Thin Solid Films 433(2003) 237–242

0040-6090/03/$ - see front matter� 2003 Elsevier Science B.V. All rights reserved.PII: S0040-6090Ž03.00290-6

Correlation between interface structure and giant magnetoresistance inelectrodeposited Co–CuyCu multilayers

A. Cziraki , M. Koteles , L. Peter , Z. Kupay , J. Padar , L. Pogany , I. Bakonyi *, M. Uhlemann ,a a b b b b b, c´ ´ ¨ ´ ´ ´ ´M. Herrich , B. Arnold , J. Thomas , H.D. Bauer , K. Wetzigc c c c c

Department of Solid State Physics, Eotvos University, P.O.B. 32, H-1518 Budapest, Hungarya ¨ ¨Research Institute of Solid State Physics & Opt., HAS., P.O.B. 49, H-1525 Budapest, Hungaryb

IFW-Dresden, Helmholtzstrasse 20, D-01069 Dresden, Germanyc

Abstract

An attempt has been made to understand the correlation between the interface structure and the giant magnetoresistance(GMR)properties of electrodeposited Co–CuyCu multilayers by measurements performed on a series produced by galvanostaticelectrodeposition under the application of different capacitances connected parallel to the electrochemical cell, and this wasexpected to increase the width of the chemically intermixed interface between the magnetic and non-magnetic layer. In contrastto expectation, the GMR values of multilayers electrodeposited in the presence of a capacitance remained nearly unchanged evenat the highest applied capacitance value, as a consequence of immiscibility of alloying elements.� 2003 Elsevier Science B.V. All rights reserved.

Keywords: Multilayers; Structural properties; Transmission electron microscopy; Giant magnetoresistance

1. Introduction

Since the discovery of giant magnetoresistance(GMR) in nanoscale multilayered thin films made ofalternate layers of ferromagnetic and paramagnetic met-als w1x, magnetic multilayers have become a widelystudied family of nanostructured materials, especiallybecause of their potential and actual applications asmagnetic field sensors in magnetic data recordingdevicesw2x. A great effort has been focused on produc-ing such nanostructured materials by applying differentmethods as high-vacuum techniquesw3x and electrodep-osition w4–6x, the latter representing a cheap and simpleprocedure. Generally, electrodeposited multilayers arenot considered to exhibit sharp interfaces between themagnetic and non-magnetic layers due to the codeposi-tion of the more noble and less noble metals from asingle bath by the usual two-pulse plating technique.

Co and Cu are weakly miscible elements, and for thisreason they are the most likely to yield chemically sharpinterfaces. Whereas pure Cu can be deposited withoutCo contamination, Cu codeposition also occurs during

*Corresponding author. Tel.:q36-1-392-2628; fax:q36-1-392-2215.

E-mail address: bakonyi@szfki.hu(I. Bakonyi).

the Co deposition cycle. Under the conditions appliedin this work, the Cu codeposition was limited to lessthan 5%, and such a small amount did not modifysignificantly the magnetic and electrical transport prop-erties of the Co-rich layer. The widths of interfaces werefound to be 0.53"0.07 nm of which 0.4"0.2 nm wasdue to roughness and only 0.3"0.2 nm was due tointerdiffusion w5x. The quality of interfaces is thereforecomparable with those grown by physical depositiontechniquesw5x. The structure of interfaces has an impor-tant role in controlling the GMR properties of multilay-ers, while the origin of GMR lies in spin-dependentscattering at the interfacesw7x, the details of which areto some extent still unclear. The interface structure isexpected to depend drastically on the preparation con-ditions and techniques, resulting in very different inter-faces from an abrupt chemical transition to an intermixedinterface of several atomic layers.

In the present work, the interface width was meant tobe increased artificially to study its influence on GMRproperties of electrodeposited Co–CuyCu multilayers.For this purpose, a capacitor was connected parallel tothe electrochemical cell, and hence the electric currentthrough the cell was expected to change less abruptlythan without the capacitor in order to see if the rate at

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Fig. 1. Current forms used for the deposition of Co–CuyCu layers,where(a) the solid line shows the ideal one and the broken line showsthe real one distorted as a consequence of switch on and off effects,and (b) the concentration profile of Cu in the multilayered films(schematic representation).

which the current switches between the low and highamplitude could impact the interfacial propertiesbetween the layers of different composition.

2. Experimental

Co–CuyCu multilayers were electrochemically depos-ited on Ti substrates placed vertically in the depositioncell. The Co–CuyCu multilayers were grown by usinga single electrolyte containing 1.00 molyl CoSO and4

25 mmolyl CuSO under current control by applying a4

pulse technique with a programmed Keithley 228Acurrent source. The total thickness of the layered filmswas 1325 repeat periods, corresponding to approximate-ly 11 mm. More details of the preparation and samplecharacterization have been described in Peter et al.w6x.´

In Fig. 1a, the continuous line shows schematicallythe change of current vs. deposition time for ideal caseunder current control. As a consequence of the transientcurrent signal caused by the capacitor, the ideal rectan-gular shape of the deposition pulses is distorted asillustrated by the broken line. Accordingly, the interfacebetween the Co and Cu layers is not sharp but ratherintermixed, leading to a trapezoidal concentration profile(Fig. 1b).

In order to regulate the width of the interface betweenthe magnetic Co-rich and non-magnetic Cu layers, acapacitance varied from 0.047 to 6.7 mF was connectedparallel to the electrochemical cell, therefore increasingthe time while the current switch takes place within thecell.

The GMR properties of the films were studied bymagnetoresistance measurements performed at roomtemperature up toHs"8 kOe.

The structure of the multilayers was characterized byX-ray diffraction (XRD), atomic force microscopy(AFM), scanning and transmission electron microscopy(SEM and TEM) as described in Peter et al.w6x and´Cziraki et al.w8x.´

3. Results and discussion

As shown by Fig. 2, the GMR values changed onlyslightly as a function of the capacitance connectedparallel with the electrolytic cell. The samples did notlose the GMR properties even at high capacitancevalues. These measurements revealed that either thewidth of the interface itself has only a small effect onGMR properties in Co–CuyCu multilayers or the capac-itive shunt of the cell is not capable of regulating theinterface thickness.

It is remarkable that the GMR value does not changemonotonously with the variation of the capacitanceconnected parallel with the cell but it rather has aminimum in its absolute value. At the smallest appliedcapacitance of 0.047 mF, the GMR of the sample hasnearly the same value as for the sample depositedwithout any capacitor. Applying a capacitor with highercapacitance than 0.1 mF, the absolute value of the GMRdecreases abruptly from the original 6% to 4.3%, but athigher capacitance it starts to increase monotonously. Apossible interpretation of this phenomenon could be thatthe value of theRC time constant becomes comparableto the deposition time of a Co layer(t s0.65 s) at 0.1Co

mF, whereas it achieves the value of the Cu depositiontime (t s5 s) at 1 mF only. However, the time constantCu

of the circuit remains well below the length of any ofthe current pulses. The resistance of the cell is expectedto be less than 10 ohms, and the time constant of thecircuit is hence obtained asRC-60 ms. This is lessthan 10% of the shorter pulse length. Therefore, thepresence of the capacitor can modify the depositionconditions of the first one or two atomic layers only.Another possibility is that the capacitor connected out-side the cell has the higher impact on the depositionwhen it is of the same order of magnitude than thedouble layer capacitance at the electrodeyelectrolyteinterface. Since the double layer capacitance in concen-trated aqueous solution is usually some 20mF cm ,y2

the overall double layer capacitance is expected to beclose to 0.1 mF. At lower external capacitances than thedouble layer capacitance, the capacitor connected out-side the cell cannot have a significant influence on thedeposition because the shunting effect of the doublelayer capacitance is much larger than that of the externalcapacitor. In contrast, at very high external capacitancethe decay of the current within the cell is determined

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Fig. 3. SEM picture taken on the Co–CuyCu multilayered sampledeposited at a capacitance of 0.047 mF applied parallel with electrode.

Fig. 2. Longitudinal and transverse GMR values measured for the samples deposited with different capacitances applied parallel to the electrolyticcell.

by the external capacitor only because the double layercapacitance is negligible in this case. The largest dropin GMR was experienced when the two capacitancesboth control the current transient. Nevertheless, thequalitative picture on the elemental steps of the deposi-tion mechanism modified by the capacitive shunting isnot yet clear.

Concerning the results of structural studies on ourmultilayers, first we present a SEM picture showing acolumnar growth perpendicular to the foil plane(seeFig. 3), which is typical for multilayers. In agreementwith our previous workw6x, the AFM study revealedthat the samples exhibit a very fine granular structure atthe substrate side and a very rough surface at the solutionside, with a large average diameter(approx. 500 nm)of the granules where the latter value corresponds to thediameter of columns. It is important to emphasize thatthese granules consist of several semi-coherently growncrystals as revealed by X-ray and TEM observations.

The surface roughness at the solution side has aminimum at 0.1 mF capacitance value, but it increasesmonotonously with increasing capacitance(compareFig. 4a,b), in correlation with the change of GMRvalues with capacitance(see Fig. 2.). Therefore, onecan say that the external capacitor somehow controlsthe surface roughness, then the layer thicknesses aredetermined by the varying effective surface area. Fromthis viewpoint, the change in GMR can also be attributedto a macroscopic roughening effect rather than only tothe intermixing at the interfaces of the individual layers.

In a mixture of randomly oriented fcc Co or fcc Cucrystals(i.e. under the condition of powder diffraction)

the ratio of (111) and (200) Bragg peak intensities is100:40 and 100:46, respectively. This ratio for themultilayers is approximately 100:17(see Table 1),showing a strong(111) preferred orientation, except thesample deposited at 0.1 mF in which the crystallites arearranged nearly randomly.

Analysing the half width of Bragg peaks, it has beenestablished that the crystal grains oriented in(111)direction are approximately four times larger than thegrains with(100) orientation.

In correlation with the GMR properties, the averagegrain size obtained from the half width of the Bragg

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Fig. 4. AFM pictures taken on the solution side of Co–CuyCu multilayered sample electrodeposited with a capacitance of(a) 0.1 mF and(b)6.7 mF.

peaks of the fcc superlattice increases with increasingcapacitance and exhibits a minimum at the same capac-itance value as the magnetoresistance(see Table 1).

Comparing the column diameter determined by AFMto the average grain size(;30 nm), it could beconcluded that the columns consist of many crystallites.

The plane-view TEM pictures reveal the real grainsize(see Fig. 5), which is much larger(a few hundredsof nanometers) than that deduced from XRD measure-ments. Taking into account the high density of the

stacking faults and twins observed inside the crystalgrains, this difference is easy to understand, while thesefaults also cause an increase in the half width of Braggpeaks in the XRD patterns.

Analysing the grain size in the samples depositedwith different capacitance values, it was found that thehighly textured samples have extraordinarily large(;500 nm) grains oriented with(111) planes parallelto the substrate plane, and they are surrounded withmuch smaller grains oriented in other directions(see

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Table 1Average Cu content, unrelaxed internal strain, average grain size andintegrated XRD intensity ratio of the two strongest fcc Bragg reflec-tions as a function of the capacitance connected parallel to the cellduring the deposition of the Co–CuyCu multilayers

Capacitance Average Cu Internal strain Grain size I yI111 200( ) ( )

(mF) content(at.%) (%) (nm)

0.047 31 0.22 21 13.60.10 28 0.24 18 32.20.47 28 0.27 22 22.71.0 34 0.33 25 17.94.7 44 0.51 37 17.96.7 48 0.42 31 17.3

Fig. 6. Cross-sectional TEM picture taken on the Co–CuyCu sampledeposited at 6.7 mF, which shows a perfectly layered structure.

Fig. 5. Plane-view TEM pictures taken on Co–CuyCu samples deposited(a) at 0.1 mF and(b) at 6.7 mF.

Fig. 5a). The sample containing nearly randomly ori-ented grains exhibits the most uniform structure withthe smallest average grain size(see Fig. 5b).

The repeat period of the layered structure observed incross-sectional TEM pictures was found to be the samein all samples investigated. Even at the highest capaci-tance value applied during the deposition, the layers areperfect, showing a sharp contrast in the conventionalTEM pictures(see Fig. 6). In the case when the interfaceis of a few atomic layer wide, the loss of contrastobtained by the conventional TEM method would surelyindicate the intermixing. However, no change in contrastcould be seen even at the highest capacitance applied.

These results are in good correlation with the GMRvalues that exhibit only a relatively small decrease inabsolute value at high capacitances in comparison withmultilayers grown at the lowest capacitance. The reason

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of the relatively sharp interfaces is probably the immis-cibility of the Co and Cu under equilibrium conditions.By decreasing the slope of the trapezoidal current form,the width of interfaces should increase in principle(seeFig. 1), whereas the intermixing of the constituentsremain limited. As a consequence of the wider interfac-es, the unrelaxed misfit strain could be of a higher levelin these samples, as it was detected by the XRDmeasurements, and it is consistent with the decreasingfault density observed by the TEM study.

The relatively small variation in GMR could beattributed to the observed changes in the size andpreferred orientation of the crystallites and in the surfaceroughness of the films deposited. These changes are aconsequence of the distortion of the trapezoidal currentform. Whereas it turns out from the present study thatthis distortion apparently could not significantly broadenthe compositionally smeared interface, such a modifi-cation of the current in the transient period does have astrong effect on the overall crystal structure andmorphology.

4. Conclusions

By artificially enlarging the transient time at theswitch between the current pulses during the deposition,the GMR properties of the Co–CuyCu multilayers donot change significantly. As a function of capacitanceapplied parallel with the electrolytic cell during deposi-tion, the GMR value decreases approximately by 20%at large capacitance values. At intermediate capacitance,whereRC is comparable with the double layer capaci-tance this decrease is a little bit higher.

According to the structural investigations, the repeatperiods and the chemical sharpness of the interfaces donot change substantially as a function of the applied

capacitance; at least, the change cannot be detected bythe resolution of the instruments applied. This meansthat although the current transient was enlarged, thecapacitive shunting could not thicken the interface. Thisis probably due to the immiscibility of the alloyingelements(Co and Cu).

The present investigations found a strong correlationbetween texture, surface roughness, grain size and GMRproperties of the multilayers.

Acknowledgments

This work was supported by the Hungarian ScientificResearch Fund(OTKA) through grants F 032046 andT 037673. We have benefited from a joint research grantsupported by the Hungarian-German(D-36y97) Inter-governmental Science and Technology Co-operationProgramme.

References

w1x M.N. Baibich, J.M. Broto, A. Fert, F. van Dau Ngyuen, F.Petroff, P. Etienne, G. Creuzert, A. Friedrich, J. Chjazels, Phys.Rev. Lett. 61(1988) 2472.

w2x S.S.P. Parkin, IBM J. Res. Dev. 42(1998) 3.w3x A.R. Modak, S.S.P. Parkin, D.J. Smith, J. Magn. Magn. Mater.

129 (1994) 415.w4x W. Schwarzacher, D.S. Lashmore, IEEE Trans. Magn. 32

(1996) 3133.w5x P. Nallet, E. Chassaing, M.G. Walls, M.J. Hytch, J. Appl. Phys.

79 (1996) 6884.w6x L. Peter, A. Cziraki, L. Pogany, Z. Kupay, I. Bakonyi, M.´´ ´ ´

Uhlemann, M. Herrich, B. Arnold, T. Bauer, K. Wetzig, J.Electrochem. Soc. 148(2001) C168.

w7x S.S.P. Parkin, Appl. Phys. Lett. 61(1992) 1358.w8x A. Cziraki, I. Gerocs, B. Fogarassy, B. Arnold, M. Reibold, K.´ ´ ´´

Wetzig, E. Toth-Kadar, I. Bakonyi, Z. Metallkde. 88(1997)´ ´ ´781.