Ag multilayer

NIM B Beam Interactions with Materials & Atoms

Nuclear Instruments and Methods in Physics Research B 244 (2006) 359–364 www.elsevier.com/locate/nimb

Microstructural investigations on ion beam mixed Fe/Ag multilayer A.K. Srivastava

a,*

, S. Amirthapandian b, B.K. Panigrahi b, Ajay Gupta c, R.V. Nandedkar

a

a

c

Synchrotron Utilisation Division, INDUS-1 Building, Centre for Advanced Technology, Indore 452 013, India b Materials Science Division, Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, India UGC-DAE Consortium for Scientific Research, Indore center, University Campus, Khandwa Road, Indore 452 017, India Received 19 April 2005; received in revised form 28 September 2005 Available online 23 November 2005

Abstract Electron beam evaporated iron/silver multilayer deposited on NaCl crystal were irradiated with 90 keV Ar+ ions with different fluence of 1 · 1016, 5 · 1016 and 7 · 1016 ions/cm2 to study ion beam mixing in Fe/Ag thermally immiscible system. Transmission electron microscopy and X-ray mapping were used to analyse the ion beam mixing in this system. We have found that as the fluence increases Ag particles disintegrate into smaller particles and finally at the highest fluence a homogeneous mixture of Ag and Fe particles is formed. The particle size of Ag and Fe for the highest fluence is found to be in the range of 10–20 nm. The decrease in the particle size is attributed to the higher chemical affinity and smaller thickness of Ag and Fe layers.
2005 Elsevier B.V. All rights reserved. PACS: 61.14.Lj; 61.80.Jh; 61.72. y Keywords: TEM; Microstructure; Bright field; Dark field

1. Introduction In recent years, there has been great interest in nanostructured materials with novel physical properties [1]. One of the major applications of these nanostructured materials is in giant magnetoresistance (GMR) [2]. A variety of magnetic superlattices exhibit GMR properties. These superlattices consist of a stack of alternating magnetic and non-magnetic (spacer) layers. In an applied magnetic field, antiferromagnetically coupled magnetic layers transform into ferromagnetic which gives rise to a reduction in spin dependent electron scattering. The reduction in the spin dependent scattering leads to decrease in resistivity. This phenomenon is called GMR. In order to avoid sophisticated techniques to prepare the multilayers, granular films were synthesised [3]. These magnetic granular films are composite materials that usually consist of nanoscale magnetic granules embedded in an immiscible *

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2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2005.10.004

metallic matrix. In granular films, the spin dependent scattering at the interfaces of magnetic granules is responsible for GMR phenomenon. These films show GMR properties when the composition of the magnetic material in the film is around 30 at.% [4]. Different methods were used to synthesise granular films, like splat-cooling [5], sputtering [6] and laser ablation [7], directly providing mixtures of thermodynamically immiscible elements. The direct ion implantation of ferromagnetic ions into a non-magnetic matrix is one among the methods. However, it has a disadvantage of limited mixing concentration [8] (a maximum of only 10% by volume) and hence the required stoichiometry for optimum GMR value cannot be reached. This limitation can be overcome by ion beam mixing of multilayers comprising thermally immiscible elements. Atomic intermixing of the different layers takes place leading to the formation of thermally non-equilibrium alloys with optimum grain size distribution. Among the granular systems, both in terms of basic understanding and the technological relevance, the Fe/Ag and Co/Ag systems are of great interest as they exhibit high values of GMR. The Fe/Ag system

2. Experimental The [Ag (8 nm)/Fe (2 nm)]·7 multilayer films were grown on a NaCl substrate in an UHV chamber at a base pressure of 8 · 10 10 mbar by using the electron beam evaporation technique. The deposition was carried out at a rate of 0.6 nm/min. The individual layer thicknesses of the elements was adjusted for an overall composition of 33 at.% Fe and 67 at.% Ag considering the density and the volume of the individual film. The reason for this choice of composition is that, in the Fe/Ag system, an optimum value of GMR has been reported for this composition [4]. The Fe/Ag samples were irradiated with 90 keV Ar+ ions with different fluences in the range of 1 · 1016– 7 · 1016 ions/cm2 at room temperature at normal incidence using a 150 kV ion accelerator. In order to avoid beam heating, the beam current was limited to less than 500 nA/cm2. The range of penetration of Ar+ ions in the multilayer sample was found to be 45 nm using Monte Carlo simulation SRIM 2003 with a straggling of 15 nm

Substrate

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Number/ion/ang.

is highly immiscible in both liquid and solid state [9] because of the differences in the surface free energy and large lattice mismatch between Fe and Ag. Ion beam mixing in Fe/Ag bilayer has been reported by Crespo-Sosa et al. [10]. These authors observed very low mixing efficiency due to de-mixing and phase separation occurring in the thermal spike phase of the collision cascade. In an another study [11] laser deposited Fe–Ag multilayer shows stress relaxation and demixing process at the interface upon 300 keV Ar+ ion irradiation at 77 K. This relaxation and demixing process is followed by grain coursing and a supersaturation of the bcc Fe phase with Ag due to ballistic mixing at higher fluence. In our earlier work [12], we have reported that low energy ion irradiation on Co/Ag multilayers leads to the formation new metastable phase while the matrix is being amorphised. The Co/Ag system is also immiscible in both liquid state and solid state. Microstructural studies on Fe/ Ag granular films prepared by co-evaporation is reported by Thangaraj et al. [13] and the authors have observed 4% of GMR in Fe/Ag granular films with both epitaxially grown and polycrystalline films. The uniform granular microstructure sample with Fe29Ag71 films shows 4% of magnetoresistance with 1.6 T at room temperature. In this paper, we present microstructural studies on Fe/Ag multilayers, coated sequentially under ultra-high vacuum (UHV) conditions, and irradiated with different fluences of 90 keV Ar+ ions. The ion irradiation of Fe/Ag multilayers leads to better mixing of Fe and Ag layers as compared to other methods of mixing like co-evaporation, sputtering, or laser ablation techniques. The microstructure has been characterised by using transmission electron microscopy and energy dispersive X-ray spectroscopy (EDX) mapping attached to TEM employed for elemental analysis. Our results indicate that irradiation with a fluence of 7 · 1016 ions/cm2 leads to a granular microstructure with uniform homogeneity and narrow size distribution.

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Ar Concentration (a.u.)

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Fig. 1. Dark curve shows the target displacement and light curve shows the depth profile as a function of depth in Fe/Ag multilayer irradiated with 90 keV Ar+ ion with a fluence of 1 · 1016 ions/cm2.

[14]. The energy has been chosen such that the implantation range covers the total film thickness and also the damage profile was calculated in order to know the depth of mixing of Fe/Ag multilayer (Fig. 1). TEM studies were carried out by floating the film on de-ionized water and lifted on carbon coated copper grids. The selected area diffraction patterns and micrographs were recorded using a Philips CM200 transmission electron microscope equipped with EDX. Energy dispersive X-ray mapping were also performed using 50 nm electron beam probe size. 3. Results The bright field image of an as-synthesised Fe/Ag multilayer is shown in Fig. 2(a). The microstructure of the film is inhomogeneous with a distribution of dark (pure Ag) and light (pure Fe) regions. The dark regions corresponding to Ag particles are quite large in size (50–100 nm). The as-synthesised samples always show Ag precipitates in Fe matrix, with large size, since Ag has a tendency to form a globular structure in insulator [15], semiconductor [16] and metal [17] matrices. In as-synthesised samples, even though Ag film thickness is more than Fe film thickness, Ag has formed island like structure. The size distribution of the Ag particles is quite wide and the sizes range from 100 to 1000 nm. The dark field image (Fig. 2(b)) from the same region also reveals the size distribution of Ag particles. However, it is very difficult to identify the small Ag particles and Fe particles separately as the inter planner spacing (d) of both fcc Ag and bcc Fe are too close to each other. The bigger (white) particles are identified with EDX, as pure Ag. The diffraction pattern (Fig. 2(c)) reveals the presence of Ag and Fe in fcc and bcc structures respectively. The diffraction pattern was indexed for Ag with a lattice parameter of a = 0.40862 nm and Fe with a lattice parameter of a = 0.28661 nm. The inner most diffraction ring contains spots, which are from the NaCl crystal, with

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Fig. 3. (a) Bright field TEM micrograph of the sample irradiated to a fluence of 1 · 106. (b) Corresponding dark field TEM micrograph. Scale length is 140 nm.

Fig. 2. (a) Bright field TEM micrograph of an as-synthesised Fe/Ag mutilayer. (b) Dark field of the same region from where bright field has been taken. (c) Selected area diffraction pattern from the corresponding region. Scale length is 140 nm.

cubic orientation (001). This may be due to some undissolved NaCl crystal. Upon irradiation with 90 keV Ar ions to a fluence of 1 · 1016 ions/cm2, the bigger Ag particles start to disintegrate into smaller particles. Since the Fe/Ag system is immiscible it is difficult to form an alloy during ion irradiation. Fig. 3(a) and (b) shows the bright field and dark field micrograph of the same region of the irradiated specimen. The bright field micrograph reveals that the size of the dark regions corresponding to Ag particles has reduced and also the corresponding dark field image shows the reduction in size of the Ag particles. The EDX from corresponding

region show the reduction in size of Ag and Fe particles as compared to the as-synthesised samples. The corresponding diffraction pattern reveals that there is no structural change in the phases of bcc Fe and fcc Ag. In order to confirm the reduction of the Ag particle size, we have carried out EDX mapping. Comparison of the EDX maps of the as-synthesised and the irradiated sample (1 · 1016 ions/cm2) (Fig. 4(a) and (b)) shows the reduction in particle size. The EDX mapping also shows that the Fe particle size has not changed much as compared to the Ag particles. According to the EDX mapping, it seems that Fe forms a matrix, where Ag particles are embedded in. The number of Ag particles in the Fe matrix has increased. But the volume fraction of pure Ag remains more or less same as in the unirradiated sample. EDX mapping, which shows Ag is not forming any metastable phase with Fe. In order to see the effect of further irradiation, we have irradiated the sample with a fluence of 5 · 1016 ions/cm2. The bright field micrograph (Fig. 5(a)) of the sample shows better homogeneity as compared to the sample irradiated with a fluence of 1 · 1016 ions/cm2. The homogeneity in the sample arises from the reduction in the Ag particle size and mixing between Fe and Ag. The size of the Ag particles has been reduced drastically (from 10 to 30 nm). The dark field image (Fig. 5(b)) reveals that the Ag particles are uniformly distributed and that their sizes are almost the same.

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Fig. 4. X-ray mapping of (a) as-synthesised, (b) irradiated to a fluence of 1 · 106, (c) irradiated to a fluence of 5 · 106, (d) irradiated to a fluence of 7 · 106. Scale length is 5000 nm.

The large reduction in the Ag particles size can also be seen from the EDX mapping (Fig. 4(c)). On further increase of the irradiation fluence to 7 · 1016 ions/cm2, the film shows a homogeneous Ag distribution. Fig. 6(a) shows the microstructure of the sample irradiated with the fluence of 7 · 1016 ions/cm2. The particle size of Ag has reduced considerably as compared to the sampled irradiated with a fluence of 5 · 1016 ions/cm2. Also, the particle size distribution found to becomes narrow and Ag particles are uniformly distributed in the Fe matrix. The EDX mapping (Fig. 4(d)) also reveals that the Ag has dissolved in Fe matrix. The dark field image (Fig. 6(b)) shows that the Ag particles are small and typically of a size ranging from 10 nm to 20 nm. The diffraction pattern (Fig. 6(c)) shows the Ag rings become broader than in the sample irradiated with a fluence of 5 · 1016 (Fig. 5(c)), indicating the reduction in Ag particle size. This broadening leads to the merging of near about rings corresponding to Ag. However, the

polycrystalline rings corresponding to Fe remains nearly unchanged. The microstructure of this sample reveals a better granular nature and homogeneity than the sample irradiated with a fluence of 5 · 1016 ions/cm2. A similar kind of microstructure was reported earlier [18], obtained with co-evaporation technique of sample preparation. 4. Discussions TEM investigations on ion beam mixed Fe/Ag multilayers show that the film transforms from a highly inhomogeneous to a homogeneous microstructure with increasing Ar+ ion fluence through reduction in Ag particle size. However, Crespo-Sosa et al. [10] found that Ag grains grew upon ion bombardment. Keeping in view that the Fe/Ag bilayer films are quite thick in their sample (Fe film is 70–150 nm thick and Ag film is 60–110 nm thick), during ion bombardment strong crystallization and grain growth

A.K. Srivastava et al. / Nucl. Instr. and Meth. in Phys. Res. B 244 (2006) 359–364

Fig. 5. (a) Bright field TEM micrograph of the sample irradiated to a fluence of 5 · 106. (b) Corresponding dark field TEM micrograph. (c) Corresponding selected area diffraction pattern. Scale length is 140 nm.

can occur. The grain growth and crystallization can happen due to local thermal spikes of few tens of nm in diameter formed inside the Ag film fusing the grain boundaries and subsequent solidification. As soon as the thermally activated long-range defect migration becomes possible, grains grow in three dimensions and become spherically shaped. In the present experiment the Fe/Ag multilayer is quite thin (Fe layer is 2 nm and Ag layer is 8 nm thick) The formation of local thermal spike within the Ag grain is most unlikely and the grain growth will not initiate instead it will disintegrate into smaller Ag particles. The Fe/Ag system is thermodynamically highly immiscible in both solid and liquid states. One needs a technique to

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Fig. 6. (a) Bright field TEM micrograph of the sample irradiated to a fluence of 7 · 106. (b) Dark field of the same region from where bright field has been taken. (c) Selected area diffraction pattern from the corresponding region. Scale length is 140 nm.

mix Fe and Ag, because quenching alone is not sufficient because of the immiscibility in the melt phase. Earlier, the Fe/Ag granular films were prepared by using rapid quenching, co-sputtering and co-evaporation techniques. In these techniques, the quenching rates are estimated to be of the order of 107–109 K/s. During ion irradiation, the necessary condition for the formation of a thermal spike is that the average atomic number (Z) should be greater than 20 [19], which in the present case is 36.5. Hence, formation of thermal spikes is expected in the Fe/Ag system, though the lifetime of the spike is in the order of 10 12 s. The estimated rise in temperature due to

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ion irradiation in these samples is around 1000 K. Hence, the ion beam mixed samples should show some better properties than the samples synthesised with conventional techniques. The homogeneity in the irradiated samples was due to the reduction in Ag particle size that indicates that Ag is mixing with Fe. In the present experiments, the ion energy has been chosen to maximize the damage with in the multilayers so that it assists the mixing (Fig. 1). The displaced atoms in the multilayer film can move to the respective other layer, and give rise to better mixing of particles. Since the Ar+ ion mass is heavier than other inert gaseous ions like He or Ne, it produces more damage as well as form dense cascade. Since the layer thickness is small and comparable with the atomic diffusion length (typically 2–4 nm), we assume that ballistic mixing might have taken place in the system and mixed atoms may produce better mixing of Ag particles. Ion irradiation not only causes damage in the film but also modifies cluster sizes [20–22]. The final size distribution depends on the attachment and detachment of atoms from the cluster. The attachment requires thermally activated diffusion. But detachment of atoms from clusters can be due to both thermal activation and displacement of Ag atoms in the collision cascade. Since the system is thermally immiscible, the reduction in cluster size is only due to the displacement of Ag atoms during collision process. The microstructure observed in the sample irradiated with the fluence of 7 · 1016 ions/cm2, was homogeneous with the narrow size distribution. For the sample irradiated up to a fluence of 7 · 1016 ions/cm2, if the implanted Ar+ ions are immobile and it results in the Ar ion concentration of at most 6 at.% at the mean range. However, noble gases diffuse in Ag by a vacancy mechanism [23] and single and di-vacancies become mobile at 240 K Ag [24]. Hence, in the present experiments, the irradiation temperature is 300 K, there will be a considerable reduction in Ar ion concentration due to gas release effect. Trapping of gas atoms by irradiation induce vacancies will result in very small bubbles which can not be resolved [25]. In fact, we have not observed any gas bubbles during TEM studies of our samples. However, the volume fraction of these small bubbles is expected to be less than 2 vol.% [24,25]. Earlier work by Makhlouf et al. prepared the granular films by co-evaporation leads to a homogeneous microstructure, which shows 4% of magnetoresistance with the magnetic field of 1.2 T at room temperature [4]. Here a similar kind of microstructure was also obtained by ion beam mixing of Fe/Ag multilayers. The study of magnetic properties of these films is underway and the results will be forthcoming. 5. Summary In the present study we have deposited iron/silver mutilayer by electron beam evaporation method. The samples

were irradiated with 90 keV Ar+ ions with different fluence of 1 · 1016, 5 · 1016 and 7 · 1016 ions/cm2. Transmission electron microscopic investigation shows that with the increase of fluence Ag particles disintegrate into smaller particles and finally at the highest fluence a homogeneous distribution of Ag and Fe particles were formed. Since, the layer thickness is small and comparable with the atomic diffusion length we assume that the ballistic mixing might be responsible for mixing of Ag and Fe particles. The reduction in the cluster size has also been observed which is due to the detachment of atoms from the clusters by displacement of atoms during collision process. References [1] H.S. Nalwa (Ed.), Handbook of Nanostructured Materials and Nanotechnology, Vols. 1–5, Academic Press, San Diego, 2000. [2] M.N. Baibich, J.M. Broto, A. Fert, N. Nguen van Dau, F. Petroff, P. Etienne, G. Greuzet, A. Friedrich, J. Chazelas, Phys. Rev. Lett. 68 (1989) 2472. [3] U. Hartmann (Ed.), Magnetic Multilayers and Giant Magnetoresistance, Springer Verlag, Berlin, 1999, p. 251. [4] S.A. Makhlouf, K. Sumiyama, K. Wakoh, K. Suzuki, K. Takanashi, H. Fujimori, J. Magn. Magn. Mater. 126 (1993) 485. [5] M.M. Pereira de Azevedo, M.S. Rogalski, J.B. Sousa, Solid State Commun. 100 (1996) 639. [6] J.Q. Wang, G. Xiao, Phys. Rev. B 49 (1992) 3982. [7] T.J. Tackson, S.B. Palmer, H.J. Blythe, A.S. Halim, J. Magn. Magn. Mater. 159 (1996) 269. [8] J.B. Sousa, M.M. Pereira de Azevedo, M.S. Rogalski, Yu.G. Pogorelov, L.M. Rendondo, C.M. de Jesus, J.G. Marques, M.F. da Silva, J.C. Soares, J.-C. Ousset, E. Snoeck, J. Magn. Magn. Mater. 196–197 (1999) 13. [9] F.R. de Boer, R. Boom, W.C. Matterns, A.R. Midema, A.K. Niessen, Cohesion in Metals, North-Holland, Amsterdam, 1998. [10] A. Crespo-Sosa, P. Schaaf, W. Bolse, K.P. Liep, Phys. Rev. B 53 (1996) 14795. [11] H.U. Krebs, Y. Luo, M. Stormer, A. Crespo, P. Schaaf, W. Bolse, Appl. Phys. A 61 (1995) 591. [12] S. Amirthapandian, B.K. Panigrahi, A.K. Srivastava, A. Gupta, K.G.M. Nair, R.V. Nandedkar, A. Narayanasamy, J. Phys. Cond. Matter. 14 (2002) L641. [13] N. Thangaraj, K.M. Krishnan, R.F. Farrow, Scripta. Metall. Mater. 33 (1995) 1667. [14] J.F. Ziegler, J.P. Biersack, U. Littmark, The Stopping and Range of Ions in Solids, Pergamon, New York, 1985. [15] G. Safram, O. Geszti, P.B. Barna, Phys. Status. Solidii A 150 (1995) 489. [16] K.E. Mello, S.R. Soss, S.P. Murakara, T.M. Lu, S.L. Lee, Appl. Phys. Lett. 68 (1996) 681. [17] S. Morz, Z. Jankowski, Surf. Sci. 322 (1995) 133. [18] S.A. Makhulof, K. Sumiya, T. Kamiyama, K. Wakoh, K. Suzuki, Mater. Sci. Eng. A 179/180 (1994) 483. [19] Y.T. Cheng, Mater. Sci. Rep. 5 (1990) 45. [20] R. Enrique, P. Bellon, Phys. Rev. B 63 (2001) 134111. [21] J. Liu, P. Bellon, Phys. Rev. B 66 (2002) 20303. [22] G.C. Rizza, M. Strobel, K.H. Heinig, H. Bernas, Nucl. Instr. and Meth. B 178 (2001) 78. [23] R.S. Nelson, Phil. Mag. 9 (1964) 343. [24] F. Pleiter, C. Hohenmser, Phys. Rev. B 25 (1982) 106. [25] F. Vasiliu, V. Teodorescu, Rad. Effects 27 (1975) 75.