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Journal of Materials Science: Materials in Electronics, 19, 1, pp. 51-59, 2008
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Preparation and characterization of electroless deposition of Co-Fe-B
thin films produced by electroless deposition
Dadvand, N.; Jarjoura, G.; Kipouros, G. J.
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https://nrc-publications.canada.ca/eng/view/object/?id=3ced0992-bfb3-4e40-9cc9-759da69b156a https://publications-cnrc.canada.ca/fra/voir/objet/?id=3ced0992-bfb3-4e40-9cc9-759da69b156a
Preparation and characterization of Co–Fe–B thin films produced
by electroless deposition
N. Dadvand Æ G. Jarjoura Æ G. J. Kipouros
Received: 2 January 2007 / Accepted: 22 May 2007 / Published online: 10 August 2007 Springer Science+Business Media, LLC 2007
Abstract The requirement for integration into silicon IC circuitry working at GHz frequencies drives the research into new development of both materials and processes. The formation of electroless Co–Fe–B coatings was investi-gated on pure copper and 304 stainless steel substrates which were pretreated in a bath containing EDTA (20 g l1), ethylene diamine (30 g l1), PdCl2 (10 g l1) and
NaHPO2(4 g l1). Electroless deposition was performed at
348 K, for 2 h, in a bath that contained sodium tartrate (0.2 M) and sodium citrate (0.05 M) as complexing agents and the bath pH was adjusted between 6.5 and 9.5 using a concentrated solution of sodium hydroxide. The main components of the bath were a source of cobalt (II) ions, a source of iron (II) ions, DMAB as reducing agent and boron source, ammonium sulfate as buffering agent, phosphorous acid as a slow-rate reducing agent, a stabi-lizer, and an accelerator. Co–Fe–B films having a variety of composition with respect to cobalt, iron, and boron con-tents were produced by varying the experimental parameters such as pH and the concentration of bath components. The coatings were characterized in terms of composition, structure and corrosion resistance by ICP and EDS, XRD and SEM, and by electrochemical instrumen-tation, respectively. The magnetic properties of the
electroless Co–Fe–B ternary alloy deposits were measured by a DC extraction method. The results indicate that tight control of process parameters enables the tailoring of coercivity and magnetic permeability of the films, and may lead to the microfabrication of innovative and efficient devices having nanofeatures such as nanowires.
Nomenclature
a Reaction order for DMAB no units b Reaction order for Co2+no units h Diffraction angle deg.
c Reaction order for Fe2+no units d Reaction order for H+no units r Reaction order for A no units e Reaction order for L no units A Accelerator
C Concentration M or mol l1 Ea Activation energy kJ mol1g1
Ecorr Corrosion potential mV/SCE
H Applied field Oe Hc Coercivity Oe
I XRD intensity A.U.
Ic Index of crystallinity no units
k Rate constant s1 k1 Rate constant s1
L Ligand or complexing agent M Metal
M Magnetization A m1
Ms Saturation of magnetization A m1
R Gas constant = 8.314 kJ mol1 Rc Deposition rate mg h1
T Temperature C or K t Time s
N. Dadvand
Accumold, London, ON, Canada G. Jarjoura (&) G. J. Kipouros
Department of Process Engineering and Applied Science, Materials Engineering Programme, Dalhousie University, Halifax, Nova Scotia, Canada B3J 2X4
e-mail: georges.kipouros@dal.ca DOI 10.1007/s10854-007-9341-2
1 Introduction
In recent years, the fabrication of new nanostructured materials and exploration of their properties have attracted the attention of materials engineers, physicists, chemists, and biologists. The rapid advances drive the need for new materials for computer and telecommunication technolo-gies. This required integration into silicon IC circuitry working at GHz frequencies, which cannot be achieved using conventional ferrite-based field amplifying compo-nents. The main requirements for the magnetic materials suitable for high-frequency applications are high saturation magnetizationcombined with a low coercivity and a small but finite anisotropy field [1]. In addition, the material should have a high electrical resistivity to reduce eddy currents induction.
Electroless deposition has attracted interest since it constitutes a process to produce uniform deposits on sub-strates having irregular shapes and large sizes. Several soft magnetic Co alloys films, such as electroless and electro-deposited Co–Fe and some of their ternary alloys have been described [2–4]. It has been reported that the Co-B films had excellent soft magnetic properties in the as-deposited state whereas the introduction of Fe in the films had a dramatic effect on the coercivity, which decreased very sharply with the initial 0 to 10 at.% of Fe [5,6]. The magnetic properties and structures of electrodeposited Ni–Fe, Ni–Co, Co–Fe, and Co–Ni–Fe have been exten-sively investigated by many researchers; however, electroless deposited coatings of Ni–Fe (B or P), Ni–Co (B or P), Co–Fe (B or P), Co–Ni–Fe (B or P) and Co–Fe–B have been less well studied [7–11]. Yokoshima et al. [12] have used electroless Co–Fe–B for writing-head cores. They have also studied functionally graded magnetic thin films prepared by electroless deposition of Co–Fe–B. To date, no extensive investigation on the effects of experi-mental variables on composition, microstructure and deposition rate of the electroless plating of Co–Fe–B ter-nary alloys have been reported. This early work showed that eletcroless deposition could be a good processing route toward the fabrication of soft magnetic structures with properties suitable for high-frequency applications in IT industry such as inductors, chokes, sensors, core-shape transformers, ultra high radio frequency telecommunica-tions, and planar transformers.
The ultimate objective of the work reported here is to develop a scalable processing based on electroless depo-sition in order to produce amorphous Co–Fe–B nanowires intended for high frequency applications. In the present work, the results obtained on the development and opti-mization of an electroless plating process for deposition of Co–Fe–B ternary alloys are reported. The effects of the boron content on surface morphology, microstructure,
corrosion resistance and coercivity were investigated. Preparation, characterization and study of structural prop-erties of soft magnetic nanostructures (patterned media and magnetic nanowires) will be discussed in a future publication.
2 Experimental 2.1 Electroless plating
Pure copper (99.5%) and stainless steel (grade 304) were used as substrates (2.5 cm2and 1 mm thick) to determine the plating rates of Co–Fe–B and to perform character-ization studies. The pre-treatment procedure [13] for copper is described in Table1. The stainless steel sub-strates were cleaned with acetone and sand blasted with a-alumina particles (50 lm diameter) having a velocity of 340 m s1 followed by an immersion step in 50% (w/w) sodium hydroxide solution for 10 min [14].
In order to provide a catalytic surface, the copper and stainless steel substrates were given a flash palladium strike [15,16] before the electroless plating process. The current density for flash palladium strike was optimized in order to obtain an adherent palladium coating onto the substrates. The plating solution contained ethylenediamine tetraacetic acid (EDTA) (20 g l1), ethylene diamine (30 g l1), PdCl2(10 g l1) and NaHPO2 (4 g l1). The optimized
current density was 0.05 A cm2, the deposition time was 30 s and the temperature was 298 K. The electroless bath contained cobalt (II) sulfate heptahydrate (0.03–0.14 M) as a source of cobalt ions; iron (II) sulfate heptahydrate (0.005–0.022 M) as a source of iron ions; dimethylamin-oborane (DMAB, 0.085–0.68 M) as a reducing agent and boron source; sodium tartrate (0.2 M) and sodium citrate (0.05 M) as complexing agents; ammonium sulfate as a buffering agent; phosphorous acid as a slow-rate reducing agent, thallium acetate (1.06 · 104M) as stabilizer, and thiourea (1.76 · 1061.25 · 105M) as accelerator. For all electroless plating solutions, the pH was adjusted between 6.5 and 9.5 using a concentrated solution of sodium hydroxide and the operating temperature was maintained at 348± 1 K with a thermostatically controlled water bath. All the samples were plated for a period of 2 h.
2.2 Alloy characterizations
The boron contents of Co–Fe–B ternary alloy deposits were determined using Inductively Coupled Plasma (ICP). Cobalt and iron contents of the deposits were determined using Electron Dispersive Spectrometry (EDS). A Hitachi S-4700 scanning electron microscope (SEM) operating at
15 kV was used to examine the surface morphology of the coatings. The various Co–Fe–B alloys were characterized by X-ray diffraction (XRD) to identify the phases present. All specimens were examined under chromium Ka
radia-tion (k = 2.2897 A˚ ) using a Bruker Advanced XRD unit (model D8 Discover) with operating parameters of 40 kV and a scan speed of 0.6 deg min1. The samples were scanned through 2h angles of 20120. Corrosion potentials (Ecorr) were measured in a 3.5% (w/w) NaCl
solution at room temperature. All experiments were con-ducted after the steady-state corrosion potential was attained, which took up to 18 h. The exposed surface area of the working electrode was 1 cm2. Corrosion potentials were evaluated versus a saturated calomel electrode (SCE) reference electrode fitted with a Luggin capillary. Magnetic properties were measured by a Physical Property Mea-surement System (PPMS) from Quantum Design using a DC extraction method. Saturation of induction, perme-ability and coercivity of the samples were evaluated from standard hysteresis curves at room temperature (300 K) and low (2 K) temperatures for both parallel and perpen-dicular alignment of the thin films in the magnetic field.
3 Results and discussion
In the electroless Co–Fe–B plating method, the concen-tration of the complexing agents were kept constant for all experiments and the effect of pH, concentration of cobalt and iron ions, DMAB and accelerator on the deposition rate and the composition of Co–Fe–B ternary alloy deposits were studied. The complexing agents were used to prevent the metal ions from precipitating as hydroxide compounds, to suppress metal ions reducing reactions and to prevent bath decomposition through rapid deposition [17]. Therefore, the complexing agent(s) and concentra-tion(s) were chosen with respect to the metal ions being reduced in the electroless plating solution. In the present work, use of a large amount (4 times) of weaker com-plexing agent (sodium tartrate) together with a small amount of stronger complexing agent (sodium citrate) provided a reasonable deposition rate while suppressing the precipitation of iron and cobalt.
In preparation of the electroless Co–Fe–B plating solution, the complexing agents (sodium citrate and
tartrate), buffering agent (ammonium sulfate) and phos-phorous acid were added into the solution before the addition of iron (II) sulfate and cobalt (II) sulfate to prevent the precipitation of metallic ions followed by DMAB addition. Adjusting the pH of electroless plating solution into alkaline region was performed after the addition of iron (II) sulfate and cobalt (II) sulfate into the solution. Aqueous iron (II) salt solutions contain the hexaaquoiron (II), which is subject to air oxidation. The oxidation pro-cess in basic solution is more rapid than that in acid solution. Upon standing, aqueous iron (II) salt solution results in precipitated iron oxides or iron hydroxides. Therefore, in order to prevent the oxidation of iron (II) to iron (III), phosphorous acid, which is a slow-rate reducing agent, was added into the electroless plating solution before addition of iron (II) sulfate.
3.1 Effect of bath composition on deposition rate
The effect of DMAB concentration (0.085–0.68 M) on deposition rate of electroless Co–Fe–B ternary alloy deposits is shown in Fig.1. As it can be seen, the depo-sition rate (Rc) increases with an increase in the
concentration of DMAB until a plateau is reached around 0.45 M, above which the bath destabilized and at 0.5 M bath decomposition became quite evident. Therefore, a higher concentration of stabilizer (thallium acetate) was required to maintain the stability of the plating bath when the concentration of DMAB exceeds 0.5 M. However, higher concentrations of thallium acetate tend to decrease the deposition rate [18,19]. The optimum concentration of Table 1 Pretreatment procedure for copper substrates
Step Composition (% v/v) T(C) Time (s)
Pickling HCl (50) 25 120
Scale dip H2SO4(15), HNO3(20) and HCl (0.5) 25 20
Bright dip H2SO4(30), HNO3(10) and HCl (0.5) 25 40
Fig. 1 Co–Fe–B deposition rate versus DMAB (•), CoSO4(m) and
FeSO4(j) concentrations. Plating bath parameters: sodium tartrate
(0.2 M), sodium citrate (0.05 M), ammonium sulfate (0.2 M) and phosphorous acid (0.06 M). pH = 6.5 for CoSO4and FeSO4curves
DMAB was chosen at 0.45 M where it could provide the highest deposition rate without destabilizing of electroless plating bath. Regarding reaction mechanisms, DMAB has three active hydrogen atoms bonded to the boron atom (CH3)2NHBH3 and therefore, should theoretically reduce
three metallic ions (Co2+ and Fe2+) for each DMAB molecule consumed. The reduction reactions [20–22] are described by the two overall Eq. 1 and 2 where L and M represent the complexing agent and the metal ion, respec-tively. Equation 1 produces the reduced metal coating while Eq. 2 results in codeposition of the reduced metal together with its boron compound.
3 LM½ 2þþ CHð 3ÞNHBH3þ 3H2O! 3Mþ CHð 3Þ2NH þ 2 þ H3BO3þ 5Hþþ 3L ð1Þ 4 LM½ 2þþ 2 CHð 3ÞNHBH3þ 3H2O! M2Bþ 2M þ 2 CHð 3Þ2NHþ2 þ H3BO3 þ 6Hþþ 0:5H2þ 4L ð2Þ
With progress of the reduction reaction, the reducing agent (DMAB), cobalt and iron ions are consumed and the hydrogen ion concentration is increased. The progress of reduction reaction is prevented by hydrogen ions adhered on the surface. Therefore, the agitation is important in order to help the diffusion of hydrogen ions from the surface into the bulk solution [20]. Figure1shows also the effect of cobalt (II) sulfate concentration (0.03–0.14 M) on the Co–Fe–B deposition rate, Rc. As it can be seen, an
increase in cobalt sulfate concentration of the bath results in an initial increase in deposition rate. Beyond the con-centration of 0.06 M, the deposition rate begins to decrease due to destabilization of the bath. The initial increase of deposition rate is expected, as an increase in cobalt sulfate concentration increases the free cobalt ions available for the reduction reaction. In the same figure, the effect of iron (II) sulfate concentration on deposition rate is also dis-played. It can be seen that Rcdecreases with an increase in
iron ions concentration in solution. Therefore, the ratio of CoSO4over FeSO4is an important variable in determining
the plating rate of electroless Co–Fe–B deposits. It is evi-dent that the deposition rate increases with the above-mentioned metallic ratio. This behavior is attributed to the fact that cobalt has a higher catalytic activity than iron.
The effect of bath pH (6.5–9.5) and thiourea concen-tration on the deposition rate of the ternary alloy deposits is depicted in Fig.2. The deposition rate was found to accelerate with an increase of pH from 6.5 to 8.2. This behavior can be attributed to the enhanced oxidation of DMAB with increase in pH. However, beyond pH = 8.2, the bath destabilizes and when operated at 9.0, the bath decomposes within a short span of time. Therefore, in the
absence of a suitable stabilizer it is difficult to operate the plating bath at high pH. In Fig.2, it can be seen that an increase in thiourea concentration in electroless bath solution increases the deposition rate. Beyond 1.3 · 105 M of thiourea concentration, the deposition rate decreases. The effect of thiourea concentration on the deposition rate of electroless nickel-phosphorus plating has been investi-gated previously. It has been reported [23] that the nickel deposition rate is dramatically reduced over the range of thiourea concentrations from 3 to 9 mg L1 and above 9 mg L1 the nickel deposition rate is flattened out and close to zero. Therefore, thiourea exhibits accelerator and inhibitor effects on plating rate below and above a specific concentration, respectively. This specific concentration is dependent on the electroless plating bath conditions and should be optimized according to the experimental parameters of electroless deposition process.
To quantify the influence of each parameter more precisely [24], the general equation of deposition rate as a function of experimental parameters was written as follows:
Rc¼ d½ M dt ¼ k1½DMABa Co2þ b Fe2þ c Hþ ½ d½ Ar½ Leexp Ea RT ð3Þ where Rcis the deposition rate, a,b,c,d,r, and e the reaction
orders, T the temperature, Eathe activation energy, ‘‘L’’ the
complexing agent and ‘‘A’’ the accelerator. Since the concentration of the complexing agents were kept constant for all experiments, the corresponding reaction order (e)is equal to zero. Equation 4 shows the possible variation of experimental parameters such as pH and the concentration of DMAB, cobalt and iron ions with deposition rate. To determine the reaction orders (a,b,c,d and r), the logarithm of deposition rates were plotted versus the logarithm of DMAB, cobalt, iron and hydrogen ion concentrations, respectively (see Eqs. 5–9). It must be mentioned that only the linear parts of the plots were considered to calculate the reaction orders. The subscripts a, b, c, d, and e indicate each set of variables, which were held constant for the corresponding partial derivative.
LogRc¼Logk1þ aLog DMAB½
þ bLog Co 2þ þ cLog Fe 2þ þ dLog H½ þ þ rLog A½ Ea 2:3RT ð4Þ a¼ dLogRc dLog DMAB½ a ð5Þ
b¼ dLogRc dLog Co 2þ ! b ð6Þ c¼ dLogRc dLog Fe 2þ ! c ð7Þ d¼ dLogRc dLog H½ þ d ð8Þ r¼ dLogRc dLog A½ e ð9Þ In summary, the reaction orders were:
Symbols a b c d r
Reaction orders 1.01 0.77 1.31 0.40 0.29
Using Eq. 10 and the different estimated reaction orders; the overall reduction reaction rate is described in Eq. 11. k¼ k1exp Ea RT ð10Þ Rc¼ d½ M dt ¼k DMAB½ 1:012Co2þ0:773 Fe2þ 1:307 Hþ ½ 0:395½ A0:287 ð11Þ As it can be seen from Eq. 11, the concentrations of cobalt sulfate, DMAB and accelerator have positive effects
on the deposition rate whereas iron sulfate and hydrogen ion concentrations have negative effects. As well, it can be seen that the reaction order for iron sulfate concentration is larger than those of other constituents. Therefore, a slight increase in iron ions concentration in plating solution can decrease the deposition rate remarkably.
The effect of the temperature on deposition rate of Co–Fe–B coatings can be seen in the insert of Fig.3. It can be seen that temperature has a strong effect upon the deposition rate. The rate of deposition is very low at temperatures below 323 K but increases rapidly with increasing temperature. However, at temperatures above 368 K the bath decomposes. Using the Arrhenius rela-tionship, the activation energy (Ea) can be obtained from
the slope of the logarithm of deposition rate vs. reciprocal temperature plot according to Eq. 12. The experimental value of the slope was evaluated through linear fitting of the curve Log Rcversus 1/T as shown in Fig. 3. Using the
slope of this graph (3358) the activation energy was calculated as Ea = 64.2 kJ mol1g1 (15.4 kcal mol1
g1). It has been reported [25] that the activation energy for electroless nickel plating lies between 13 and 23 kcal mol1g1depending on the type of bath components such as complexing agent and reducing agent. These values are comparable to the above-calculated Ea for the Co–Fe–B
deposits. dLog Rð Þc d T1 ¼ Ea 2:3R ð12Þ Fig. 2 Co–Fe–B deposition rate versus thiourea (() and H+ ()
concentrations. Plating bath parameters: sodium tartrate (0.2 M), sodium citrate (0.05 M), ammonium sulfate (0.2 M), phosphorous acid (0.06 M), iron sulfate (0.01 M), cobalt sulfate (0.06 M) and DMAB (0.26 M). pH = 6.5 for thiourea plot
Fig. 3 Arrhenius plot and deposition rate vs. temperature (insert) for bath composition: DMAB (0.26 M), sodium tartrate (0.2 M), sodium citrate (0.05 M), ammonium sulfate (0.2 M), phosphorous acid (0.06 M), cobalt sulfate (0.06 M), iron sulfate (0.01 M) and pH = 6.5
3.2 Effect of bath component concentration on coating composition
The effects of bath components concentration on the composition of Co–Fe–B ternary alloy deposits were also investigated. Figure4 shows the effect of DMAB con-centration on the composition of Co–Fe–B deposits. Concentrations of cobalt sulfate and iron sulfate were kept constant at 0.06 and 0.01 M, respectively, while pH was maintained at 5.5. It can be seen that the boron content of the deposits increases with DMAB concentration. Beyond the DMAB concentration of 0.45 M, the boron content does not change and reaches a plateau. In order to inves-tigate the effect of iron ions concentration on the boron content of the coatings, several deposits were produced while maintaining the cobalt sulfate and DMAB concen-trations constant at 0.01 and 0.26 M, respectively, whereas the pH was adjusted at 6.5.
Figure5 shows the effect of iron sulfate concentration on boron, cobalt and iron content of the deposits. It can be seen that the iron ions concentration in electroless plating solution has a very slight effect. As well, the effect of cobalt sulfate concentration on cobalt and iron contents of the Co–Fe–B deposits was studied while maintaining the iron sulfate and DMAB concentrations constant at 0.01 and 0.26 M, respectively and the pH at 6.5.
Figure6 shows the influence of cobalt ions concentra-tion in plating soluconcentra-tion on cobalt and iron contents of the deposits. For these experiments, only iron and cobalt were analyzed and the data were reported without taking the boron content into consideration. As can be seen, cobalt
content of the coatings increases almost linearly with cobalt sulfate concentration from 72% ([Co(II)] = 0.03 M) to 91% ([Co(II)] = 0.14 M) while the iron content decreases from 27% to 8%.
Figure7shows the effect of pH on the composition of the deposits. The cobalt sulfate, iron sulfate and DMAB concentrations were kept constant at 0.06, 0.01 and 0.26 M respectively. It can be seen that with an increase in pH from 6.5 to 9, the boron content of the deposits increases from 3.5 to 7 atomic percent. Further increase in pH decreases the boron content of the coatings. The effect of Fig. 4 Cobalt (D), iron (() and boron () atomic percentage of the
Co–Fe–B alloy coating versus DMAB concentration. Plating bath parameters: sodium tartrate (0.2 M), sodium citrate (0.05 M), ammonium sulfate (0.2 M), phosphorous acid (0.06 M), cobalt sulfate (0.06 M) and iron sulfate (0.01 M). pH = 6.5 for CoSO4and FeSO4
curves and pH = 5.5 for DMAB
Fig. 5 Cobalt (D), iron (() and boron () atomic percentage of the Co–Fe–B alloy coating versus iron (II) sulfate concentration. Plating bath parameters: sodium tartrate (0.2 M), sodium citrate (0.05 M), ammonium sulfate (0.2 M), phosphorous acid (0.06 M), cobalt sulfate (0.06 M), DMAB (0.26 M) and pH = 5.5
Fig. 6 Cobalt (D) and iron (() atomic percentage of the Co–Fe–B alloy coating versus cobalt (II) sulfate concentration. Plating bath parameters: sodium tartrate (0.2 M), sodium citrate (0.05 M), ammonium sulfate (0.2 M), phosphorous acid (0.06 M), iron sulfate (0.01 M), DMAB (0.26 M) and pH = 6.5
thiourea concentration (accelerator) on composition of electroless Co–Fe–B ternary alloy coatings was also stud-ied. The bath contained sodium tartrate (0.2 M), sodium citrate (0.05 M), ammonium sulfate (0.2 M), phosphorous acid (0.06 M), cobalt sulfate (0.06 M), iron sulfate (0.01 M) and DMAB (0.26 M). The pH was adjusted at 6.5. Varying the concentration of thiourea between 1.7 · 106 and 1.3 · 105 M resulted in slight variations of cobalt and iron atomic percentage.
In summary, the pH of the electroless Co–Fe–B plating bath was the most effective parameter on boron content of the deposits whereas variation of DMAB, cobalt and iron ions concentration in the plating solution had slight effects on the boron content of the coatings.
3.3 Microstructure and corrosion behavior of the deposits
To examine the effect of the boron contents of the electroless Co–Fe–B ternary alloy deposits on the surface morphology, SEM micrographs of the surface were taken for the samples having 3.7, 4.5, 5.5, 6.5 and 7.0 boron at.% contents (see Fig.8). Each as-plated deposit had nodular structure and the surface roughness increased with the increase of boron content (see Fig.8a1 to 8e1). Figure8a2 to 8e2 show the SEM micrograph of one nodule on the surface of the samples having 3.7, 4.5, 5.5, 6.5, and 7.0 at.% B. Comparison of Fig.8a2 with Fig.8b2, shows that with an increase in boron content each nodule on the surface of the sample is divided into smaller sub-nodules and new boundaries begin to appear. From Fig.8c2 it can be seen that with a further increase in boron content (5.5 at.%), the sub-nodules con-fined within the new boundaries as shown in Fig.8b2 are further divided into smaller nodules. Figure8d2 and 8e2 show that with further increase in the boron content, each nodule is subdivided and the number of nodules produced per surface area increases while their size decreases. In sum-mary, the surface morphology of the Co–Fe–B ternary alloy deposits changes dramatically with slight variations in boron content of the coatings. Therefore, for a specific industrial application, the boron content of alloy deposits is a crucial factor to be considered.
The corrosion potential measurements, Ecorr, are
dis-played in Fig.9. The Ecorrvalue for the coating containing 7
atomic percent of boron was 116 mV more negative than that of 3.7% of boron. The other two Ecorrvalues (4.5 and 5.5% of
boron) were intermediate values and were 63 mV and 60 mV, respectively, more negative than that of 3.7% of boron. Several factors such as surface roughness, crystallinity and composition may explain this corrosion behavior. For instance, corrosion current density measure-ments on polished samples together with electrochemical impedance spectroscopy studies to evaluate the real surface
area of the exposed samples will be relevant experiments to assess thoroughly the corrosion behavior.
3.4 XRD characterization of the deposits
In order to investigate the influence of boron content of the Co–Fe–B coatings on the crystal structure, the XRD pat-terns of electroless Co–Fe–B ternary alloy deposits prepared at different pH baths (6.5–9.0) were obtained. Figure10 shows the XRD patterns of the stainless steel substrate (Fig.10a) and Co–Fe–B deposits having 3.7, 4.5, 5.5, 6.5, and 7 atomic percent boron contents (Fig.10b–f). All XRD patterns for the electroless coatings show com-mon broad diffraction halos at 2h ranges of approximately 70 and 109. Two of the XRD peaks for the Co7Fe3phase
are indicated in Fig.10(vertical lines) and correspond to 2h1 = 69.605 and 2h2= 107.486. The peaks related to
this phase have shifted towards larger angles; the differ-ences for each spectrum are indicated in Table2. These shifts indicate the existence of a solid solution in accor-dance with the peak shifts shown in Fig. 10, which may be related to boron. To make a relative comparison of the amorphous component present in each coating, the indexes of crystallinity, Ic, were calculated using the ratio between
the areas of the Bragg peaks (crystalline material) and the total areas of the spectrum for 2h comprised between 20 and 120, see Table2. The values obtained for Icare not
absolute but can be used to relatively rank the materials. According to this approach, the larger the value of Ic, the
higher the fraction of crystalline material and the lower the amorphous content in the coating. It can be seen that with an increase in boron content of deposits to 5.5 atomic percent, the crystallinity increases and then decreases. Fig. 7 Cobalt (D), iron (() and boron () atomic percentage of the Co–Fe–B alloy coating. versus pH. Plating bath parameters: sodium tartrate (0.2 M), sodium citrate (0.05 M), ammonium sulfate (0.2 M), phosphorous acid (0.06 M), cobalt sulfate (0.06 M), iron sulfate concentration (0.01 M) and DMAB (0.26 M)
3.5 Magnetic properties
The magnetic properties of Co–Fe–B films were studied as a function of boron content ranging from 3.7 to 7 at.%. Typical easy-axis (magnetic field in the plane of the sam-ple) and hard-axis hysteresis loops for a Co–Fe–B film are shown in Fig.11. Figure12 shows the coercivity as a function of boron content for both in-plane and out-of-plane measurements. As it can be seen, the coercivity shows a peak around 6% boron content. The coercivities obtained in the present work show that the magnetic properties can be tailored by the boron content in a sys-tematic way and can be correlated with the changes in the film morphology as revealed by the X-ray diffraction
measurements (Fig.10). The values are comparable with the ones obtained by Osaka et al. [25].
4 Conclusions
From the effects that variables such as pH and the concentration of metallic ions in solution have on the composition of electroless Co–Fe–B ternary alloy deposits, it is evident that variation in the experimental parameters provides a mean of preparing deposits with varying content of cobalt, iron and boron. XRD patterns suggest that the crystallinity of electroless Co–Fe–B ternary alloy deposits Fig. 8 SEM micrographs of Co–Fe–B coatings having different
boron atomic percentage: 3.7% (a1and a2), 4.5% (b1and b2), 5.5% (c1
and c2), 6.5% (d1and d2) and 7.0% (e1and e2)
Fig. 9 Atomic percentage of boron content of Co–Fe–B deposits versus corrosion potential in 3.5% NaCl at room temperature
Fig. 10 XRD spectra for the stainless steel substrate (a) and Co–Fe– B deposits obtained at different boron atomic percentage content: (b) 3.7%, (c) 4.5%, (d) 5.5%, (e) 6.5% and (b) 7%
changes with change in pH electroless bath. Crystallinity plays an important role in the design of soft-magnetic materials. The tight control of process parameters enables
the tailoring of coercivity and magnetic permeability of the films, hence allows the microfabrication of innovative and efficient devices having nanofeatures such as nanowires. Acknowledgments The authors wish to acknowledge financial assistance from the Natural Sciences and Engineering Research Council (NSERC) of Canada, technical assistance from Francois Normardin and Michel Thibodeau for their technical assistance in Magnetic measurement, XRD, and SEM micrographs of nanowires.
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Ic 0.0249 0.5095 0.5186 0.1372 0.1384
Fig. 11 Room temperature magnetization measurements with the magnetic field in the plane (() of the sample (easy-axis) and perpendicular () to the film plane (hard-axis) for a Co–Fe–B film having 3.7 at.% of boron
Fig. 12 Room temperature in-plane (() and out of the plane () coercive fields for Co–Fe–B samples as function of the boron content
