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Magnetic properties of nanocrystalline iron group thin film alloys electrodeposited from sulfate and chloride baths
Electrochimica Acta 48 (2003) 819 /830 www.elsevier.com/locate/electacta
Magnetic properties of nanocrystalline iron group thin film alloys electrodeposited from sulfate and chloride baths Daheum Kim 1, D.-Y. Park, B.Y. Yoo, P.T.A. Sumodjo 2,3, N.V. Myung2 * Department of Chemical Engineering, University of California, Los Angeles, CA 90095-1592, USA Received 31 July 2002; received in revised form 23 October 2002
Abstract Systematic studies of iron group binary (NiCo and CoFe) and ternary (CoNiFe) thin film alloys relating their magnetic properties with film composition, grain size and the corresponding crystal structure were investigated. Anions influence current efficiencies, magnetic properties, surface morphology and phases of electrodeposited films. Higher current efficiencies in chloride baths compared to sulfate baths were observed for CoFe, NiCo and CoNiFe alloys. The higher deposition current efficiencies in chloride baths were attributed to a catalytic effect. Anion types in CoFe and CoNiFe thin film alloys influenced the microstructures and the resulting magnetic properties (coercivity and squareness). The microstructures of NiCo alloys depend on the deposit Co contents rather than anion types. The surface morphologies of CoFe, NiCo and CoNiFe thin films were independent of anion types. CoFe deposits exhibited relatively smooth surface morphology and turned into fine crystallites with increasing solution Fe 2 concentration. NiCo deposits showed very smooth surface morphology. CoNiFe deposits had the surface morphology of polyhedral crystallites. The deposit Fe content in CoFe electrodeposits linearly increased with increasing solution Fe 2 concentration for both chloride and sulfate baths. Similar linear behavior of deposit Co contents was observed in NiCo electrodeposits. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: Electrodeposition; Magnetic films; NiCo alloys; CoFe alloys; CoNiFe alloys; Magnetic properties
1. Introduction Electrodeposited magnetic thin films of the irongroup metals (Fe, Co and Ni) have been developed because of potential applications in computer read/write heads [1,2] and microelectromechanical systems (MEMS) [3 /5]. Current computer read/write heads are separately fabricated using physical deposition methods for GMR spin valve read heads, and electrodeposition for write heads. Electrodeposited permalloy (80Ni20Fe)
* Corresponding author. Present address: MEMS Technology Group, Jet Propulsion Laboratory, 4800 Oak Glove Blvd., Pasadena, CA 91109, USA. Tel.: /1-818-393-3239; fax: /1-818-3934540. E-mail address: [email protected] (N.V. Myung2). 1 Department of Chemical Engineering, Kwangwoon University, Wolgye-dong, Nowon-gu, Seoul, Republic of Korea. 2 ISE member. 3 Visiting scholar from Institute of Chemistry, University of Sa˜o Paulo, Sa˜o Paulo, Brazil.
is the best known iron group thin film alloy in magnetic thin film recording heads [1,2] and MEMS applications [3 /5] with magnetic saturation of 0.97 T, low coercivity, and low magnetostriction. However, new soft magnetic materials with higher performance are needed because of the dramatic increase of the areal density (60% per year) in computer drives [2,6] and further miniaturization and/or better performance of electromagnetic devices in MEMS [5,6]. Various CoFe- and CoNi-based ternary and quarternary alloys including CoFeB [7], CoFeCu [2], CoNiFe [5,6,8 /11], CoNiFeS [12], CoFeP [13] and CoFeSnP [13] have been considered as possible candidates. Among those alloy systems, the ternary CoNiFe alloys have been demonstrated as one of the promising systems due to their high magnetic saturation flux density (BS /2.0 /2.1 T) combined with reasonably low coercivity field strength (B/2 Oe) [6,14,15]. Liao [7] suggested the fabrication of read/write heads using CoFeB alloys with 0.1 /2 wt.% B and 7/12 wt.% Fe contents. He used a sulfate bath with sodium citrate
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and dimethyl amine borane (DMAB) as reducing agent for boron at pH 3.5. He reported a decrease of coercivity to less than 1 Oe due to boron addition into CoFe alloy, while other properties remained the same as the properties of the binary CoFe alloy. Andricacos and Robertson [2] reviewed the requirements, including: (i) high magnetic saturation (MS /1 T); (ii) low coercivity HC B/1 Oe; (iii) optimal anisotropy field (HK) for high permeability; (iv) close to zero saturation magnetostriction (l); (v) high electrical resistivity (r); (vi) good corrosion resistance (Rp), for improved thin-film recording heads. They studied the dependence of magnetic properties of CoFeCu films on composition and suggested that addition of Cu causes a decrease ( B/1.5 Oe) in the coercivity of the film. Chessnutt [10] studied electrodeposited CoNiFeB alloys for recording head. He obtained a magnetic saturation of 1.5 T in Co80Ni10Fe10 alloy. It was observed that addition of boron in CoNiFe alloy decreased the coercivity from 1.5 to 0.6 Oe. Ohashi et al. [11] suggested electrodeposited Co65Ni12Fe23 film from sulfate bath as one of the promising materials for high density recording MR heads. They reported that Co65Ni12Fe23 films with mixed bcc and fcc phases exhibit higher magnetic saturation (MS /2.0 /2.1 T), improved corrosion resistance and good thermal stability than other Co65Ni12Fe23 films with bcc phases. In a subsequent study [14], Osaka et al. obtained electrodeposited Co65Ni12Fe23 alloys (fine grain sizes /10 /20 nm) with high magnetic saturation (MS /2.1 T) and low coercivity (B/1.2 Oe) using sulfate baths without sulfurcontaining additives such as saccharin and thiourea. Takai et al. [12] studied soft magnetic CoNiFe films electrodeposited from sulfate bath with thiourea as additive. They obtained (Co73Ni12Fe15)99.1S0.9 with small grain size (5 /10 nm), high magnetic saturation (MS /1.7 T), high resistivity up to 51 mVcm and low saturation magnetostriction (lS /4.4 /10 6). They suggested that low coercivity and high resistivity resulted from the formation of nano grains (5 /10 nm). Hironaka and Uedaira [13] investigated amorphous CoFeP and CoFeSnP films electrodeposited from sulfamate bath. CoFeP and CoFeSnP films exhibit a magnetic saturation of 1 /1.35 T. They observed that the addition of Sn into CoFeP increases the corrosion resistance without decreasing magnetic saturation. Although numerous studies had been carried out to investigate the binary (CoFe, NiCo, NiFe) and ternary (CoNiFe) iron group magnetic thin films, they mostly focused on the mechanism of anomalous codeposition [16 /24], the effects of various additives [15,25,26], the effects of pulse plating [27,28], corrosion properties [29,30], and performance studies for recording head [10,31]. There is a lack of systematic studies relating magnetic properties of electrodeposited iron group thin films including CoFe, NiCo, NiFe and CoNiFe alloys with deposit composition, grain size and the correspond-
ing crystal structures. This paper is the extended study based on our previous works [30]. In this study, we discuss the influence of solution compositions and electrodeposition parameters on the film compositions using chloride and sulfate baths. The resulting microstructures and magnetic properties of Co, Ni, Fe, CoFe, NiCo and CoNiFe alloys were also studied.
2. Experimental Co, Ni, Fe, CoFe, NiCo and CoNiFe alloys were electrodeposited from chloride and sulfate baths. Table 1 gives the plating solution compositions investigated. Two different anions (chloride and sulfate electrolytes)
Table 1 Bath compositions for Co, Ni, Fe, CoFe, CoNi and CoNiFe electrodeposits Metal/ alloy
Anion type
Composition
Co
Chloride
0.2 M CoCl2/0.7 M NaCl/0.4 M H3BO3/ 0.0075 M saccharin 0.2 M CoSO4/0.7 M Na2SO4/0.4 M H3BO3/ 0.0075 M saccharin 0.2 M CoCl2/0.7 M NaCl/0.4 M H3BO3/ 0.0075 M saccharin/0.05 M L’ascorbic acid 0.2 M CoSO4/0.7 M Na2SO4/0.4 M H3BO3/ 0.0075 M saccharin/0.05 M L’ascorbic acid
Sulfate Chloride Sulfate Ni
Chloride Sulfate
Fe
Chloride Sulfate
CoFe
Chloride
Sulfate
NiCo
Chloride Sulfate
CoNiFe Chloride
Sulfate
0.2 M NiCl2/0.7 M NaCl/0.4 M H3BO3/ 0.0075 M saccharin 0.2 M NiSO4/0.7 M Na2SO4/0.4 M H3BO3/ 0.0075 M saccharin 0.2 M FeCl2/0.7 M NaCl/0.4 M H3BO3/ 0.0075 M saccharin/0.05 M L’ascorbic acid 0.2 M FeSO4/0.7 M Na2SO4/0.4 M H3BO3/ 0.0075 M saccharin/0.05 M L’ascorbic acid 0.2 M CoCl2/x M FeCl2/0.7 M NaCl/0.4 M H3BO3/0.0075 M saccharin/0.05 M L’ascorbic acid 0.2 M CoSO4/x M FeSO4/0.7 M Na2SO4/ 0.4 M H3BO3/0.0075 M saccharin/0.05 M L’ascorbic acid 0.2 M NiCl2/x M CoCl2/0.7 M NaCl/0.4 M H3BO3/0.0075 M saccharin 0.2 M NiSO4/x M CoSO4/0.7 M Na2SO4/ 0.4 M H3BO3/0.0075 M saccharin 0.2 M NiCl2/0.15 M CoCl2/y M FeCl2/0.7 M NaCl/0.4 M H3BO3/0.0075 M saccharin/ 0.05 M L’ascorbic acid 0.2 M NiSO4/0.15 M CoSO4/y M FeSO4/ 0.7 M Na2SO4/0.4 M H3BO3/0.0075 M saccharin/0.05 M L’ascorbic acid
x : Fe 2 concentration in CoFe and Co 2 concentration in NiCo, 0.01 5/x 5/0.16 M; y : Fe 2 concentration in CoNiFe, 0.0055/y 5/ 0.16 M.
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Fig. 1. Dependence of deposit Fe content and current efficiency with Fe2 solution concentration in binary CoFe electrodeposits (a) chloride bath and (b) sulfate bath ([Co 2] /0.2 M).
were used for comparison. NaCl and Na2SO4 were used as supporting electrolytes in chloride and sulfate baths, respectively. Boric acid was added as pH buffer. Saccharin was used to reduce deposit stress and L’ascorbic acid to minimize Fe2 oxidation in CoFe and CoNiFe solutions. Solutions were exposed to air and solution pH was adjusted to 3 by adding HCl (chloride baths), H2SO4 (sulfate baths) or NaOH
(chloride and sulfate baths); experiments were conducted at 10 mA/cm2, pH 3, 10 coulombs/cm2, and room temperature without stirring. The effect of the solution [Co 2]/[Fe2] ratio on the deposit composition of binary CoFe alloy films was conducted by varying the Fe 2 concentration from 0.01 to 0.16 M, with Co 2 concentration set at 0.2 M. The effect of the solution [Ni 2]/[Co 2] ratio on the deposit
Fig. 2. Dependence of magnetic saturation (MS), coercivity (HC) and squareness (S ) of binary CoFe electrodeposits with deposit Fe content from chloride and sulfate baths (a) magnetic saturation, (b) coercivity, and (c) squareness ([Co 2] /0.2 M).
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Fig. 4. Comparison of deposit Fe content for chloride and sulfate baths as a function of solution Fe 2 concentration in binary CoFe electrodeposits ([Co2] /0.2 M).
deposits were examined with a scanning electron microscope (Stereoscan 250, Cambridge, Scientific Instruments Ltd.). Magnetic properties such as magnetic saturation (MS), coercivity (HC) and squareness (S / Mr/Ms) of electrodeposited films were determined using a vibrating sample magnetometer (Model 880, ADE Technologies Inc.).
3. Results and discussion 3.1. CoFe alloys Fig. 3. XRD patterns of Fe, Co and CoFe electrodeposits from: (a) chloride and (b) sulfate baths.
composition of binary NiCo films was investigated by varying the Co 2 concentration from 0.01 to 0.16 M, with Ni 2 concentration set at 0.2 M. The electrodeposition of ternary CoFeNi alloys was carried out with fixed Ni 2 concentration of 0.2 M, Co 2 concentration of 0.15 M, and variable concentrations of Fe 2 from 0.005 to 0.16 M. Brass was used as substrate; Fe, Co or Ni was used as soluble anode. Deposit Co, Ni and Fe contents were analyzed using atomic absorption spectroscopy (AA) (Model 280, Perkin/Elmer). Alloy compositions are given in weight percent. X-ray diffractometer (XRD) (Model 42202, Norelco, North American Philips Company Inc.) with CuKa radiation was used for the identification of the phases and the measurement of grain size in the electrodeposits. The conditions of XRD were a scanning range (40 /808) with 0.038 increments and 1 s dwell time. Microstructures of CoFe, NiCo and CoNiFe electro-
To study anion effects on current efficiencies and magnetic properties, binary CoFe thin films were electrodeposited from chloride and sulfate baths. Fig. 1 shows dependence of deposit Fe contents and current efficiencies with solution Fe 2 concentration. Deposit Fe contents practically increased linearly with increasing solution Fe2 concentration for chloride and sulfate baths. Current efficiencies (approximately /52%) was independent of solution Fe2 concentrations in chloride baths, but decreased to level at /18% in sulfate baths. Current efficiency (/52%) in chloride baths were higher than that (/18%) in sulfate baths. Hokans [17,32] investigated hydrogen evolution using Na2SO4, NaCl and NaClO4 solutions during the electrodeposition of NiFe alloys. She obtained higher limiting currents of H reduction in Na2SO4 than in NaCl and NaClO4. She observed a significant decrease in the reduction potential of Ni, Fe and NiFe in chloride baths and suggested that chloride catalyses deposition of Ni, Fe, and NiFe through the formation of an ion bridge between the electrode and the metal ion being discharged. Myung and Nobe [30] also observed that
Fig. 5. Surface morphology of Co, Fe and CoFe electrodeposits from chloride [(a) /(d)] and sulfate baths [(e) /(f)] containing L’ascorbic acid (a) pure Co, (b) 74Co24Fe, (c) 52Co48Fe, (d) pure Fe, (e) pure Co, (f) 78Co22Fe, (g) 52Co48Fe, and (h) pure Fe.
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Fig. 5
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current efficiencies in chloride baths were higher than in sulfate baths during the electrodeposition of NiFe thin film alloys. Therefore, higher current efficiency for CoFe electrodeposition in chloride baths compared to sulfate baths in our experiments may result from a similar catalytic mechanism. Fig. 2 shows the effect of deposit Fe content from chloride and sulfate baths on magnetic saturation (MS), coercivity (HC) and squareness (S ). Magnetic saturation of CoFe electrodeposits was independent of anion types and increased with increasing deposit Fe content. The observed MS variation is reasonable from the fact that MS is an intrinsic magnetic property, which is dependent only on the film composition. In CoFe alloys, when the solution Fe 2 concentrations in chloride and sulfate baths are equal, nearly same amounts of Fe were electrodeposited as shown in Fig. 1. Coercivity of CoFe electrodeposits from chloride baths increased from 17 (at 0 wt.% Fe) to 40 Oe (/ /18 wt.% Fe), and then reached a plateau. Coercivity of CoFe electrodeposits from sulfate baths increased from /17 to /60 Oe with increasing deposit Fe content with B/ /22 wt.%, and then decreased to /18 Oe at deposit Fe content of 22/50 wt.%. Squareness of CoFe alloys from chloride baths increased from /0.4 to /0.6 with increasing deposit Fe contents of B/ /20 wt.% Fe, and then reached a plateau with deposit Fe contents of / /20 wt.%. Squareness from sulfate baths decreased from 0.8 to 0.3 at B/ /20 wt.% Fe, and then increased to be 0.5 at /50 wt.% Fe. To study the influence of anion type on the microstructure of CoFe electrodeposits, XRD measurements were carried out as shown in Fig. 3 (XRD measurements of pure Co and Fe deposits were conducted for comparison). The grain size was calculated from the peak broadening using the Scherrer formula [33]. In chloride baths, CoFe electrodeposits containing deposit 32 wt.% Fe (0.08 M Fe2 concentration in the bath) exhibited a coercivity of /41 Oe. From the XRD patterns (Fig. 3a), this film consisted of hcp (002) [or bcc (110)] and bcc (200) phases with an average grain size of /63 nm. For deposits with higher Fe content, the microstructures did not changed, but the intensities of hcp (002) [or bcc (110)] phase decreased (as shown in Fig. 3a for the alloy containing 48 wt.% Fe). However, fairly constant coercivities (/40 Oe) are observed. Co22 wt.% Fe films plated from sulfate baths (0.06 M Fe 2 concentration), which had a maximum coercivity of /60 Oe, consisted of hcp (002) [or bcc (110)] planes with the average grain size of /40 nm. The coercivity of CoFe electrodeposits containing 48 wt.% Fe (0.14 M Fe2 concentration) was B/ /30 Oe. The microstructure of this film showed hcp (002) [or bcc (110)] and bcc (200) planes with an average grain size of /33 nm. Therefore, anion types did not affect on the phases of CoFe deposits, but influenced on the magnetic proper-
ties. There exist difficulties to exactly index the peak at /45.18 [either hcp (002) or bcc (110)] for CoFe alloys with Fe contents (22 wt.%) from chloride and (32 wt.%) sulfate baths (Fig. 3). The difficulties of indexing result from two reasons. One comes from that JCPDS files for the CoFe alloys are not available and other results from a shift of peak positions due to the slight difference of atomic radius between Co (0.125 nm) and Fe (0.124 nm). From the phase diagram of CoFe alloys [34], Co and Fe form a complete substitutional solid solution at room temperature. As the deposit Fe amount in CoFe electrodeposits increase, the peak position will shift to higher angle according to the following Eqs. (1) and (2) [33]; 2u of hcp (002) is 44.7628 from JCPDS file #5-727, l2 (h2 k2 l 2 ) for cubic (1) 4a2 l2 h2 hk l 2 l2 × l 2 2 for hexagonal (2) sin u 3 a2 4 × c2 sin2 u
where, l is the wavelength of radiation, a is the size of unit cell, and h, k , l are the Miller indices of reflecting plane. The peaks at /45.18 for CoFe electrodeposits containing 22/32 wt.% Fe from chloride and sulfate baths can be attributed to the hcp (002) planes rather than bcc (110) planes, because the deposit Co contents were higher than the deposit Fe contents. The deposit Fe contents was analyzed to be 22/32 wt.%. However, it is obscure to index the peaks at /45.18 for 0.14 M Fe2 concentration for chloride and sulfate baths due to the nearly same amount of deposit Co and Fe (52 and 48) as shown in Fig. 4. Pure Co electrodeposits from chloride baths containing L’ascorbic acid showed acicular morphology as shown in Fig. 5a. As the deposit Fe content in CoFe deposits increased from 24 to 48 wt.% (0.06/0.14 M Fe 2 concentration), crystalline size decreased (Fig. 5b and c). Pure Fe electrodeposits were not compact and
Fig. 6. Dependence of deposit Co content and current efficiency with Co 2 solution concentration in binary CoNi electrodeposits from sulfate bath ([Ni 2] /0.2 M).
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exhibited polyhedral crystallites. On the other hand, pure Co deposits (Fig. 5e) from sulfate baths with L’ascorbic acid had very smooth surface. As the deposit Fe content increased, similar feature (Fig. 5f and g) of the surface morphology compared to those from chloride baths was observed. The needle shape morphology at the right and upper corner part (marked with an arrow in Fig. 5g) resulted from a fast rusting of CoFe electrodeposits. Pure Fe deposits from sulfate bath (Fig. 5h) showed more compact and dense polyhedral crystallites compared to that from chloride bath (Fig. 5d).
Fig. 7. Effect of binary CoNi electrodeposits with deposit Co contents on magnetic saturation (MS), coercivity (HC) and squareness (S ) from chloride and sulfate baths (a) magnetic saturation, (b) coercivity, and (c) squareness ([Ni 2] /0.2 M).
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3.2. CoNi alloys Fig. 6 shows the dependence of deposit Co content and current efficiency with Co2 solution concentration during the electrodeposition of CoNi thin films from sulfate baths. Deposit Co content increased linearly with increasing solution Co 2 concentration. Unlike NiCo electrodeposits from chloride bath reported previously [30], current efficiency in sulfate baths showed a maximum ( /45%) for Co 2 concentrations around 0.9 M. The magnetic saturation, coercivity, and squareness of electrodeposited NiCo films from chloride and sulfate baths are shown in Fig. 7. The magnetic saturation (MS) increased linearly with increasing deposit Co content (Fig. 7a). The coercivity of Ni electrodeposit from chloride bath was /60 Oe. NiCo films electrodeposited from chloride bath exhibited a decrease in coercivity with increasing Co content up to approximately 55 wt.% Fe. Higher Co content resulted in an increase in coercivity. The same trend was also observed when sulfate bath was used. A minimum coercivity of /20 Oe was observed for the Ni and Co ratio (50:50). Fig. 7c
Fig. 8. XRD patterns of Ni, Co and CoNi electrodeposits: (a) chloride and (b) sulfate baths.
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Fig. 9
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Fig. 10. Dependence of deposit Co, Fe and Ni contents and current efficiencies with Co 2 solution concentration in ternary CoNiFe electrodeposits (a) chloride and (b) sulfate baths ([Co 2] /0.2 M).
shows the squareness of NiCo thin film alloys from sulfate and chloride baths. Squareness increased from /0.1 to /0.7 with increasing deposit Co content for B/ /70 wt.% for sulfate and chloride baths. The NiCo films electrodeposited from chloride and sulfate baths with 49/53 wt.% Co (0.08 M Co concentration) exhibited coercivities of /16 Oe for sulfate and /22 Oe for chloride baths. This film consisted of hcp (002) [or fcc (111)] and fcc (200) phases with the average grain size of /18 nm, as shown in Fig. 8. However, the NiCo electrodeposit with higher Co content, which have different coercivities (/60 Oe for sulfate and /121 Oe for chloride baths), consisted of hcp (100), hcp (002) [or fcc (111)] and hcp (101) phases with the average grain size of /13 nm. Hence, it is clear that the microstructures of NiCo alloy are independent of anion types. However, the microstructures changed with increasing deposit Co content for both chloride and sulfate baths. The NiCo electrodeposits (containing 0.15 M Co 2 concentration; deposit Co content of /80 wt.%) from
Fig. 11. Dependence of magnetic saturation (MS), coercivity (HC) and squareness (S ) of ternary CoNiFe electrodeposits with deposit Fe content from chloride and sulfate baths (a) magnetic saturation, (b) coercivity, and (c) squareness ([Co2] /0.2 M).
chloride bath exhibits higher coercivity (120 Oe) than the coercivity of 60 Oe from sulfate baths. Pure Co and NiCo thin films (Fig. 9a/c and e /g) electrodeposited from chloride and sulfate baths exhibited very smooth surface morphologies. Relatively smooth surface morphologies were observed on the surfaces of pure Ni electrodeposits (Fig. 9d and h) from chloride and sulfate baths.
Fig. 9. Surface morphology of Co, Ni and NiCo electrodeposits from chloride [(a) /(d)] and sulfate baths [(e) /(f)]; (a) pure Co, (b) 51Ni49Co, (c) 22Ni78Co, (d) pure Ni, (e) pure Co, (f) 47Ni53Co, (g) 19Ni81Co, and (h) pure Ni.
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3.3. CoNiFe alloys Fig. 10 shows the dependence of deposit Co, Fe and Ni contents and current efficiency with solution Fe 2 concentration during the electrodeposition of CoNiFe thin films from chloride and sulfate baths. In chloride baths, with increasing solution Fe 2 concentration, and thus with increasing deposit Fe content up to /40 wt.%, Co content decreased from /88 to /50 wt.% and deposit Ni content remained practically constant. Current efficiency was /75% and independent of solution Fe 2 concentration. In sulfate baths, deposit Co, Ni and Fe contents have the similar behavior of observed deposit Co, Ni and Fe contents compared to those from chloride baths. Current efficiency of binary CoNi alloys without solution Fe 2 concentration was measured to be /52%. However, current efficiency was practically constant, /75%, with increasing solution Fe2 concentration up to B/0.06 M. For higher Fe 2 concentration, it decreased to /52%. Fig. 11 shows the effect of deposit Fe contents on magnetic saturation, coercivity and squareness from
chloride and sulfate baths. Binary NiCo alloys exhibited high coercivity of /70 Oe for sulfate and chloride baths. When chloride bath is used to deposit the CoNiFe alloy, the presence of iron for deposit Fe content less than 10 wt.%, caused a sharp decrease in the coercivity down to 3 Oe. For deposit Fe content in the range 10/40 wt.%, the 80wt.%Co-5wt.%Ni15wt.%Fe alloy exhibited the maximum attained coercivity of /26 Oe. Coercivity from sulfate baths was independent of deposit Fe contents, /3 Oe. Squareness from sulfate and chloride baths decreased from 0.6 to 0.2 with increasing deposit Fe content. 80Co5Ni15Fe alloys from chloride bath, which has a maximum coercivity of 26 Oe, consisted of hcp (100), fcc (111) and hcp (002) [or bcc (110)] with the average grain size of /22 nm as shown in Fig. 12. The microstructures changed to hcp (002) [or bcc (110)] and bcc (200) with increasing the solution Fe2 concentration from 15 (0.04 M Fe 2 concentration) to 42 wt.% Fe (0.14 M Fe 2 concentration). 76wt.%Co-7wt.%Ni-17wt.%Fe from sulfate bath, which has a low coercivity of 9 Oe, consisted of hcp (002) [or bcc (110)] and bcc (200) with the average grain size of /42 nm. CoNiFe deposits with 46 wt.% Fe (0.1 M Fe concentration) exhibited hcp (002) [or bcc (110)] phases. Therefore, anion type in CoNiFe alloys influenced on the microstructures and the resulting coercivities. The addition of Fe into electrodeposited CoNi alloys for chloride and sulfate baths caused the decrease of the coercivity. Binary CoNi from chloride and sulfate baths showed very smooth surfaces as shown in Fig. 13. As solution Fe 2 concentration in CoNiFe electrodeposits increased, the decrease of the size of polyhedral crystallites were observed.
4. Conclusion
4.1. CoFe alloys
Fig. 12. XRD patterns of CoNi and CoNiFe electrodeposits (a) chloride and (b) sulfate baths.
Current efficiencies of the electrodeposited CoFe thin film alloys from chloride baths (/52%) were independent of the Fe 2 solution concentrations. The current efficiencies from sulfate baths decreased to be B/ /20% with increasing the solution Fe 2 concentrations. Current efficiencies of the electrodeposited CoFe thin film alloys from chloride baths were significantly higher than that from sulfate baths. The higher current efficiencies in chloride baths may be the results of a low H2 limiting current compared to those from sulfate baths. Magnetic saturation (MS) of electrodeposited CoFe films both in chloride and sulfate baths increased from /1.5 to /2.3 T as deposit Fe content increased. Coercivities of the electrodeposited CoFe thin films increased from /20 to /60 Oe in sulfate baths and from /20 to /40 Oe in
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Fig. 13. Surface morphology of CoNi and CoNiFe electrodeposits from chloride [(a) /(c)] and sulfate baths [(d) /(f)]: (a) 80Co20Ni, (b) 80Co5Ni15Fe, (c) 49Co5Ni46Fe, (d) 80Co20Ni, (e) 76Co7Ni17Fe, (f) 49Co5Ni46Fe.
chloride baths as the deposit Co content increased up to 20 wt.%. A minimum squareness of 0.2 was observed in sulfate baths at /18 wt.% Co, while a maximum squareness of 0.7 was observed in chloride solution at /18 wt.% Co. Anion types affected the magnetic properties. However, no effects of anion types on the phases and surface morphology in CoFe electrodeposits were observed. Surface morphology were affected by solution Fe 2 concentration.
4.2. NiCo alloys Current efficiencies (/75%) from chloride baths were higher than that (25 /45%) from sulfate baths. Magnetic saturation of the alloys with increasing deposit Co contents exhibited similar linear behavior for chloride and sulfate baths. The electrodeposits of NiCo thin film alloys from chloride baths showed higher coercivities (120 Oe) than that from sulfate baths (/60 Oe) at the
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deposit Co contents of /80 wt.%. Electrodeposited NiCo thin film alloys from both chloride and sulfate baths exhibited the similar linear behavior of coercivities up to /50 wt.% Co. The microstructure of NiCo deposits did not depend on anion types, but on deposit Co contents. NiCo electrodeposits from chloride and sulfate baths exhibited very smooth surface morphology. 4.3. NiCoFe alloys Current efficiencies from chloride solutions (/75%) were independent of solution Fe 2 concentrations, whereas those from sulfate solutions gradually decreased from /75 to /50% as solution Fe 2 concentrations increased. Coercivity of electrodeposited CoNi thin films from chloride and sulfate baths measured to be /70 Oe. Coercivities of CoNiFe deposits from sulfate baths sharply decreased to be 3 Oe with increasing the deposit Fe contents. The squarenesses in chloride and sulfate baths showed gradual decreasing tendency from /0.6 to /0.2 in sulfate and 0.1 in chloride baths. Microstructures depend on anion types and the addition of Fe into CoNi deposits caused the decrease of the coercivity. CoNeFe electrodeposits had the surface morphology of polyhedral crystallites.
Acknowledgements This work was supported by the DARPA MEMS program DAB63-99-1-0020 and the Research Grant of Kwangwoon University in 2001.
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[4] C. Liu, T. Tsao, G.-B. Lee, J.T.S. Leu, Y.W. Yi, Y.-C. Tai, C.-M. Ho, Sens. Actuators 78 (1999) 190. [5] F.E. Rasmussen, J.T. Ravnkilde, P.T. Tang, O. Hansen, S. Bouwstra, Sens. Actuators A 92 (2001) 242. [6] T. Osaka, M. Takai, K. Hayashi, K. Ohashi, M. Saito, K. Yamada, Nature 392 (1998) 796. [7] S.H. Liao, U.S. Pat. 5,168,410 (1992). [8] X. Liu, G. Zangari, L. Shen, J. Appl. Phys. 87 (2000) 5410. [9] I. Tabakovic, V. Inturi, S. Riemer, J. Electrochem. Soc. 149 (2002) C18. [10] R. Chesnutt, J. Appl. Phys. 73 (1993) 6223. [11] K. Ohashi, Y. Yasue, M. Saito, K. Yamada, T. Osaka, M. Takai, K. Hayashi, IEEE Trans. Magn. 34 (1998) 1462. [12] M. Takai, K. Hayashi, M. Aoyaki, T. Osaka, J. Electrochem. Soc. 144 (1997) L203. [13] K. Hironaka, S. Uedaira, IEEE Trans. Magn. 26 (1990) 2421. [14] T. Osaka, M. Takai, Y. Sogawa, T. Momma, K. Ohashi, M. Saito, K. Yamada, J. Electrochem. Soc. 146 (1999) 2092. [15] I. Tabakovic, S. Riemer, V. Inturi, P. Jallen, A. Thayer, J. Electrochem. Soc. 147 (2000) 219. [16] H. Dahms, I.M. Croll, J. Electrochem. Soc. 112 (1965) 771. [17] J. Horkans, J. Electrochem. Soc. 128 (1981) 45. [18] S. Hessami, C.W. Tobias, J. Electrochem. Soc. 136 (1989) 3611. [19] W.C. Grande, J.B. Talbot, J. Electrochem. Soc. 140 (1993) 669. [20] W.C. Grande, J.B. Talbot, J. Electrochem. Soc. 140 (1993) 675. [21] K.Y. Sasaki, J.B. Talbot, J. Electrochem. Soc. 142 (1995) 775. [22] N. Zech, E.J. Podlaha, D. Landolt, J. Appl. Electrochem. 28 (1998) 1251. [23] N. Zech, E.J. Podlaha, D. Landolt, J. Electrochem. Soc. 146 (1999) 2886. [24] N. Zech, E.J. Podlaha, D. Landolt, J. Electrochem. Soc. 146 (1999) 2892. [25] T. Osaka, T. Sawaguchi, F. Mizutani, T. Yokoshima, M. Takai, Y. Okinaka, J. Electrochem. Soc. 146 (1999) 3295. [26] L. Ricq, F. Lallemand, M.P. Gigandet, J. Pagetti, Surf. Coat. Tech. 138 (2001) 278. [27] X. Liu, G. Zangari, IEEE Trans. Magn. 37 (2001) 1764. [28] T. Nakanishi, M. Ozaki, H.-S. Nam, T. Yokoshima, T. Osaka, J. Electrochem. Soc. 148 (2001) C627. [29] M. Saito, K. Yamada, K. Ohashi, Y. Yasue, Y. Sogawa, T. Osaka, J. Electrochem. Soc. 146 (1999) 2845. [30] N.V. Myung, K. Nobe, J. Electrochem. Soc. 148 (2001) C136. [31] S.H. Liao, IEEE Trans. Magn. 26 (1990) 328. [32] J. Hokans, J. Electrochem. Soc. 126 (1979) 1861. [33] B.D. Cullity, Elements of X-ray Diffraction, 2nd Ed., AddisonWesley Publishing Company Inc, Reading, MA, 1978, p. 91, 501. [34] M. Hansen, K. Anderk (Eds.), Constitution of Binary Alloys, 2nd ed., Genium Publishing Corporation, Schenectady, NY, 1989, pp. 471 /474.
Magnetic properties of nanocrystalline iron group thin film alloys electrodeposited from sulfate and chloride baths Daheum Kim 1, D.-Y. Park, B.Y. Yoo, P.T.A. Sumodjo 2,3, N.V. Myung2 * Department of Chemical Engineering, University of California, Los Angeles, CA 90095-1592, USA Received 31 July 2002; received in revised form 23 October 2002
Abstract Systematic studies of iron group binary (NiCo and CoFe) and ternary (CoNiFe) thin film alloys relating their magnetic properties with film composition, grain size and the corresponding crystal structure were investigated. Anions influence current efficiencies, magnetic properties, surface morphology and phases of electrodeposited films. Higher current efficiencies in chloride baths compared to sulfate baths were observed for CoFe, NiCo and CoNiFe alloys. The higher deposition current efficiencies in chloride baths were attributed to a catalytic effect. Anion types in CoFe and CoNiFe thin film alloys influenced the microstructures and the resulting magnetic properties (coercivity and squareness). The microstructures of NiCo alloys depend on the deposit Co contents rather than anion types. The surface morphologies of CoFe, NiCo and CoNiFe thin films were independent of anion types. CoFe deposits exhibited relatively smooth surface morphology and turned into fine crystallites with increasing solution Fe 2 concentration. NiCo deposits showed very smooth surface morphology. CoNiFe deposits had the surface morphology of polyhedral crystallites. The deposit Fe content in CoFe electrodeposits linearly increased with increasing solution Fe 2 concentration for both chloride and sulfate baths. Similar linear behavior of deposit Co contents was observed in NiCo electrodeposits. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: Electrodeposition; Magnetic films; NiCo alloys; CoFe alloys; CoNiFe alloys; Magnetic properties
1. Introduction Electrodeposited magnetic thin films of the irongroup metals (Fe, Co and Ni) have been developed because of potential applications in computer read/write heads [1,2] and microelectromechanical systems (MEMS) [3 /5]. Current computer read/write heads are separately fabricated using physical deposition methods for GMR spin valve read heads, and electrodeposition for write heads. Electrodeposited permalloy (80Ni20Fe)
* Corresponding author. Present address: MEMS Technology Group, Jet Propulsion Laboratory, 4800 Oak Glove Blvd., Pasadena, CA 91109, USA. Tel.: /1-818-393-3239; fax: /1-818-3934540. E-mail address: [email protected] (N.V. Myung2). 1 Department of Chemical Engineering, Kwangwoon University, Wolgye-dong, Nowon-gu, Seoul, Republic of Korea. 2 ISE member. 3 Visiting scholar from Institute of Chemistry, University of Sa˜o Paulo, Sa˜o Paulo, Brazil.
is the best known iron group thin film alloy in magnetic thin film recording heads [1,2] and MEMS applications [3 /5] with magnetic saturation of 0.97 T, low coercivity, and low magnetostriction. However, new soft magnetic materials with higher performance are needed because of the dramatic increase of the areal density (60% per year) in computer drives [2,6] and further miniaturization and/or better performance of electromagnetic devices in MEMS [5,6]. Various CoFe- and CoNi-based ternary and quarternary alloys including CoFeB [7], CoFeCu [2], CoNiFe [5,6,8 /11], CoNiFeS [12], CoFeP [13] and CoFeSnP [13] have been considered as possible candidates. Among those alloy systems, the ternary CoNiFe alloys have been demonstrated as one of the promising systems due to their high magnetic saturation flux density (BS /2.0 /2.1 T) combined with reasonably low coercivity field strength (B/2 Oe) [6,14,15]. Liao [7] suggested the fabrication of read/write heads using CoFeB alloys with 0.1 /2 wt.% B and 7/12 wt.% Fe contents. He used a sulfate bath with sodium citrate
0013-4686/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 3 - 4 6 8 6 ( 0 2 ) 0 0 7 7 3 - 9
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and dimethyl amine borane (DMAB) as reducing agent for boron at pH 3.5. He reported a decrease of coercivity to less than 1 Oe due to boron addition into CoFe alloy, while other properties remained the same as the properties of the binary CoFe alloy. Andricacos and Robertson [2] reviewed the requirements, including: (i) high magnetic saturation (MS /1 T); (ii) low coercivity HC B/1 Oe; (iii) optimal anisotropy field (HK) for high permeability; (iv) close to zero saturation magnetostriction (l); (v) high electrical resistivity (r); (vi) good corrosion resistance (Rp), for improved thin-film recording heads. They studied the dependence of magnetic properties of CoFeCu films on composition and suggested that addition of Cu causes a decrease ( B/1.5 Oe) in the coercivity of the film. Chessnutt [10] studied electrodeposited CoNiFeB alloys for recording head. He obtained a magnetic saturation of 1.5 T in Co80Ni10Fe10 alloy. It was observed that addition of boron in CoNiFe alloy decreased the coercivity from 1.5 to 0.6 Oe. Ohashi et al. [11] suggested electrodeposited Co65Ni12Fe23 film from sulfate bath as one of the promising materials for high density recording MR heads. They reported that Co65Ni12Fe23 films with mixed bcc and fcc phases exhibit higher magnetic saturation (MS /2.0 /2.1 T), improved corrosion resistance and good thermal stability than other Co65Ni12Fe23 films with bcc phases. In a subsequent study [14], Osaka et al. obtained electrodeposited Co65Ni12Fe23 alloys (fine grain sizes /10 /20 nm) with high magnetic saturation (MS /2.1 T) and low coercivity (B/1.2 Oe) using sulfate baths without sulfurcontaining additives such as saccharin and thiourea. Takai et al. [12] studied soft magnetic CoNiFe films electrodeposited from sulfate bath with thiourea as additive. They obtained (Co73Ni12Fe15)99.1S0.9 with small grain size (5 /10 nm), high magnetic saturation (MS /1.7 T), high resistivity up to 51 mVcm and low saturation magnetostriction (lS /4.4 /10 6). They suggested that low coercivity and high resistivity resulted from the formation of nano grains (5 /10 nm). Hironaka and Uedaira [13] investigated amorphous CoFeP and CoFeSnP films electrodeposited from sulfamate bath. CoFeP and CoFeSnP films exhibit a magnetic saturation of 1 /1.35 T. They observed that the addition of Sn into CoFeP increases the corrosion resistance without decreasing magnetic saturation. Although numerous studies had been carried out to investigate the binary (CoFe, NiCo, NiFe) and ternary (CoNiFe) iron group magnetic thin films, they mostly focused on the mechanism of anomalous codeposition [16 /24], the effects of various additives [15,25,26], the effects of pulse plating [27,28], corrosion properties [29,30], and performance studies for recording head [10,31]. There is a lack of systematic studies relating magnetic properties of electrodeposited iron group thin films including CoFe, NiCo, NiFe and CoNiFe alloys with deposit composition, grain size and the correspond-
ing crystal structures. This paper is the extended study based on our previous works [30]. In this study, we discuss the influence of solution compositions and electrodeposition parameters on the film compositions using chloride and sulfate baths. The resulting microstructures and magnetic properties of Co, Ni, Fe, CoFe, NiCo and CoNiFe alloys were also studied.
2. Experimental Co, Ni, Fe, CoFe, NiCo and CoNiFe alloys were electrodeposited from chloride and sulfate baths. Table 1 gives the plating solution compositions investigated. Two different anions (chloride and sulfate electrolytes)
Table 1 Bath compositions for Co, Ni, Fe, CoFe, CoNi and CoNiFe electrodeposits Metal/ alloy
Anion type
Composition
Co
Chloride
0.2 M CoCl2/0.7 M NaCl/0.4 M H3BO3/ 0.0075 M saccharin 0.2 M CoSO4/0.7 M Na2SO4/0.4 M H3BO3/ 0.0075 M saccharin 0.2 M CoCl2/0.7 M NaCl/0.4 M H3BO3/ 0.0075 M saccharin/0.05 M L’ascorbic acid 0.2 M CoSO4/0.7 M Na2SO4/0.4 M H3BO3/ 0.0075 M saccharin/0.05 M L’ascorbic acid
Sulfate Chloride Sulfate Ni
Chloride Sulfate
Fe
Chloride Sulfate
CoFe
Chloride
Sulfate
NiCo
Chloride Sulfate
CoNiFe Chloride
Sulfate
0.2 M NiCl2/0.7 M NaCl/0.4 M H3BO3/ 0.0075 M saccharin 0.2 M NiSO4/0.7 M Na2SO4/0.4 M H3BO3/ 0.0075 M saccharin 0.2 M FeCl2/0.7 M NaCl/0.4 M H3BO3/ 0.0075 M saccharin/0.05 M L’ascorbic acid 0.2 M FeSO4/0.7 M Na2SO4/0.4 M H3BO3/ 0.0075 M saccharin/0.05 M L’ascorbic acid 0.2 M CoCl2/x M FeCl2/0.7 M NaCl/0.4 M H3BO3/0.0075 M saccharin/0.05 M L’ascorbic acid 0.2 M CoSO4/x M FeSO4/0.7 M Na2SO4/ 0.4 M H3BO3/0.0075 M saccharin/0.05 M L’ascorbic acid 0.2 M NiCl2/x M CoCl2/0.7 M NaCl/0.4 M H3BO3/0.0075 M saccharin 0.2 M NiSO4/x M CoSO4/0.7 M Na2SO4/ 0.4 M H3BO3/0.0075 M saccharin 0.2 M NiCl2/0.15 M CoCl2/y M FeCl2/0.7 M NaCl/0.4 M H3BO3/0.0075 M saccharin/ 0.05 M L’ascorbic acid 0.2 M NiSO4/0.15 M CoSO4/y M FeSO4/ 0.7 M Na2SO4/0.4 M H3BO3/0.0075 M saccharin/0.05 M L’ascorbic acid
x : Fe 2 concentration in CoFe and Co 2 concentration in NiCo, 0.01 5/x 5/0.16 M; y : Fe 2 concentration in CoNiFe, 0.0055/y 5/ 0.16 M.
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Fig. 1. Dependence of deposit Fe content and current efficiency with Fe2 solution concentration in binary CoFe electrodeposits (a) chloride bath and (b) sulfate bath ([Co 2] /0.2 M).
were used for comparison. NaCl and Na2SO4 were used as supporting electrolytes in chloride and sulfate baths, respectively. Boric acid was added as pH buffer. Saccharin was used to reduce deposit stress and L’ascorbic acid to minimize Fe2 oxidation in CoFe and CoNiFe solutions. Solutions were exposed to air and solution pH was adjusted to 3 by adding HCl (chloride baths), H2SO4 (sulfate baths) or NaOH
(chloride and sulfate baths); experiments were conducted at 10 mA/cm2, pH 3, 10 coulombs/cm2, and room temperature without stirring. The effect of the solution [Co 2]/[Fe2] ratio on the deposit composition of binary CoFe alloy films was conducted by varying the Fe 2 concentration from 0.01 to 0.16 M, with Co 2 concentration set at 0.2 M. The effect of the solution [Ni 2]/[Co 2] ratio on the deposit
Fig. 2. Dependence of magnetic saturation (MS), coercivity (HC) and squareness (S ) of binary CoFe electrodeposits with deposit Fe content from chloride and sulfate baths (a) magnetic saturation, (b) coercivity, and (c) squareness ([Co 2] /0.2 M).
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Fig. 4. Comparison of deposit Fe content for chloride and sulfate baths as a function of solution Fe 2 concentration in binary CoFe electrodeposits ([Co2] /0.2 M).
deposits were examined with a scanning electron microscope (Stereoscan 250, Cambridge, Scientific Instruments Ltd.). Magnetic properties such as magnetic saturation (MS), coercivity (HC) and squareness (S / Mr/Ms) of electrodeposited films were determined using a vibrating sample magnetometer (Model 880, ADE Technologies Inc.).
3. Results and discussion 3.1. CoFe alloys Fig. 3. XRD patterns of Fe, Co and CoFe electrodeposits from: (a) chloride and (b) sulfate baths.
composition of binary NiCo films was investigated by varying the Co 2 concentration from 0.01 to 0.16 M, with Ni 2 concentration set at 0.2 M. The electrodeposition of ternary CoFeNi alloys was carried out with fixed Ni 2 concentration of 0.2 M, Co 2 concentration of 0.15 M, and variable concentrations of Fe 2 from 0.005 to 0.16 M. Brass was used as substrate; Fe, Co or Ni was used as soluble anode. Deposit Co, Ni and Fe contents were analyzed using atomic absorption spectroscopy (AA) (Model 280, Perkin/Elmer). Alloy compositions are given in weight percent. X-ray diffractometer (XRD) (Model 42202, Norelco, North American Philips Company Inc.) with CuKa radiation was used for the identification of the phases and the measurement of grain size in the electrodeposits. The conditions of XRD were a scanning range (40 /808) with 0.038 increments and 1 s dwell time. Microstructures of CoFe, NiCo and CoNiFe electro-
To study anion effects on current efficiencies and magnetic properties, binary CoFe thin films were electrodeposited from chloride and sulfate baths. Fig. 1 shows dependence of deposit Fe contents and current efficiencies with solution Fe 2 concentration. Deposit Fe contents practically increased linearly with increasing solution Fe2 concentration for chloride and sulfate baths. Current efficiencies (approximately /52%) was independent of solution Fe2 concentrations in chloride baths, but decreased to level at /18% in sulfate baths. Current efficiency (/52%) in chloride baths were higher than that (/18%) in sulfate baths. Hokans [17,32] investigated hydrogen evolution using Na2SO4, NaCl and NaClO4 solutions during the electrodeposition of NiFe alloys. She obtained higher limiting currents of H reduction in Na2SO4 than in NaCl and NaClO4. She observed a significant decrease in the reduction potential of Ni, Fe and NiFe in chloride baths and suggested that chloride catalyses deposition of Ni, Fe, and NiFe through the formation of an ion bridge between the electrode and the metal ion being discharged. Myung and Nobe [30] also observed that
Fig. 5. Surface morphology of Co, Fe and CoFe electrodeposits from chloride [(a) /(d)] and sulfate baths [(e) /(f)] containing L’ascorbic acid (a) pure Co, (b) 74Co24Fe, (c) 52Co48Fe, (d) pure Fe, (e) pure Co, (f) 78Co22Fe, (g) 52Co48Fe, and (h) pure Fe.
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Fig. 5
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current efficiencies in chloride baths were higher than in sulfate baths during the electrodeposition of NiFe thin film alloys. Therefore, higher current efficiency for CoFe electrodeposition in chloride baths compared to sulfate baths in our experiments may result from a similar catalytic mechanism. Fig. 2 shows the effect of deposit Fe content from chloride and sulfate baths on magnetic saturation (MS), coercivity (HC) and squareness (S ). Magnetic saturation of CoFe electrodeposits was independent of anion types and increased with increasing deposit Fe content. The observed MS variation is reasonable from the fact that MS is an intrinsic magnetic property, which is dependent only on the film composition. In CoFe alloys, when the solution Fe 2 concentrations in chloride and sulfate baths are equal, nearly same amounts of Fe were electrodeposited as shown in Fig. 1. Coercivity of CoFe electrodeposits from chloride baths increased from 17 (at 0 wt.% Fe) to 40 Oe (/ /18 wt.% Fe), and then reached a plateau. Coercivity of CoFe electrodeposits from sulfate baths increased from /17 to /60 Oe with increasing deposit Fe content with B/ /22 wt.%, and then decreased to /18 Oe at deposit Fe content of 22/50 wt.%. Squareness of CoFe alloys from chloride baths increased from /0.4 to /0.6 with increasing deposit Fe contents of B/ /20 wt.% Fe, and then reached a plateau with deposit Fe contents of / /20 wt.%. Squareness from sulfate baths decreased from 0.8 to 0.3 at B/ /20 wt.% Fe, and then increased to be 0.5 at /50 wt.% Fe. To study the influence of anion type on the microstructure of CoFe electrodeposits, XRD measurements were carried out as shown in Fig. 3 (XRD measurements of pure Co and Fe deposits were conducted for comparison). The grain size was calculated from the peak broadening using the Scherrer formula [33]. In chloride baths, CoFe electrodeposits containing deposit 32 wt.% Fe (0.08 M Fe2 concentration in the bath) exhibited a coercivity of /41 Oe. From the XRD patterns (Fig. 3a), this film consisted of hcp (002) [or bcc (110)] and bcc (200) phases with an average grain size of /63 nm. For deposits with higher Fe content, the microstructures did not changed, but the intensities of hcp (002) [or bcc (110)] phase decreased (as shown in Fig. 3a for the alloy containing 48 wt.% Fe). However, fairly constant coercivities (/40 Oe) are observed. Co22 wt.% Fe films plated from sulfate baths (0.06 M Fe 2 concentration), which had a maximum coercivity of /60 Oe, consisted of hcp (002) [or bcc (110)] planes with the average grain size of /40 nm. The coercivity of CoFe electrodeposits containing 48 wt.% Fe (0.14 M Fe2 concentration) was B/ /30 Oe. The microstructure of this film showed hcp (002) [or bcc (110)] and bcc (200) planes with an average grain size of /33 nm. Therefore, anion types did not affect on the phases of CoFe deposits, but influenced on the magnetic proper-
ties. There exist difficulties to exactly index the peak at /45.18 [either hcp (002) or bcc (110)] for CoFe alloys with Fe contents (22 wt.%) from chloride and (32 wt.%) sulfate baths (Fig. 3). The difficulties of indexing result from two reasons. One comes from that JCPDS files for the CoFe alloys are not available and other results from a shift of peak positions due to the slight difference of atomic radius between Co (0.125 nm) and Fe (0.124 nm). From the phase diagram of CoFe alloys [34], Co and Fe form a complete substitutional solid solution at room temperature. As the deposit Fe amount in CoFe electrodeposits increase, the peak position will shift to higher angle according to the following Eqs. (1) and (2) [33]; 2u of hcp (002) is 44.7628 from JCPDS file #5-727, l2 (h2 k2 l 2 ) for cubic (1) 4a2 l2 h2 hk l 2 l2 × l 2 2 for hexagonal (2) sin u 3 a2 4 × c2 sin2 u
where, l is the wavelength of radiation, a is the size of unit cell, and h, k , l are the Miller indices of reflecting plane. The peaks at /45.18 for CoFe electrodeposits containing 22/32 wt.% Fe from chloride and sulfate baths can be attributed to the hcp (002) planes rather than bcc (110) planes, because the deposit Co contents were higher than the deposit Fe contents. The deposit Fe contents was analyzed to be 22/32 wt.%. However, it is obscure to index the peaks at /45.18 for 0.14 M Fe2 concentration for chloride and sulfate baths due to the nearly same amount of deposit Co and Fe (52 and 48) as shown in Fig. 4. Pure Co electrodeposits from chloride baths containing L’ascorbic acid showed acicular morphology as shown in Fig. 5a. As the deposit Fe content in CoFe deposits increased from 24 to 48 wt.% (0.06/0.14 M Fe 2 concentration), crystalline size decreased (Fig. 5b and c). Pure Fe electrodeposits were not compact and
Fig. 6. Dependence of deposit Co content and current efficiency with Co 2 solution concentration in binary CoNi electrodeposits from sulfate bath ([Ni 2] /0.2 M).
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exhibited polyhedral crystallites. On the other hand, pure Co deposits (Fig. 5e) from sulfate baths with L’ascorbic acid had very smooth surface. As the deposit Fe content increased, similar feature (Fig. 5f and g) of the surface morphology compared to those from chloride baths was observed. The needle shape morphology at the right and upper corner part (marked with an arrow in Fig. 5g) resulted from a fast rusting of CoFe electrodeposits. Pure Fe deposits from sulfate bath (Fig. 5h) showed more compact and dense polyhedral crystallites compared to that from chloride bath (Fig. 5d).
Fig. 7. Effect of binary CoNi electrodeposits with deposit Co contents on magnetic saturation (MS), coercivity (HC) and squareness (S ) from chloride and sulfate baths (a) magnetic saturation, (b) coercivity, and (c) squareness ([Ni 2] /0.2 M).
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3.2. CoNi alloys Fig. 6 shows the dependence of deposit Co content and current efficiency with Co2 solution concentration during the electrodeposition of CoNi thin films from sulfate baths. Deposit Co content increased linearly with increasing solution Co 2 concentration. Unlike NiCo electrodeposits from chloride bath reported previously [30], current efficiency in sulfate baths showed a maximum ( /45%) for Co 2 concentrations around 0.9 M. The magnetic saturation, coercivity, and squareness of electrodeposited NiCo films from chloride and sulfate baths are shown in Fig. 7. The magnetic saturation (MS) increased linearly with increasing deposit Co content (Fig. 7a). The coercivity of Ni electrodeposit from chloride bath was /60 Oe. NiCo films electrodeposited from chloride bath exhibited a decrease in coercivity with increasing Co content up to approximately 55 wt.% Fe. Higher Co content resulted in an increase in coercivity. The same trend was also observed when sulfate bath was used. A minimum coercivity of /20 Oe was observed for the Ni and Co ratio (50:50). Fig. 7c
Fig. 8. XRD patterns of Ni, Co and CoNi electrodeposits: (a) chloride and (b) sulfate baths.
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Fig. 9
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Fig. 10. Dependence of deposit Co, Fe and Ni contents and current efficiencies with Co 2 solution concentration in ternary CoNiFe electrodeposits (a) chloride and (b) sulfate baths ([Co 2] /0.2 M).
shows the squareness of NiCo thin film alloys from sulfate and chloride baths. Squareness increased from /0.1 to /0.7 with increasing deposit Co content for B/ /70 wt.% for sulfate and chloride baths. The NiCo films electrodeposited from chloride and sulfate baths with 49/53 wt.% Co (0.08 M Co concentration) exhibited coercivities of /16 Oe for sulfate and /22 Oe for chloride baths. This film consisted of hcp (002) [or fcc (111)] and fcc (200) phases with the average grain size of /18 nm, as shown in Fig. 8. However, the NiCo electrodeposit with higher Co content, which have different coercivities (/60 Oe for sulfate and /121 Oe for chloride baths), consisted of hcp (100), hcp (002) [or fcc (111)] and hcp (101) phases with the average grain size of /13 nm. Hence, it is clear that the microstructures of NiCo alloy are independent of anion types. However, the microstructures changed with increasing deposit Co content for both chloride and sulfate baths. The NiCo electrodeposits (containing 0.15 M Co 2 concentration; deposit Co content of /80 wt.%) from
Fig. 11. Dependence of magnetic saturation (MS), coercivity (HC) and squareness (S ) of ternary CoNiFe electrodeposits with deposit Fe content from chloride and sulfate baths (a) magnetic saturation, (b) coercivity, and (c) squareness ([Co2] /0.2 M).
chloride bath exhibits higher coercivity (120 Oe) than the coercivity of 60 Oe from sulfate baths. Pure Co and NiCo thin films (Fig. 9a/c and e /g) electrodeposited from chloride and sulfate baths exhibited very smooth surface morphologies. Relatively smooth surface morphologies were observed on the surfaces of pure Ni electrodeposits (Fig. 9d and h) from chloride and sulfate baths.
Fig. 9. Surface morphology of Co, Ni and NiCo electrodeposits from chloride [(a) /(d)] and sulfate baths [(e) /(f)]; (a) pure Co, (b) 51Ni49Co, (c) 22Ni78Co, (d) pure Ni, (e) pure Co, (f) 47Ni53Co, (g) 19Ni81Co, and (h) pure Ni.
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3.3. CoNiFe alloys Fig. 10 shows the dependence of deposit Co, Fe and Ni contents and current efficiency with solution Fe 2 concentration during the electrodeposition of CoNiFe thin films from chloride and sulfate baths. In chloride baths, with increasing solution Fe 2 concentration, and thus with increasing deposit Fe content up to /40 wt.%, Co content decreased from /88 to /50 wt.% and deposit Ni content remained practically constant. Current efficiency was /75% and independent of solution Fe 2 concentration. In sulfate baths, deposit Co, Ni and Fe contents have the similar behavior of observed deposit Co, Ni and Fe contents compared to those from chloride baths. Current efficiency of binary CoNi alloys without solution Fe 2 concentration was measured to be /52%. However, current efficiency was practically constant, /75%, with increasing solution Fe2 concentration up to B/0.06 M. For higher Fe 2 concentration, it decreased to /52%. Fig. 11 shows the effect of deposit Fe contents on magnetic saturation, coercivity and squareness from
chloride and sulfate baths. Binary NiCo alloys exhibited high coercivity of /70 Oe for sulfate and chloride baths. When chloride bath is used to deposit the CoNiFe alloy, the presence of iron for deposit Fe content less than 10 wt.%, caused a sharp decrease in the coercivity down to 3 Oe. For deposit Fe content in the range 10/40 wt.%, the 80wt.%Co-5wt.%Ni15wt.%Fe alloy exhibited the maximum attained coercivity of /26 Oe. Coercivity from sulfate baths was independent of deposit Fe contents, /3 Oe. Squareness from sulfate and chloride baths decreased from 0.6 to 0.2 with increasing deposit Fe content. 80Co5Ni15Fe alloys from chloride bath, which has a maximum coercivity of 26 Oe, consisted of hcp (100), fcc (111) and hcp (002) [or bcc (110)] with the average grain size of /22 nm as shown in Fig. 12. The microstructures changed to hcp (002) [or bcc (110)] and bcc (200) with increasing the solution Fe2 concentration from 15 (0.04 M Fe 2 concentration) to 42 wt.% Fe (0.14 M Fe 2 concentration). 76wt.%Co-7wt.%Ni-17wt.%Fe from sulfate bath, which has a low coercivity of 9 Oe, consisted of hcp (002) [or bcc (110)] and bcc (200) with the average grain size of /42 nm. CoNiFe deposits with 46 wt.% Fe (0.1 M Fe concentration) exhibited hcp (002) [or bcc (110)] phases. Therefore, anion type in CoNiFe alloys influenced on the microstructures and the resulting coercivities. The addition of Fe into electrodeposited CoNi alloys for chloride and sulfate baths caused the decrease of the coercivity. Binary CoNi from chloride and sulfate baths showed very smooth surfaces as shown in Fig. 13. As solution Fe 2 concentration in CoNiFe electrodeposits increased, the decrease of the size of polyhedral crystallites were observed.
4. Conclusion
4.1. CoFe alloys
Fig. 12. XRD patterns of CoNi and CoNiFe electrodeposits (a) chloride and (b) sulfate baths.
Current efficiencies of the electrodeposited CoFe thin film alloys from chloride baths (/52%) were independent of the Fe 2 solution concentrations. The current efficiencies from sulfate baths decreased to be B/ /20% with increasing the solution Fe 2 concentrations. Current efficiencies of the electrodeposited CoFe thin film alloys from chloride baths were significantly higher than that from sulfate baths. The higher current efficiencies in chloride baths may be the results of a low H2 limiting current compared to those from sulfate baths. Magnetic saturation (MS) of electrodeposited CoFe films both in chloride and sulfate baths increased from /1.5 to /2.3 T as deposit Fe content increased. Coercivities of the electrodeposited CoFe thin films increased from /20 to /60 Oe in sulfate baths and from /20 to /40 Oe in
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Fig. 13. Surface morphology of CoNi and CoNiFe electrodeposits from chloride [(a) /(c)] and sulfate baths [(d) /(f)]: (a) 80Co20Ni, (b) 80Co5Ni15Fe, (c) 49Co5Ni46Fe, (d) 80Co20Ni, (e) 76Co7Ni17Fe, (f) 49Co5Ni46Fe.
chloride baths as the deposit Co content increased up to 20 wt.%. A minimum squareness of 0.2 was observed in sulfate baths at /18 wt.% Co, while a maximum squareness of 0.7 was observed in chloride solution at /18 wt.% Co. Anion types affected the magnetic properties. However, no effects of anion types on the phases and surface morphology in CoFe electrodeposits were observed. Surface morphology were affected by solution Fe 2 concentration.
4.2. NiCo alloys Current efficiencies (/75%) from chloride baths were higher than that (25 /45%) from sulfate baths. Magnetic saturation of the alloys with increasing deposit Co contents exhibited similar linear behavior for chloride and sulfate baths. The electrodeposits of NiCo thin film alloys from chloride baths showed higher coercivities (120 Oe) than that from sulfate baths (/60 Oe) at the
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deposit Co contents of /80 wt.%. Electrodeposited NiCo thin film alloys from both chloride and sulfate baths exhibited the similar linear behavior of coercivities up to /50 wt.% Co. The microstructure of NiCo deposits did not depend on anion types, but on deposit Co contents. NiCo electrodeposits from chloride and sulfate baths exhibited very smooth surface morphology. 4.3. NiCoFe alloys Current efficiencies from chloride solutions (/75%) were independent of solution Fe 2 concentrations, whereas those from sulfate solutions gradually decreased from /75 to /50% as solution Fe 2 concentrations increased. Coercivity of electrodeposited CoNi thin films from chloride and sulfate baths measured to be /70 Oe. Coercivities of CoNiFe deposits from sulfate baths sharply decreased to be 3 Oe with increasing the deposit Fe contents. The squarenesses in chloride and sulfate baths showed gradual decreasing tendency from /0.6 to /0.2 in sulfate and 0.1 in chloride baths. Microstructures depend on anion types and the addition of Fe into CoNi deposits caused the decrease of the coercivity. CoNeFe electrodeposits had the surface morphology of polyhedral crystallites.
Acknowledgements This work was supported by the DARPA MEMS program DAB63-99-1-0020 and the Research Grant of Kwangwoon University in 2001.
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