Effects of organic additives on electroplated soft magnetic CoFeCr films

Applied Surface Science 225 (2004) 59–71

Effects of organic additives on electroplated soft magnetic CoFeCr films F. Lallemanda,*, D. Comteb,1, L. Ricqa, P. Renauxc,2, J. Pagettia, C. Dieppedalec,2, P. Gaudc,2 a

Laboratoire de Chimie des mate´riaux et Interfaces, poˆle Corrosion, Traitements de Surface et Syste`mes e´lectrochimiques, UFR-ST, 16 Route de Gray, 25030 Besanc¸on Cedex, France b Alditech, CEA-LETI, 17 rue des Martyrs, 38054 Grenoble Cedex 9, France c De´partement des Technologies Silicium, STMS, LETI, CEA/Grenoble, 17 rue des Martyrs, 38054 Grenoble Cedex 9, France Received 16 June 2003; received in revised form 23 September 2003; accepted 23 September 2003

Abstract Soft magnetic CoFeCr films produced with high saturation magnetic flux density, Bs ¼ 1:7 T, low coercivity, Hc ¼ 0:6 Oe, and high film resistivity, r ¼ 40 mO cm, are potentially useful in high density magnetic recording head. The electrodeposition of CoFeCr ternary alloy is investigated in the presence of two sulfur containing organic additives, saccharin (SAC) and o-toluene sulfonamide (oTOL). The results demonstrated that the CoFeCr films produced with SAC or oTOL have the same structural and magnetic properties. However, a corrosion behavior study shows that the CoFeCr deposits prepared with oTOL are very attractive for their stability in corrosive conditions. This additive (oTOL) can be considered as a good candidate for electroplated soft magnetic CoFeCr films. # 2003 Elsevier B.V. All rights reserved. Keywords: Organic additives; Anti-corrosion properties; Electrodeposition; CoFeCr films; Magnetic properties

1. Introduction The trend of accelerated increase in magnetic recording density requires a high-performance soft magnetic material. The conventional Permalloy (Ni80Fe20) has been widely used for essential parts of magnetic recording heads and microelectromecha* Corresponding author. Present address: Laboratoire de Chimie des Materiaux et Interfaces, University ST de Franche Comt, Pole Corrosion, 16 route de gray, 25000 Besancon Cedex, France. Tel.: þ33-3-81-66-20-30; fax: þ33-3-81-66-20-33. E-mail addresses: [email protected] (F. Lallemand), [email protected] (D. Comte). 1 Tel.: þ33-4-38-78-57-14; fax: þ33-4-38-78-58-95. 2 Tel.: þ33-4-38-78-43-04; fax: þ33-4-38-78-94-14.

nical systems (MEMS) since 1979 [1]. In spite of a good corrosion resistance, this film has an insufficient saturation magnetic flux density (Bs ¼ 1 T). It was then necessary to develop a new electroplated soft magnetic film with a high saturation magnetic flux density value (Bs > 1:6 T) and a low coercivity value (Hc < 2 Oe). Liao [2] prepared a binary alloy Co90Fe10 with a magnetostriction near zero, a high permeability and Bs ¼ 1:9 T. However, the coercivity was too high, Hc ¼ 4 Oe. Chang and Andricacos [3] have demonstrated that there is a relation between the Hc and the iron content. Easy-axis coercive field exhibits a minimum 3 Oe at the composition of 11.5% Fe. This observation can be explained by grain size and phase structure. The microstructural changes that occur have

0169-4332/$ – see front matter # 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2003.09.033

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F. Lallemand et al. / Applied Surface Science 225 (2004) 59–71

a predictable effect on the magnetic properties. The problem of this CoFe magnetic material was the high Hc and the insufficient corrosion resistance [4]. This is the reason why, the electrodeposition of CoFeCr alloy has been considered to meet the challenges of improved corrosion resistance with superior soft magnetic properties. This work is a continuation of our previous work [5], which reported the effect of the structure of organic additives in the electrochemical preparation of CoFe deposits. It was reported that even if the molecular structure of the phthalimide was similar to that of saccharin, the behavior during the electrodeposition of the CoFe alloy was different. It was concluded that the presence of organic additives containing the sulfonamide group (R–SO2–NH2) seems to be responsible for the best anti-corrosion properties of the CoFe binary alloy. In this paper, the electrodeposition conditions were not discussed because they have been studied by Maire [6] and Comte [7]. They have shown that few percents of chromium can be added in binary CoFe alloy. With more than 2.5 wt.% of chromium the resistivity of the alloy can reach more than 50 mO cm, but chromium decreases the saturation magnetization of the CoFeCr alloy. So, the optimum composition integrates 2 wt.% of chromium in the alloy to obtain the CoFe10–11Cr2.0 optimized material. The present paper is undertaken to investigate, in the light of our previous study, the effect of a new additive o-toluene sulfonamide (oTOL) on the polarization behaviour of cobalt, iron and chromium, deposit morphology, crystallographic orientation, electric and magnetic properties of CoFeCr films. The results are compared with films obtained from plating bath containing saccharin (SAC), which is a common additive used in industries. Indeed, SAC is known as a brightening and anti stress agent [8]. The additive oTOL is considered because of the sulfonamide group present in its molecular structure. The oTOL structure is similar to SAC one. The corrosion

properties of CoFeCr films containing S inclusion electrodeposited from the baths containing SAC and oTOL are also studied to provide a comparison.

2. Experimental 2.1. CoFeCr electrodeposition CoFeCr thin films have been electrodeposited with an approximate thickness of 1 mm on blanket or patterned 4 in. silicon wafers with a sputtered Permalloy (80 nm) interlayer. In addition, a sputtered chromium (20 nm) adhesion layer have been deposited between Si and Ni–Fe. The plating setup included a paddle plating-cell system which was used for the agitation during the electrodeposition, an electrolyte recycling system, pH controllers and a titanium–platinum bar as a counter electrode. To prepare CoFeCr alloy, electroplating of chromium requires high overpotentials. The use of pulsed current with high cathodic density for a short time, followed by a certain off time, allows the codeposition of cobalt, iron and chromium. The galvanostat is controlled by a computer that can generate multiple combinations of signals envisaged by a software (Oscar–Solyred). The electroplating conditions are summarized in Table 1. The operating parameters have been optimised by plotting the potential–time transient curves [6]. The plating electrolyte have been prepared from reagent grade chemicals using ultrapure water (Millipore MilliQ system) and consisted of H3BO3 4  101 mol l1, NaCl 5  101 mol l1, CoCl2 2:5  101 mol l1, FeCl2 2:5  102 mol l1, CrCl3 4  104 mol l1 and organic additives whose structures and symbols are detailled in Fig. 1. These compound were added to the electrolyte with concentrations from 2:5  103 mol l1 to 2  102 mol l1. The pH was adjusted by the addition of an aqueous

Table 1 Electrodeposition parameters Temperature

Anode

Current mode

Current parameters

30  1 8C

Titanium-platinum

Pulsed

10 < current density < 285 mA cm2 0.1 < pulse time < 1000 ms 0.25 < frequency < 250 Hz

F. Lallemand et al. / Applied Surface Science 225 (2004) 59–71 O

CH3 NH

NH 2

S O

S O

O

O

(B)

(A)

Fig. 1. Chemical structure of (A) saccharin (SAC) and (B) o-Toluenesulflonamide (oTOL).

HCl solution until a value of 3:00  0:05 was reached. The bulk pH was measured before and after every deposition. In the event of a pH change, the value was corrected by adding chloride acid. 2.2. Electrochemical studies 2.2.1. Voltammetry analysis The experimental set-up for voltammetric measurements is a 200 ml three-electrode cell consisting of a platinum counter electrode, a mercury sulfate electrode (SSE) immersed in a separated compartment filled with a K2SO4 solution and a silicon wafer plate working electrode. The potential–current curves are recorded in the potential range between 600 and 2000 mV/SSE at a scanning rate of 0.5 mV s1 using a potentiostat (PGP-301 (Tacussel, France)) connected to a computer. The Voltamaster software compensates the ohmic drop between working and reference electrodes using electrochemical impedance spectroscopy. The investigated electrolyte compositions are summarized in Table 2. The chloride ions concentration is kept constant in all electrolytes. 2.2.2. Corrosion analysis The corrosion properties are determined by anodic polarization in stired borate medium composed of Table 2 Composition of the various studied bath without additive (mol l1) N8 Bath Bath Bath Bath Bath

A B C D E

H3BO3

NaCl

CoCl2

FeCl2

CrCl3

0.40 0.40 0.40 0.40 0.40

73.05 42.61 42.61 30.00 30.00

0.0 0.25 0.0 0.0 0.25

0.0 0.0 0.25 0.0 0.025

0.0 0.0 0.0 0.25 0.00375

61

boric acid 3  101 mol l1, sodium borate 7:5 103 mol l1 and potassium chloride 102 mol l1. Many papers refer to the use of poorly aggressive borate electrolytes in the corrosion study of electronic and magnetic materials [9,10]. The polarization curves are recorded in the potential range between the corrosion potential and 2500 mV/SSE at a scanning rate of 1 mV s1 with a potentiostat (PGP 301 (Tacussel, France)) under computer control operated (Voltamaster 4 programmer). The electrolyte used for studies was desaered with N2 bubbling for 60 min prior to use and maintained over the solutions during all the experiments in order to inhibit the absorption of oxygen. The CoFeCr sample (1 cm2) used as working electrode was beforehand cleaned with ethanol and rinsed with milli-Q water. Then, the electrode was cleaned by imposing a cathodic current of 10 mA cm2 to obtain reproducible results. The following parameters are extracted from the anodic polarization curves: (i) Jcrit and Ecrit the critical passivation current density and potential respectively, (ii) Epit the pitting potential. Four independent anodic polarization curves are recorded for each electrolyte-material combination. Only, one typical curve is reported in each case. The estimated errors are reported in Tables 3 and 4. 2.3. Deposit examination The examination of the preferential orientation of the crystals are performed by X-ray diffraction (XRD) (Philips MPD), using Co Ka radiation. The different phase and texture are be determined with the JCPDS card of cobalt. The roughness is evaluated by profilometry with a white-light phase shifting method. This method allows unambiguous profilometry with a nanometric resolution [11]. The experimental set-up is described in details in literature [12]. The working surface was 675:8  72:2 mm2. We can characterize the CoFeCr surface by the Ra and Rt values. The Ra value is the arithmetic means roughness and the Rt value is the variation between the highest and the lowest point of the profile of roughness on the entire working surface. The curvature of the silicon substrate is evaluated before and after film deposition by profilometry (K200 Si). Then, the residual stress of the CoFeCr film is

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Table 3 Quantification of CoFeCr corrosion test in presence of SAC varying content SAC concentration (mol l1)

Potential of the activity peak, Ecrit (mV)

Current density of the activity peak, Jcrit (mA cm2)

Charge determination by activity peak integration (mC cm2) (E free to 750)

Passivation range  20 (mV)

Pitting potential Epit (mV)

0.019 0.015 0.01 0.005 0.0025

840 840 840 840 840

90–110 60–100 40–100 35–45 3545

10.5 6–9 4–9 3–5 3–5

750; 750; 750; 750; 750;

0 0 0 0 0

    

10 10 10 10 10

0 0 0 0 0

    

20 20 20 20 20

Table 4 Quantification of CoFeCr corrosion test in presence of oTOL varying content oTOL concentration (mol l1)

Potential of the activity peak, Ecrit (mV)

Current density of the activity peak, Jcrit (mA cm2)

Charges determination by activity peak integration (mC cm2) (E free to 750)

Passivation field  20 (mV)

Pitting potential Epit (mV)

0.005 0.0025

840  10 840  10

75–90 18–22

7–9 1–3

750; 0 750; 0

0  20 0  20

calculated with the Stoney equation [13,14]: e2  w 4 Esub  2sub s¼  3 1  nsub D  edep

(1)

where s is the residual stresses (Pa); Esub and nsub are the elastic constants of the substrate; Esub ðSiÞ ¼ 150 GPa at nsub ðSiÞ ¼ 0:28; esub is the thickness of the substrate (m); D is the measured diameter on the substrate (m); edep is the thickness of the deposit; w ¼ D2 =8R with R is the radius of curvature (m). After each electrodeposition, the ternary alloy was studied by energy dispersive spectroscopy EDS (Idfix Fondis). The estimated composition of alloys are an average on various points (the confidence intervals is 5 wt.%). The sulfur content of the alloy was determined by the evolution method given by SIMS (CAMAECA IMS 5F). Samples with known sulfur content is used as reference (the confidence intervals is 10 wt.%). We have developed a specific annealing treatment of the electrodeposited alloy, in the presence of an inplane magnetic field parallel to the deposition magnetic field, so in the easy axis. This annealing is carried out during a few hours, at a temperature below 300 8C. After annealing, CoFeCr film exhibits very good soft magnetic properties and a high saturation magnetization. The Bs values were measured by vibrating sample magnetometer (VSM) (EG&G park model

4500, Soft Lake Shore Cryogenics model 7300), which can apply a field of 20 kOe. However, Hc values are more precisely measured with a L.D.J. model BH 10 000 fluxmeter apparatus. The resistivity r is measured by a four-point probe method using Veeco FPP5000.

3. Experimental results 3.1. Voltametry analysis Voltammetry is carried out to characterize the organic additive influence on the electrodeposition process. The Fig. 2 shows the steady-state polarization curve for the bath A. We can distinguish three stages. Two weak reduction stages with an approximate current density of 5 mA cm2 at 600 mV/SSE and 50 mA cm2 at 900 mV/SSE are observed. They are due to the presence of dissolved residual oxygen. Two reactions can describe these two stages: O2 þ 4Hþ þ 4e ! 2H2 O þ



O2 þ 2H þ 2e ! 2H2 O2

(2) (3)

The third stage for potentials below 1150 mV/SSE represents the hydrogen release at the electrode. 2Hþ þ 2e ! H2

(4)

F. Lallemand et al. / Applied Surface Science 225 (2004) 59–71

63

Fig. 2. Polarization curves of bath A with SAC (&), oTOL (~) and without additive (*).

After the addition of 2:5  103 mol l1 SAC or oTOL in the bath A, we can observe that the presence of SAC shifts the potential of hydrogen evolution reaction and consequently decreases the kinetic of the hydrogen release. On the contrary, the presence of oTOL does not significantly influence this reaction (Fig. 2). This hydrogen decrease in the presence of SAC was already observed by Tabakovic and coworkers for CoFeNi bath [15]. Cobalt (II) is added to the bath A (bath B). To study the influence of SAC on the reduction of the metal specie, its content is changed. The Fig. 3 shows the obtained results. The oxygen stage and the hydrogen

release are at the same potential as for previous curves. The cobalt deposition may be considered as the result of two-step discharge of the Co (II) species via an intermediate adion CoOHads [16–19]. Co2þ þ OH þ e ! CoOHads þ



CoOHads þ H þ e ! Cos þ H2 O

(5) (6)

When the SAC concentration increases, the polarization curves shift towards more negative values. Slower electrolytic deposition kinetic of cobalt are observed for stronger concentration in SAC. In the presence of oTOL (Fig. 4), the same phenomenon is E / (V vs. SSE)

-2,0

-1,9

-1,8

-1,7

-1,6

-1,5

-1,4

-1,3

-1,2

-1,1

-1,0

0 -5 -10

J (mA cm-2)

-15 -20 -25 -30 -35 -40 -45 -50

Fig. 3. Polarization curves of bath B with SAC 2:5  103 (&), 5  103 (~), 102 (*), 1:5  102 (&), 2  102 mol l1 (~) and without additive (*).

64

F. Lallemand et al. / Applied Surface Science 225 (2004) 59–71 E / (V vs. SSE) -2,0 -1,9 -1,8 -1,7 -1,6 -1,5 -1,4 -1,3 -1,2 -1,1 -1,0 -0,9 -0,8 -0,7 -0,6 0 -5 -10

-2

J (mA cm )

-15 -20 -25 -30 -35 -40 -45 -50

Fig. 4. Polarization curves of bath B with oTOL 2:5  103 (&), 5  103 mol l1 (~) and without additive (*).

observed, the oTOL concentration does not influence the shift on the contrary to SAC’s case. Iron (II) is added to basic solution A (bath C). The curves for cathodic polarization are shown in Fig. 5. The reduction stages of oxygen and hydrogen are still present at the same potentials. On the other hand, a cathodic current density increase can be identically localized at 1300 mV/SSE. This stage corresponds to the reduction of iron (II) ions in twostep discharge: Fe2þ þ OH þ e ! FeOHads þ

(7)



FeOHads þ H þ e ! Fes þ H2 O

(8)

When SAC is added in the bath C, we can note for iron deposition kinetic the same effect as for cobalt. Indeed, the reduction potential of iron species is shifted towards negative values when the SAC concentration increases. On the other hand, this effect is not detected in the presence of oTOL. This organic compound does not affect the iron reduction kinetic. Similar experiments are performed with the chromium metal ions in the presence of SAC and oTOL (Fig. 6). Chromium in solution presents an oxidation level of þ3. It can be reduced in two steps (Cr2þ þ 2e ! Cr and Cr3þ þ 3e ! Cr). In our case, we cannot differentiate these two steps. The polarization

E / (V vs. SSE) -1,5 0

-1,4

-1,3

-1,2

-1,1

-1,0

J (mA cm-2)

-1 -2 -3 -4 -5 -6

Fig. 5. Polarization curves of bath C with SAC 2:5  103 (&), 5  103 (~), 102 (*), 1:5  102 (&), 2  102 mol l1 (~) and without additive (*).

F. Lallemand et al. / Applied Surface Science 225 (2004) 59–71

65

E / (V vs. SSE) -2,0 0

-1,8

-1,6

-1,4

-1,2

-1,0

-0,8

-0,6

-5 -10

-2

J (mA cm )

-15 -20 -25 -30 -35 -40 -45

Fig. 6. Polarization curves of bath D with SAC (&), oTOL (~) and without additive (*).

curves with or without additives do not exhibit significant changes, whatever the organic additives concentration used (Fig. 6). As a consequence, the organic compounds do not influence the chromium deposition kinetic. The difference between these curves seems to be due to the difficulty to reduce chromium metal ions. Indeed, it is necessary to use a high current density. This is the reason why the pulsed current is used to elaborate the CoFeCr deposits.

peaks present on this figure are due to the fcc aCo phase. We can note that the phase aFe is not distinguished even if the contents of iron are higher than 10%. We can note a asymmetry of the principal peak Table 5 Mass percentage of sulfur in electroplated CoFeCr film with SAC and oTOL varying content % S  0.02

Additive concentration 3

3.2. Composition and structural analysis of CoFeCr films When we evaluate the alloy compositions by EDS, we can note that the addition of various amounts of SAC or oTOL in the plating bath do not change the elemental composition of CoFeCr alloy. We obtain a CoFe11–10Cr1.7–1.6 film. The additives that contain the sulfur element are used as efficient stress reducing agent. We determinate the S content in the deposits in order to analyze its influence. The results are reported in the Table 5. The S value stays constant. Indeed, the confidence intervals do not allow to differentiate the different values. Consequently, SAC or oTOL enables the introduction of the same sulfur content about 0:14  0:03 wt.% of the CoFeCr film. The Fig. 7 shows the crystalline structures of CoFeCr films electrodeposited from baths with SAC obtained by X-ray diffraction. Maire et al. [6] have already observed the structural analysis of CoFeCr deposits with X-ray diffraction and TEM. The various

SAC 2.5  10 SAC 5  103 SAC 10  103 SAC 15  103 SAC 20  103 oTOL 2.5  103 oTOL 5  103

0.14 0.16 0.13 0.13 0.12 0.12 0.10

αCo (111)

Si (400)

αCo (222)

αCo (200)

20

30

40

50

60

αCo (311)

70

80

90

100

2θ (˚) Fig. 7. X-ray diffraction pattern of CoFeCr alloys produced with 2:5  103 mol l1 saccharin using Co Ka radiation.

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F. Lallemand et al. / Applied Surface Science 225 (2004) 59–71

aCo(1 1 1). A digital processing of the signal indicates a convolution of two peaks, one located at 43.98 (aCo) and the other 44.68 (aFe). Thus, the peaks aCo and aFe are shifted towards the small angles which means that the cristalline lattice parameters increases. Maire propose that chromium is probably inserted in the fcc phase aCo and the bcc phase aFe [6]. The objective of this paper is not to determine the structure but to analyze if organic additives change the orientation of crystals in the deposit. When we study the different results, we can note that the crystalline structures are similar for all CoFeCr deposits prepared with SAC or oTOL. The Fig. 7 shows one of the graphs. The organic compounds do not affect the fcc structures. In order to evaluate if the studied organic additive exhibit any levelling effect, the roughness of the CoFeCr deposits is investigated. The Fig. 8 represents the roughness of the CoFeCr alloy deposited with 2:5  103 mol l1 SAC. We cannot distinguish significant effect of the varying organic additive amount. We obtain, for all deposits electroplated with SAC or oTOL, a Ra value of 1:0  0:2 nm and Rt value of 11:0  1 nm. 3.3. Stress in CoFeCr deposits During the deposition of thin films residual elastic stress almost always appears. This residual stress can come from intrinsic stress in the film and from

interfacial stress between the deposit and the plating seed layer. The coatings electrodeposited without organic additives peel-off during plating. The CoFeCr thin film falls apart of the silicon wafer. This is the reason why it is impossible to provide a value of the CoFeCr stress produced without additives. To have low stress CoFeCr alloy deposit, a stress reducing agent is necessary. SAC was known for cobalt– iron deposit as the most efficient stress reducing agent [20,21]. The two additives are tested individually to evaluate their effectiveness for reducing stress. The results are represented in the Fig. 9. We can note that there is no relation between stress and organic additive concentration. The minimum stress is obtained with 5  103 mol l1 of SAC and oTOL. 3.4. Electric and magnetic properties The saturation magnetic flux density (Bs) and the resistivity (r) of the Permalloy is insufficient. To increase Bs and r, a ternary alloy composed with cobalt and iron is evaluated. After the electrodeposition, the CoFeCr films are considered as magnetically isotropic. These deposits are also annealed in order to induce uniaxial anisotropy. After the annealing treatment, the magnetic properties are analyzed. Maire has shown that the chromium and iron contents influences the saturation magnetization Bs. To keep a Bs higher than 1.7 T with chromium contents of about 2%, the

Fig. 8. Roughness of CoFeCr alloy electroplated with 2:5  103 mol l1 saccharin.

F. Lallemand et al. / Applied Surface Science 225 (2004) 59–71

67

450 400

Stress / (Mpa)

350

SAC

300 oTOL

250 200 150 100 50 0 0

5

10

15 -3

20

25

-1

Concentration / (10 mol L )

Fig. 9. Effect of SAC () and oTOL (*) content on average stress for CoFeCr deposits.

content of iron must exceed 10.0%. This is the reason why we elaborate CoFeCr deposits with a iron content included between 10.5 and 11 wt.%. The hysteresis cycle for a CoFeCr prepared with SAC is represented on the Fig. 10. The values of Bs, coercitive field Hc and anisotropy field Hk determinated from the hysteresis cycle are indentical for the CoFeCr films produced with SAC or oTOL, whatever the organic additives concentration used in the plating electrolyte. We obtain a CoFeCr deposit with a higher saturation magnetic flux density, 1:7  0:1 T, and a lower coercitivity, 0:6  0:1 Oe, than the ones that can be obtained with the concentional permalloy film. Maire shown that the addition of a few percentages of chromium to the binary CoFe alloy increases

strongly the resistivity of the deposits. In our case, the addition of 1.7% of chromium increases the resistivity of CoFe from 15 to 40 mO cm [22]. The influence of SAC and oTOL on the resistivity of CoFeCr films is represented in the Fig. 11. We can note that the two additives SAC and oTOL influence in the same way the resistivity. On the other hand, we can say that the resistivity value increases slightly up to a value of 45 mO cm when the concentration of SAC increases. 3.5. Corrosion properties: anodic polarization This study consists in comparing the CoFeCr alloy prepared with varying content of SAC and oTOL in the

20 000 15 000

Bs / (Gauss)

10 000 5 000

Hk = 23 Oe

0 -5 000

-40

-30

-20

-10

0

10

Hc = 0,6 Oe

20

30

40

-10 000 -15 000 -20 000

Appied field / (Oe) Fig. 10. Hyteresis cycle of annealed CoFeCr films prouced with 102 mol l1 of SAC easy axis (*), and hard axis (&).

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60

Resistivity / (µΩ Ω cm)

50

oTOL

40

SAC 30 20 10 0 0

5

10

15 -3

20

25

-1

Concentration / (10 mol L ) Fig. 11. Effect of SAC () and oTOL (*) content on CoFeCr deposit resistivity.

plating bath. The main objective is to obtain an alloy having the best ability to passivation in order to use its in magnetic heads. The trends of the chromium presence to increase the free potential and to decrease current density Jcrit are known [22]. To understand the influence of additives containing sulfur, the material resistance corrosion is studied with CoFeCr alloys, that have similar composition. Anodic polarization curves of the CoFeCr films electroplated with varying SAC and oTOL content are shown in Figs. 12 and 13, respectively. These curves show an intense dissolution

of the material, which extends between 920 and 750 mV/SSE. The passivation action of the CoFeCr electrodeposit in the relevant environment is observed on the contrary to CoFeNi’s and NiFe’s cases [22,23]. We can calculate the charge required for the formation of passive film by the integration of this current peak (Tables 3 and 4). The critical passivation current density was lower with oTOL than with SAC for the concentration of 2:5  103 mol l1. An opposite behavior is observed for the 5  103 mol l1 concentration. For higher concentration than 0.01 mol l1,

0.14

J (mA cm-2)

0.12 0.10 0.08 0.06 0.04 0.02 0.00 -1000

-800

-600

-400

-200

0

200

E / (mV vs SSE) Fig. 12. Anodic polarization curves of electroplated CoFeCr films with varying SAC content, 2:5  103 (&), 5  103 (~), 102 (*), 1:5  102 (&), 2  102 mol l1 (~).

F. Lallemand et al. / Applied Surface Science 225 (2004) 59–71

69

0.14

J (mA cm-2)

0.12 0.10 0.08 0.06 0.04 0.02 0.00 -1000 -900 -800 -700 -600 -500 -400 -300 -200 -100

0

100

200

300

E / (mV vs SSE) Fig. 13. Anodic polarization curves of electroplated CoFeCr films with varying oTOL content, 2:5  103 (&), 5  103 mol l1 (~).

the values of charge increase when SAC content increases. The range between 750 and 0 mV/SSE corresponds to the passive range. The rupture potential Epit of the CoFeCr alloy in this solution is near 0 mV. The oxygen release appears at potentials higher than þ600 mV/SSE. The pitting-corrosion potential for a CoFeCr film is not impacted by the varying content of SAC and oTOL, whereas it was in the case of CoNiFe films [15]. In addition we can observe that these curves are characteristic of a cavernous corrosion. We can conclude that the bigger concentration of organic additives in the plating bath, the more dissolution of CoFeCr film is observed to passivate the material. Consequently, the smaller concentration in organic additives is prefered to prepare CoFeCr deposit.

4. Discussion The electrodeposition of CoFeCr alloy by pulsed current requires the introduction of small amounts of organic substances into the plating bath to decrease stress. Otherwise, CoFeCr coatings are unsable because they simply peel-off during plating. However, the small amounts of organic additives can also have harmful effects on the deposit. The mechanism of action of additives in electrodeposition is then studied

focusing on their ability to modify the crystallization of the metal. Indeed, one of the primary areas of interest in practical plating is the use of the additives as brighteners. However, additives affect the rate of the other steps in the electrodeposition process. The presence of the additives increases the polarization of the cathode, i.e. decreases the current density obtained for a given electrode potential. First, we have observed that SAC influences the hydrogen release. The mechanism of hydrogen release is Hþ þ e ! Hads Hads ! H2

(9) (10)

SAC is supposed to block the association of the electrolytically generated hydrogen atoms (Eq. (9)), allowing to increase the concentration of Hads and thus increasing the rate of adsorption (Eq. (10)) [24]. However, changing the type of additive is expected not to alter the slope of this curve but this is not true in the presence of oTOL (Fig. 2), indicating that there is another factor besides the ability to block the electrode. We can consider an additional factor as the charge density on the electrode surface introduced by the adsorbed additive. Thus, the higher the amount of negative charge introduced per unit area of the electrode the faster, the adsorption rate [24]. It was known that the changes in the amount of hydrogen cause changes in the rate of metal deposition and in the

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crystallization of the metal. Indeed, hydrogen can be a factor in the formation of pits and in the prevention of leveling. Porosity of deposits is most commonly attributed to the presence of dirt on the electrode surface. But one can easily see that part of the dirt is hydrogen bubbles. Plating under reduced pressure, that would decrease the amount of adsorbed hydrogen, produces a denser and less porous deposit. In our case, we have not observed a different morphology of the CoFeCr deposits produced with SAC or oTOL in the plating bath. We have observed that SAC can influence the deposition process of cobalt, iron contrary to oTOL that has only an impact on the cobalt reduction. This may be explained by the strong adsorption of the additives on the surface of the electrode. In addition, the cathode polarization increases with the SAC concentration in the electrolyte. We can also conclude that the extent of the cathodic polarization dependes on the degree of adsorption of the additive at the electrode surface. Osaka et al. [25] showed that SAC was physically and reversibly adsorbed on the gold electrode and Moskute et al. proposed that the SAC was adsorbed at the electrode surface by carbonyl and sulfo groups [26]. Even if the molecular structures of additives are close, oTOL structure does not contain the carbonyl group. We can then suppose that oTOL is adsorbed at the electrode surface by only the sulfonamide group. In addition, we have observed that the anti corrosion properties are sensitive to the additives content. Comparison of the curves for the films prepared with SAC or oTOL shows that the anodic current increases with the additives concentration. The crystalline structure, the composition and the S inclusion are similar in the presence of SAC or oTOL. The anodic current increase could be explained by microstructural effect (grain and grain boundary). To affirm this hypothesis, a further study would be required to understand more deeply the anodic current increase. However, we can conclude that the smaller concentration in organic additives is adapted for a good corrosion resistance. Concerning the pitting-corrosion potential of a CoFeCr film, we can note that the various contents of SAC and oTOL do not influence it. Concerning the sulfur inclusion in the deposit, we have observed that the sulfur content remains approximately constant. The additives concentration increase in the electrolysis bath do not increase the sulfur content. Similar effects have been analyzed by Osaka

[25] in CoNiFe films prepared with SAC and by Edwards in Ni deposits [27]. They assume a two step process of sulfur inclusion in CoNiFe. The first step is the reversible adsorption of saccharin molecules, which is rate-determining, and the second step is the breakage of the S–C bond accompanied by the reduction of S to S2. Indeed, the action of additives can be monitored by a cathodic reaction and the main products of cathodic reaction of saccharin have been studied by Mockute [26]. The cathodic reaction of SAC products benzamide, oTOL and sulfur incorporated in the electrodeposits. As a consequence, we can suppose the same mechanism for oTOL. To confirm this assumption, the main products of cathodic reaction of oTOL have to be studied.

5. Conclusion Even if the materials used for writer head in industry at the present time have Bs value close to 2.4 T, the obtained 1.7 T CoFeCr alloy can be used for Bottom Shield or Shared Pole material. The CoFeCr can be used thanks to high Bs, high resistivity and good corrosion resistance. In this paper, the choice of organic additives to elaborate the CoFeCr soft magnetic films has been investigated and in particular their electrochemical behavior and magnetic properties. We have shown that the introduction of a small amounts of SAC or oTOL in the plating bath is required to prepare the CoFeCr alloy by pulsed current. Indeed, without additives the deposits peel-off during the plating. These two additives can be considered as stress-revealing agent. With the concentration used, the addition of organic additives do not cause significant changes of the elemental CoFeCr composition, the magnetic properties, the crystalline structure and the sulfur inclusion content. We obtain a CoFe11–10Cr1.7–1.6 film with Bs ¼ 1:7 T, Hc ¼ 0:6 Oe and approximately 40 mO cm. However, in the presence of SAC, the polarization curves of cobalt and iron are shifted toward more negative values whereas in the presence of oTOL only the cobalt kinetic is impacted. The polarization curves of chromium is not changed whatever the additives used. The cathodic polarization may be explained by the strong adsorption of the additives on the surface of the electrode. SAC is adsorbed at the surface electrode physically and reversibly by the carbonyl

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and sulfonamide group. OTOL includes sulfur in the deposits and the polarization curves of cobalt reduction is shifted towards more negative values. Consequently, we can suppose an adsorption of oTOL at the surface electrode by only the sulfonamide group. This consumption has to be verified by a study of the main products of cathodic reaction of oTOL. Moreover, the presence of both organic additives increases the dissolution of CoFeCr to form the passive layer. The anodic current increase could be explained by the effect of microstructure (grain and grain boundary). To confirm this hypothesis, a further study would be required to interpret more into the details the anodic current increase. Consequently, it has been shown that the inclusion of a small amount of into the deposits from SAC or oTOL plays a benefic role on elaborating CoFeCr magnetic films. A lower dissolution of CoFeCr was also observed in the presence of 2:5  103 mol l1 oTOL. Regarding the different experimental results and considering its anti corrosion properties, o-TOL can be integrated in the plating bath of an industrial CoFeCr electroplating deposition process. Acknowledgements The author would like to acknowledge the Microscopy Center and the optic laboratory P.M. Duffieux of Besanc¸on for its assistance. References [1] L.T. Romankiw, M.I. Croll, M. Hatzakis, IEEE Trans. Magn. 6 (1970) 597.

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