Al-Fe multilayers have been mixed by thermal treatment and their evolution followed by conversion electron Mossbauer spectroscopy. The initial and final states have been characterized by Rutherford backscattering spectrometry. The results are compared with those previously obtained in the ion beam mixing of similar systems.Facultad de Ciencias Exacta

Mendoza Zélis, Luis Alejandro

Meyer, Marcos

Sánchez, Francisco Homero

Traverse, Agnès

Hyperfine Interactions

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Hyperfine Interactions 83 (1994) 327-331 327 Thermal mixing ofA1-Fe multilayers M. Meyer 1, L. Mendoza Z61is 2, F.H. S/mchez 2 and A. Traverse a Departamento de Fisica, Facultad de Ciencias Exactas, Universidad Nacional de La Plata, Argentina a Centre de Spectrorn~trie Nucl~aire et de Spectrombtrie de Masse, Universitb Paris XI, Orsay, France A1-Fe multilayers have been mixed by thermal treatment and their evolution followed by conversion electron M6ssbauer spectroscopy. The initial and final states have been character- ized by Rutherford backscattering spectrometry. The results are compared with those pre- viously obtained in the ion beam mixing of similar systems. 1. I n t roduc t ion  Ion beam mixing of Al-Fe multilayers produces amorphous, quasicrystaUine or crystalline phases depending on the target temperature during irradiation [1]. During this process an effective local temperature, which depends on the target temperature and on the rate of energy deposition, would determine the formed phase. We present here results on the thermal mixing of A1-Fe multilayers made with the aim of determining which phases are formed and how the local order around Fe sites evolves during the process. 2. Exper imenta l  Al-Fe multilayers consisting of four bilayers of 220 A Al and 35 A Fe were pre- pared by electron gun evaporation on a quartz substrate at a base pressure of 1.3 x 10 -8 mbar. Additionally 30 A of A1 were deposited on top of the multilayer as a barrier for O penetration. Prior to evaporation iron was enriched in 57Fe in order to enhance the M6ssbauer absorption. From a total of four samples two were used for the present study (samples I and II). The multilayers were analyzed by Rutherford scattering of a-particles at 93 and 173 degrees. Also a search for O contamination was made using the 160(a, I],)O 16 resonance at 3.05 MeV. I Fellow of CONICET. 2 Member of CONICET. © J.C. Baltzer AG, Science Publishers 328 M. Meyer et al. / Thermalmixing of AI-Fe multilayers The heat treatments were performed in a heating device continuously evacuated by a turbomolecular pump at a pressure lower than 5 × 10 -5 mbar. Conversion electron M6ssbauer spectra (CEMS) were obtained in the constant acceleration mode using a 57CoRh source and an electron counter with a constant flow of He-8 at% CH4. 3. Results and discussion 3.1. AS-PREPARED MULTILAYERS The layered structure of the samples is dearly displayed by the RBS spectrum of sample II, mainly in the energy region corresponding to scattering by the Fe atoms (fig. I). The M6ssbauer spectrum of both samples (fig. 2) shows, apart from several magnetic sextets, an important absorption at low velocities, revealing that 13% of the probes are in a paramagnetic environment. Therefore the spectrum was fitted with a superposition of three or four magnetic interactions of varying mag- netic field and isomeric shift, plus a quadrupole doublet. The magnetic interactions could readily be associated with pure Fe and Fe with one or two AI atoms in the first two coordination spheres, as expected for a dilute solid solution of A1 in Fe (the mean composition would be about 5 at%). There is, however, an excess population of Fe without non-magnetic neighbors with respect to that expected for a random solid solution. The quadrupole interaction corresponds to a broad distribution of doublets cen- 0.6 80 - I 50 -o 7~ 40 >- -1D ~ 30 ~ 2o Z 10 0 2OO Energy (MeV) 0.7 0.8 0.9 I i 250 400 Y T I 300 350 Channel  Fig. 1. RBS spectra of the starting multilayer (sample II). The full line represents the simulation described in the text. M. Meyer et al. / Thermalmixing of Al-Fe multilayers 1,68x 10';' J ' ""' [ . . . .  I 329 1.67 1.66 1.64 1,63 - 8  8 - 6  - 4  - 2  0 2 4 6 v(mm/s) Fig. 2, ME spectra of the starting multilayer (sample II), The resulting fit and the individual compo- nents are also shown, tered at (QS ~ 0.5 mm/s ,  IS ~ 0.26 mm/s).  It gives no evidence for the formation of Fe aggregates in A1 (like dimers or clusters) or some known compounds like A16Fe or All 3Fe4 [2]. Its parameters are similar to those obtained in the orthorhom- bic compound AlsFe2 and in amorphous Al-rich alloys. This facts were taken into account to simulate the RBS spectra by allowing for the presence of pure A1 layers and nearly pure Fe ones separated by regions of an approximate composition: AlsFe2. The experimental RBS spectrum is very well reproduced in this manner (see fig. 1), resulting in an average Fe concentration of 11.5 at%. In conclusion, the starting multilayer seems to be partially mixed, with a few per- cent Al penetrating in Fe on one hand and intermediate disordered regions with 20-30 at% Fe on the other. 3.2. THERMAL MIXING The samples were thermally treated under the conditions described above, and their evolution observed by CEMS. In the first stages of mixing the spectra could still be interpreted in terms of a dilute solid solution of A1 in Fe and a paramagnetic phase. The observed changes in the relative fraction of each magnetic component are consistent with an increase in A1 concentration in the dilute alloy region, while the overall fraction of this phase decreases monotonically. Accordingly the para- magnetic phase develops at the interphase. Both the QS and the IS of this phase decrease and the doublet narrows. This phase grows linearly with v'7 as expected for a diffusion controlled process. From the results of isothermal treatments at 330 M. Meyer et aL / Thermal mixing of  Al-Fe multilayers 2.32x106 2.30 m 2.28 0 ~-) 2.26 2.24 2.22 u.. r i + , I ' i ' ' ' I ' ' ' ' . ~ " t l r  I ' t  " l ~ " I t l f r l ' l i q ~ r l l  ~ - 2  - 1  0 t v(~/~)  Fig. 3. ME spectra of sample I after 270 rain at 550 K. 220°C (sample II) an effective interdiffusion coefficient of  about 10 -15 cm2/s was obtained. Finally, sample I was sequentially treated at 280°C. All the magnetic sextets col- lapse in a broad distribution of magnetic fields centered at 305 kOe like that observed in amorphous Fe-metalloid alloys. This phase changes into a paramag- netic one. The IS of this phase remains constant but its QS decreases. After 270 min at this temperature no magnetic interactions could be observed in the spectrum (fig. 3). Accordingly, RBS measurements on this sample show overall composi- tional uniformity: the Fe concentration decreases slightly from 12.5 at% on top to 10.5 at% on bottom. The results also indicate a slight penetration of Fe in the SiO2 substrate. Concerning the paramagnetic phase, as it was already said, the doublet narrows with the thermal treatments (see table 1), and becomes asymmetric. The QS Table 1 Quadrupole parameters obtained from the ME spectra after several heat treatments. Fraction QS (ram/s) IS (ram / s) FWHM (ram / s) as-made 132 0.47948 0.27323 0.69886 10 rain, 423 K 182 0.45824 0.24315 0.4994] 10 min, 463 K 233 0.45323 0.28314 0.4854t 30 min, 463 K 314 0.38517 0.24110 0.44632 I0 min, 523 K 273 0.42814 0.2338 0.49425 30 min, 553 K 416 0.3966 0.2254 0.41112 90 rain, 553 K 745 0.3605 0.2364 0.3798 150 min, 553 K 100 0.3322 0.2285 0.3013 30 min, 623 K 100 0.321z 0.2311 0.2913 M. Meyer et al. / Thermalrnixing of  Al-Fe multilayers 331 decreases monotonically from 0.5 to 0.33 mm/s,  and the IS changes from 0.26 to 0.23 mm/s  after the first stages of mixing and then remains constant. The observed quadrupole parameters are alike to those obtained for amorphous and quasicrystalline A1Fe phases of similar composition [3], obtained by ion beam mixing of A1Fe multilayers at 70 and 520 K respectively. Furthermore, during the thermal mixing our values seem to evolve from "amorphous" to "quasicrystalline" ones. However, it is important to note that practically the same values were observed for metastable A16Fe (crystalline) and quasicrystals of the same composi- tion. Nevertheless the observed asymmetry is not consistent with a crystalline com- pound and would be an indication that a quasicrystalline phase was formed. This needs to be confirmed by X-ray diffraction techniques. 4. C o n c l u s i o n  Our results are consistent with a diffusion controlled mixing mechanism, through an Al-rich paramagnetic region already present in the starting material. In the nearly pure Fe regions the A1 concentration increases, forming a highly disordered ferromagnetic phase, presumably amorphous, before its transforma- tion to the paramagnetic phase. The latter evolves during the mixing toward meta- stable A1Fe alloys similar to those obtained by ion beam mixing at lower temperatures. During the mixing at 280°C the formation of quasicrystalline phases would com- pete with that of crystalline ml6Fe [4] and probably both phases were formed. References  [1] B. Ranschenbach and V. Heera, J. Mater. Sci. Lett. 6 (1987)401. [2] S. Nasu, U. Gonser and R.S. Preston, J. Phys. (Paris) 41 (1980) C1-385. [3] J.C.Plenet, Thesis, Universit6 Claude Bernard, LyonI, France (1991). [4] A. Csnady, J.R. Giinther, P.B. Barna and J. Mayer, Thin Solid Films 167 (1988) 203. 

Thermal mixing of Al-Fe multilayers

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Thermal mixing of Al-Fe multilayers

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