Interlayer segregation of Cu atoms in Ta/NiFe/Cu/NiFe/FeMn/Ta spin-valve multilayersand its influence on magnetic propertiesG. H. Yu, M. H. Li, F. W. Zhu, Q. K. Li, Y. Zhang, C. L. Chai, H. W. Jiang, and W. Y. Lai Citation: Journal of Applied Physics 91, 3759 (2002); doi: 10.1063/1.1450033 View online: http://dx.doi.org/10.1063/1.1450033 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/91/6?ver=pdfcov Published by the AIP Publishing Articles you may be interested in NiFe/CoFe/Cu/CoFe/MnIr spin valves studied by ferromagnetic resonance J. Appl. Phys. 113, 17D713 (2013); 10.1063/1.4798615 Interlayer segregation of nonmagnetic metal spacers and its influence on exchange coupling in magneticmultilayers J. Appl. Phys. 101, 063912 (2007); 10.1063/1.2713949 Interlayer segregation in magnetic multilayers and its influence on exchange coupling Appl. Phys. Lett. 82, 94 (2003); 10.1063/1.1533121 Segregation of the Cu atom at the ferromagnetic/antiferromagnetic interlayer in spin-valve structures J. Appl. Phys. 92, 2620 (2002); 10.1063/1.1495092 Physical properties of spin-valve films grown on naturally oxidized metal nano-oxide surfaces J. Appl. Phys. 91, 8560 (2002); 10.1063/1.1455615

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Interlayer segregation of Cu atoms in Ta ÕNiFeÕCuÕNiFeÕFeMnÕTa spin-valvemultilayers and its influence on magnetic properties

G. H. Yu,a) M. H. Li, F. W. Zhu, Q. K. Li, and Y. ZhangDepartment of Materials Physics, University of Science and Technology Beijing, Beijing, 100083, China

C. L. ChaiInstitute of Semiconductor, Chinese Academy of Sciences, Beijing, 100083, China

H. W. Jiang and W. Y. LaiInstitute of Physics, Chinese Academy of Sciences, Beijing, 100080, China

~Received 31 July 2001; accepted for publication 17 December 2001!

Experimental results show that the exchange coupling field (Hex) of NiFe/FeMn for Ta/NiFe/FeMn/Ta multilayers is higher than that for spin-valve multilayers Ta/NiFe/Cu/NiFe/FeMn/Ta. In order tofind out the reason, the composition and chemical states at the surface of Ta~12 nm!/NiFe~7 nm!,Ta~12 nm!/NiFe~7 nm!/Cu~4 nm!, and Ta~12 nm!/NiFe~7 nm!/Cu~3 nm!/NiFe~5 nm! were studiedusing x-ray photoelectron spectroscopy. The results show that no elements from lower layers floatout or segregate to the surface in the first and second samples. However, Cu atoms segregate to thesurface of Ta~12 nm!/NiFe~7 nm!/Cu~3 nm!/NiFe~5 nm! multilayers, i.e., Cu atoms segregate to theNiFe/FeMn interface for Ta/NiFe/Cu/NiFe/FeMn/Ta multilayers. We believe that the presence of Cuatoms at the interface of NiFe/FeMn is one of the important factors which causes the exchangecoupling field (Hex) of Ta/NiFe/Cu/NiFe/FeMn/Ta to be weaker than that of Ta/NiFe/FeMn/Ta.© 2002 American Institute of Physics.@DOI: 10.1063/1.1450033#

I. INTRODUCTION

Exchange coupling between ferromagnetic~FM! and an-tiferromagnetic~AF! thin film has been known for more than40 years.1 Due to this interfacial coupling, the hysteresis loopof the FM layer is displaced from zero field relative to that ofa single layer by an amount termed the exchange fieldHex.The FM film also shows an enhanced coercivityHc . Re-cently, considerable interest in the FM/AF exchange cou-pling has been revived because of its application to giantmagnetoresistive spin-valve heads for high density recordingsystems.2 Despite extensive experimental studies3,4 and nu-merous theoretical efforts,5,6 the mechanism of exchangecoupling is not well understood.

Fundamentally, exchange coupling is an interfacialphenomenon,6 so any modification of the interface structuremay have a significant effect on it. The factors studied whichcould influence the exchange coupling are~1! interfacialroughness7,8 and its slopes,9 ~2! interfacial interdiffusion ofatoms,10 and ~3! the texture11 and grain size12,13 of AF thinfilms. Recently, we found that there are reactions at the NiO/NiFe interface, and the interface chemical reaction is an im-portant factor influencing exchange coupling.14 Interestingly,different researchers often gave totally different experimentalresults. Sometimes, even opposite conclusions werereached.5,15,16On the other hand, experimental results appearto depend sensitively on differences in sample preparation aswell as on the specific system being studied. All these reflectthe complexity of exchange coupling and the importance of

probing for more details of the microstructure of the inter-face.

NiFe/FeMn may be the most extensively studied ex-change coupling system in recent years, however, we foundthat a higher exchange coupling fieldHex can be obtained forNiFe/FeMn multilayers than for spin-valve multilayers NiFe/Cu/NiFe/FeMn on an average. For example, it was reportedthat theHex of Ta/NiFe/FeMn/Ta multilayers and Ta/NiFe/Cu/NiFe/FeMn/Ta multilayers were in the range of 33.4–47.8 kA/m ~i.e., 420–600 Oe!13,17–19 and 22.3–33.4 kA/m~i.e., 280–420 Oe!,20–24 respectively, in prior research. Forcomparison, theHex had been normalized for a NiFe/~4 nm!film based on their 1/tFM dependence, wheretFM is the thick-ness of the pinned magnetic layer.17 However, these twokinds of multilayers were prepared by different authors indifferent instruments. Therefore, in this article, in order tocheck the difference of theirHex, the aforementioned twokinds of multilayers were first prepared under the same ex-perimental conditions and their magnetic properties weretested as well. Then, the surface composition and chemicalstates of Ta/NiFe, Ta/NiFe/Cu, and Ta/NiFe/Cu/NiFe filmswere studied using x-ray photoelectron spectroscopy~XPS!.We found that Cu atoms could float out or segregate to theNiFe surface in the third sample, which resulted in a fall ofthe Hex of Ta/NiFe/Cu/NiFe/FeMn/Ta multilayers.

II. EXPERIMENT

In order to prove that theHex of NiFe/FeMn filmwas higher than that of NiFe/Cu/NiFe/FeMn film,Ta(12 nm) / Ni81Fe19(11 nm) / Fe50Mn50(15 nm) / Ta (9 nm),and Ta(12 nm)/Ni81Fe19(7 nm)/Cu(3 nm)/Ni81Fe19(5 nm)/Fe50Mn50(15 nm)/Ta(9 nm) were deposited on glass sub-a!Electronic mail: guanghua–yu@263.net

JOURNAL OF APPLIED PHYSICS VOLUME 91, NUMBER 6 15 MARCH 2002

37590021-8979/2002/91(6)/3759/5/$19.00 © 2002 American Institute of Physics

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strates in regular order in magnetron sputtering systems. Thebase pressure was less than 431025 Pa and the argon sput-tering pressure was 0.5 Pa. The substrates were cooled bywater. A permanent magnet which produced a magnetic fieldof 20 kA/m ~i.e., ;250 Oe! along the substrate surface waspresent during the deposition process. This field produced aneasy axis in the NiFe film and defined the exchange-couplingaxis. The hysteresis loops were obtained from a JDJ9600vibrating sample magnetometer. The exchange coupling field(Hex) and coercivity (Hc) in these two films could be ob-tained from the hysteresis loops.

In order to detect whether there were foreign atoms seg-regated from lower layers to the surface of the films by theuse of XPS, Ta~12 nm!/NiFe~7 nm!, Ta~12 nm!/NiFe~7 nm!/Cu~4 nm!, and Ta~12 nm!/NiFe~7 nm!/Cu~3 nm!/NiFe~5 nm!were fabricated in the same way as the aforementioned twofilms used in magnetic properties tests. The samples wereintroduced into a MICROLAB MK II x-ray photoelectronspectroscopy system within 1 h after being taken out of thedeposition system. The vacuum of the analysis chamber wasless than 331027 Pa. A MgKa line at 1253.6 eV was usedwith the x-ray source run at 14.5 kV. An energy analyzer wasoperated at a constant pass energy of 50 eV. All bindingenergies have been corrected for sample charging effect withreference to the C 1s line at 284.6 eV. The XPS peak areasand peak decomposition~i.e., ‘‘curve fitting’’! were deter-mined using Gaussian~80%!–Lorentzian~20%! curve fittingsoftware~including the atomic sensitivity factor! provided bythis XPS system. Peak areas were measured with a precisionof 5% or better. Angle-resolved XPS was used to study thedifferent depth information of the surfaces for the threesamples. The XPS detectable sampling depth isd53l sina,25 where l and a are inelastic mean-free paths~IMFPs! for photoelectrons and the take off angle for photo-electrons with respect to the samples surface plane respec-tively. About 95% of the total photoelectron signal will arisefrom this sampling depth. The IMFPs can be obtained byusing the table compiled by Tanumaet al.26 For a Mg Karadiation source, the IMFPs for Fe 2p in Fe, Ni 2p in Ni, Cu2p in Cu and Ta 4f in Ta are 10.3 Å, 7.9 Å, 7.8 Å, and 17.3Å, respectively. Whena changed from 15° to 90°, the dif-ferent depth information from the top layers of the threesamples could be acquired. The signals from the interfacesunder top layers could not be detected since the XPS detect-able sampling depth wasd53l sina. The maximum valueof d for Fe 2p in Fe, Ni 2p in Ni, Cu 2p in Cu and Ta 4f inTa are 30.9 Å, 23.7 Å, 23.4 Å, and 51.9 Å, respectively.

III. RESULTS AND DISCUSSION

Figures 1~a! and 1~b! show the hysteresis loops ofTa(12 nm) / Ni81Fe19(11 nm) / Fe50Mn50(15 nm) / Ta(9 nm)and Ta(12 nm)/Ni81Fe19(7 nm)/Cu(3 nm)/Ni81Fe19(5 nm)/Fe50Mn50(15 nm)/Ta(9 nm), respectively. From the hyster-esis loops the exchange coupling fieldHex ~11.9 and 21.5kA/m! and coercivityHc ~0.9 and 2.4 kA/m! could be ob-tained respectively. For comparison, theHex could be nor-malized for a NiFe~4 nm! film based on their 1/tFM depen-dence. After normalization, the two samples exhibitedHex’s

of 32.8 and 26.8 kA/m, respectively. Evidently, theHex forthe former is higher than that for the latter, in agreement withthe change trend ofHex reported in many articles.13,17–24

Angle-resolved XPS was used to study different depthinformation of the top layers for the three samples and thetakeoff angle for photoelectrons asa changed from 15° to90°. Figures 2~a!–2~d! show Cu 2p high-resolution XPSspectra obtained for Ta~12 nm!/NiFe~7 nm!/Cu~3 nm!/NiFe~5nm! multilayers ata515°, 35°, 60°, and 90°. The relativesignal intensity of Cu 2p high-resolution XPS spectra dimin-ishes with increasing detectable sampling depth. For ex-ample, the relative signal intensity of Cu 2p high-resolutionXPS spectrum fora515° is obviously larger than that fora590°. The information in about 6 and 23 Å can be ob-tained fora515° and 90°, respectively.25,26 Ni 2p and Cu2p high-resolution XPS spectra acquired fora515°, 35°,60°, and 90° are integrated. Simultaneously, integration re-sults were corrected using the atomic sensitivity factor. Then,the Cu/Ni atomic ratio could be estimated. Figure 3~a! showsthat Cu content decreases with increasing detectable sam-pling depth. Figure 3~b! represents a computer fitted curve ofa Cu 2p3/2 high-resolution XPS spectrum fora515°, it canactually be fitted with two components. From the XPShandbook,27 we know that peak 1 at 932.30 eV is character-istic of a metallic Cu 2p3/2 peak, peak 2 at 933.60 eV corre-sponds to the Cu21 2p3/2 peak in CuO. CuO was formed due

FIG. 1. Hysteresis loops of ~a! Ta(12 nm)/Ni81Fe19(11 nm)/Fe50Mn50(15 nm)/Ta(9 nm) and~b! Ta(12 nm)/Ni81Fe19(7 nm)/Cu(3 nm)/Ni81Fe19(5 nm)/Fe50Mn50(15 nm)/Ta(9 nm).

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to Cu atoms which had segregated to the surface and beenoxidized in the air after the sample was taken out of thedeposition system. All these results show Cu atoms had seg-regated to the surface of the deposited Ta/NiFe/Cu/NiFe mul-tilayers. These experiments on magnetic tests and angle-resolved XPS analyses were repeatedly done, which had thesame experimental results. In other words, Cu atoms reallysegregated to the NiFe/FeMn interface for Ta/NiFe/Cu/NiFe/FeMn/Ta multilayers, which resulted in a fall of theHex ofTa/NiFe/Cu/NiFe/FeMn/Ta multilayers. The other twosamples were studied using the same method. XPS experi-mental results show Ta atoms were not detected in the sur-face of Ta~12 nm!/NiFe~7 nm!. This indicates that Ta atomsdid not segregate to the NiFe layer surface, which explainswhy the magnetic properties of Ta/NiFe/FeMn are not af-fected. Ni and Fe atoms were not detected in the surface ofTa~12 nm!/NiFe~7 nm!/Cu~4 nm!. This indicates that Ni andFe atoms did not segregate to the Cu layer surface, whichexplains why the magnetic properties of Ta/NiFe/Cu/FeMn/Ta are not effected as well.

While studying the spin valve with a NiO pinning layer,Egelhoff Jr.et al. found that a small amount of Pb, In, etc. assurfactants can segregate to the sample surface.28,29A bilayersystem~NiFe film deposited on Cu!, includes NiFe/Cu inter-face energy (sNiFe/Cu) and surface energy of NiFe layer(sNiFe). If Cu atoms segregate to the surface of the NiFelayer, the surface of NiFe will be replaced by a Cu/NiFeinterface and a Cu layer surface. WhensCu1sCu/NiFe

,sNiFe, the segregation of Cu atoms to the surface of NiFeis thermodynamically favorable, but the experimental dataabout sNiFe and sCu/NiFe are not available yet. Therefore,molecular dynamics simulation was used to calculate the en-ergy for a NiFe/Cu bilayer system by employing embeddedatom method potential.30,31 In order to verify the validity of

FIG. 2. Cu 2p high-resolution XPSspectra of Ta~12 nm!/NiFe~7 nm!/Cu~3nm!/NiFe~5 nm! multilayers for~a! a515°, ~b! a530°, ~c! a560°, and~d! a590°, a is a take off angle forphotoelectrons with respect to thesamples surface plane.

FIG. 3. ~a! Cu/Ni atomic ratio as a function ofa and ~b! computer fittingcurve of Cu 2p3/2 high-resolution XPS spectrum fora515°.

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the method used to calculate the surface energy for metals,we calculated the surface energy of~100!, ~110!, and ~111!for Ni, Fe, and Cu. The results are basically in agreementwith the experimental data~see Table I!. As to the ~100!,~110!, and ~111! of Cu/NiFe system, when all other condi-tions are the same, the energy difference between the systemthat uses periodic boundary condition in the normal directionof the surface~i.e., the Cu layer deposited on the NiFe layer!and the system that uses nonperiodic boundary condition inthe same direction~i.e., the NiFe layer deposited on the Culayer! are21962 mJ/m2, 22124 mJ/m2, and21809 mJ/m2,respectively. Therefore, Cu atoms segregating to the NiFesurface facilitate a decrease in the energy of the Cu/NiFesystem, i.e., it is thermodynamically favorable that Cu atomssegregate to the NiFe surface.

It can be estimated from the Cu/Ni atomic ratio and theXPS detectable sampling depthd that a monolayer of Cuatoms approximately segregate to the NiFe surface. In orderto directly investigate the Cu/Ni atomic ratio at the NiFe/FeMn interface, Ta~12 nm!/NiFe~7 nm!/Cu~3 nm!/NiFe~5nm!/FeMn~15 nm! were fabricated in the same way as thefilms used in magnetic properties tests. The sample wasetched by lower energy Ar1 ~the sputter rate of Ar1 to FeMnlayer had been calibrated!. The Ar1 gun was operated at 0.5kV at a pressure of 131024 Pa, and the Ar1 ion currentdensity was 50mA/cm2. XPS data were received once every10 s at each depth by using a 5° take off angle for photoelec-trons with respect to the samples surface plane to monitor theappearance of the pinned Ni81Fe19 layer. Then, a 15° take offangle was used to study the Cu/Ni atomic ratio at the NiFe/FeMn interface. The experimental results indicate that theratio was 0.30. In other words, a half monolayer of Cu atomsapproximately segregate to the NiFe/FeMn interface. In ad-dition, the trace Cu~the Cu/Fe atomic ratio is 0.048! on theFeMn surface was detected. However, the Cu 2p spectra wasnot found during etching FeMn layer process. The aforemen-tioned XPS analysis results indicate that for the spin-valveTa/NiFe/Cu/NiFe/FeMn/Ta, Cu atoms migrate to the surfaceof Ta/NiFe/Cu/NiFe. Some of them stop at the NiFe/FeMninterface. Moreover, trace Cu atoms segregate to the FeMnlayer surface during the deposition process.

There are four layers under the NiFe/FeMn interface inTa/NiFe/Cu/NiFe/FeMn/Ta. Thus, the roughness of the NiFe/FeMn interface in Ta/NiFe/Cu/NiFe/FeMn/Ta multilayers isdifferent from that in Ta/NiFe/FeMn/Ta. Most investigationsof the roughness role on exchange coupling agree that themagnitude of Hex decreases with increasing roughness.9

However, it is worth noticing that the magnitude ofHex

seems to be insensitive to roughness when an AF layer is inthe polycrystal form.29

For the spin-valve Ta/NiFe/Cu/NiFe/FeMn/Ta, Cu atomswhich segregate to the NiFe/FeMn interface during the depo-sition process greatly diminish actual contact areas of NiFewith FeMn, and these impurities could also hinder domainwalls in the NiFe layer from moving. We believe that theformer leads to a decrease in the exchange coupling field(Hex), and the latter gives rise to an increase in the coerciv-ity (Hc). For Ta/NiFe/FeMn/Ta, no Ta atoms were detectedon the NiFe surface~i.e., in the NiFe/FeMn interface!. There-fore, theHex andHc of the multilayers are relatively higherand weaker, respectively. Go¨kemeijeret al.34 have recentlyshown that theHex will decrease when a small number ofnonmagnetic metal atoms such as Cu, Ag, or Au are speciallydeposited in the AF/FM interface. This is in agreement withour experimental results. Fujiwaraet al.35 have reported thatthe Hex of Ta/NiFe/FeMn/Ta is higher than that of Ta/Cu/NiFe/FeMn/Ta, where the NiFe thickness is the same. Theysuggested that the reason may be because the former has aslightly better perpendicular texturing of@111# direction ofthe NiFe and/or FeMn layers than the latter. We fabricatedtwo kinds of multilayers, Ta/NiFe/FeMn/Ta and Ta/Cu/NiFe/FeMn/Ta, under the same experimental conditions. The re-sults of x-ray diffraction show that the textures of the twokinds of multilayers have no evident differences. The mag-netic properties tests indicate theHex for the latter is reallylower than that for the former. The main reason should be thesegregation of Cu atoms to the NiFe/FeMn interface. Thedetails of the experimental procedure and discussion will bereported in another article. Therefore, the interlayer segrega-tion of Cu atoms is an important factor influencing the NiFe/FeMn exchange coupling.

IV. CONCLUSION

In summary, magnetic properties tests indicate theHex ofTa/NiFe/FeMn/Ta is higher than that of Ta/NiFe/Cu/NiFe/FeMn/Ta. Ta/NiFe, Ta/NiFe/Cu, and Ta/NiFe/Cu/NiFe havebeen studied using angle-resolved XPS. No elements fromlower layers segregate to the surface of films for the first andsecond samples. However, Cu atoms segregate to the surfaceof Ta/NiFe/Cu/NiFe. We belive that the presence of Cu at-oms in the NiFe/FeMn interface is an important factor influ-encing exchange coupling.

ACKNOWLEDGMENTS

The authors would like to thank the National ScienceFoundations of China and Beijing for their support underGrant Nos. 19890310 and 2012011, respectively.

1W. H. Meiklejohn and C. P. Bean, Phys. Rev.102, 1423~1956!.2B. Dieny, V. S. Speriosu, S. S. P. Parkin, B. A. Gurney, D. R. Wilhojt, andD. Mauri, Phys. Rev. B43, 1297~1991!.

3R. Jungblut, R. Coehoorn, M. T. Johnson, J. aan de Stegge, and A. Re-inders, J. Appl. Phys.75, 6659~1994!.

4J. Nogues, D. Ledermn, T. J. Moran, and I. Schuller, Phys. Rev. Lett.76,4624 ~1997!.

5N. C. Koon, Phys. Rev. Lett.78, 4865~1997!.6T. C. Schulthess and W. H. Butler, Phys. Rev. Lett.81, 4516~1998!.

TABLE I. Calculated surface energiesa ~mJ/m2!.

Element ~100! ~110! ~111! Exp. value

Ni 1847 2005 1695 2380Fe 1822 1974 1678 2200Cu 1477 1621 1348 1790Ni81Fe19 1953 2128 1817

aExperimental data are the averaged values of polycrystal surfaces with anerror of about 10%.~See Refs. 32 and 33!.

3762 J. Appl. Phys., Vol. 91, No. 6, 15 March 2002 Yu et al.

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128.59.171.71 On: Wed, 10 Dec 2014 12:43:19

7J. Nogues, D. Ledermn, T. J. Moran, I. K. Schuller, and K. V. Rao, Appl.Phys. Lett.68, 3186~1996!.

8C. X. Liu, C. T. Yu, H. M. Jiang, L. Y. Shen, C. Alexander, and G. J.Mankey, J. Appl. Phys.87, 6644~2000!.

9D. G. Hwang, S. S. Lee, and C. M. Park, Appl. Phys. Lett.72, 2162~1998!.

10Z. H. Qian and J. M. Sivertsen, J. Appl. Phys.83, 6825~1998!.11G. Choe and S. Gupta, Appl. Phys. Lett.70, 1764~1997!.12H. Uyama, Y. Otani, K. Fukamichi, O. Kitakami, Y. Shimada, and J.

Echigoya, Appl. Phys. Lett.71, 1258~1997!.13K. Takano, R. H. Kodama, A. E. Berkowitz, W. Cao, and G. Thomas,

Phys. Rev. Lett.79, 1130~1997!.14G. H. Yu, C. L. Chai, F. W. Zhu, J. M. Xiao, and W. Y. Lai, Appl. Phys.

Lett. 78, 1706~2001!.15A. P. Malozemoff, Phys. Rev. B35, 3679~1987!.16D. Mauri, H. C. Siegmann, P. S. Bagus, and E. Kay, J. Appl. Phys.62,

3047 ~1987!.17J. Fujikata, K. Hayashi, H. Yamamoto, and M. Nakada, J. Appl. Phys.83,

7210 ~1998!.18T. Lin, C. Tsang, R. E. Fontana, and J. K. Howard, IEEE Trans. Magn.31,

2585 ~1995!.19Y. K. Kim, K. Ha, and L. L. Rea, IEEE Trans. Magn.31, 3823~1995!.20K. Nishioka, S. Gangopadhyay, H. Fujiwara, and M. Parker, IEEE Trans.

Magn.31, 3949~1995!.21K. Nishioka, T. Iseki, H. Fujiwara, and M. R. Parker, J. Appl. Phys.79,

4970 ~1996!.

22B. Dieny, V. S. Speriosu, J. S. Metin, S. S. P. Parkin, B. A. Gurney, P.Baumgart, and D. R. Wilhoit, J. Appl. Phys.69, 4774~1991!.

23H. Kanai, J. Kane, K. Aoshima, M. Kanamine, and Y. Uehara, IEEE Trans.Magn.31, 2612~1995!.

24T. P. A. Hase, B. K. Tanner, P. Pyan, C. H. Marrows, and B. J. Hickey,IEEE Trans. Magn.34, 831 ~1998!.

25E. Atanassova, T. Dimitrova, and J. Koprinarova, Appl. Surf. Sci.84, 193~1995!.

26S. Tanuma, C. J. Powell, and D. R. Penn, Surf. Interface Anal.11, 577~1988!.

27C. D. Wagner, W. M. Riggs, L. E. Davis, and J. F. Moulder,Handbook ofX-ray Photoelectron Spectroscopy~Perkin–Elmer, 1979!, p. 81 and p.144.

28W. F. Egelhoff, P. J. Chen, C. J. Powell, M. D. Stiles, R. D. McMichael, C.-L. Lin, J. M. Sivertsen, J. H. Judy, K. Takano, and A. E. Berkowitz, J.Appl. Phys.80, 5183~1996!.

29W. F. Egelhoff, Jr., P. J. Chen, C. J. Powell, M. D. Stiles, and R. D.McMichael, J. Appl. Phys.79, 2491~1996!.

30R. A. Johnson, Phys. Rev. B39, 12554~1989!.31R. A. Johnson, Phys. Rev. B37, 3924~1988!.32M. I. Baskes, Phys. Rev. B46, 2727~1992!.33M. W. Finnis and J. E. Sinclair, Philos. Mag. A50, 45 ~1984!.34N. J. Gokemeijer, T. Ambrose, and C. L. Chien, Phys. Rev. Lett.79, 4270

~1997!.35H. Fujiwara, K. Nishioka, C. Hou, M. R. Parker, S. Gangopadhyay, and R.

Metzger, J. Appl. Phys.79, 6286~1996!.

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