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Electrochimica Acta 52 (2007) 3813–3821
Electrodeposition of Co–Ni–Cu/Cu multilayers1. Composition, structure and magnetotransport properties
Laszlo Peter a,∗,1, Jozsef Padar a, Eniko Toth-Kadar a, Agnes Cziraki b,Peter Soki b,2, Lajos Pogany a, Imre Bakonyi a
a Research Institute for Solid State Physics and Optics, Hungarian Academy of Sciences, P.O. Box 49, 1525 Budapest, Hungaryb Department of Materials Physics, Eotvos University, P.O. Box 32, H-1518 Budapest, Hungary
Received 21 August 2006; received in revised form 25 October 2006; accepted 29 October 2006Available online 4 December 2006
bstract
Co–Ni–Cu/Cu multilayers have been deposited from a sulfate/sulfamate bath by controlling the current and the potential for the deposition of theagnetic and the copper layer, respectively. The magnetic layer thickness was varied between 1.2 nm and 12 nm, while the Cu layer thickness was
n the range of 1.1–2.3 nm. Alloys with direct current have also been produced for comparison. All multilayer deposits exhibited a face centeredubic structure, while a small amount of hexagonal close packed phase was also present in the d.c. deposited alloys. The composition analysishowed that the codeposition of Co and Ni can be classified as anomalous codeposition for both d.c. and compositionally modulated deposits,egardless of the simultaneous deposition of Cu. The composition of the magnetic layer was affected by its layer thickness. This finding led to
he conclusion that there must be a composition gradient in the magnetic layer in the growth direction. The magnetoresistance of the multilayersncreased with increasing copper layer thickness, while the superparamagnetic contribution in the magnetoresistance increased with decreasingeposition current density of the magnetic layers.2006 Elsevier Ltd. All rights reserved.
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eywords: Multilayer; Anomalous codeposition; Giant magnetoresistance; Pul
. Introduction
Nanoscale multilayers have been in the focus of researchince the discovery of the giant magnetoresistance (GMR)ffect in this family of nanostructures. The first observationf GMR on sputtered magnetic/non-magnetic multilayers wasollowed by the detection of the same effect in electrode-osited Co–Ni–Cu/Cu multilayers within half a decade [1].o–Ni–Cu/Cu multilayers were electrodeposited by using var-
ous deposition systems and electrolyte compositions [1–12].ernary Ni–Fe–Cu/Cu [13–15], Co–Fe–Cu/Cu [16] and qua-
ernary Co–Fe–Ni–Cu/Cu [17–22] magnetic multilayers werelso obtained by electrodeposition. The possible application ofhese structures as magnetic field sensors has been the main
∗ Corresponding author. Tel.: +36 1 392 2222x3614; fax: +36 1 392 2215.E-mail address: [email protected] (L. Peter).
1 ISE member.2 Undergraduate student at the Eotvos University of Budapest during the
ourse of this work.
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013-4686/$ – see front matter © 2006 Elsevier Ltd. All rights reserved.oi:10.1016/j.electacta.2006.10.058
ting; Deposit structure
riving force of the research of electrodeposited nanoscaleultilayers.Although some calculations anticipated that multilayers with
ure Co as the magnetic layer have superior magnetoresistanceo mixed Co–Ni layers, several arguments offer groundsor electrodeposition of Co–Ni–Cu/Cu multilayers: (i) somexperimental data [23] indicate that in the Co–Ni system theaximum magnetoresistance is obtained at about 2:1 Co–Ni
atio; (ii) the coercive field of the magnetic layers decreases withhe inclusion of Ni, hence leading to an enhanced sensitivity23]; (iii) the corrosion resistance of the sample obtained isigher if Ni is present, making the treatment of the samplesasier after the electrodeposition process.
It has long been known that the simultaneous electrodepo-ition of the iron group metals takes place in the so-callednomalous manner [24]. The diagnostic criterion of the anoma-
ous codeposition is that the metallic component of lower stan-ard potential can be discharged together with the more noblene at potentials where the less noble component alone can-ot be deposited onto a substrate composed of the parent metal;
3 ica Acta 52 (2007) 3813–3821
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814 L. Peter et al. / Electrochim
oreover, the less noble metal is deposited preferentially. Abun-ant up-to-date literature data can be found on the anomalousodeposition of Co as the less noble metal besides Ni as the moreoble metal [25–34], and various theoretical approaches havelso been published on the possible origin of the phenomenon35–39].
In spite of the long history of research on both electrode-osited multilayers and anomalous codeposition, systematictudies on the impact of the anomalous codeposition on mul-ilayer composition showed up quite recently only [20,40]. Bothxperimental results and theoretical simulation of the depositionrocess revealed that the composition of the alloyed layers sub-tantially changes during the initial stage of the layer deposition.his composition variation may persist over several nanometers.he composition change along the growth direction is due to theepletion of the ions of the preferentially deposited less nobleetal in the electrolyte close to the cathode surface as a result
f the fairly high current density.The aim of this work was to develop a bath for the elec-
rodeposition of Co–Ni–Cu/Cu multilayers with GMR. Theomposition of the multilayers and bulk alloys was analysednd the influence of anomalous codeposition on the sampleomposition is discussed in detail. Results on structure and mag-etotransport properties are also given. The direct observationf the composition gradient along the growth direction will beublished separately [41].
. Experimental
The composition of the electrolytes used is presented inable 1. Analytical grade chemicals and double distilled waterere used for the preparation of all electrolytes. All metal saltsere added to the electrolyte in the easily available and inexpen-
ive sulfate form. The combination of the chemicals accordingo Table 1 makes the solution preparation economic, while theeneficial effect of all components is exploited, similarly tohe typical sulfamate Ni baths. Although it is known that theurther decrease in pH may lead to higher magnetoresistance6], the technical optimization of the system used revealed thatydrogen evolution is diminished above pH ≈ 3, the depositsere less fragile and more uniform, and self-supporting samples
ere easily obtained. The relative Co content of the electrolyteith respect to the total amount of the magnetic elementsill be denoted as follows: σ = c(Co2+)/{c(Co2+) + c(Ni2+)},hile a similar notation is introduced also for the deposit:at1w
able 1omposition of the electrolytes used for the deposition of alloys and multilayers
Chemicals used for elec
etal sources (magnetic) Ni NiSO4·7H2OCo CoSO4·7H2O
etal source (non-magnetic) Cu CuSO4·5H2O
thers Na2SO4 Na2SO4·10H2OH3BO3 H3BO3
HSO3NH2 HSO3NH2
NaOH NaOH solution, 3 mol d
ig. 1. The cross-sectional view of the electrochemical cell and the electroderrangement.
= y(Co)/{y(Co) + y(Ni)} (y is the molar fraction in the solidhase).
A modified version of the tubular cell shown in Fig. 2 of Ref.42] with 60 ml internal volume was used in all experimentssee Fig. 1). The mechanically polished titanium cathode with a0 mm × 8 mm exposed surface area was placed to the bottomf the cell. The saturated calomel reference electrode (SCE) wasonnected to the cell with a Luggin capillary at a fixed distanceo the working electrode in order to ensure the uniformity ofhe electric current close to the cathode surface. A Cu sacrificialnode (counter electrode) was placed close to the top of the cell.
A fully computer-controlled EF453 type combined potentio-tat/galvanostat (Elektroflex, Hungary) was used as a currentource. The multilayer samples were deposited in the so-called/P mode [42] where the magnetic layer was produced with axed current (galvanostatic or G mode), while the non-magneticu layer was deposited under potentiostatic conditions (poten-
iostatic or P mode). The potential during the deposition of Cuas optimized by the method suggested in Ref. [43], and all Cu
ayers were deposited at −585 mV. This ensures that Co does notissolve when the high-current pulse ends and the deposition ofure Cu starts, as a result of which the actual layer thicknessesre equal to nominal ones. The potentials throughout this paperre referred to the SCE reference electrode without any correc-
ion for the ohmic drop. The total sample thickness was typically.5 �m for both the multilayers and the alloys, the latter obtainedith a constant cathodic current. The current efficiency of thetrolyte preparation Electrolyte concentration (mol dm−3)
0.5c(Ni2+) = 0.075–0.475c(Co2+) = 0.5–c(Ni2+)
0.015
0.300.250.15
m−3 pH adjustment: pH = 3.25 ± 0.03
ica Acta 52 (2007) 3813–3821 3815
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3.1.2. Multilayer composition as a function of thedeposition rate of the magnetic layer
The composition diagram obtained for the Co–Ni–Cu/Cumultilayers is shown in Fig. 3. The electrolyte composition and
L. Peter et al. / Electrochim
eposition of magnetic layers was established only for σ = 0.2nd j = −15 mA cm−2 d.c. current and was obtained as 96%.he current efficiency during the d.c. deposition decreased with
ncreasing Ni content of the deposit, and hence no continuouseposit could be obtained for σ = 0.05 and j = −45 mA cm−2.he current efficiency was assumed to be 100% for the Cu layereposition.
The sample composition was determined by the electronrobe microanalysis (EPMA) facility of a JEOL JSM 840 scan-ing electron microscope. The ORTEC System 5000 EPMA unitas operated at 25 kV. Composition was determined for spots of
bout 1 mm2 surface area each. Composition data given are theverages of measurements on three different spots for every sam-le. The structure of the multilayers was investigated by X-rayiffraction (XRD) measurements using a Philips X’Pert type X-ay diffractometer with Cu K� radiation (wavelength: 1.5406 A).he XRD peaks were fitted and decomposed applying the ProFitommercial software.
Magnetoresistance measurements were performed in theeld-in-plane/current-in-plane mode. An approximately 2 mmide strip of the multilayer sample was placed in the probeith four electric contact points in a linear arrangement. Theeasurements were carried out in both longitudinal (L) and
ransverse (T) directions, i.e., with the external magnetic fieldnd the test current parallel and perpendicular to each other,espectively. The following formula was used for calculating theagnetoresistance ratio: �R/R0 = [R(H) − R0]/R0 where R(H) is
he resistance in the magnetic field H and R0 is the resistanceaximum value around H = 0.
. Results and discussion
.1. Sample composition from the EPMA method
.1.1. Composition of bulk alloys prepared with d.c. platingThe cobalt ratio with respect to the total amount of the mag-
etic elements in the deposit (γ) as a function of the sameariable in the electrolyte (σ) is presented in Fig. 2. The d.c.eposits were produced from an electrolyte of identical com-osition as used for the multilayer preparation; hence, the d.c.lloys contained copper as well. The Cu content of the d.c.eposits was a function of the current density, and no system-tic dependence on the ratio of the magnetic elements in thelectrolyte was observed. The Cu content of the d.c. depositsaried from 12 at.% (at j(CoNiCu) = −15 mA cm−2) to 4 at.% (at(CoNiCu) = −45 mA cm−2), corresponding to a deposition cur-ent of approximately j(Cu) = −1.8 mA cm−2, independently ofhe overall cathodic current. Fig. 2 clearly shows that the anoma-ous nature of the codeposition of Co and Ni is unaffected byhe simultaneously codeposited Cu at the concentration levelnvestigated.
The composition diagram in Fig. 2 is also informativebout the relative importance of the surface kinetics and
ass transport limitations. The composition data obtained for(CoNiCu) = −45 mA cm−2 are all very close to the composi-ion reference line (γ = σ), which means that the major factoretermining the deposit composition is the transport of the metal
FCaγ
ig. 2. Composition diagram of the d.c. deposits. The continuous line is theeference line (γ = σ). Dashed and dotted lines are a guide for the eye only.otations: σ = c(Co2+)/{c(Co2+) + c(Ni2+)} and γ = y(Co)/{y(Co) + y(Ni)}.
ons in the electrolyte at this current density. However, the depo-ition at j(CoNiCu) = −15 mA cm−2 is significantly affectedy the kinetics of charge transfer as well, which is indicatedy the larger distance of the data points from the referenceine.
It also has to be noted that the transport control of Ni2+
nd Co2+ is not identical to the diffusion-controlled nature ofhe deposition since the transference number of the ions is farrom zero. However, ions of similar sizes (hexaqua complexesf both ions), of almost equal weights and of identical chargesre assumed to exhibit similar transport coefficients, even if theransport process involves both diffusion and migration.
ig. 3. Composition diagram for multilayers deposited with variouso2+:Ni2+ ratios and current densities. Layer thicknesses: d(Cu) = 1.1 nmnd d(CoNiCu) = 2.3 nm. Notations: σ = c(Co2+)/{c(Co2+) + c(Ni2+)} and= y(Co)/{y(Co) + y(Ni)}.
3 ica Acta 52 (2007) 3813–3821
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Fig. 4. Composition diagram for multilayers deposited with variousCo2+:Ni2+ ratios and magnetic layer thicknesses. Copper layer thickness:djσ
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the minimum Cu layer thickness applied) lasted over 4.6 s on theaverage. This time period can also be regarded as the interruptionof the high-current pulse and the re-establishment period of theCu “substrate” surface. It is obvious that the Cu pulse was suffi-
Table 2Comparison of the ratio of the magnetic elements in a few multilayer sampleswith “thin” and “thick” Cu layer
σ γ
d(Cu) = 1.1 nm d(Cu) = 2.3 nm
816 L. Peter et al. / Electrochim
he magnetic layer deposition current density were varied, whilehe layer thickness of both sublayers was fixed.
There is a striking difference between the composition dia-ram of the multilayers with fixed sublayer thicknesses and thatf the d.c. deposits. While the current density in the case of.c. deposition strongly affects the ratio of the magnetic ele-ents, for multilayers at a fixed magnetic layer thickness it is
ractically independent of the current density, especially whenhe Co2+:Ni2+ ratio is high enough (σ ≥ 0.2). The independencef the ratio of the magnetic elements from the current densityndicates that the kinetics of the magnetic layer deposition ishe same at each current density applied, and the electrolyte isepleted with respect to the salts of both magnetic elements tohe same extent during the deposition of the magnetic layer.
The Cu content of the magnetic layers can be calculated ashe overall Cu content minus the Cu content of the non-magneticayers. The Cu content of the magnetic layers is significantlyigher than the Cu content of the d.c. deposit obtained with theame current density. At −15 and −45 mA cm−2, the Cu contentf the magnetic layer is 20 at.% and 6 at.%, respectively; i.e.,bout 1.5 times higher than for the d.c. deposits.
The Cu deposition becomes diffusion limited at about400 mV versus SCE, while it was carried out at −585 mV
uring this work. Under these circumstances, the Cu depositionan be considered as a steady-state process taking place at theass transport limited current density during both pulses. This
ndicates that the partial current density of the Cu depositionuring pulse-plating has to be larger than in the case of d.c.lating.
The increase in c(Cu2+) as a result of the dissolution of theacrificial anode cannot be the source of the enlarged Cu con-ent because its effect is smaller by an order of magnitude thanhat was observed. It is possible that at high current density, the
ransport of the Cu2+ ions takes place with migration besideshe diffusion, and hence the ion flux is larger than at low currentensity where the diffusion is the only way of the ionic trans-ort. Therefore, the reason of the increase in the Cu content ofhe magnetic layer is yet to be explained.
.1.3. Multilayer composition as a function of the magneticayer thickness
Fig. 4 presents the composition diagram of the multilayershen the magnetic layer thickness and the electrolyte compo-
ition were varied at fixed d(Cu) and j(CoNiCu). It is obviousrom Fig. 4 that the higher the magnetic layer thickness, the morehe average magnetic layer composition approaches the compo-ition of the d.c. deposit. This clearly indicates that there muste a concentration gradient within the magnetic layer at leastp to d(CoNiCu) ≈ 12 nm and σ < 0.3. In most previous stud-es, this concentration gradient within the magnetic layer alonghe growth direction has usually been neglected, and the mag-etic layers of the magnetic/non-magnetic multilayers have beenharacterized by the average composition of the layers only. An
ttempt to simulate such composition change can be found inef. [20].The composition change along the growth direction withinhe magnetic layer is the higher, the lower is the concentration
000
j
(Cu) = 1.1 nm and current density during the deposition of the magnetic layer:(CoNiCu) = −15 mA cm−2. Lines are a guide for the eye only. Notations:= c(Co2+)/{c(Co2+) + c(Ni2+)} and γ = y(Co)/{y(Co) + y(Ni)}.
f the preferentially deposited metal ion (in this case, Co2+).t σ = 0.05 (i.e., 5% Co content in the electrolyte), the 1.2 nm
hick magnetic layers are 4.5 times more rich in Co than the d.c.eposit, but at σ = 0.70, the compositions of the bulk alloy andhe magnetic layers in the multilayers are almost the same.
The current efficiency during the deposition pulse waseported to increase with the length of the high-current pulse forlectrodeposited Co–Cu/Cu multilayers [44]. Although a sim-lar effect may also lead to a composition change with pulseength when two magnetic elements are codeposited, this can bexcluded with the help of the XRD study (see the estimation ofhe bilayer thicknesses in Section 3.2.2).
.1.4. Multilayer composition as a function of the copperayer thickness
A few samples were also made for which the only differenceas the Cu layer thickness. Table 2 shows that no composi-
ion change was found when the Cu layer thickness was ateast 1.1 nm, i.e., the same thickness as applied for most of theomposition study.
The deposition of the copper layer with d(Cu) = 1.1 nm (i.e.,
.05 0.42 0.41
.15 0.68 0.68
.3 0.86 0.83
(CoNiCu) = −15 mA cm−2; d(CoNiCu) = 2.3 nm.
ica Acta 52 (2007) 3813–3821 3817
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Fig. 6. Highlight of the satellite peaks in the X-ray diffractograms of a mul-tilayer sample series with varying Cu layer thickness. Deposition parameters:σ
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L. Peter et al. / Electrochim
iently long so that the Co2+ and Ni2+ ion concentration close tohe cathode surface could recover, and hence their values can beaken as the bulk values when the next high-current pulse wastarted. This explains well why the composition of the magneticayer is independent of the length of the Cu pulse in the rangenvestigated.
.2. Structure of the samples as obtained from XRD study
.2.1. Bulk alloys obtained with d.c. currentDeposits obtained with d.c. current from electrolytes of low
o content exhibit mostly a face centered cubic (fcc) structure.wo representative diffractograms are shown in Fig. 5. At highurrent density (−45 mA cm−2), only electrolytes with high Cooncentration (σ ≥ 0.7) result in deposits that contain a smallmount of the hexagonal close packed (hcp) phase. At low cur-ent density (−15 mA cm−2), the hcp phase appears already at= 0.3. In other word, it means that the hcp phase can form if> 0.65 (see Fig. 2).In the case of d.c. deposits obtained with j = −15 mA cm−2,
he amount of the hcp phase increases with σ. The intensity ratiof the main hcp line to the fcc (1 1 1) peak is 1.6, 3.7 and 14.2t σ = 0.3, 0.5 and 0.7, respectively. It is not surprising that themount of the hcp phase increases with the Co content of theeposit, i.e., with the molar fraction of the only component thatxhibits a stable hcp crystal structure.
The texture of the d.c. samples strongly depends on the cur-ent density and hence also on composition. At low currentensity, a slight (1 1 1) texture can be observed, while for sam-les obtained at high current density the crystals mostly growlong the 〈0 1 1〉 direction. The variation of the texture can belearly seen in Fig. 5. At random orientation, the ratio of theeak intensities for an fcc structure is I(1 1 1):I(2 0 0) = 100:47 [45],hile we obtained I(1 1 1):I(2 0 0) = 100:26 for −15 mA cm−2. On
he other hand, at −45 mA cm−2 the strongest line is the (2 2 0)eflection and, thus, we have a different texture since we obtain(1 1 1):I(2 2 0) = 73:100 whereas for an fcc powder pattern withoutexture the expected ratio is I(1 1 1):I(2 2 0) = 100:27 [45].
ig. 5. X-ray diffractogram obtained for two d.c. deposits. Electrolyte composi-ion: σ = 0.2; deposit compositions: Co40.4Ni46.7Cu12.9 for j = −15 mA cm−2 ando23.4Ni70.8Cu5.8 for j = −45 mA cm−2 (subscripts denote atomic percentages).eflections shown refer to the fcc planes.
wwtaelTttcbnmfwaCt
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= 0.2; j = −45 mA cm−2; d(CoNiCu) = 2.3 nm. Arrows indicate the position ofhe satellite peaks on the top curve. Diffraction curves are displaced along the-axis for sake of clarity.
.2.2. Multilayer samplesAll multilayer samples studied by XRD are mainly of the fcc
tructure, independently of both the current density during theeposition of the magnetic layer and the electrolyte concentra-ion. In most of the cases, the amount of the hcp phase is belowhe detection limit of the XRD method. The presence of the hcphase in the multilayers can only be detected at the substrateide of the samples when the thickness of the magnetic layers isigher than 5 nm, but even in this case the hcp fraction does notxceed 2–3%.
In contrast to the d.c. samples, all the multilayers exhibitedvery pronounced (1 1 1) texture, regardless of the deposition
urrent density of the magnetic layer. Comparing samples withdentical layer thicknesses, it could be established that the higherhe deposition current density of the magnetic layer, the moreronounced the texture was.
In Fig. 6, the XRD patterns are shown for a multilayer seriesith varying Cu layer thickness whereby the magnetic layeras deposited with j(CoNiCu) = −45 mA cm−2. It can be seen
hat satellite peaks appear symmetrically around both the (1 1 1)nd (2 0 0) Bragg reflections. The mean bilayer thickness can bextracted from the position of the satellite peaks [46]. The repeatength values obtained from the XRD data are summarized inable 3. Although the multilayer quality is clearly indicated by
he presence of the satellite reflections around several peaks,he repeat lengths measured by the XRD method are systemati-ally higher than the nominal values. A very similar differenceetween the repeat periods obtained from nominal layer thick-esses and from XRD has also been observed for Co–Cu/Cuultilayers [47]. Interestingly, cross-sectional TEM studies per-
ormed on the latter system yielded a very good agreementith the nominal values in most cases. In spite of this discrep-
ncy, the variation of the XRD repeat periods for the presento–Ni–Cu/Cu multilayers clearly exhibited the same trend as
he nominal repeat periods (see Table 3).
Fig. 6 also shows that the intensity of the satellite peaks isarger for thicker Cu layers. This can be due to the levellingffect of the fairly thick Cu layers, which was also establishedrom magnetoresistance data [48,49].
3818 L. Peter et al. / Electrochimica Acta 52 (2007) 3813–3821
Table 3Comparison of the modulation wavelength established from the XRD data with the nominal values for the same series as shown in Fig. 6
d(Cu) (nm) Modulation length (nm)
From the (1 1 1) peak From the (2 0 0) peak From Faraday’s law (nominal)
Lower angle Higher angle Lower angle Higher angle
1.0 4.7 4.6 – – 3.41.3 4.9 4.8 4.9 5.6 3.61.5 5.5 5.3 5.3 5.6 3.91.9 5.7 5.5 5.4 5.4 4.22.4 5.9 5.7 – – 4.73.0 6.0 6.0 5.9 5.9 5.3
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ultilayer preparation parameters: j(CoNiCu) = −45 mA cm−2. The typical err
The XRD patterns for a series with the same layerhicknesses but with the magnetic layers prepared with(CoNiCu) = −15 mA cm−2 have also been recorded. In theatter case, much weaker satellites were found than for(CoNiCu) = −45 mA cm−2. This indicates that the layer struc-ure of the samples can be improved by the appropriate selectionf the current density. The consequences of the structural imper-ections will be discussed in the next section together with theagnetoresistance properties.As shown by Michaelsen [46], the appearance of the satellite
eaks is strongly influenced by the sharpness of the chemicalodulation, the layer thickness fluctuation, the undulation of
he layer interfaces and the inclination of the layer planes tohe substrate. The appearance of the satellites on the presentamples undoubtedly indicate the good structural quality of theseultilayers.
The grain size was also established from the XRD datay the Scherrer formula [45] for multilayer samples with(CoNiCu) = 2.3 nm, j(CoNiCu) = −15 and −45 mA cm−2 andarious Cu layer thicknesses. The grain size data show that
oF
ig. 7. Room temperature magnetoresistance curves for multilayer samples(CoNiCu) = 2.3 nm and σ = 0.2 for all samples.
the modulation length determined by X-ray diffraction is ±0.5 nm [45].
he majority (1 1 1)-oriented grains are “large” with an averageateral grain diameter of 31 nm and the minority (1 0 0)-riented grains are small with a diameter below 20 nm. Foroth j(CoNiCu) values, the size of the grains with (1 1 1) ori-ntation increases with d(Cu): typical values are 29 nm and3 nm for d(Cu) = 1.0 nm and d(Cu) = 2.6 nm, respectively. Noimilar dependence on d(Cu) was observed for the (1 0 0) ori-nted grains, the average diameter being 19 nm and 16 nm for(CoNiCu) = −15 mA cm−2 and −45 mA cm−2, respectively. Itlso has to be noted that the grain size as established from theRD data usually significantly underestimates the actual grain
ize as observed in the transmission electron microscope becausehe presence of dislocation and stress in the grains also lead toine broadening.
.3. Magnetoresistance study
Fig. 7 shows the room temperature magnetoresistance curvesf a few selected multilayer samples. It can be seen in bothig. 7a and b that the total magnetoresistance increases with the
with (a) j(CoNiCu) = −15 mA cm−2 and (b) j(CoNiCu) = −45 mA cm−2.
ica Acta 52 (2007) 3813–3821 3819
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Fig. 8. (a) Ferromagnetic (MRFM) and (b) superparamagnetic (MRSPM) con-tributions of the total room temperature magnetoresistance as a function of theCσ
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L. Peter et al. / Electrochim
u layer thickness in the 1.0–2.3 nm range. This observation isn good agreement with earlier results obtained for Co–Cu/Cu
ultilayers [50,51].It can be seen from Fig. 7 that the degree of saturation of
he magnetoresistance up to H = 8 kOe is significantly smalleror samples prepared with lower deposition current density. Theower rate of saturation is related to at least two factors: (i) theu content of the magnetic layers prepared with low currentensity is higher; (ii) according to the structural studies, theigher current density leads to a multilayer structure of betteruality, as shown by the higher intensity of the X-ray satelliteeflections. This comparison is especially relevant since the ratiof the magnetic elements is the same in the samples prepared atoth low and high current densities (γ ≈ 0.8, see Fig. 3).
Both the higher Cu concentration in the magnetic layers andhe presence of structural imperfections affect the magnetore-istance properties in the same way, namely, via the formationf superparamagnetic (SPM) clusters. It is known from earlierorks that the Cu content of the magnetic layer enhances therobability of the segregation in the magnetic layers [51]. Thebserved magnetoresistance behaviour is in agreement with thessumption that the lateral segregation of the elements in theagnetic layer leads to the appearance of magnetic entities that
re magnetically decoupled from the main (ferromagnetic, FM)art of the same layer, and their size is in the range where SPMehaviour occurs. It has been shown recently [49,52] that theagnetoresistance contribution of these SPM entities can be
uantitatively established from an appropriate analysis of theeld dependence of the magnetoresistance.
The main steps of the separation of the FM and SPM con-ributions are as follows: Magnetoresistance data measured atH| > HS are considered only where HS is the saturation field ofhe FM part (both magnetization and magnetoresistance loopsre closed at this field). The data are fitted with the follow-ng function: MR(H) = MRFM + MRSPML(μH/kT) where MRFMnd MRSPM are the saturation values of the FM and SPM mag-etoresistance contributions, respectively; L is the Langevin-unction and μ is the average magnetic moment of the SPMlusters. Further details of the analysis are given elsewhere [52].
Data obtained for the present multilayer series by using thisind of magnetoresistance analysis show that the segregationlso occurs when the magnetic layer contains some 20% Niesides Co. It is clear from the data shown in Fig. 7 that Ni in suchconcentration is still not enough to completely eliminate theiscibility gap between Co and Cu (which metals are immiscible
nder equilibrium conditions).The roughness of the interfaces has a similar effect as the
nitial roughness of the substrate. The impact of the substrateoughness for Co–Cu/Cu multilayers was demonstrated in aecent work [47], which evidenced that the magnetic satura-ion takes place at a significantly lower magnetic field when theubstrate has a low roughness. The common conclusion of thearlier [47] and the current work is that the presence of satel-
ite peaks in the X-ray diffractograms is a good indication ofhe layer quality, originating from either a low substrate rough-ess, the favourable levelling effect of the electrolyte or the largehickness of the Cu layer.aj
f
u layer thickness of the multilayers. For all samples, d(CoNiCu) = 2.3 nm and= 0.2.
The results of the quantitative analysis of the magnetoresis-ance behaviour as a function of the Cu layer thickness and theeposition current density of the magnetic layer are depictedn Fig. 8. It can be seen that the magnetoresistance contribu-ion MRFM related to the electron travel pathways “FM region1) → non-magnetic region → FM region (2)” increases with theopper layer thickness and no oscillating behaviour can be estab-ished. Similar dependence on d(Cu) was obtained for Co–Cu/Cu
ultilayers [50,51].The magnetoresistance contribution MRSPM related to
he electron travel pathways “FM region → non-magneticegion → SPM region” (or in the reverse order) strongly dependsn the deposition current density of the magnetic layer. Forower current densities, the MRSPM contribution is 2.5% onhe average, while it is reduced to about 1% as the cur-ent density is tripled. Since the apparent cluster size ofhe SPM regions does not show a systematic dependencen d(Cu), it is concluded that the shape of the magnetore-istance curves is determined by the relative weight of the
RSPM and MRFM contributions. The SPM cluster size showsfairly large scatter for both deposition current densities
pplied, being about (1800 ± 800) μ and (3000 ± 600) μ for
B B(CoNiCu) = −15 mA cm−2 and −45 mA cm−2, respectively.The origin of the SPM contribution can be attributed to theormation of magnetically isolated islands within the magnetic
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ayers or to the intermixing of components at the layer interfaces49]. The increase in the cluster size and the decrease of the SPMontribution to GMR with decreasing Cu content of the magneticayers is in agreement with the island model.
It has to be noted that the typical SPM moment is smallery about two orders of magnitude than the magnetic momentf a single magnetic layer within a crystallite. This observationupports the view that the occurrence of the SPM regions iselated to local concentration fluctuations within the grains.
. Summary
Co–Ni–Cu alloys and Co–Ni–Cu/Cu multilayers have beeneposited from a sulfate/sulfamate bath. The nature of the code-osition of Co and Ni is anomalous and not affected by theimultaneous codeposition of Cu at the concentration level inves-igated. The average composition of the magnetic layers provedo be fairly independent of the deposition current density at con-tant magnetic layer thickness. However, the composition of theagnetic layers tends to approach the composition of d.c. plated
lloys as the layer thickness increases. This fact indicates thathere must be a concentration gradient within the magnetic layerlong the growth direction.
The structure of the d.c. plated alloys and the multilayersere very different. While the d.c. plated samples contained a
mall amount of hcp face besides the fcc phase, the multilayersonsisted of the fcc phase almost exclusively. The d.c. platedlloys showed various textures depending on the sample com-osition, but the multilayers all exhibited a strong (1 1 1) texturendependently of the composition and the deposition conditions.
The multilayer samples showed giant magnetoresistance. Theecomposition of the GMR curves into ferromagnetic and super-aramagnetic contributions showed that the higher Cu content ofhe magnetic layers is favourable for the formation of the SPMegions. This finding was in good agreement with the earlierbservation on Co–Cu/Cu multilayers [51]. Another importantonclusion was that the SPM contribution is smaller in multi-ayer samples with better structural quality, again in accord withur recent findings on electrodeposited Co–Cu/Cu multilayers47].
cknowledgements
The support of the research fund of the Research Institute forolid State Physics and Optics (#3322) and that of the Hungariancientific Research Fund (OTKA, grant K-60821) are gratefullycknowledged.
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