DOI: 10.1007/s00339-003-2206-5

Appl. Phys. A 78, 1229–1233 (2004)

Materials Science & ProcessingApplied Physics A

g.h. yu1,�

f.w. zhu1

m.h. li1

h.w. jiang2

w.y. lai2

Suppression of interlayer segregationin spin-valve NiFe///Cu///NiFe///FeMn multilayers1 Department of Materials Physics, University of Science and Technology Beijing, Beijing 100083, P.R. China2 Institute of Physics, Chinese Academy of Sciences, Beijing 100080, P.R. China

Received: 17 December 2002/accepted: 16 April 2003Published online: 11 June 2003 • © Springer-Verlag 2003

ABSTRACT Experimental results show that Cu atoms canfloat out to or segregate to the NiFe/FeMn interface forTa/NiFe/Cu/NiFe/FeMn/Ta spin-valve multilayers, whichresults in a drop of the exchange-coupling field (Hex) ofNiFe/FeMn in the spin-valve multilayers. However, whena small amount of Bi atoms is deposited between the Cu andthe pinned NiFe layers, Cu segregation to the NiFe/FeMn inter-face can be suppressed. At the same time, Hex of NiFe/FeMnin the spin-valve multilayers with a Bi interfacial layer can beeffectively increased.

PACS 75.70.Cn; 82.80.Pv

1 Introduction

Exchange coupling between ferromagnetic (FM)and antiferromagnetic (AF) thin films has been known formore than 40 years [1]. Due to this interfacial coupling, thehysteresis loop of the FM layer is displaced from zero fieldrelative to that of a single layer by an amount termed theexchange field Hex. The FM film also shows an enhanced co-ercivity Hc. Recently, considerable interest in the FM/AF ex-change coupling has been revived, because of its applicationto giant magnetoresistive spin-valve heads for high-densityrecording systems [2]. Despite extensive experimental stud-ies [3, 4] and numerous theoretical efforts [5, 6], the mechan-ism of exchange coupling is not well understood.

Fundamentally, exchange coupling is an interfacial phe-nomenon [6], so any modification of the interface struc-ture may have a significant effect on it. The factors studiedwhich could influence the exchange coupling are (i) interfa-cial roughness [7] and its slopes [8], (ii) interfacial interdiffu-sion of atoms [9], and (iii) the texture [10] and grain size [11]of AF thin films. Recently, we found that there are reactionsat the NiO/NiFe interface, and the interface chemical reactionis an important factor influencing exchange coupling [12]. In-terestingly, different researchers often gave totally differentexperimental results. Sometimes, even opposite conclusions

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were reached [5, 13, 14]. On the other hand, experimental re-sults appear to depend sensitively on differences in samplepreparation as well as on the specific system being studied. Allthese reflect the complexity of exchange coupling and the im-portance of studying in more detail the microstructure of theinterface.

NiFe/FeMn may be the most extensively studied ex-change-coupling system in recent years; however, we foundthat a higher Hex can be obtained for NiFe/FeMn multi-layers than for spin-valve NiFe/Cu/NiFe/FeMn multilay-ers on average. For example, it was reported that Hex ofTa/NiFe/FeMn/Ta and Ta/NiFe/Cu/NiFe/FeMn/Ta multi-layers was in the range of 33.4–47.8 kA/m [11, 15, 16] and22.3–33.4 kA/m [17–19], respectively, in prior research. Forcomparison, Hex had been normalized for a NiFe (4 nm)film based on its 1/tFM dependence, where tFM is the thick-ness of the pinned magnetic layer [15]. However, these twokinds of layer stacks were prepared by different authors indifferent instruments. Therefore, in order to check the dif-ference of their Hex values and find out the reason whichresulted in the phenomenon, we have carried out a seriesof studies [20]. The experimental results indicate that Cuatoms can float out to or segregate to the NiFe/FeMn inter-face for Ta/NiFe/Cu/NiFe/FeMn/Ta top spin valves, whichresults in a drop of Hex of the multilayers. Therefore, atpresent, an important subject is to study how to suppressthe segregation of Cu atoms and increase Hex of the spin-valve multilayers. Surfactants such as Bi, In, Pb, Ag, etc.tend to exhibit rapid surface diffusion and low surface en-ergies, properties that should favor their floating out to thesurface during overlayer deposition [21]. A suppression ofthe Cu segregation would be expected for these surfactants,since they would probably lower the surface free energyand eliminate the driving force for Cu segregation. Thus,they can be tentatively deposited between the Cu and thepinned Ni81Fe19 layers to improve Hex of the spin-valvemultilayers.

In this work, in order to suppress the segregation of Cuatoms in the NiFe/FeMn interface, a small amount of Biatoms (or other surfactants such as In, Pb, Ag, etc.) wasdeposited between the Cu and the pinned NiFe layers ofTa/NiFe/Cu/NiFe/FeMn/Ta spin valves. The results indi-cate that Cu segregation to the NiFe/FeMn interface can be

1230 Applied Physics A – Materials Science & Processing

suppressed. At the same time, Hex of the spin-valve multilay-ers can be effectively increased.

2 Experimental

Ta (12 nm)/Ni81Fe19 (7 nm)/Cu (3 nm)/Ni81Fe19

(5 nm)/Fe50Mn50(15 nm)/Ta(9 nm) multilayers were pre-pared in magnetron sputtering systems. At the same time,a small amount of Bi atoms (or other surfactants such as In,Pb, Ag, etc.) was tentatively deposited between the Cu andthe pinned Ni81Fe19 layers because the surface free energy ofthe surfactant Bi (382 mJ/m2) is much lower than that of thesurfactant Cu (1934 mJ/m2) [22]. The base pressure was lessthan 4 ×10−5 Pa and the argon sputtering pressure was 0.5 Pa.The substrates were cooled by water. The sample temperatureduring film growth was lower than 30 ◦C. A permanent mag-net which produced a magnetic field of ∼ 250 Oe along thesubstrate surface was present during the deposition process.This field produced an easy axis in the NiFe film and definedthe exchange-coupling axis. The hysteresis loops were ob-tained from a JDJ9600 vibrating sample magnetometer. Thevalue of Hex of NiFe/FeMn in the spin-valve multilayers withinsertion layers could be obtained from the hysteresis loops.The error of the magnetic tests was less than 5%.

In order to analyze the distribution of Bi atoms in the spin-valve multilayers, sample I: Ta(12 nm)/Ni81Fe19(7 nm)/Cu(3 nm)/Bi(1.2 nm)/Ni81Fe19(6 nm), sample II: Ta(12 nm)/

Ni81Fe19 (7 nm)/Cu(3 nm)/Bi(1.2 nm)/Ni81Fe19 (6 nm)/

Fe50Mn50(15 nm), and sample III: Ta(12 nm)/Bi(x nm)

Ni81Fe19 (11 nm)/Fe50Mn50 (15 nm)/Ta(3 nm) were fabri-cated in the same way as the samples used in the magneticproperties tests, where the thickness of the Bi layer x wasvaried in the range from 0 nm to 2 nm. The samples wereintroduced into a MICROLAB MK II X-ray photoelectronspectroscopy (XPS) system within 1 h after being taken outof the deposition system. The vacuum of the analysis cham-ber was less than 3 ×10−7 Pa. A Mg Kα line at 1253.6 eV wasused with the X-ray source run at 14.5 kV. An energy analyzerwas operated at a constant pass energy of 50 eV. All bindingenergies have been corrected for the sample-charging effectwith reference to the C 1s line at 284.6 eV. The XPS peakareas and peak decomposition (i.e. “curve fitting”) were deter-mined using Gaussian(80%)–Lorentzian(20%) curve-fittingsoftware (including the atomic sensitivity factor) providedby this XPS system. Peak areas were measured with a preci-sion of 5% or better. S/N ratios in XPS measurements weremore than 20. Angle-resolved XPS was used to study the dif-ferent depth information of the surfaces of the samples. TheXPS detectable sampling depth is d = 3λ sin α, where λ andα are inelastic mean-free paths (IMFPs) for photoelectronsand the take-off angle for photoelectrons with respect to thesamples’ surface plane, respectively [23]. About 95% of thetotal photoelectron signal will arise from this sampling depth.The IMFPs can be obtained by using the table compiled byTanuma et al. [24]. When α changed from 15◦ to 90◦, the dif-ferent depth information from the top layers of the samplescould be acquired. The signals from the interfaces under thetop layers could not be detected since the XPS detectable sam-pling depth was d = 3λ sin α. In view of this, a 6-nm Ni81Fe19

pinned layer was chosen in sample I and sample II, in order

to make the Ni81Fe19 thickness more than the XPS detectablesampling depth d of the Bi layer. The value of d for Bi is about6 nm. Moreover, a 11-nm Ni81Fe19 pinned layer was chosenin sample III for increasing the signal intensity in the hystere-sis loops tests. During the XPS experimental process, first, inorder to detect whether foreign atoms had segregated fromlower layers to the surface of the films, angle-resolved XPSwas used to study the different depth information of the sur-faces for the first and second samples. Then, depth profilingwas carried out for the second and third samples to analyze thedistribution of Bi atoms in the films.

3 Results and discussion

Figure 1 shows the exchange-coupling field (Hex)

of NiFe/FeMn in spin-valve multilayers Ta(12 nm)/Ni81Fe19

(7 nm ) /Cu (3 nm ) /Bi ( x nm )Ni81Fe19 (5 nm ) /Fe50Mn50(15 nm)/Ta(9 nm) as a function of the thickness of the Biinterfacial layer. The experimental results show that Hexgradually becomes stronger as the thickness of the Bi layer in-creases and reaches a maximum value at a thickness of about1.2 nm. Then, Hex gradually decreases with a further increaseof the thickness of the Bi layer. Therefore, the Bi interfaciallayer with the proper thickness can effectively increase Hex. Inorder to find out the reason for this, the composition at the sur-face of the Ta(12 nm)/Ni81Fe19(7 nm)/Cu(3 nm)Bi(1.2 nm)/

Ni81Fe19(6 nm) sample was studied using angle-resolvedXPS. The take-off angle for photoelectrons α changed from15◦ to 90◦ during the experimental process. The peak areasof Bi 4 f and Ni 2p high-resolution XPS spectra acquiredfor α = 15◦, 35◦, 60◦, and 90◦ are integrated. Simultaneously,the integration results are corrected using the atomic sensi-tivity factor. Then, the Bi/Ni atomic number ratios can beestimated. The curve I in Fig. 2a shows that the Bi content de-creases with increasing sampling depth. This indicates that Biatoms had segregated to the NiFe surface after the sample wasprepared. Figure 3a and b represent typical XPS spectra overthe entire energy range for α = 15◦ and 90◦, respectively. Itcan be seen from Fig. 3a and b that the relative signal inten-sity of the Bi 4 f high-resolution XPS spectrum for α = 15◦ is

0.0 0.5 1.0 1.5 2.020

22

24

26

28

30

32

34

36

Hex(K

A/m

)

Bi Layer Thickness (nm)FIGURE 1 A plot of the exchange-coupling field Hex of Ta(12 nm)/

Ni81Fe19(7 nm)/Cu(3 nm)/Bi(x nm)Ni81Fe19(5 nm)/Fe50Mn50(15 nm)/Ta(9 nm) multilayers vs. the thickness of Bi deposited between Cu and pinnedNiFe layers

YU et al. Suppression of interlayer segregation in spin-valve NiFe/Cu/NiFe/FeMn multilayers 1231

FIGURE 2 a The atomic molar ratio as a function of α, (I), Bi/Ni;(II), Bi/Mn; α is a take-off angle for photoelectrons with respect tothe samples’ surface plane. b The exchange-coupling field (Hex) ofTa(12 nm)/Bi(x nm)Ni81Fe19(11 nm)/Fe50Mn50(15 nm)/Ta(3 nm) multi-layers as a function of the thickness of the Bi insertion layer

much larger than that for α = 90◦. Therefore, Bi atoms obvi-ously floated out to the NiFe surface. However, Cu 2p peakswere not detected in the XPS spectra over the entire energyrange and Cu 2p high-resolution XPS spectra (Fig. 3c) ofTa(12 nm)/Ni81Fe19(7 nm)/Cu(3 nm)Bi(1.2 nm)/Ni81Fe19

(6 nm) for α = 15◦ and 90◦. However, we have also carriedout angle-resolved XPS studies of Ta(12 nm)/NiFe(7 nm)/

Cu(3 nm)/NiFe(6 nm). Figure 4 shows Cu 2p high-resolutionXPS spectra obtained for Ta(12 nm)/NiFe(7 nm)/Cu(3 nm)/

NiFe(6 nm) multilayers at α = 15◦, 35◦, 60◦, and 90◦. Therelative signal intensity of Cu 2p high-resolution XPS spec-tra diminishes with increasing detectable sampling depth. TheCu/Ni atomic ratios calculated indicate that the ratio rapidlydecreases with increasing detectable sampling depth, whichshows Cu atoms had segregated to the surface of the depositedTa/NiFe/Cu/NiFe multilayers [20]. These results above indi-cate that Cu segregation to the NiFe surface (or NiFe/FeMninterface) had been suppressed due to the presence of Bi.We think that if the glass substrates were heated during thedeposition process or after the deposition, the segregationphenomenon should be more obvious. In this work, we onlydiscuss the segregation at normal temperature.

Surfactant atoms have lower surface energy. When atomswith the higher surface energy are deposited onto them, theywill float out to the surface to decrease the total surface and in-terface energy of the system. Therefore, for spin-valve multi-layers Ta/NiFe/Cu/Bi/NiFe/FeMn/Ta, when the NiFe layerwith the higher surface energy is deposited onto the Bi layerwith the lower surface energy, it is thermodynamically pos-

FIGURE 3 XPS spectra over the entire energy range (0–1000 eV) ob-tained at the surface of Ta(12 nm)/Ni81Fe19(7 nm)/Cu(3 nm)Bi(1.2 nm)/Ni81Fe19(6 nm) for a α = 15◦ and b α = 90◦. c Cu 2p high-resolution XPSspectra for α = 15◦ and α = 90◦

sible that Bi atoms float out to the NiFe surface. The Biatoms were deposited onto the Cu layer. Since the surfaceenergy of Cu (1934 mJ/m2) is much higher than that of Bi(382 mJ/m2) [22], Cu atoms cannot segregate to the surface ofthe Bi layer. Therefore, the Bi atoms receive more kinetic en-ergy from the Ni (or Fe) atoms sputtered onto the surface andcan easily segregate to the surface of the NiFe layer.

1232 Applied Physics A – Materials Science & Processing

FIGURE 4 Cu 2 p high-resolution XPS spectra of Ta(12 nm)/NiFe(7 nm)/Cu(3 nm)/NiFe(6 nm) multilayers for α = 15◦, 35◦, 60◦, and 90◦, respec-tively. α is a take-off angle for photoelectrons with respect to the samples’surface plane

For the case of Ta(12 nm)/NiFe(7 nm)/Cu(3 nm)/

Bi(1.2 nm)/NiFe(6 nm)/FeMn(15 nm) multilayers, in orderto find out whether the Bi atoms which segregated to the NiFesurface stayed in the NiFe/FeMn interface, angle-resolvedXPS was used to study the surface information of the FeMnlayer. Depth profiling was also carried out for the FeMn layerin the multilayers. The FeMn layer was etched by lower-energy Ar+ (the sputter rate of Ar+ to the FeMn layer had beencalibrated). The Ar+ gun was operated at 0.5 kV at a pres-sure of 1 ×10−4 Pa, and the Ar+-ion current density was50 µA/cm2. XPS data were received once every ten secondsat each depth by using a 5◦ take-off angle for photoelectronswith respect to the samples’ surface plane to monitor the ap-pearance of the pinned Ni81Fe19 layer. Then, a 15◦ take-offangle was used to study the Bi/Mn atomic ratio. The curve IIin Fig. 2a shows the Bi/Mn atomic ratio as a function of thetake-off angle α of the photoelectrons. It can be seen thatthe Bi content slightly increases with increasing samplingdepth. Moreover, the depth profiling indicates that the ratiohardly changed in the profiling process [25]. All these re-sults show that the Bi atoms which segregated to the NiFesurface further migrated into the FeMn layer, and probablyformed Fe-Mn-Bi ternary alloys with FeMn. Therefore, theantiferromagnetic properties of the FeMn layer were dam-aged. Simultaneously, Bi atoms stayed in the NiFe/FeMninterface as impurities decreased the actual contact areas of

the NiFe with the FeMn layers, which also resulted in a de-crease of Hex of the Ta/Bi/NiFe/FeMn/Ta multilayers.

In order to further investigate the influence of a Bi in-sertion layer on the exchange coupling, Ta(12 nm)/Bi(x nm)

Ni81Fe19(11 nm) / Fe50Mn50(15 nm)/ Ta(3 nm) multilayerswere also studied. Figure 2b shows Hex of the multilayersas a function of the thickness of the Bi insertion layer. Theexperimental results show that Hex gradually decreases withan increase of the Bi thickness. Angle-resolved XPS wasused to study the different depth information of the surfacesof Ta(12 nm)/Bi(1.2 nm)/NiFe(11 nm). Depth profiling wasalso carried out for Ta(12 nm)/Bi(1.2 nm)/NiFe(11 nm)/

FeMn(15 nm) to analyze the distribution of Bi atoms inthe films. We found that Bi atoms first segregated to theNiFe/FeMn interface, and then migrated into the FeMn layer.The antiferromagnetic properties of the FeMn layer weredamaged as well. Simultaneously, Bi atoms stayed in theNiFe/FeMn interface as impurities decreased the actual con-tact areas of the NiFe with the FeMn layers. Therefore, Hexof Ta/Bi/NiFe/FeMn/Ta was reduced. However, when a Bilayer with the proper thickness is deposited between the Cuand the pinned NiFe layers for Ta/NiFe/Cu/NiFe/FeMn/Tamultilayers, Hex can indeed be increased effectively.

In view of the above analyses of the magnetic propertiesand microstructures, we think the reason that the Bi inser-tion layer can influence the NiFe/FeMn exchange-couplingfield results from the following three factors. (i) The Bi atomscan effectively suppress the segregation of Cu atoms sinceBi atoms have lower surface free energy and eliminate thedriving force for Cu segregation. From the point of view ofsuppressing Cu segregation, Bi as an insertion layer favorsthe increase of Hex. (ii) From the point of view of the growthof the multilayers, in the presence of the surfactant Bi thecoherent growth mode of Ta/NiFe/Cu layers was disrupted,leading to a fine grain size in the pinned NiFe layer [26].This also favors the increase of Hex [27]. (iii) The experi-mental results of Ta/Bi/NiFe/FeMn multilayers show thatBi atoms could migrate into the FeMn layer, which resultedin a decrease of Hex of Ta/Bi/NiFe/FeMn/Ta multilayers.Therefore, from the point of view of Bi as impurities at the in-terface, an insertion layer of Bi between the Cu and the pinnedNiFe layers will decrease Hex. In general, the aforementionedthree factors work together to make Hex increase to a max-imum if the proper-thickness Bi is deposited between the Cuand the pinned NiFe layers. However, excessive Bi results ina drop of Hex. Other surfactants such as In, Pb, Ag, etc. werealso deposited between the Cu and pinned NiFe layers forTa/NiFe/Cu/NiFe/FeMn/Ta multilayers and studied by thesame method. We obtain the same conclusion as for Bi. Asfor the effect of insertion layers on giant magnetoresistancevalues, we will report on this in another paper.

4 Conclusions

In summary, when a small amount of Bi atomsis deposited between the Cu and the pinned NiFe layers forTa/NiFe/Cu/NiFe/FeMn/Ta spin-valve multilayers, the in-terlayer segregation of Cu atoms can be suppressed. More-over, the NiFe/FeMn exchange-coupling field can be effec-tively increased. The experimental results provide a method

YU et al. Suppression of interlayer segregation in spin-valve NiFe/Cu/NiFe/FeMn multilayers 1233

by which Hex of the spin-valve multilayers with Cu spacerlayers can be improved.

ACKNOWLEDGEMENTS The authors would like to thank theNatural Science Foundations of China and Beijing for their support underGrant Nos. 50271007 and 2012011, respectively.

REFERENCES

1 W.H. Meiklejohn, C.P. Bean: Phys. Rev. 102, 1423 (1956)2 B. Dieny, V.S. Speriosu, S.S.P. Parkin, B.A. Gurney, D.R. Wilhojt,

D. Mauri: Phys. Rev. B 43, 1279 (1991)3 R. Jungblut, R. Coehoorn, M.T. Johnson, J. aan de Stegge, A. Reinders:

J. Appl. Phys. 75, 6659 (1994)4 J. Nogues, D. Lederman, T.J. Moran, I. Schuller: Phys. Rev. Lett. 76,

4624 (1997)5 N.C. Koon: Phys. Rev. Lett. 78, 4865 (1997)6 T.C. Schulthess, W.H. Butler: Phys. Rev. Lett. 81, 4516 (1998)7 J. Nogues, D. Lederman, T.J. Moran, I.K. Schuller, K.V. Rao: Appl.

Phys. Lett. 68, 3186 (1996)8 D.G. Hwang, S.S. Lee, C.M. Park: Appl. Phys. Lett. 72, 2162 (1998)9 Z.H. Qian, J.M. Sivertsen: J. Appl. Phys. 83, 6825 (1998)

10 G. Choe, S. Gupta: Appl. Phys. Lett. 70, 1764 (1997)11 K. Takano, R.H. Kodama, A.E. Berkowitz, W. Cao, G. Thomas: Phys.

Rev. Lett. 79, 1130 (1997)

12 G.H. Yu, C.L. Chai, F.W. Zhu, J.M. Xiao, W.Y. Lai: Appl. Phys. Lett. 78,1706 (2001)

13 A.P. Malozemoff: Phys. Rev B 35, 3679 (1987)14 D. Mauri, H.C. Siegmann, P.S. Bagus, E. Kay: J. Appl. Phys. 62, 3047

(1987)15 J. Fujikata, K. Hayashi, H. Yamamoto, M. Nakada: J. Appl. Phys. 83,

7210 (1998)16 Y.K. Kim, K. Ha, L.L. Rea: IEEE. Trans. Magn. 31, 3823 (1995)17 K. Nishioka, S. Gangopadhyay, H. Fujiwara: IEEE. Trans. Magn. 31,

3949 (1995)18 B. Dieny, V.S. Speriosu, J.S. Metin, S.S.P. Parkin, B.A. Gurney, P. Baum-

gart, D.R. Wilhoit: J. Appl. Phys. 69, 4774 (1991)19 H. Kanai, J. Kane, K. Aoshima, M. Kanamine, Y. Uehara: IEEE. Trans.

Magn. 31, 2612 (1995)20 G.H. Yu, M.H. Li, F.W. Zhu, Q.K. Li, C.L. Chai, H.W. Jiang, W.Y. Lai:

J. Appl. Phys. 91, 3759 (2002)21 W.F. Egelhoff, Jr., P.J. Chen, C.J. Powell, M.D. Stiles, R.D. McMichael,

C.-L. Lin, J.M. Sivertsen, J.H. Judy, K. Takano, A.E. Berkowitz: J. Appl.Phys. 80, 5183 (1996)

22 L.Z. Mezey, J. Giber: Jpn. J. Appl. Phys. 21, 1569 (1982)23 E. Atanassova, T. Dimitrova, J. Koprinarova: Appl. Surf. Sci. 84, 193

(1995)24 S. Tanuma, C.J. Powell, D.R. Penn: Surf. Interface Anal. 11, 577 (1988)25 G.H. Yu: Doctoral thesis, University of Science and Technology, Beijing

(2001)26 H.D. Chopra, E.J. Repetski, H.J. Brown, P.J. Chen, L.J. Swartzendruber,

W.F. Egelhoff, Jr.: Acta Mater. 48, 3501 (2000)27 W.F. Egelhoff, Jr., P.J. Chen, C.J. Powell, M.D. Stiles, R.D. McMichael:

J. Appl. Phys. 79, 2491 (1996)