Electrochemical and Solid-State Letters, 7 共10兲 C121-C124 共2004兲
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1099-0062/2004/7共10兲/C121/4/$7.00 © The Electrochemical Society, Inc.
Compositional Control in Electrodeposition of FePt Films J. J. Mallett,a,z E. B. Svedberg,a,b S. Sayan,a,c A. J. Shapiro,a L. Wielunski,c T. E. Madey,c W. F. Egelhoff, Jr.,a and T. P. Moffata,* a
National Institute of Standards and Technology, Gaithersburg, Maryland 20899, USA Seagate Technology, Pittsburgh, Pennsylvania 15222, USA Rutgers, The State University, Piscataway, New Jersey 08854, USA
b c
Fe-Pt thin-film alloys have been grown by electrodeposition at potentials positive to that required to deposit elemental Fe. X-ray diffraction studies indicate the formation of fine grained face centered cubic alloys, while Rutherford backscattering spectrscopy and energy-dispersive X-ray spectroscopy reveal substantial incorporation of oxygen in the FePt deposits. The Fe-Pt codeposition process is driven by the negative enthalpy associated with alloy formation. The experimentally determined relationship between alloy composition and the iron group underpotential was found to be in reasonable agreement with free energy calculations for the binary alloy system, based on thermochemical data. © 2004 The Electrochemical Society. 关DOI: 10.1149/1.1792251兴 All rights reserved. Manuscript submitted January 7, 2004; revised manuscript received March 4, 2004. Available electronically September 15, 2004.
There is currently considerable interest in FePt as a high-density perpendicular recording medium, due to the high magnetocrystalline anisotropy of the L1 0 phase. The significant challenges of achieving an appropriately oriented L1 0 phase, while maintaining the required grain 共or particle兲 size of less than 5 nm, remain unsolved, despite considerable effort.1-3 FePt has attracted additional interest due to its shape-memory properties, and Invar effects, both of potential utility in microelectromechanical systems 共MEMS兲.4 In addition to these useful physical properties Fe-Pt and related alloys have potential application as CO-tolerant electrocatalyst in polymer electrolyte fuel cells.5,6 In all the above applications, process control during synthesis is of central importance. A variety of means have been used to produce Fe-Pt and similar alloys ranging from vacuum methods like MBE and sputtering2,3,7,8 to electrodepositon9-13 of thin films or fine particle production by solution phase chemical reduction.1,14-16 One particular advantage of electrochemical methods is the ability to easily specify and control the supersaturation while monitoring its effect on growth kinetics. Herein we examine the factors affecting alloy composition during electrodeposition from an aqueous electrolyte containing chlorocomplexes of platinum and iron. Traditional alloy deposition studies largely focus on growth in the overpotential domain.17 In this case, the composition is controlled by the relative rate of reduction of the constituents occurring in a potential regime where both species can be deposited in their elemental form. The desired differential activity, required for a particular alloy composition, is achieved by judicious choice of component concentrations and complex forming ligands. In contrast, in this study the use of the free energy of alloy formation to control alloy composition is demonstrated. The thermodynamic basis for alloy formation is well established. In fact, high temperature electrochemical potential 共emf兲 measurements have contributed significantly toward the understanding of phase equilibria and the construction of phase diagrams. A necessary condition for binary alloy A 1⫺x B x formation is equality of the electrochemical potential of the respective constituents EA ⫹ A ⫽ EB ⫹ B
关1兴
where E i is the Nernst potential given by E i ⫽ E io ⫹
a iion RT ln alloy zF ai
关2兴
The free energy of alloy formation is reflected in the activity of the denominator while i represents the kinetic overpotential or degree of supersaturation. Inspection reveals that alloying 共i.e., an activity
* Electrochemical Society Active Member. z
E-mail:
[email protected]
less than 1兲 results in a positive shift of the reversible potential for each constituent away from that characteristic of the elemental state. This is equivalent to the formal description of the underpotential deposition 共upd兲 phenomenon.18 Historically, this term has been used to describe the deposition of submonolayer quantities of a metal onto a foreign substrate wherein the activity was less than unity. More generally, it is recognized that deviations from a simple linear activity-composition relationship can arise from a combination of effects ranging from alloying interactions to composition driven changes in the surface energy and associated double layer effects. Several early studies of upd systems revealed the occurrence of alloy formation by interdiffusion of the upd overlayer and substrate 共see, for example, Ref. 18-20兲. The subject of Fe group upd on Pt and other metals has received very limited study;21 the most recent being a report of upd of Fe, Co, and Ni on Pt and Au in a nonaqueous electrolyte, although no significant evidence of intermixing or alloying was observed.22 In contrast to traditional upd studies, the role of upd in the direct formation of thin film alloys by codeposition has received less attention. Two prototypical cases where the composition of the solid has been correlated with the free energy of phase formation are CdTe compound formation23,24 and NiAl alloy deposition.25 This approach has also been used to produce Cu-Cd,26 Cu-Sn,27 Cu-Au,28 Cu-Pt,29 Cu-Pd,29 and ZnFe,30 as well as a variety of aluminum31 alloys. This approach is clearly distinct from traditional alloy deposition studies that focus on growth in the overpotential domain. The quantitative connection between underpotential alloy deposition and thermochemical data is given by the combination of Eq. 2 and a ialloy ⫽ ␥ iX i ⫽ exp
冉
⫺⌬G iM RT
冊
关3兴
where the activity coefficient, ␥ i , reflects deviations from ideality of the partial molar free energy of alloying constituent i, ⌬G iM , for the particular phase. The latter may be evaluated from tabulated integral free energy data or related constitutive equations that are available for a wide range of alloys. In the Fe-Pt system, high-temperature measurements of the equilibria between Fe-Pt alloys, Fe-oxides, and oxygen 共1473-1673 K兲 reveal that fcc Fe-Pt solid solutions exhibit strong negative deviations from ideality that may be described by an asymmetric regular solution model alloy RT ln ␥ Fe ⫽ 共 W G1 ⫹ 2 共 W G2 ⫺ W G1 兲 x Fe兲共 1 ⫺ x Fe兲 2
关4兴
where the temperature and composition-independent constants are W G1 ⫽ ⫺138.0 ⫾ 3.3 kJ/mol and W G2 ⫽ ⫺90.8 ⫾ 24.0 kJ/mol. 32
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Electrochemical and Solid-State Letters, 7 共10兲 C121-C124 共2004兲
Figure 1. The separated cell designed to maintain a low activity of Fe3⫹ and O2 in the working compartment.
In the present work this result is extrapolated to 298 K for comparison to the experimentally determined potential dependence of alloy formation. Ordering is an important aspect for systems with strong A-B interactions. Calorimetric measurements for FePt reveal the facecentered cubic 共fcc兲 to L1 o transformation, involves an enthalpy of ⫺10.2 ⫾ 2.1 kJ/mol, although this only proceeds at higher temperatures, e.g., 673 K for heating rates of 20 K/min.33 Kinetic limitations on the development of long range order during low temperature synthesis results in fcc solid solutions rather than the lower energy ordered phases.3,7-16 This effect is one of the key obstacles to the production of ordered L1 o Fe-Pt thin films as media for magnetic storage. Experimental Fe-Pt alloys were electrodeposited from an aqueous electrolyte consisting of 0.11 mol/L FeCl2 共99%兲, 0.3-30 mmol/L PtCl4 共99.99%兲 and 0.5 mol/L NaCl 共99% min兲. The solutions were made using 18 M⍀ water and adjusted to pH 2.5 (⬇3.2 mmol/L H3 O⫹). by the addition of hydrochloric acid. The electrolyte was stable against spontaneous Pt decomposition as no visible colloids formed during the experiments. However, to establish a stable environment for the deposition of iron, specific measures were taken to avoid or minimize the production of Fe3⫹. A membrane separated cell, shown in Fig. 1, enabled the use of a Fe2⫹-free NaCl anolyte (⬇0.4 L) while a Pt auxiliary electrode in the cathode compartment (⬇0.4 L) was used to reduce dissolved O2 along with any Fe3⫹. A saturated calomel reference electrode 共SCE兲 was held at a fixed position relative to the working electrode and all quoted potentials refer to this scale. A bipotentiostat was used to independently control the working electrode and auxiliary Pt grid electrode. In a typical experiment the FeCl2 -NaCl catholyte was preelectrolyzed at ⫺0.4 V for an extended period prior to the addition of PtCl4 . The electrolyte was first de-aerated by bubbling N2 for 30 min, followed by blanketing the head space of the cell with flowing N2 for the duration of the experiment. After 2 min of pre-electrolysis a cathodic current of 10 mA was measured at the auxiliary Pt electrode that diminished to 20 A after 10 h, indicating a decrease of the combined 关 Fe3⫹兴 and 关 O2 兴 of almost three orders of magnitude. The PtCl4 salt was then added to the cell and, simultaneously, the Pt auxiliary grid potential was increased to 0 V in an attempt to minimize the loss of platinum due to plating on the grid. The loss was estimated to be under 2.5 mg/h, compared to the 300 mg 共3 mmol/L兲 initially added to the cell. This was balanced by the equivalent addition of PtCl4 to the cell every 2 h. FePt films were deposited by immersing substrates into the cell with the potential applied. After a fixed period of time the specimens
Figure 2. CVs collected at 10 mV/s in 共a兲 0.5 mol/L NaCl containing 共b兲 3 mmol/L PtCl4 , 共c兲 0.11 mol/L FeCl2 , and 共d兲 3 mmol/L PtCl4 and 0.11 mol/L FeCl2 .
were quickly removed and rinsed before disconnecting from the potentiostat. Deposition was performed at room temperature (⬇294 K) under quiescent conditions. Substrates were prepared by electron-beam evaporation of ⬇100 nm of copper, or silver, onto Si共100兲 wafers which either had a ⬇9 nm Ti adhesion layer or for purposes of texturing had been H-terminated with a 10% HF solution. Results and Discussion Cyclic voltammetry 共CV兲 was used to investigate the deposition process on mechanically polished Pt plate working electrodes. Figure 2a shows the voltammetric response of a Pt electrode in deaerated 0.5 mol/L NaCl. The current rise at ⫺0.2 V is due to proton reduction (E H ⫹ /H2 ⫽ ⫺0.213 V for aH2 ⫽ 10⫺6 ) that is diffusion limited below ⫺0.45 V. This is followed by water reduction at ⫺ 0.8 V. On the positive going sweep the reverse reactions are evident, with H2 oxidation occurring at ⫺0.5 V. The shift in the H⫹/H2 potential toward more negative values is due to a change in the interfacial pH (⬇4.4) associated with proton depletion combined with enrichment of H2 near the interface 共i.e., gas bubbles attached to the electrode兲. The addition of 3 mmol/L PtCl4 results in the onset of platinum deposition below 0.3 V. As shown in Fig. 2b the reaction is independent of potential below 0.0 V, most likely due
Electrochemical and Solid-State Letters, 7 共10兲 C121-C124 共2004兲 to transport limitation. Likewise, proton reduction is transport limited beyond ⫺0.35 V; a slightly more positive value than that observed in the absence of Pt deposition reflecting the enhanced catalytic nature of freshly deposited platinum. Interestingly, bright specular Pt films, greater than 1 m in thickness, were obtained by potentiostatic growth between 0.0 and ⫺0.7 V from an electrolyte containing 30 mmol/L PtCl4 . This observation is surprising in light of the Mullins-Sekerka instability that is usually associated with transport limited deposition reactions.34 Deposition at more negative potentials results in rough black deposits. Voltammetry in the FeCl2 -NaCl electrolyte 共Fig. 2c兲 reveals the onset of diffusion limited proton reduction at ⫺0.45 followed by iron deposition and water reduction below ⫺0.8 V. During the reverse sweep iron dissolution occurs between ⫺0.75 and ⫺0.4 V and involves at least two processes as reflected by the two peaks in the stripping wave. At positive potentials beyond ⫹0.3 V oxidation of Fe2⫹ to Fe3⫹ is evident. To obtain an estimate of the reversible Fe/Fe2⫹ potential the voltammetric behavior of an iron wire electrode 共not shown兲 was examined. The open-circuit potential 共OCP兲 was ⫺0.72 V, that is close to the zero-current potential, ⫺0.75 V observed in Fig. 2c. This mixed potential is associated with the balance between iron dissolution and proton reduction. Small potential voltammetric excursions of the iron wire about the OCP indicate that the reversible potential for the Fe/Fe2⫹ reaction is close to ⫺0.75 V. Figure 2d shows the voltammetric behavior for the complete Fe-Pt plating electrolyte. At potentials below 0 V the current exceeds that associated with platinum deposition and proton reduction. The disparity increases with decreasing potential and is attributed to iron codeposition with platinum. A small peak is observed at ⫺0.53 V followed by a marked increase in the deposition rate at potentials below ⫺0.7 V. On the reverse sweep the onset of iron dissolution is evident at ⫺0.68 V although the stripping wave is displaced to more positive potentials, ⬇0.5 V, compared to pure iron. Inhibition of iron dissolution is due to (i) stabilization provided by alloy formation combined with the kinetic resistance associated with dissolution through the platinum enriched overlayer that develops with dealloying and/or (ii) continued platinum deposition that occurs during the positive-going sweep. Knowledge of the current efficiency of the partial deposition reactions is required to calculate the alloy composition from these measurements. Alloy formation was unambiguously demonstrated by growing a series of FePt films by potentiostatic deposition over a range of potentials followed by compositional and structural analysis. Films deposited between ⫺0.2 and ⫺0.7 V were specular even for thicknesses of several micrometers corresponding to several hours of deposition. In contrast, films deposited at more negative potential, i.e., ⫺0.8 V, were black due to high surface roughness. This is similar to the results described earlier for the deposition of elemental platinum. The compositions of the thin films were measured by energy dispersive X-ray spectroscopy 共EDS兲 and Rutherford backscattering spectrometry 共RBS兲. An 2 MeV He2⫹ RBS spectrum for a FePt film deposited on a Cu共poly兲/Ti/Si共100兲 substrate at ⫺0.7 V is shown in Fig. 3. A substantial quantity of oxygen was incorporated in the films. The oxygen levels appear to scale with the iron concentration in the deposit. The quantitative fit indicates the film is 107 nm thick with an iron content of 0.53 atomic fraction with respect to Pt although the film contains ⬇0.25 atomic fraction of O with respect to Fe and Pt. The metal composition, i.e., Fe and Pt, determined by EDS and RBS differ by less than 0.10 atomic fraction. This is ascribed to a systematic error associated with the finite film thickness algorithm used for the quantitative EDS analysis. Structural characterization was performed by symmetric X-ray diffraction 共XRD兲. In Fig. 4, a -2 XRD spectrum is shown for a ⬇100 nm thick FePt film grown on a Cu共100兲//Si共100兲兲 substrate at ⫺0.7 V. The film has an fcc structure and is slightly textured in the
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Figure 3. RBS spectrum of a 107 nm thick FePt film deposited at ⫺0.7 V SCE and a four-layer FePt/Cu/Ti/Si model fit.
共100兲 direction as suggested by the FePt 共200兲 shoulder on the Cu共200兲 substrate peak. The peaks corresponding to FePt 共111兲 and 共311兲 scattering are also evident. A coherence length of 3.5 nm was calculated from the peak widths using the Scherrer formula. This indicates that the film has a very fine grain size. For different films the peak positions shift to higher diffraction angles as the film growth potential was decreased 共not shown兲. This reflects the iron content in the films and was quantified by combining lattice parameter measurements 共from the 共111兲 peak兲 with Vegard’s linear approximation. For the same samples the coherence length varied between 8.0 and 1.9 nm. No evidence for the formation of crystalline iron oxides was observed despite the high oxygen content in the films. Importantly, annealing the films grown on copper substrates at ⫺0.65 V results in the formation of L1 o phase. A detailed report of the effects of annealing on the structural and magnetic properties will be given elsewhere. The RBS, EDS, and XRD measurements reveal the potential dependence of FePt film composition as summarized in Fig. 5. By referencing the potential to the estimated Fe/Fe2⫹ reversible poten-
Figure 4. XRD spectrum of a 100 nm thick FePt film deposited at ⫺700 mV.
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Electrochemical and Solid-State Letters, 7 共10兲 C121-C124 共2004兲 electrochemical atomic layer epitaxy 共ECALE兲36 may offer an interesting alternative for obtaining layer-by-layer growth of ordered L1 0 FePt and related phases. Conclusions A series of bright fine grained fcc Fe-Pt thin film alloys have been grown at potentials positive to that required to deposit elemental Fe. The codeposition process is driven by the negative enthalpy associated with alloy formation. The relationship between alloy composition and the iron group underpotential was established experimentally and found to be in reasonable agreement with free energy calculations based on available thermochemical data. The construct is broadly applicable and of particular interest for alloy systems such as Co-Pt, Fe-Pd, and related combinations. The National Institute of Standards and Technology assisted in meeting the publication costs of this article.
References
Figure 5. The experimentally determined composition of FePt alloys, neglecting the oxygen content, compared to prediction based on thermochemical data 共Eq. 4兲.
tial (⫺0.75 V) the top scale shows that codeposition of Fe takes place by underpotential deposition. The solid curve is the result of free energy calculations for the Fe-Pt binary system, based on Eq. 4. The favorable agreement between fcc FePt thermochemical data and the electroplating experiments, despite the presence of oxygen in the film, suggests that thermodynamic factors dominate over kinetics in determining alloy composition. Significant oxygen incorporation has been noted previously for iron and nickel deposition from simple chloride solution at pH of 3.0.35 The apparent insensitivity to oxygen may simply be a fortuitous outcome of an overly simplest comparison or may reflect minimal perturbation of the interaction between Fe-Pt by interstitial oxygen. We also note for the sake of completeness that the interstitial oxygen content of the alloys used in Ref. 32, i.e., Eq. 4, was not reported. The argument for thermochemically controlled codeposition is further supported by RBS measurements demonstrating insensitivity of alloy composition to variations in the PtCl4 concentration. Specifically, a series of film were grown at ⫺0.6 V while PtCl4 was varied over two orders of magnitude resulting in variations of the iron content of less than 0.05 atomic fraction of metal. Thus, alloy formation proceeds at a rate determined by the reduction of the platinum complex accompanied by kinetically facile iron codeposition. Related alloy systems, such as Co-Pt and Ni-Pt, exhibit similar negative deviations from ideal solution behavior and are expected to demonstrate similar codeposition behavior. Preliminary experiments provide solid support for this conclusion. A significant limitation on calculating the potential dependence of codeposition is the lack of reliable low-temperature bulk phase thermochemical data. This is further hampered by limited knowledge of interfacial energies, segregation effects, etc. Nevertheless, the codeposition process is clearly relevant to determining alloy composition in related processes such as particle production by chemical reduction. In this case, the electrochemical potential is determined by the strength of the reducing agent and concentration of the reactants as opposed to an external potentiostat. In a related fashion, the strong interactions between iron group metals and Pt or Pd suggests caution in interpreting prior reports of Fe-Pt, Co-Pt multilayer production from single electrolyte systems. Many of these studies are incorrectly based on the linear combination of elemental behaviors. In contrast
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