C = 624 K) and after partial (T C = 674 K) and complete (T C = 595 K) crystallization. The existence of three well differentiated temperature regions is observed, corresponding to the ferro to paramagnetic transition of the amorphous, nanocrystallization of bcc Fe(Si) and the ferro to superpara or paramagnetic transition of the crystalline phase. The crystallization kinetics at 763 K and 805 K was studied by two different approaches. At 763 K spectra were recorded every two hours up to a total time of 117 h. At 805 K scans were used due to the much higher transformation rate. Both results were analized with theoretical contributions representing the amorphous, very small crystals or embryos and grown up crystals. The evolution of their relative abundances obtained with the two approaches was consistent. The embryo nucleation was almost completed after two and eight hours at 763 and 805 K, respectively. Introduction The Mössbauer effect discovered by Rudolph Ludwig Mössbauer became a versatile and powerful non-destructive analysis tool soon after its discovery in 1957[1]. Nowadays, the applications of this effect are numerous and extend well beyond the fields of physics and chemistry. The phenomenon is the base of a spectroscopic technique, the Mössbauer spectroscopy (MS), where energy is swept by means of the Doppler effect[2]. Among several derivations of this technique, Mössbauer scans performed at fixed Doppler velocity, and very especially the Mössbauer thermal scans (MTS), were used by many researchers to perform rapid explorations of a system temperature response[3]. Mössbauer scans remained in the status of a semiquantitative method, but even so nice examples of its potential can be found in the relevant literature[4] Recently, it was reported that Mössbauer scans can be developed and formalized as a fully quantitative technique to study magnetic phase transitions[5]. The method can be extended to investigate other transformations which occur as a function of temperature, time, pressure, etc.. In this contribution we apply MTS to the study of the temperature and time evolution of melt spun amorphous Fe 73.5 Si 13.5 Nb 6 Cu 1 B 9, a well known system that undergoes a nanoscopic scale crystallization process which conferes to it outstanding soft magnetic properties[6]. This alloy was chosen to test the merits of MTS when applied to complex systems, as opposed to an already tested simple case[5]. Here it is shown that an appropriate combination of MS and MTS brings a deeper insight into the system and allows to extend Mössbauer studies to time scales shorter than those usually accessible to MS. Licensed to Francisco H. Sánchez (sanchez@fisica.unlp.edu.ar) - Universidad Nacional de la Plata - Argentina All rights reserved. No part of the contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of the publisher: Trans Tech Publications Ltd, Switzerland, (ID: /09/05,11:13:29)

2 40 Journal of Metastable and Nanocrystalline Materials: Winter e-volume 2004 Title of Publication (to be inserted by the publisher) Experimental details Experiments were performed using a standard equipment with appropriate modifications designed to carry on MTS and MIT (Mössbauer Isothermal runs) at the Physics Department of the University of La Plata. For MS the linear motor was driven with a conventional triangular velocity wave (constant acceleration mode) while for MS a wave composed by two square pulses with opposite polarities and different amplitudes and durations (null mean displacement) was used, the longer pulse corresponding to the useful part of the cycle. A standard 57 CoRh radioactive source with Fig. 1. Simulated Mössbauer velocity-temperature 2d surface (a) and its intersections with constant temperature (b) and constant velocity (c and d) planes. about 35 mci activity was employed. During heating (at constant rates), time, temperature, and counts of gamma rays transmitted through the absorber were automatically recorded during equal sequential periods of 1.4 seconds duration. Afterwards data was integrated within appropriate time or temperature intervals as to build up enough statistics to bring relative errors below reasonable figures. Gaussian distributions of hyperfine interactions which allowed linear correlations between main parameters were used as basic analysis units. Therefore, fitting functions consisted of Gaussian distributions of Lorentzian sextets or doublets (Voigtian patterns), simulated with the Puerta-Martín approximation [7]. The Fe 73.5 Si 13.5 Nb 6 Cu 1 B 9 amorphous alloy was prepared by melt spinning at the Madrid Institute of Materials Science. Resulting ribbons were typically 1 mm wide and 20 µm thick. The MTS mode The Mössbauer spectrum (Sp) temperature dependence can be regarded as a 2d surface Sp(v,T) in the Doppler velocity and temperature space as the one shown in Fig. 1a for a substance with a unique magnetic site for the Mössbauer probes (in the case 57 Fe). Intersections of this pattern with constant T or constant v planes (indicated by the dark lines in the Figure) give rise to spectra (Fig. 1b) or thermal a) 302 K 376 K 422 K 468 K 508 K 589 K 637 K 679 K 724 K 758 K 787 K 803 K mm/seg b) c) d) Tc = 624 K T(K) Fig. 2. Spectra measured at the labelled temperatures (a), heating up (b) and cooling down scans (c and d) at v = mm/s and 2 K/min.

3 Journal of Metastable and Nanocrystalline Materials Vol Journal Title and Volume Number (to be inserted by the publisher) scans (Figs. 1c, 1d), respectively by choosing appropriately the Doppler velocity the scans are especially sensitive to the ordering temperature (Fig. 1c) or to the hyperfine field B temperature dependence (Fig. 1d). In fact, Fig. 1d can be regarded as a temperature image of the spectral lines 5 and 6. From this discussion it becomes clear that scans must be analyzed with the same theoretical functions as spectra. However for the first case the temperature dependence of the relevant parameters: Mössbauer-Lamb factor, isomer shift, hyperfine field, etc., must be explicitly included, and saturation effects derived from the superposition of probability absorption at a given velocity be taken into account. An example of this as well as the application of MTS to the determination of the hyperfine field temperature dependence in FeSn 2 can be found in [5]. Fig. 3. Set of spectra taken every two hours at 763 K h h 0-20 h 0-10 h crystals embryos amorphous fit v(mm/s) Fig. 4. Analysis of the spectra taken while keeping the sample at 763 K during the indicated time periods, with contributions representing amorphous (doublet), embryos (broad magnetic hyperfine distribution), and grown up crystals. Results Fig. 2a shows the spectra of Fe 73.5 Si 13.5 Nb 6 Cu 1 B 9 taken at temperatures between 302 and 803 K. Three temperature regions can be distinguished. The first two correspond to the amorphous alloy in its ferromagnetic and paramagnetic states and the third one to a stage where magnetic bcc Fe(Si) nanocrystals form and growth. In correspondence to these, a thermal scan taken at v = mm/s and 2K/min also presents three well differentiated regions (Fig. 2b). In addition, some characteristic temperatures become evident, such as the amorphous Curie point (624 K) and the one where the nanocrystallization becomes the dominant feature (about 803 K). The positive slope of the scan above 624 K is due to temperature induced phonon softening (diminution of the sample Mössbauer- Lamb factor) and crystallization while the negative one above 780 K is the result of competition among phonon softening, crystallization and bcc Fe(Si) hyperfine field reduction. For the last part of the curve ( K) the effect of crystallization must be negligible. Hence, the small slope of this region indicates the existence of a distribution of ordering or blocking temperatures originated in a distribution of crystallite sizes.

4 42 Journal of Metastable and Nanocrystalline Materials: Winter e-volume 2004 Title of Publication (to be inserted by the publisher) Fig. 2c and 2d were obtained at the same Doppler velocity and a cooling rate of 2 K/min just after reaching 873 K (3b), and after staying at this temperature for several hours(3c). Therefore, they correspond to samples with different crystallized fractions. It can be concluded that the Curie temperature of the remnant amorphous changes during the crystallization process (674 K at an intermediate stage and 595 for the final state). The magnetic ordering of the nanocrystals becomes sharper with increasing crystallized fraction, indicating that crystallites have reached a more homogeneous size distribution and consequently more similar ordering/blocking temperatures. Fig. 7. Isothermal scan at 805 K. The inset indicates the spectral position of the employed constant velocity. fraction t(h) crystals embryos amorphous Fig. 6. Time evolution of the spectral contributions shown in Fig. 5. Fig. 3 shows a set of MS taken every 2 h from the as-spun sample at 763 K. The transition from the amorphous paramagnetic phase to the mixture of magnetic nanocrystals and paramagnetic remnant amorphous phase is apparent. The spectra were fitted with a superposition of three subspectra: a paramagnetic one (amorphous), a highly distributed magnetic one (smaller crystals or embryos) and a well defined crystalline pattern (grown up crystals) (Fig. 4). The model used to describe the crystalline pattern was based on what it is expected for a Fe 77.5 Si 22.5 ordered alloy[8]. In turns, this composition was deduced from RT spectra of crystallized samples (not shown). Hyperfine parameters were kept fixed for all spectra, and the relative areas of the three interactions were obtained as a function of time (Fig. 6). It can be seen that crystallization is almost complete after 8 h in agreement with other authors findings [9], but crystal growth takes times one order of magnitude longer. At higher temperatures (803 K), the procedure described above is no longer applicable because the transformation becomes too fast to allow the recording of MS with enough information. We have then performed an isothermal experiment (Fig. 7) at v=-1.76 mm/s, a velocity where the magnetic signals make the most important spectral contribution (inset in Fig. 7). The scan Relative abundance (a) Amorphous (b) Embryos (c) Crystals t(h) Figure 8. Relative abundance of amorphous, embryos and grown up crystals, obtained from data shown in Fig. 7. (c) (b) (a)

5 Journal of Metastable and Nanocrystalline Materials Vol Journal Title and Volume Number (to be inserted by the publisher) reflects the kinetics of the crystallization. It was analyzed asuming that the process occurs through two coupled steps: amorphous embryos crystals. The result of this analysis is presented in Fig. 8 and is conceptually consistent with the results obtained at 763 K (notice that the amorphous abundance represented in Fig. 8 corresponds to the total of the Fe atoms which undergo the crystallization process). In this case embryo nucleation is almost complete after about two hours. Conclusions We have performed Mössbauer scans (2 K/min) to study thermal and isothermal transformations of the clasical amorphous finemet-type precursor Fe 73.5 Si 13.5 Nb 6 Cu 1 B 9 alloy. The thermal scans clearly show three differentiated temperature regions. In the first one amorphous undergo a ferro to paramagnetic transition at 624 K, the second one is dominated by the crystallization process and the third one by the ferro to superpara or paramagnetic transition of the crystalline phase. It was found that the amorphous Curie temperature strongly depends on the degree of crystallization being 624 K for the original sample, higher (about 674 K) for the partially crystallized alloy, and lower (595 K) for the fully crystallized system. The crystallization kinetics was studied at 763 K and 805 K. In the firt case Mössbauer spectra were recorded every two hours up to a total time of 117 h. In the second one the much higher transformation rate did not allow the recording of spectra with sufficient information. Therefore one scans was performed during 50 h. The results of both experiments were analized with theoretical contributions representing the amorphous phase, crystals in their initial formation stage (embryos) and grown up crystals. It was found that the embryo nucleation was almost completed in two and eigth hours at 763 and 805 K, respectively. This work demonstrates the advantage of complementing Mössbauer spectra with thermal or isothermal scans to get a more deep insight on materials transformations. The scans become particularly advantageous when the time scale of the observed transitions is to short as two allow conventional Mösbauer studies. Acknowledgements The authors wish to thank Engs. Nolberto Martínez and Alejandro Veiga (UNLP) for helping with the development of the MTS facility and providing continuous professional support. Partial economic support from CONICET and UNLP is acknowledged. References [1] R. L. Mössbauer: Z. Phys. Vol. 151 (1958), p [2] A. Vértes, L. Korecz, and K. Burger: Mössbauer Spectroscopy (Elsevier Scientific Publishing Company, USA, 1979). [3] U. Gonser, C.J. Meechan, A.H. Muir, H. Wiedersich: J. Appl. Phys. Vol. 34 (1963), p [4] C.L. Chien: Phys. Rev. B Vol. 18 (1978), p [5] P. Mendoza Zélis, G. Paquevich, F.H. Sánchez, N. Martínez and A. Veiga: Phys. Lett. Vol. A 298 (2002), p. 55. [6] Y. Yoshizawa, S. Oguma, and K. Yamauchi: J. Appl. Phys. Vol. 64 (1998), p [7] J. Puerta and P. Martin: Appl. Opt. Vol. 20 (1981), p [8] A.F. Cabrera and F.H. Sánchez, Phys Rev B Vol. 65, (2002) p [9] N. Lecaude and J. C. Perron: Mat. Sci. Eng. Vol. A (1997), p. 581.