- Email: [email protected]
Zr multilayers
Nuclear Instruments and Methods in Physics Research B 148 (1999) 176±183
Atomic mixing induced by swift heavy ion irradiation of Fe/Zr multilayers C. Jaouen a
a,*
, A. Michel a, J. Pacaud a, C. Dufour b, Ph. Bauer c, B. Gervais
d
Laboratoire de M etallurgie Physique, UMR CNRS 6630, SP2MI, Universit e de Poitiers, BP 179, F-86960 Chasseneuil-Futuroscope Cedex, France b Laboratoire de Physique des Mat eriaux, Universit e Henri Poincar e Nancy I, BP 239, F-54506 Vandoeuvre Cedex, France c Laboratoire de M etrologie des Interfaces Techniques, Universit e de Franche-Comt e, BP 427, F-25211 Montb eliard Cedex, France d CIRIL, BP 5133, F-14070 Caen Cedex 05, France
Abstract The mechanism of ion induced mixing and phase change was studied for Fe/Zr multilayers, and speci®cally for the case of swift heavy ions giving rise to a very large electronic excitation of the target. The multilayers had a modulation of 7.6 nm and an overall composition Fe69 Zr31 . The Zr layers were amorphous whereas the Fe ones were crystalline (bcc) with a very strong (1 1 0) texture in the growth direction. The phase transformation and the composition changes were analysed using the structural and magnetic properties of the Fe component by means of a detailed analysis of the X-ray diraction pro®les and with the aid of backscattering M ossbauer spectroscopy. A complete mixing was observed at a ¯uence of 1013 U/cm2 . Both phenomena, the dose dependence of the ion beam mixed amorphous non-magnetic phase and the quantitative evolution of the crystalline iron layer thickness, suggest that mixing occurs in a two-stage process. At an initial stage, an anisotropic diusion of iron atoms in the amorphous zirconium layers takes place along the interface, while subsequent ion bombardment leads to a generalised transformation through the whole of the Fe layer. Finally, the implications of these observations are discussed in comparison to the plastic deformation phenomena reported for amorphous alloys. Ó 1999 Elsevier Science B.V. All rights reserved. PACS: 61.80jh; 75 70; 75.30k; 61.43d Keywords: Ion irradiation eects; Metallic multilayer; Ion beam mixing; Magnetic phase transition; Amorphous metallic alloys
1. Introduction Recent experiments have shown the occurrence of nanoscale mixing eects induced by
* Corresponding author. Tel.: +33 5 49 49 67 30; fax: +33 5 49 49 66 92; e-mail: [email protected]
electronic excitation in multilayers irradiated by swift heavy ions [1]. In the present investigation, we report the observation of atomic mixing in nanometric Fe/Zr multilayers submitted to 811 MeV 238 U ion irradiation. The Fe/Zr system is particularly interesting because of the high sensitivity of its two constituents to electronic excitations [2±5]. In this system atomic mixing is also
0168-583X/98/$ ± see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 8 ) 0 0 7 2 5 - 3
C. Jaouen et al. / Nucl. Instr. and Meth. in Phys. Res. B 148 (1999) 176±183
expected due to the negative mixing enthalpy of the system ()0.35 eV/at), which leads to a fast solid state reaction. Furthermore, even by referring to usual methods, the formation of metastable amorphous Fex Zr1ÿx alloys is expected almost in the complete compositional range (0.20 < x < 0.93). This structural transition at room temperature involves the loss of ferromagnetism either for paramagnetic or for diamagnetic states, depending on the composition. The magnetic properties of these amorphous alloys are very well characterised [6±8]. The investigations presented here concern a Fe/Zr multilayer system with a period of about 7.6 nm and layer thicknesses having a ratio 1:1, thus providing the crystalline growth for the iron layers but an amorphous structure for the Zr ones. Two methods, X-ray diraction and M ossbauer spectroscopy, were used for the analysis of the irradiation induced atomic transport. The X-ray diraction is highly sensitive to phase change as well as to the interfacial reaction, thus making the method suitable for measurement of small amounts of mixing. On the other hand, M ossbauer spectroscopy provides additional and complementary information on the local chemical and structural environment of the 57 Fe atoms. The formation of the amorphous phase due to the mixing process reveals, when measured at room temperature, a quadrupole-split doublet growth in the M ossbauer spectra that is clearly in contrast to the ferromagnetic a±Fe and the crystalline interfacial Fe±(Zr) which exhibit a magnetic hyper®ne structure [9,10].
2. Experimental details The Fe/Zr multilayers were produced in a high vacuum NORDIKO-3000 sputtering deposition system equipped with an RF-plasma source and having an accelerating beam voltage of 1.2 kV. The multilayers consisted of 35 alternating Fe and Zr layers having a thickness ratio 1:1 for the two elements. This corresponds, roughly, to a 2:1 composition ratio. The modulation length K tFe + tZr , where tFe and tZr are the thickness of
177
the Fe and Zr layer respectively, was 7.6 nm, as it was determined by small-angle X-ray diraction. That gives a total thickness for the system of about 266 nm. The used substrates were singlecrystal (1 0 0) silicon wafers, and the deposition was carried out at room temperature. Energy Dispersive X-ray spectroscopy (EDX) indicated an overall composition for the multilayer system of Fe69 Zr31 (within an uncertainty of 1%), pointing out a slight enrichment in Fe, relatively to the nominal layer thickness. Consequently, the pattern of the layers may be noted [Fe3:9 /Zr3:7 ] ´ 35 (the subscripts represent the thickness in nanometers). The samples were irradiated, at a normal angle of incidence, with 811 MeV 238 U, at GANIL in Caen. In the case of this energy range, the projected ion range exceeds, about two orders of magnitude, the total ®lm thickness. Thus it is considered that the excitation is nearly uniform throughout the thickness of the sample. The exchange mechanism is mainly dominated by electronic excitations and ionisation while the loss due to nuclear elastic collisions (dE/dx)n can be considered very low (75 eV/nm). The average value of the electronic energy loss, (dE/dx)e , calculated using the computer simulation program TRIM 96 [11], is 57 keV/nm for U ions. The irradiation of the samples was carried out at 77 K and the incident ion ¯ux was kept at a low value (about 2 ´ 108 ±3 ´ 108 ions/cm2 /s) to prevent possible thermal assisted diusion or defect recombination. The characterisation of the structural and magnetic properties of the Fe/Zr multilayers, as deposited and after irradiation with swift heavy ions, was performed by means of X-ray diraction and M ossbauer spectroscopy. The X-ray diraction spectra were obtained using a Siemens D500 and a Bruker D5005 diffractometer. Both experimental con®gurations use the Cu Ka radiation and are equipped with a backside monochromator and a point detector. Two geometries were used to study the crystallinity of the samples: the Bragg±Brentano geometry (h/2h scan), which gives information on the stacking along the growth direction, and the rocking geometry (x-scan), which allows to esti-
178
C. Jaouen et al. / Nucl. Instr. and Meth. in Phys. Res. B 148 (1999) 176±183
mate the mosaic distribution of the growth planes in the sample. Conversion Electron M ossbauer spectroscopy (CEMS) was used to determine the magnetic state ossbauer spectra were of the 57 Fe atoms. M obtained at room temperature using a source of 57 Co in Rh and a gas ¯ow proportional counter for the conversion and the electron detection. The analysis of the experimental data was performed with the aid of standard least-square ®tting routines. A six-line pattern with lorentzian line shape was assumed for bulk-like Fe component; the magnetic and non-magnetic parts of the interfacial component are respectively accounted for by a hyper®ne ®eld distribution (smoothed histogram method)and an asymmetrical broad doublet featuring a quadrupole distribution.
3. Results 3.1. As deposited Fe/Zr multilayers The [Fe3:9 /Zr3:7 ] ´ 35 system studied demonstrated alternating crystalline Fe layers and
Fig. 1. X-ray diraction pattern of an as-deposited Fe/Zr multilayer (K 7.6 nm). The calculated diraction pattern of an amorphous-Zr/crystalline-Fe bilayer with ®nite size of the Fe crystalline layers (with 19 Fe(1 1 0) planes parallel to the specimen surface) is shown for comparison. Intensity is plotted on a logarithmic scale.
amorphous Zr layers. Fig. 1 shows the h/2h diffraction spectrum for the as deposited material. The main peak corresponds to the Fe (1 1 0) re¯ection. The Zr layers are amorphous and, as expected, no signal was generated from those layers. The Fe lattice spacing is 0.2049 nm in comparison to the bulk value of 0.2027 nm. This can be ascribed to the existence of a compressive stress in the Fe layers parallel to the Fe/Zr interface, as it is often observed in crystalline ®lms deposited by sputtering. The rocking curve obtained for the (1 1 0) Fe re¯ection has a full width at half maximum (FWHM) of 9°, indicating a well-de®ned texture for the Fe layers despite the amorphous state of Zr. Amorphous Zr or Fe layers have been observed already in the Fe/Zr multilayer system in the case of small layer thickness for both constituents [12]. An amorphous to crystalline phase transition is observed when the thickness is increased above a critical thickness which corresponds to a number n 10 of atomic planes. It has been suggested that the large lattice mis®t between the two elements could be held responsible for this eect. Our study of the Fe/Zr multilayer system shows that the critical number of atomic planes above which the Fe layers are crystalline is about n 10, whereas a number n 20 is required for the Zr ones. The latter value does not agree with previously published results [9,10]. A possible explanation might be found in the presence of iron impurities in the zirconium layers (x 2%, as deduced from EDX analysis) due to the target screens. This assumption is supported by a comparable retardation in the formation of crystalline Fe with increasing Zr concentration in the Fe layers, as it has been reported [13]. The striking feature of the X-ray spectrum is the occurrence of secondary, small, peaks around the main Fe (110) Bragg peak. These secondary peaks have to be treated as ®nite size peaks and not as superlattice re¯ections. The sample can then be described as a stacking of Fe layers of wellde®ned size separated by non diracting amorphous Zr layers. The simulation of the experimental pro®le using this description is shown in Fig. 1. When the number of Fe monolayers is nFe 19, a good agreement between experiment
C. Jaouen et al. / Nucl. Instr. and Meth. in Phys. Res. B 148 (1999) 176±183
and simulation is obtained, as far as peak intensity and width are concerned. The asymmetry in the intensity of the satellites at both sides of the main peak is correctly reproduced by taking into account the Debye±Waller eects. Hence, the determination of the coherence length for the Fe layers is very accurate since both, the pro®le of the main Fe (110) Bragg peak and the position of the secondary peaks, are reproducible by simulation. That means that an estimation of the size Lz of the diracting domains in a direction perpendicular to the diracting planes (here Lz is equal to the Felayer thickness) may be obtained from both, the position of the maxima/minima of the Laue function, and the peak broadening. For the present multilayer system it is found to be Lz 3.89 0.09 nm, in good agreement with the value derived from the overall composition and the period of the multilayer system. The M ossbauer spectrum of the as deposited multilayer system obtained at room temperature is shown in Fig. 2(a). This spectrum shows a magnetic contribution, with the existence of a Zeeman splitting, and a non-magnetic quadrupole contribution in the middle part. Three Fe site components were necessary to interpret the spectra: (a) A six-line pattern with spectral contribution of 41% (with a magnetic hyper®ne ®eld of 32.75 T, slightly lower than the characteristic value of bulk Fe) corresponding to the crystalline a-Fe phase. (b) A hyper®ne ®eld distribution with a spectral contribution of 35.5% (with a mean hyper®ne ®eld depleted at an average value of 26 T) ascribed to a crystalline interfacial contribution. (c) An asymmetrical quadrupole doublet with large lines corresponding to an amorphous Fex Zr1ÿx and contributing to 23.5% of the total area spectra (from the mean quadrupole splitting of 0.52 mm/s, the average composition is x 0.25 [6]). The results obtained represent an important extension of the magnetic interface, since 59% of the Fe atoms belong either to the interfacial amorphous component or to a crystalline component characterised by a signi®cant reduction (20%) of the Fe hyper®ne ®eld. The presence of the latter contribution cannot be related directly to the structural properties of the Fe layers which exhibit a remarkable coherency. The as deposited
179
Fig. 2. CEMS spectra obtained with Fe/Zr multilayers before and after ion irradiation at 77 K with high energy U ions : (a) as-deposited multilayer; (b) after 1 ´ 1012 U/cm2 ; (c) after 2 ´ 1012 U/cm2 ; (d) after 5 ´ 1012 U/cm2 .
amorphous phase, with an average composition x 0.25, suggests a diusion of the Fe atoms in the Zr layers.
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3.2. Evolution under irradiation with 811 MeV U ions Fig. 2 presents, through the characteristic M ossbauer spectra, the main features of the evolution observed under irradiation. The growth of the non-magnetic amorphous component is clearly observed. At 5 ´ 1012 U/cm2 , a paramagnetic amorphous phase is obtained having related spectral parameters quite consistent with the overall composition of the multilayers: the quadrupole splitting (0.32 mm/s) obtained at higher irradiation dose is quite consistent with the formation of a Fe-rich amorphous alloy [6]. The spectrum obtained after 1013 U/cm2 (not presented here) is quite similar. The relative fraction (f) versus the ion ¯uence / obtained by computer simulation for the dierent components is shown in Fig. 3. It is worth mentioning that above 2 ´ 1012 U/cm2 an increase in the transformation rate is observed leading to the complete transformation of the Fe layers into an amorphous state. The fraction of Fe atoms introduced in the nonmagnetic phase deduced from the M ossbauer spectra is: avol Mossbauer
fFe magn 0 ÿ fFe magn / fFe magn 0
and is presented in Fig. 4.
Fig. 3. Relative fraction of the bulklike a-Fe, interfacial crystalline Fe±(Zr) and amorphous non-magnetic Fe±Zr interface components as deduced from the CEMS analysis versus the ion ¯uence.
Fig. 4. Fraction of the Fe atoms introduced in the non-magnetic amorphous phase versus the U ion ¯uence. The fractions corresponding to an interfacial process, as deduced from the decrease of the Fe crystalline layer thickness (by ®tting the diraction pro®le with a Laue function), and to the total transformation, as deduced from X-ray and M ossbauer data, can be compared.
Fig. 5 shows the evolution of the X-ray diffraction spectra for the samples after irradiation. First, it should be noted that neither the position nor the shape of the Fe peak is altered by the irradiation; this means that no relaxation is introduced, and that the Fe lattice parameter remains signi®cantly greater than the bulk value. By contrast, important changes to both, the intensity and the pro®le, are clearly observed. The strong decrease of the peak intensity is an evidence of the transformation under irradiation of the Fe layers into an amorphous Fex Zr1ÿx alloy, which is complete at a dose of 1 ´ 1013 U/cm2 . The integrated intensity, Iint , of the Bragg peak is proportional to the volume of the remaining crystalline material in the Fe layers, provided that no change, in the average scattering factors per atom or in the texture, occurs. These last two conditions are here well ful®lled since the lattice parameter deduced from the position of the Bragg peak and the rocking curves remain the same during the transformation, at least until 5 ´ 1012 U/cm2 . Thus, the average volume fraction, avol of the Fe layers transformed into an amorphous Fex Zr1ÿx alloy, for a given irradiation ¯uence /, can be deduced directly from the variation of the integrated intensity:
C. Jaouen et al. / Nucl. Instr. and Meth. in Phys. Res. B 148 (1999) 176±183
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the main peak, inversely proportional to Lz , increases. The fraction, aint , of the crystalline Fe layers consumed upon irradiation at a given ¯uence /, by a progressive ion induced diusion at the interfaces, can be deduced from the ratio: aint
Fig. 5. X-ray diraction patterns obtained before and after irradiation at 77 K with high energy U ions at dierent doses. The dashed part is shown in an expanded 2h-scale as insert, the angular shift of the Laue satellites in the ®rst stage of the irradiation is visible. The small peak observed at 2h 38° may be attributed to crystalline Zr oxide.
avol XRD
Iint 0 ÿ Iint / : Iint 0
The corresponding evolution is presented in Fig. 4 and can be compared to the evolution of the mixed amorphous fraction a vol (Mossbauer). It is worth emphasising the good agreement of the values for the total amorphous part obtained from both investigating techniques, X-ray diraction and M ossbauer spectroscopy. This attests the validity of the results. The other visible evolution in the spectra concerns the secondary peaks, which remain distinguishable up to a dose of 2 ´ 1012 U/cm2 . The positions of these peaks change relatively to the main peak position and, according to the analysis described above, this means that the thickness of the crystalline Fe layers decreases. In a consistent way, the full width at half-maximum intensity of
Lz 0 ÿ Lz / : Lz 0
By comparing the progress of amorphisation, using the ratios avol and aint , it becomes possible to distinguish the part of the solid transformation that occurs in the bulk from the one occurring at the interface. Such an approach, already applied to the study of amorphisation by solid state reactions [14], is able to provide information about the mechanisms of the ion induced mixing-phase change. The plot in Fig. 4 represents the evolution of both quantities. The importance of this plot lies in the existence of two successive stages in the evolution under irradiation: In the ®rst stage (up to 1012 U/cm2 ), the dominant mechanism is the interfacial reaction limited to three Fe planes, and it is represented by aint , whereas in the second stage the saturation of aint suggests that, for subsequent ion impacts, amorphisation would be generalised throughout the whole Fe layer.
4. Discussion As described above, the ®rst stage of the mixing kinetics (up to 1012 U/cm2 ) is dominated by interdiusion at the interface. This takes place for doses similar to the doses required for defect creation in amorphous materials. This stage corresponds to a diusion of iron in zirconium at the interfaces in order to create an amorphous FeZr alloy. Indeed, in a multilayer system with larger period (both elements being crystalline), we observed the amorphisation of nanocrystalline zirconium during this ®rst stage, showing the fast diusion of iron into the Zr layers. Therefore, we can say that a spontaneous movement of the iron atoms occurs during the ®rst impacts of the ions leading to a densi®cation of the Zr layers. During the second stage, the absence of further shift of the Laue satellites leads to the conclusion
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that a diusion of the zirconium throughout the whole Fe layer in the wake of the ions is the dominant mechanism. This mixing leads to the formation of a paramagnetic amorphous alloy. However, no de®nite conclusion can be drawn for the homogeneity of the formed phase. This two stage process can be compared to the one observed during high-energy irradiation of amorphous alloys [15±17]. As already mentioned, the ®rst stage may correspond to a densi®cation of the amorphous Zr phase with interdiusion of iron. However, the link between the zirconium diusion observed in the second stage and the anisotropic deformation of the amorphous materials does not appear straightforward to prove. We can suggest that both the diusion and the phase change observed in the Fe layers can be an alternative way to relax the deformation and the subsequent stresses induced by intense electronic excitations in the dense amorphous alloy. Finally, it is interesting to compare these results to mixing eects induced in Fe/Zr multilayers (with a similar period) by low energy He [9] or Ar ions [10], i.e. in an energy range where interfacial mixing is attributed to ballistic atomic displacements. First, in contrast to the step ¯uence dependence obtained here with irradiation by swift heavy ions, the mixed iron fraction varies according to the square root of the low energy ion dose, in good agreement with collisional or spike models. Such a dierence is all the better demonstrated since amorphisation and mixing have been also studied using M ossbauer spectroscopy. Another dierence arises from the fact that a saturation of the amorphisation and the mixing process is observed at about 70% and 95% for He and Ar low energy ion irradiation, while, in the case of swift heavy ion irradiation, the occurring amorphisation is complete. Such contrasting behaviours suggest that the mixing and the amorphisation introduced by the two processes involve dierent mechanisms. 5. Conclusion It is worth pointing out the main features of the above mentioned experiments. In the case of irra-
diation by swift heavy ions, a mixing and an amorphous transformation, which are complete for doses around 1013 811 MeV U/cm2 , are induced in the Fe/Zr multilayers. No stress relaxation is observed in the remaining crystalline iron layers during the process. The reaction can be explained by a two stage mechanism: (a) An interdiusion at the interfaces for low irradiation doses; (b) A subsequent complete or almost complete mixing in the wake of the ions concomitantly to the saturation of the ®rst eect. We must emphasise that a one-stage-only full mixing in the ion tracks cannot explain our results. The origin of this two stage process and particularly the in¯uence of the amorphous state of the Zr layer on the observed atomic motion are still to be determined. Further investigations should include the study of the damaging process during the ®rst stage, especially under very low dose irradiation, and investigations for possible period variations during irradiation.
References [1] Ph. Bauer, C. Dufour, C. Jaouen, G. Marchal, J. Pacaud, J. Grilhe, J.C. Jousset, J. Appl. Phys. 81 (1997) 116. [2] A. Dunlop, D. Lesueur, J. Dural, Nucl. Instr. and Meth. B 42 (1989) 412. [3] A. Dunlop, D. Lesueur, J. Morillo, J. Dural, R. Spohr, J. Vetter, Nucl. Instr. and Meth. B 48 (1990) 419. [4] A. Dunlop, D. Lesueur, P. Legrand, H. Dammak, Nucl. Instr. and Meth. B 90 (1994) 330. [5] H. Dammak, D. Lesueur, A. Dunlop, P. Legrand, J. Morillo, Radiation Eects Def. in Solids 126 (1993) 111. [6] K.M. Unruh, C.L. Chien, Phys. Rev. B 30 (1984) 4968. [7] C. Michaelsen, H.A. Wagner, H.C. Freyhardt, J. Phys. F 16 (1986) 109. [8] D.H. Ryan, J.M.D. Coey, E. Batalla, Z. Altounian, J.O. Str om-Olsen, Phys. Rev. B 35 (1987) 8630. [9] M. Kopcewicz, D.E. Williamson, J. Appl. Phys. 74 (1993) 4363. [10] M. Kopcewicz, J. Jagielski, T. Stobiecki, G. Gawlik, J. Appl. Phys. 76 (1994) 5233. [11] J.F. Ziegler, J.P. Biersack, U. Littmark, The Stopping and Range of Ions in Matter, Pergamon Press, New York, 1985. [12] D.L. Williamson, B.M. Clemens, Hyper®ne Interactions 42 (1988) 967. [13] H. Geisler, U. Herr, T. Lorentz, K. Samwer, Thin Solid Films 275 (1996) 176.
C. Jaouen et al. / Nucl. Instr. and Meth. in Phys. Res. B 148 (1999) 176±183 [14] M.A. Hollanders, B.J. Thijsse, E.J. Mittemeijer, Phys. Rev. B 42 (1990) 5481. [15] S. Klaum unzer, G. Schumacher, Phys. Rev. Lett. 51 (1983) 1987.
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[16] A. Audouard, E. Balanzat, S. Bouard, J.C. Jousset, D. Lesueur, L. Thome, Europhys. Lett. 3 (1987) 327. [17] A. Audouard, E. Balanzat, S. Bouard, J.C. Jousset, D. Lesueur, L. Thome, Europhys. Lett. 5 (1988) 241.
Atomic mixing induced by swift heavy ion irradiation of Fe/Zr multilayers C. Jaouen a
a,*
, A. Michel a, J. Pacaud a, C. Dufour b, Ph. Bauer c, B. Gervais
d
Laboratoire de M etallurgie Physique, UMR CNRS 6630, SP2MI, Universit e de Poitiers, BP 179, F-86960 Chasseneuil-Futuroscope Cedex, France b Laboratoire de Physique des Mat eriaux, Universit e Henri Poincar e Nancy I, BP 239, F-54506 Vandoeuvre Cedex, France c Laboratoire de M etrologie des Interfaces Techniques, Universit e de Franche-Comt e, BP 427, F-25211 Montb eliard Cedex, France d CIRIL, BP 5133, F-14070 Caen Cedex 05, France
Abstract The mechanism of ion induced mixing and phase change was studied for Fe/Zr multilayers, and speci®cally for the case of swift heavy ions giving rise to a very large electronic excitation of the target. The multilayers had a modulation of 7.6 nm and an overall composition Fe69 Zr31 . The Zr layers were amorphous whereas the Fe ones were crystalline (bcc) with a very strong (1 1 0) texture in the growth direction. The phase transformation and the composition changes were analysed using the structural and magnetic properties of the Fe component by means of a detailed analysis of the X-ray diraction pro®les and with the aid of backscattering M ossbauer spectroscopy. A complete mixing was observed at a ¯uence of 1013 U/cm2 . Both phenomena, the dose dependence of the ion beam mixed amorphous non-magnetic phase and the quantitative evolution of the crystalline iron layer thickness, suggest that mixing occurs in a two-stage process. At an initial stage, an anisotropic diusion of iron atoms in the amorphous zirconium layers takes place along the interface, while subsequent ion bombardment leads to a generalised transformation through the whole of the Fe layer. Finally, the implications of these observations are discussed in comparison to the plastic deformation phenomena reported for amorphous alloys. Ó 1999 Elsevier Science B.V. All rights reserved. PACS: 61.80jh; 75 70; 75.30k; 61.43d Keywords: Ion irradiation eects; Metallic multilayer; Ion beam mixing; Magnetic phase transition; Amorphous metallic alloys
1. Introduction Recent experiments have shown the occurrence of nanoscale mixing eects induced by
* Corresponding author. Tel.: +33 5 49 49 67 30; fax: +33 5 49 49 66 92; e-mail: [email protected]
electronic excitation in multilayers irradiated by swift heavy ions [1]. In the present investigation, we report the observation of atomic mixing in nanometric Fe/Zr multilayers submitted to 811 MeV 238 U ion irradiation. The Fe/Zr system is particularly interesting because of the high sensitivity of its two constituents to electronic excitations [2±5]. In this system atomic mixing is also
0168-583X/98/$ ± see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 8 ) 0 0 7 2 5 - 3
C. Jaouen et al. / Nucl. Instr. and Meth. in Phys. Res. B 148 (1999) 176±183
expected due to the negative mixing enthalpy of the system ()0.35 eV/at), which leads to a fast solid state reaction. Furthermore, even by referring to usual methods, the formation of metastable amorphous Fex Zr1ÿx alloys is expected almost in the complete compositional range (0.20 < x < 0.93). This structural transition at room temperature involves the loss of ferromagnetism either for paramagnetic or for diamagnetic states, depending on the composition. The magnetic properties of these amorphous alloys are very well characterised [6±8]. The investigations presented here concern a Fe/Zr multilayer system with a period of about 7.6 nm and layer thicknesses having a ratio 1:1, thus providing the crystalline growth for the iron layers but an amorphous structure for the Zr ones. Two methods, X-ray diraction and M ossbauer spectroscopy, were used for the analysis of the irradiation induced atomic transport. The X-ray diraction is highly sensitive to phase change as well as to the interfacial reaction, thus making the method suitable for measurement of small amounts of mixing. On the other hand, M ossbauer spectroscopy provides additional and complementary information on the local chemical and structural environment of the 57 Fe atoms. The formation of the amorphous phase due to the mixing process reveals, when measured at room temperature, a quadrupole-split doublet growth in the M ossbauer spectra that is clearly in contrast to the ferromagnetic a±Fe and the crystalline interfacial Fe±(Zr) which exhibit a magnetic hyper®ne structure [9,10].
2. Experimental details The Fe/Zr multilayers were produced in a high vacuum NORDIKO-3000 sputtering deposition system equipped with an RF-plasma source and having an accelerating beam voltage of 1.2 kV. The multilayers consisted of 35 alternating Fe and Zr layers having a thickness ratio 1:1 for the two elements. This corresponds, roughly, to a 2:1 composition ratio. The modulation length K tFe + tZr , where tFe and tZr are the thickness of
177
the Fe and Zr layer respectively, was 7.6 nm, as it was determined by small-angle X-ray diraction. That gives a total thickness for the system of about 266 nm. The used substrates were singlecrystal (1 0 0) silicon wafers, and the deposition was carried out at room temperature. Energy Dispersive X-ray spectroscopy (EDX) indicated an overall composition for the multilayer system of Fe69 Zr31 (within an uncertainty of 1%), pointing out a slight enrichment in Fe, relatively to the nominal layer thickness. Consequently, the pattern of the layers may be noted [Fe3:9 /Zr3:7 ] ´ 35 (the subscripts represent the thickness in nanometers). The samples were irradiated, at a normal angle of incidence, with 811 MeV 238 U, at GANIL in Caen. In the case of this energy range, the projected ion range exceeds, about two orders of magnitude, the total ®lm thickness. Thus it is considered that the excitation is nearly uniform throughout the thickness of the sample. The exchange mechanism is mainly dominated by electronic excitations and ionisation while the loss due to nuclear elastic collisions (dE/dx)n can be considered very low (75 eV/nm). The average value of the electronic energy loss, (dE/dx)e , calculated using the computer simulation program TRIM 96 [11], is 57 keV/nm for U ions. The irradiation of the samples was carried out at 77 K and the incident ion ¯ux was kept at a low value (about 2 ´ 108 ±3 ´ 108 ions/cm2 /s) to prevent possible thermal assisted diusion or defect recombination. The characterisation of the structural and magnetic properties of the Fe/Zr multilayers, as deposited and after irradiation with swift heavy ions, was performed by means of X-ray diraction and M ossbauer spectroscopy. The X-ray diraction spectra were obtained using a Siemens D500 and a Bruker D5005 diffractometer. Both experimental con®gurations use the Cu Ka radiation and are equipped with a backside monochromator and a point detector. Two geometries were used to study the crystallinity of the samples: the Bragg±Brentano geometry (h/2h scan), which gives information on the stacking along the growth direction, and the rocking geometry (x-scan), which allows to esti-
178
C. Jaouen et al. / Nucl. Instr. and Meth. in Phys. Res. B 148 (1999) 176±183
mate the mosaic distribution of the growth planes in the sample. Conversion Electron M ossbauer spectroscopy (CEMS) was used to determine the magnetic state ossbauer spectra were of the 57 Fe atoms. M obtained at room temperature using a source of 57 Co in Rh and a gas ¯ow proportional counter for the conversion and the electron detection. The analysis of the experimental data was performed with the aid of standard least-square ®tting routines. A six-line pattern with lorentzian line shape was assumed for bulk-like Fe component; the magnetic and non-magnetic parts of the interfacial component are respectively accounted for by a hyper®ne ®eld distribution (smoothed histogram method)and an asymmetrical broad doublet featuring a quadrupole distribution.
3. Results 3.1. As deposited Fe/Zr multilayers The [Fe3:9 /Zr3:7 ] ´ 35 system studied demonstrated alternating crystalline Fe layers and
Fig. 1. X-ray diraction pattern of an as-deposited Fe/Zr multilayer (K 7.6 nm). The calculated diraction pattern of an amorphous-Zr/crystalline-Fe bilayer with ®nite size of the Fe crystalline layers (with 19 Fe(1 1 0) planes parallel to the specimen surface) is shown for comparison. Intensity is plotted on a logarithmic scale.
amorphous Zr layers. Fig. 1 shows the h/2h diffraction spectrum for the as deposited material. The main peak corresponds to the Fe (1 1 0) re¯ection. The Zr layers are amorphous and, as expected, no signal was generated from those layers. The Fe lattice spacing is 0.2049 nm in comparison to the bulk value of 0.2027 nm. This can be ascribed to the existence of a compressive stress in the Fe layers parallel to the Fe/Zr interface, as it is often observed in crystalline ®lms deposited by sputtering. The rocking curve obtained for the (1 1 0) Fe re¯ection has a full width at half maximum (FWHM) of 9°, indicating a well-de®ned texture for the Fe layers despite the amorphous state of Zr. Amorphous Zr or Fe layers have been observed already in the Fe/Zr multilayer system in the case of small layer thickness for both constituents [12]. An amorphous to crystalline phase transition is observed when the thickness is increased above a critical thickness which corresponds to a number n 10 of atomic planes. It has been suggested that the large lattice mis®t between the two elements could be held responsible for this eect. Our study of the Fe/Zr multilayer system shows that the critical number of atomic planes above which the Fe layers are crystalline is about n 10, whereas a number n 20 is required for the Zr ones. The latter value does not agree with previously published results [9,10]. A possible explanation might be found in the presence of iron impurities in the zirconium layers (x 2%, as deduced from EDX analysis) due to the target screens. This assumption is supported by a comparable retardation in the formation of crystalline Fe with increasing Zr concentration in the Fe layers, as it has been reported [13]. The striking feature of the X-ray spectrum is the occurrence of secondary, small, peaks around the main Fe (110) Bragg peak. These secondary peaks have to be treated as ®nite size peaks and not as superlattice re¯ections. The sample can then be described as a stacking of Fe layers of wellde®ned size separated by non diracting amorphous Zr layers. The simulation of the experimental pro®le using this description is shown in Fig. 1. When the number of Fe monolayers is nFe 19, a good agreement between experiment
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and simulation is obtained, as far as peak intensity and width are concerned. The asymmetry in the intensity of the satellites at both sides of the main peak is correctly reproduced by taking into account the Debye±Waller eects. Hence, the determination of the coherence length for the Fe layers is very accurate since both, the pro®le of the main Fe (110) Bragg peak and the position of the secondary peaks, are reproducible by simulation. That means that an estimation of the size Lz of the diracting domains in a direction perpendicular to the diracting planes (here Lz is equal to the Felayer thickness) may be obtained from both, the position of the maxima/minima of the Laue function, and the peak broadening. For the present multilayer system it is found to be Lz 3.89 0.09 nm, in good agreement with the value derived from the overall composition and the period of the multilayer system. The M ossbauer spectrum of the as deposited multilayer system obtained at room temperature is shown in Fig. 2(a). This spectrum shows a magnetic contribution, with the existence of a Zeeman splitting, and a non-magnetic quadrupole contribution in the middle part. Three Fe site components were necessary to interpret the spectra: (a) A six-line pattern with spectral contribution of 41% (with a magnetic hyper®ne ®eld of 32.75 T, slightly lower than the characteristic value of bulk Fe) corresponding to the crystalline a-Fe phase. (b) A hyper®ne ®eld distribution with a spectral contribution of 35.5% (with a mean hyper®ne ®eld depleted at an average value of 26 T) ascribed to a crystalline interfacial contribution. (c) An asymmetrical quadrupole doublet with large lines corresponding to an amorphous Fex Zr1ÿx and contributing to 23.5% of the total area spectra (from the mean quadrupole splitting of 0.52 mm/s, the average composition is x 0.25 [6]). The results obtained represent an important extension of the magnetic interface, since 59% of the Fe atoms belong either to the interfacial amorphous component or to a crystalline component characterised by a signi®cant reduction (20%) of the Fe hyper®ne ®eld. The presence of the latter contribution cannot be related directly to the structural properties of the Fe layers which exhibit a remarkable coherency. The as deposited
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Fig. 2. CEMS spectra obtained with Fe/Zr multilayers before and after ion irradiation at 77 K with high energy U ions : (a) as-deposited multilayer; (b) after 1 ´ 1012 U/cm2 ; (c) after 2 ´ 1012 U/cm2 ; (d) after 5 ´ 1012 U/cm2 .
amorphous phase, with an average composition x 0.25, suggests a diusion of the Fe atoms in the Zr layers.
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3.2. Evolution under irradiation with 811 MeV U ions Fig. 2 presents, through the characteristic M ossbauer spectra, the main features of the evolution observed under irradiation. The growth of the non-magnetic amorphous component is clearly observed. At 5 ´ 1012 U/cm2 , a paramagnetic amorphous phase is obtained having related spectral parameters quite consistent with the overall composition of the multilayers: the quadrupole splitting (0.32 mm/s) obtained at higher irradiation dose is quite consistent with the formation of a Fe-rich amorphous alloy [6]. The spectrum obtained after 1013 U/cm2 (not presented here) is quite similar. The relative fraction (f) versus the ion ¯uence / obtained by computer simulation for the dierent components is shown in Fig. 3. It is worth mentioning that above 2 ´ 1012 U/cm2 an increase in the transformation rate is observed leading to the complete transformation of the Fe layers into an amorphous state. The fraction of Fe atoms introduced in the nonmagnetic phase deduced from the M ossbauer spectra is: avol Mossbauer
fFe magn 0 ÿ fFe magn / fFe magn 0
and is presented in Fig. 4.
Fig. 3. Relative fraction of the bulklike a-Fe, interfacial crystalline Fe±(Zr) and amorphous non-magnetic Fe±Zr interface components as deduced from the CEMS analysis versus the ion ¯uence.
Fig. 4. Fraction of the Fe atoms introduced in the non-magnetic amorphous phase versus the U ion ¯uence. The fractions corresponding to an interfacial process, as deduced from the decrease of the Fe crystalline layer thickness (by ®tting the diraction pro®le with a Laue function), and to the total transformation, as deduced from X-ray and M ossbauer data, can be compared.
Fig. 5 shows the evolution of the X-ray diffraction spectra for the samples after irradiation. First, it should be noted that neither the position nor the shape of the Fe peak is altered by the irradiation; this means that no relaxation is introduced, and that the Fe lattice parameter remains signi®cantly greater than the bulk value. By contrast, important changes to both, the intensity and the pro®le, are clearly observed. The strong decrease of the peak intensity is an evidence of the transformation under irradiation of the Fe layers into an amorphous Fex Zr1ÿx alloy, which is complete at a dose of 1 ´ 1013 U/cm2 . The integrated intensity, Iint , of the Bragg peak is proportional to the volume of the remaining crystalline material in the Fe layers, provided that no change, in the average scattering factors per atom or in the texture, occurs. These last two conditions are here well ful®lled since the lattice parameter deduced from the position of the Bragg peak and the rocking curves remain the same during the transformation, at least until 5 ´ 1012 U/cm2 . Thus, the average volume fraction, avol of the Fe layers transformed into an amorphous Fex Zr1ÿx alloy, for a given irradiation ¯uence /, can be deduced directly from the variation of the integrated intensity:
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the main peak, inversely proportional to Lz , increases. The fraction, aint , of the crystalline Fe layers consumed upon irradiation at a given ¯uence /, by a progressive ion induced diusion at the interfaces, can be deduced from the ratio: aint
Fig. 5. X-ray diraction patterns obtained before and after irradiation at 77 K with high energy U ions at dierent doses. The dashed part is shown in an expanded 2h-scale as insert, the angular shift of the Laue satellites in the ®rst stage of the irradiation is visible. The small peak observed at 2h 38° may be attributed to crystalline Zr oxide.
avol XRD
Iint 0 ÿ Iint / : Iint 0
The corresponding evolution is presented in Fig. 4 and can be compared to the evolution of the mixed amorphous fraction a vol (Mossbauer). It is worth emphasising the good agreement of the values for the total amorphous part obtained from both investigating techniques, X-ray diraction and M ossbauer spectroscopy. This attests the validity of the results. The other visible evolution in the spectra concerns the secondary peaks, which remain distinguishable up to a dose of 2 ´ 1012 U/cm2 . The positions of these peaks change relatively to the main peak position and, according to the analysis described above, this means that the thickness of the crystalline Fe layers decreases. In a consistent way, the full width at half-maximum intensity of
Lz 0 ÿ Lz / : Lz 0
By comparing the progress of amorphisation, using the ratios avol and aint , it becomes possible to distinguish the part of the solid transformation that occurs in the bulk from the one occurring at the interface. Such an approach, already applied to the study of amorphisation by solid state reactions [14], is able to provide information about the mechanisms of the ion induced mixing-phase change. The plot in Fig. 4 represents the evolution of both quantities. The importance of this plot lies in the existence of two successive stages in the evolution under irradiation: In the ®rst stage (up to 1012 U/cm2 ), the dominant mechanism is the interfacial reaction limited to three Fe planes, and it is represented by aint , whereas in the second stage the saturation of aint suggests that, for subsequent ion impacts, amorphisation would be generalised throughout the whole Fe layer.
4. Discussion As described above, the ®rst stage of the mixing kinetics (up to 1012 U/cm2 ) is dominated by interdiusion at the interface. This takes place for doses similar to the doses required for defect creation in amorphous materials. This stage corresponds to a diusion of iron in zirconium at the interfaces in order to create an amorphous FeZr alloy. Indeed, in a multilayer system with larger period (both elements being crystalline), we observed the amorphisation of nanocrystalline zirconium during this ®rst stage, showing the fast diusion of iron into the Zr layers. Therefore, we can say that a spontaneous movement of the iron atoms occurs during the ®rst impacts of the ions leading to a densi®cation of the Zr layers. During the second stage, the absence of further shift of the Laue satellites leads to the conclusion
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that a diusion of the zirconium throughout the whole Fe layer in the wake of the ions is the dominant mechanism. This mixing leads to the formation of a paramagnetic amorphous alloy. However, no de®nite conclusion can be drawn for the homogeneity of the formed phase. This two stage process can be compared to the one observed during high-energy irradiation of amorphous alloys [15±17]. As already mentioned, the ®rst stage may correspond to a densi®cation of the amorphous Zr phase with interdiusion of iron. However, the link between the zirconium diusion observed in the second stage and the anisotropic deformation of the amorphous materials does not appear straightforward to prove. We can suggest that both the diusion and the phase change observed in the Fe layers can be an alternative way to relax the deformation and the subsequent stresses induced by intense electronic excitations in the dense amorphous alloy. Finally, it is interesting to compare these results to mixing eects induced in Fe/Zr multilayers (with a similar period) by low energy He [9] or Ar ions [10], i.e. in an energy range where interfacial mixing is attributed to ballistic atomic displacements. First, in contrast to the step ¯uence dependence obtained here with irradiation by swift heavy ions, the mixed iron fraction varies according to the square root of the low energy ion dose, in good agreement with collisional or spike models. Such a dierence is all the better demonstrated since amorphisation and mixing have been also studied using M ossbauer spectroscopy. Another dierence arises from the fact that a saturation of the amorphisation and the mixing process is observed at about 70% and 95% for He and Ar low energy ion irradiation, while, in the case of swift heavy ion irradiation, the occurring amorphisation is complete. Such contrasting behaviours suggest that the mixing and the amorphisation introduced by the two processes involve dierent mechanisms. 5. Conclusion It is worth pointing out the main features of the above mentioned experiments. In the case of irra-
diation by swift heavy ions, a mixing and an amorphous transformation, which are complete for doses around 1013 811 MeV U/cm2 , are induced in the Fe/Zr multilayers. No stress relaxation is observed in the remaining crystalline iron layers during the process. The reaction can be explained by a two stage mechanism: (a) An interdiusion at the interfaces for low irradiation doses; (b) A subsequent complete or almost complete mixing in the wake of the ions concomitantly to the saturation of the ®rst eect. We must emphasise that a one-stage-only full mixing in the ion tracks cannot explain our results. The origin of this two stage process and particularly the in¯uence of the amorphous state of the Zr layer on the observed atomic motion are still to be determined. Further investigations should include the study of the damaging process during the ®rst stage, especially under very low dose irradiation, and investigations for possible period variations during irradiation.
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