Ion beam effects FeNi bilayers

Materials Science and Engineering, A 115 ( 1989) 161 - 164

161

Ion Beam Effects on Fe-Ni Bilayers* L. M. GRATTON

Dipartimento di Fisica, Universit6 di Trento, 38050 Povo, Trento (Italy) A JAY GUPTAt

Sezione Materiali, Dipartimento di lngegneria Meccanica, Universitti di Padova, Via Marzolo 9, 35100 Padova (Italy) W. KEUNE

Laboratorium fiir A ngewandte Physik, Universitiit Duisburg, Lotharstrasse 1, 41100 Duisburg (F.R.G.) S. LO RUSSO

Unit~i GN8M-C1SM, Dipartimento di Fisica, Universitd di Padova, Via Marzolo 8, 35100 Padova (Italy) J. PARELLADA

Instituto de Fisica Atomica y Molecular, Universidad de Barcelona, Diagonal 645, 08028 Barcelona (Spain) G. PRINCIPI

Sezione Materiali, Dipartimento di Ingegneria Meccanica, Universitgt diPadova, Via Marzolo 9, 35100 Padova (Italy) C. TOSELLO

Dipartimento di Fisica, Universit6 di Trento, 38050 Povo, Trento (Italy) (Received September 16.1988)

Abstract

Ion beam mixing induced by 100 keV Ar + irradiation in Fe-Ni bilayers has been studied. Conversion electron Mdssbauer spectroscopy has been used for structural analysis of the irradiated specimens. An Fe-Ni Invar alloy formed in the mixed region, among other phases, exhibits deviation of the average iron hyperfine field from the Slater-Pauling curve. Two other distinct phases have been identified as solid solutions of nickel in b.c.c, iron and iron in fc.c. nickel. I. Introduction

Fe-Ni alloys form an interesting magnetic system, showing Invar behaviour around the composition Fe0.6sNi0.35, characterized by an almost zero thermal expansion coefficient over a wide temperature range and by anomalies in the magnetic moment and other properties [1 ]. In the *Paper presented at the Sixth International Conference on Surface Modification of Metals by Ion Beams, Riva del Garda, Italy, September 12-16, 1988. tOn leave from Physics Department, Devi Ahilya University, lndore 452001, India. 0921-5093/89/$3.50

Invar region, the concentration dependence of the saturation magnetization and of the room temperature STFe hyperfine field Hh~ for bulk Fe-Ni alloys displays strong deviations from the Slater-Pauling curve [2]. This curve predicts an almost linear dependence of the average magnetic moment per atom (and hence of the hyperfine field) on the average number of outer electrons per atom. In an Fe-Ni alloy the number of outer electrons per atom is directly proportional to the nickel concentration. Recently, it has been found that the structural and magnetic properties of thin films of Fe-Ni alloys can differ from that of bulk alloys and strongly depend on the preparation technique [2-5]. No discontinuity in the saturation magnetization has been observed around the Invar composition for sputtered films 113]and for unannealed evaporated films [5], while evaporated films after annealing show magnetic behaviour similar to the bulk alloys. The large magnetic moment of evaporated f.c.c. Fe-Ni films in the Invar region has been interpreted as being due to the stabilization of Y2 state of iron [5]. For a better understanding of this unexpected magnetic behaviour other thin film alloying techniques are of high interest. © Elsevier Sequoia/Printed in The Netherlands

162 Ion beam mixing is a potential technique for alloying thin metallic films. The processes responsible for redistribution of atoms include radiation enhanced diffusion of atoms and defects, as well as collisional mechanisms which are athermal and fast compared to diffusional processes. Therefore, the technique may result in formation of metastable alloy phases which are not obtainable using the conventional techniques [6]. In the present work ion beam induced alloying of Fe-Ni bilayers has been studied using conversion electron M6ssbauer spectroscopy (CEMS) and Rutherford backscattering spectrometry (RBS). CEMS can be effectively used to identify various products of mixing in Ni-Fe thin films, as this system has already been extensively studied over the whole range of compositions using M6ssbauer spectroscopy, and thus, a database exists which can be used to interpret the M6ssbauer spectra of ion beam mixed samples. Due to the small mass difference between nickel and iron atoms, RBS in the present case can give only qualitative but useful information on the ion induced mixing and permits quantitative evaluation of the amount of ion induced sputtering.

2. Experimental procedure Ni-Fe bilayers were prepared by successive evaporation of layers of nickel (25 nm), iron 95% enriched in 57Fe (3 nm) and natural iron (40 nm) on a glass substrate held at room temperature in an oil free vacuum chamber. The vacuum during evaporation was better than 10 6 Pa. The deposition rate as well as the total thickness of each layer was monitored using a precalibrated quartz crystal thickness monitor. Total thickness of the iron overlayer was chosen such that the maximum of the energy deposition profile of 100 keV Ar + ions roughly corresponded to the position of the Fe-Ni interface. The 57Fe-enriched layer 3 nm thick allowed M6ssbauer spectroscopy to selectively study the interface region. Argon ion implantation was performed using the 120 keV accelerator at Trento University, equipped with an Ortec Duoplasmatron ion source. All the implantations were carried out with 100 keV Ar + ions at a current density of 5 # A cm -2. As the specimen temperature was not controlled during implantation, it could have risen by a few hundred degrees. Three different implantation doses were employed: 5x1015,

1x1016 and 5 x 1 0 ~6 Ar ÷ cm -2. The pressure in the implantation chamber was kept at about 2 x 10 -5 Pa. RBS was performed with a 2.3 MeV 4He+ beam with normal incidence and scattering angle of 135 °. The conversion electron M6ssbauer spectra were obtained using a conventional spectrometer with a flowing gas (95% helium, 5% CH4) proportional counter and a source of 57Co in a rhodium matrix. The M6ssbauer spectra were analysed using the programme A M F I R developed by Le Caer and Dubois [7] to obtain a distribution of hyperfine fields, as well as a standard fitting programme assuming a superposition of lorentzian lines.

3. Results and discussion Figure 1 shows the RBS spectra of as-evaporated and irradiated specimens at doses of i x 1016 and 5 x 1016Ar + cm z. Due to the small difference between the atomic weights of iron and nickel, and also due to the geometry of our specimens (the iron layer on top of the nickel one), the RBS signals of iron and nickel strongly overlap, making it difficult to analyse separately the different atomic species in the samples. Nevertheless, evident changes in the RBS profile induced by Ar + irradiation can be observed as an indication of intermixing induced by the irradiation. These spectra can be used to estimate the

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Fig. 1. RBS spectra (4He+, 2.3 MeV, 0=135 °) of asevaporated as well as Ar+-irradiated Fe-Ni layers. Signals fromthe glass substrate are not reported. Arrows indicatethe iron, nickeland argon surfacepositions.

163

sputtering effect: the total area of the overlapped signals of iron and nickel is reduced to 83% of the initial value after irradiation at the highest dose of 5 x 10 t6 Ar ÷ cm -2. This reduction corresponds to a removal of about 12 nm of the outer surface layer. For lower doses the sputtering effect is negligible. Figure 2 (curves a-d) shows the conversion electron M6ssbauer spectra of the specimens irradiated with various doses of Ar +. From the literature one finds that the hyperfine field in a Fe-Ni alloy, depending upon its composition and structure, can vary from 0 to about 35 T [2-5]. Therefore, as a result of mixing produced by Ar +, one expects a distribution of hyperfine fields in the M6ssbauer spectra of irradiated specimens. Accordingly, the spectra were analysed by the A M F I R program to obtain a distribution of hyperfine fields. Spectra were also fitted with the standard fitting program assuming an overlap of sextets with broadened lorentzian lines. The results of the two methods were

102

100 102

u°

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108 101

consistent, since the standard fitting program gave convergence with components corresponding to the most intense peaks of the distribution obtained by the A M F I R program. Figure 2 (curves a'-d") shows the field distribution at the iron nucleus in as-evaporated and irradiated specimens. The hyperfine field distribution has been obtained as a superposition of two distributions in the field ranges 0-32 T and 32-36 T respectively. The first range covers the hyperfine fields in the possible f.c.c, alloys of Fe-Ni, while the second range covers the hyperfine fields in the b.c.c, alloy. The published data [2-4] on composition and structure dependence of average hyperfine fields in Fe-Ni alloys, summarized in Fig. 3, have been used to identify various phases present in these specimens. From Fig. 3 it may be noted that as a function of composition, the b.c.c, phase shows variation of Hhf in the range 33-35 T. In the f.c.c, phase, Hhr varies from 27 to about 32 T, except in the Invar composition range, where it exhibits a sharp drop associated with deviation of the average magnetic moment from the Slater-Pauling curve. In the as-evaporated specimen, the peak in the hyperfine field distribution at 33.3 T is due to the a-Fe; additional peaks'at lower field values can be attributed to the iron atoms at the Fe-Ni interface. In the irradiated specimens, the peak in the distribution around 33.5 T can be attributed to a b.c.c. phase. Moreover, irradiation results in development of two additional peaks in the field distribution centred around 20 T and 29 T respectively, indicating the formation of some new phases. One can see that the broad peak in the hyperfine field distribution around 20 T is attributable to a Fe-Ni alloy in the Invar composition range, with hyperfine field evidently showing deviation from the Slater-Pauling curve. With increasing Ar + dose, this peak gets more and more broadened,

188

-4 velocity

0

4 [mm/s}

80

10 28 30 33 35

40

H (T}

Fig. 2. M6ssbauer spectra of various specimens and corresponding hypeffine field distributions. (a)-(d) M6ssbauer spectra of the as-evaporated specimen, and the specimens implanted with doses of 5x1015 A r + cm 2 l z l ( ) 16 A r + cm 2 and 5 x l ( ) ~" A t * cm = respectively. The hyperfine field distribution has been obtained as a superposition of two distributions in the field ranges 0 - 3 2 T and 3 2 - 3 6 T respectively. (a')-(d') Corresponding hyperfine field distributions in the first range, while (a")-(d") give the field distributions in the second range. It should be noted that the plots of field distribution in the two ranges have different horizontal as well as vertical scales.

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Fig. 3. Structure and composition dependence of the average hyperfine field in Fe-Ni alloys taken from ref. 4.

164 TABLE 1 Parameters of the peak in the hyperfine field distribution corresponding to the b.c.c, phase. Uncertainities on the last significant figure are reported in the brackets

Irradiation dose

( Hht)

AHh,,

(Ar ÷ era- 2)

(T)

(T)

0 5 x l O '5 1 × 10 ~6 5 x 10 '6

33.3 (2) 33.5(2) 33.6 (2) 33.8 (2)

0.75 (5) 1.50(5) 1.60 (5) 1.90 (5)

Fraction of the b.c.c, phase

(%)

93 (2) 89(2) 84 (2) 52 (2)

indicating an increased spread in the composition of the mixed region. However, even after the highest irradiation dose, the width of this peak corresponds to a rather narrow composition range (Fe~_xNix with 0.32 < x <0.34). The peak around 29 T may be associated with a solid solution of iron in f.c.c, nickel formed as a result of diffusion of iron into the nickel layer. The involved mechanisms cannot be definitively ascertained due to the simultaneous presence of radiation and thermal effects. Table 1 gives the parameters of the peak in the hyperfine field distribution corresponding to the b.c.c, phase in different specimens. From Table 1 it can be seen that the area under this peak decreases with increasing dose of Ar + . This is an indication that the mixing is an increasing function of the irradiation dose. The average hypeffine field of this component increases with increasing implantation dose. This may be attributed to diffusion of nickel atoms into the b.c.c. iron layer, causing a change in the overall composition of this layer. The incorporation of nickel into the unreacted iron layer is expected to be a combined effect of radiation-enhanced diffusion as well as an enhanced thermal diffusion.

4. Conclusions In conclusion, Ar + ion beam induced alloying of Fe-Ni bilayers has been studied using conversion electron M6ssbauer spectroscopy as the main experimental technique. Irradiation by Ar ÷ results in the formation of Fe-Ni alloy in the Invar composition range, exhibiting deviations in the iron hyperfine field from the Slater-Pauling curve. Two other distinct phases in the ion-beam mixed specimens have been identified as solid solutions of nickel in b.c.c, iron and iron in f.c.c. nickel.

Acknowledgments This work is a part of a research project supported by the Commission of the European Communities; Contract SCI-0024-C(A). One of the authors (A.G.) would like to acknowledge a fellowship by ICTP, Trieste.

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Materials, Sendai, Japan, 1987. 6 J. W. Mayer, B. Y. Tsaur, S. S. Lau and L.-S. Hung, Nucl. Instrum. Methods, 181/183 ( 1981 ) 1; B. X. Liu, W. L. Johnson, M.-A. Nicolet and S. S. Lau, Appl. Phys. Lett., 42 (1983) 45. 7 G. Le Caer and J. M. Dubois, J. Phys. E. Sci. lnstrum., 12 (1979) 1083.