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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,*

aNational Institute of Standards and Technology, Gaithersburg, Maryland 20899, USAb

Seagate Technology, Pittsburgh, Pennsylvania 15222, USAcRutgers, The State University, Piscataway, New Jersey 08854, USA

Fe-Pt thin-film alloys have been grown by electrodeposition at potentials positive to that required to deposit elemental Fe. X-raydiffraction studies indicate the formation of fine grained face centered cubic alloys, while Rutherford backscattering spectrscopyand energy-dispersive X-ray spectroscopy reveal substantial incorporation of oxygen in the FePt deposits. The Fe-Pt codepositionprocess is driven by the negative enthalpy associated with alloy formation. The experimentally determined relationship betweenalloy composition and the iron group underpotential was found to be in reasonable agreement with free energy calculations for thebinary alloy system, based on thermochemical data. 2004 The Electrochemical Society. DOI: 10.1149/1.1792251 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-densityperpendicular recording medium, due to the high magnetocrystalline

anisotropy of the L10 phase. The significant challenges of achieving

an appropriately oriented L10 phase, while maintaining the requiredgrain or particle size of less than 5 nm, remain unsolved, despiteconsiderable effort.1-3 FePt has attracted additional interest due to itsshape-memory properties, and Invar effects, both of potential utilityin microelectromechanical systems MEMS .4 In addition to theseuseful physical properties Fe-Pt and related alloys have potentialapplication as CO-tolerant electrocatalyst in polymer electrolyte fuelcells.5,6 In all the above applications, process control during synthe-sis is of central importance.

A variety of means have been used to produce Fe-Pt and similaralloys ranging from vacuum methods like MBE and sputtering 2,3,7,8

to electrodepositon9-13 of thin films or fine particle production bysolution phase chemical reduction.1,14-16 One particular advantage ofelectrochemical methods is the ability to easily specify and controlthe supersaturation while monitoring its effect on growth kinetics.

Herein we examine the factors affecting alloy composition dur-

ing electrodeposition from an aqueous electrolyte containing chloro-complexes of platinum and iron. Traditional alloy deposition studieslargely focus on growth in the overpotential domain. 17 In this case,the composition is controlled by the relative rate of reduction of theconstituents occurring in a potential regime where both species canbe deposited in their elemental form. The desired differential activ-ity, required for a particular alloy composition, is achieved by judi-cious choice of component concentrations and complex formingligands. In contrast, in this study the use of the free energy of alloyformation to control alloy composition is demonstrated.

The thermodynamic basis for alloy formation is well established.In fact, high temperature electrochemical potential emf measure-ments have contributed significantly toward the understanding ofphase equilibria and the construction of phase diagrams. A necessarycondition for binary alloy A 1 xBx formation is equality of the elec-trochemical potential of the respective constituents

EA A EB B 1

where Ei is the Nernst potential given by

Ei Eio

RT

zFln

a iion

a ialloy 2

The free energy of alloy formation is reflected in the activity of thedenominator while i represents the kinetic overpotential or degreeof supersaturation. Inspection reveals that alloying i.e., an activity

less than 1 results in a positive shift of the reversible potential foreach constituent away from that characteristic of the elemental state.This is equivalent to the formal description of the underpotential

deposition upd phenomenon.18 Historically, this term has beenused to describe the deposition of submonolayer quantities of ametal onto a foreign substrate wherein the activity was less thanunity. More generally, it is recognized that deviations from a simplelinear activity-composition relationship can arise from a combina-tion of effects ranging from alloying interactions to compositiondriven changes in the surface energy and associated double layereffects. Several early studies of upd systems revealed the occurrenceof alloy formation by interdiffusion of the upd overlayer and sub-strate see, for example, Ref. 18-20 . The subject of Fe group upd onPt and other metals has received very limited study;21 the mostrecent being a report of upd of Fe, Co, and Ni on Pt and Au in anonaqueous electrolyte, although no significant evidence of inter-mixing or alloying was observed.22

In contrast to traditional upd studies, the role of upd in the directformation of thin film alloys by codeposition has received less at-

tention. Two prototypical cases where the composition of the solidhas been correlated with the free energy of phase formation areCdTe compound formation23,24 and NiAl alloy deposition.25 Thisapproach 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 depo-sition studies that focus on growth in the overpotential domain.

The quantitative connection between underpotential alloy depo-sition and thermochemical data is given by the combination of Eq. 2and

a ialloy

iXi expG i

M

RT 3

where the activity coefficient, i , reflects deviations from ideality ofthe partial molar free energy of alloying constituent i , G i

M , for theparticular phase. The latter may be evaluated from tabulated integralfree energy data or related constitutive equations that are availablefor a wide range of alloys.

In the Fe-Pt system, high-temperature measurements of the equi-libria between Fe-Pt alloys, Fe-oxides, and oxygen 1473-1673 Kreveal that fcc Fe-Pt solid solutions exhibit strong negative devia-tions from ideality that may be described by an asymmetric regularsolution model

RT ln Fealloy

WG1 2 WG2 WG1 xFe 1 xFe2 4

where the temperature and composition-independent constants areWG1 138.0 3.3 kJ/mol and WG2 90.8 24.0 kJ/mol.

32* Electrochemical Society Active Member.z E-mail: jon.mallett@nist.gov

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In the present work this result is extrapolated to 298 K for compari-

son to the experimentally determined potential dependence of alloyformation.

Ordering is an important aspect for systems with strong A-Binteractions. Calorimetric measurements for FePt reveal the face-centered cubic fcc to L1o transformation, involves an enthalpy of10.2 2.1 kJ/mol, although this only proceeds at higher tempera-tures, e.g., 673 K for heating rates of 20 K/min.33 Kinetic limitationson the development of long range order during low temperaturesynthesis results in fcc solid solutions rather than the lower energyordered phases.3,7-16 This effect is one of the key obstacles to the

production of ordered L1o Fe-Pt thin films as media for magneticstorage.

Experimental

Fe-Pt alloys were electrodeposited from an aqueous electrolyteconsisting of 0.11 mol/L FeCl2 99% , 0.3-30 mmol/L PtCl4

99.99% and 0.5 mol/L NaCl 99% min . The solutions were madeusing 18 M water and adjusted to pH 2.5 (3.2 mmol/L H3O

).by the addition of hydrochloric acid. The electrolyte was stableagainst spontaneous Pt decomposition as no visible colloids formedduring the experiments. However, to establish a stable environmentfor the deposition of iron, specific measures were taken to avoid orminimize 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 Fe

3 . Asaturated calomel reference electrode SCE was held at a fixedposition relative to the working electrode and all quoted potentialsrefer to this scale. A bipotentiostat was used to independently con-trol the working electrode and auxiliary Pt grid electrode.

In a typical experiment the FeCl2-NaCl catholyte was pre-

electrolyzed at 0.4 V for an extended period prior to the addition

of PtCl4 . The electrolyte was first de-aerated by bubbling N2 for 30min, followed by blanketing the head space of the cell with flowing

N2 for the duration of the experiment. After 2 min of pre-electrolysisa cathodic current of 10 mA was measured at the auxiliary Pt elec-trode that diminished to 20 A after 10 h, indicating a decrease ofthe combined Fe3 and O2 of almost three orders of magnitude.

The PtCl4 salt was then added to the cell and, simultaneously, the Ptauxiliary grid potential was increased to 0 V in an attempt to mini-mize the loss of platinum due to plating on the grid. The loss wasestimated to be under 2.5 mg/h, compared to the 300 mg 3 mmol/Linitially added to the cell. This was balanced by the equivalent ad-dition of PtCl4 to the cell every 2 h.

FePt films were deposited by immersing substrates into the cellwith the potential applied. After a fixed period of time the specimens

were quickly removed and rinsed before disconnecting from thepotentiostat. Deposition was performed at room temperature

(294 K) under quiescent conditions. Substrates were prepared byelectron-beam evaporation of 100 nm of copper, or silver, ontoSi 100 wafers which either had a 9 nm Ti adhesion layer or forpurposes of texturing had been H-terminated with a 10% HFsolution.

Results and Discussion

Cyclic voltammetry CV was used to investigate the depositionprocess on mechanically polished Pt plate working electrodes. Fig-ure 2a shows the voltammetric response of a Pt electrode in de-

aerated 0.5 mol/L NaCl. The current rise at 0.2 V is due to proton

reduction (EH /H2 0.213 V for aH2 106) 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 inthe interfacial pH (4.4) associated with proton depletion com-bined with enrichment of H2 near the interface i.e., gas bubblesattached to the electrode . The addition of 3 mmol/L PtCl4 results inthe onset of platinum deposition below 0.3 V. As shown in Fig. 2bthe reaction is independent of potential below 0.0 V, most likely due

Figure 1. The separated cell designed to maintain a low activity of Fe3 and

O2 in the working compartment.

Figure 2. CVs collected at 10 mV/s in a 0.5 mol/L NaCl containing b 3

mmol/L PtCl4 , c 0.11 mol/L FeCl2 , and d 3 mmol/L PtCl4 and 0.11mol/L FeCl2 .

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to transport limitation. Likewise, proton reduction is transport lim-

ited beyond 0.35 V; a slightly more positive value than that ob-served in the absence of Pt deposition reflecting the enhanced cata-lytic nature of freshly deposited platinum. Interestingly, brightspecular Pt films, greater than 1 m in thickness, were obtained bypotentiostatic growth between 0.0 and 0.7 V from an electrolyte

containing 30 mmol/L PtCl4 . This observation is surprising in lightof the Mullins-Sekerka instability that is usually associated withtransport limited deposition reactions.34 Deposition at more negativepotentials results in rough black deposits.

Voltammetry in the FeCl2-NaCl electrolyte Fig. 2c reveals theonset of diffusion limited proton reduction at 0.45 followed by

iron deposition and water reduction below 0.8 V. During the re-

verse sweep iron dissolution occurs between 0.75 and 0.4 V andinvolves 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 thevoltammetric behavior of an iron wire electrode not shown wasexamined. The open-circuit potential OCP was 0.72 V, that isclose to the zero-current potential, 0.75 V observed in Fig. 2c.

This mixed potential is associated with the balance between irondissolution and proton reduction. Small potential voltammetric ex-cursions 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 ex-ceeds that associated with platinum deposition and proton reduction.The disparity increases with decreasing potential and is attributed toiron 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 pureiron. Inhibition of iron dissolution is due to ( i) stabilization pro-vided by alloy formation combined with the kinetic resistance asso-ciated with dissolution through the platinum enriched overlayer that

develops with dealloying and/or ( ii ) continued platinum depositionthat occurs during the positive-going sweep. Knowledge of the cur-rent efficiency of the partial deposition reactions is required to cal-culate the alloy composition from these measurements.

Alloy formation was unambiguously demonstrated by growing aseries of FePt films by potentiostatic deposition over a range ofpotentials followed by compositional and structural analysis. Films

deposited between 0.2 and 0.7 V were specular even for thick-nesses of several micrometers corresponding to several hours ofdeposition. In contrast, films deposited at more negative potential,

i.e., 0.8 V, were black due to high surface roughness. This issimilar to the results described earlier for the deposition of elementalplatinum.

The compositions of the thin films were measured by energydispersive X-ray spectroscopy EDS and Rutherford backscatteringspectrometry RBS . An 2 MeV He2 RBS spectrum for a FePt film

deposited on a Cu poly /Ti/Si 100 substrate at 0.7 V is shown inFig. 3. A substantial quantity of oxygen was incorporated in thefilms. The oxygen levels appear to scale with the iron concentrationin the deposit. The quantitative fit indicates the film is 107 nm thickwith an iron content of 0.53 atomic fraction with respect to Pt al-

though the film contains 0.25 atomic fraction of O with respect toFe and Pt. The metal composition, i.e., Fe and Pt, determined byEDS and RBS differ by less than 0.10 atomic fraction. This is as-cribed to a systematic error associated with the finite film thicknessalgorithm used for the quantitative EDS analysis.

Structural characterization was performed by symmetric X-raydiffraction XRD . In Fig. 4, a -2 XRD spectrum is shown for a100 nm thick FePt film grown on a Cu 100 //Si 100 substrate at0.7 V. The film has an fcc structure and is slightly textured in the

100 direction as suggested by the FePt 200 shoulder on theCu 200 substrate peak. The peaks corresponding to FePt 111 and 311 scattering are also evident. A coherence length of 3.5 nm wascalculated from the peak widths using the Scherrer formula. Thisindicates that the film has a very fine grain size. For different filmsthe peak positions shift to higher diffraction angles as the filmgrowth potential was decreased not shown . This reflects the ironcontent in the films and was quantified by combining lattice param-eter measurements from the 111 peak with Vegards linear ap-proximation. For the same samples the coherence length varied be-tween 8.0 and 1.9 nm. No evidence for the formation of crystallineiron oxides was observed despite the high oxygen content in thefilms. 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 propertieswill be given elsewhere.

The RBS, EDS, and XRD measurements reveal the potentialdependence of FePt film composition as summarized in Fig. 5. By

referencing the potential to the estimated Fe/Fe2 reversible poten-

Figure 3. RBS spectrum of a 107 nm thick FePt film deposited at 0.7 VSCE and a four-layer FePt/Cu/Ti/Si model fit.

Figure 4. XRD spectrum of a 100 nm thick FePt film deposited at

700 mV.

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tial ( 0.75 V) the top scale shows that codeposition of Fe takesplace by underpotential deposition. The solid curve is the result offree energy calculations for the Fe-Pt binary system, based on Eq. 4.The favorable agreement between fcc FePt thermochemical data andthe electroplating experiments, despite the presence of oxygen in thefilm, suggests that thermodynamic factors dominate over kinetics indetermining alloy composition. Significant oxygen incorporation hasbeen noted previously for iron and nickel deposition from simplechloride solution at pH of 3.0.35 The apparent insensitivity to oxy-gen may simply be a fortuitous outcome of an overly simplest com-parison or may reflect minimal perturbation of the interaction be-tween Fe-Pt by interstitial oxygen. We also note for the sake ofcompleteness that the interstitial oxygen content of the alloys usedin Ref. 32, i.e., Eq. 4, was not reported.

The argument for thermochemically controlled codeposition isfurther supported by RBS measurements demonstrating insensitivity

of alloy composition to variations in the PtCl4 concentration. Spe-

cifically, a series of film were grown at 0.6 V while PtCl4 wasvaried over two orders of magnitude resulting in variations of theiron content of less than 0.05 atomic fraction of metal. Thus,alloy formation proceeds at a rate determined by the reductionof the platinum complex accompanied by kinetically facile ironcodeposition.

Related alloy systems, such as Co-Pt and Ni-Pt, exhibit similarnegative deviations from ideal solution behavior and are expected todemonstrate similar codeposition behavior. Preliminary experimentsprovide solid support for this conclusion. A significant limitation oncalculating the potential dependence of codeposition is the lack ofreliable low-temperature bulk phase thermochemical data. This is

further hampered by limited knowledge of interfacial energies, seg-regation effects, etc. Nevertheless, the codeposition process isclearly relevant to determining alloy composition in related pro-cesses such as particle production by chemical reduction. In thiscase, the electrochemical potential is determined by the strength ofthe reducing agent and concentration of the reactants as opposed toan external potentiostat. In a related fashion, the strong interactionsbetween iron group metals and Pt or Pd suggests caution in inter-preting prior reports of Fe-Pt, Co-Pt multilayer production fromsingle electrolyte systems. Many of these studies are incorrectlybased on the linear combination of elemental behaviors. In contrast

electrochemical atomic layer epitaxy ECALE 36 may offer an inter-esting alternative for obtaining layer-by-layer growth of ordered L1 0FePt 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 elemen-tal Fe. The codeposition process is driven by the negative enthalpyassociated with alloy formation. The relationship between alloycomposition and the iron group underpotential was established ex-perimentally and found to be in reasonable agreement with freeenergy calculations based on available thermochemical data. Theconstruct is broadly applicable and of particular interest for alloysystems 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

1. K. E. Elkins, T. S. Vedantam, J. P. Liu, H. Zeng, S. Sun, Y. Ding, and Z. L. Wang,Nano Lett., 3, 1647 2003 .

2. K. R. Coffey, M. A. Parker, and J. K. Howard, IEEE Trans. Magn., 31, 2737 1995 .

3. R. A. Ristau, K. Barmak, L. H. Lwis, K. R. Coffey, and J. K. Howard, J. Appl.

Phys., 86, 4527 1999 .4. S. Inoue, T. Namazu, S. Fujita, K. Koterazawa, and K. Inoue, Mater. Res. Soc.

Symp. Proc., 426-4, 2213 2003 .5. M. Watanabe, in Handbook of Fuel Cells, Vol. 2, Electrocatalysis, W. Vielstich, A.

Lamm, and H. A. Gasteiger, Editors, p. 408, John Wiley & Sons, West Sussex,U.K. 2003 .

6. V. Stamenkovic, T. J. Schmidt, P. N. Ross, and N. N. Markovic, J. Phys. Chem.,106, 11970 2002 .

7. T. Shima, T. Moriguchi, T. Seki, S. Mitani, and K. Takanashi, J. Appl. Phys., 93,7238 2003 and reference therein.

8. C. J. Sun, G. M. Chow, and J. P. Wang, Appl. Phys. Lett., 82, 1902 2003 .9. F. M. F. Rhen, G. Hinds, C. OReilly, and J. M. D. Coey, IEEE Trans. Magn., 39,

2699 2003 .10. I. Zana and G. Zangari, Electrochem. Solid-State Lett., 6, C153 2003 .11. Y. H. Huang, H. Okumura, G. C. Hadjipanayis, and D. Weller, J. Appl. Phys., 91,

6869 2002 .12. S. Z. Chu, S. Inoue, K. Wada, and Y. Kanke, Paper 90 presented at The Electro-

chemical Society Meeting, Orlando, FL, Oct 12-16, 2003.13. N. Yasui, A. Imoda, and T. Den, Appl. Phys. Lett., 83, 3347 2003 .14. S. Sun, C. B. Murray, D. Weller, L. Folks, and A. Moser, Science, 287, 1989

2000

; and references therein.

15. B. Jeyadevan, A. Hobo, K. Urakawa, C. N. Chinnasamy, K. Shinoda, and K. Tohji,J. Appl. Phys., 93, 7574 2003 .

16. S. Sun, S. Anders, T. Thomson, J. E. E. Baglin, M. F. Toney, H. F. Hamann, C. B.Murray, and B. D. Terris, J. Phys. Chem. B, 107, 5419 2003 .

17. D. Landolt, Electrochim. Acta, 39, 1075 1994 .18. D. M. Kolb, in Advances in Electrochemistry and Electrochemical Engineering,

Vol. 11, H. Gerischer and C. W. Tobias, Editors, Wiley-Interscience, New York 1978 .

19. H. Bort, K. Juttner, W. J. Lorenz, and G. Staikov, Electrochim. Acta, 28, 993 1983 .

20. A. Vaskevich, M. Rosenblum, and E. Gileadi, J. Electroanal. Chem., 383, 167 1995 .

21. E. Herrero, L. J. Buller, and H. Abruna, Chem. Rev. (Washington, D.C.), 101, 1897 2001 ; E. Herrero, J. Li, and H. D. Abruna, Electrochim. Acta, 44, 2385 1999 .

22. A. Vaskevich and I. Rubinstein, Private communication.23. M. P. R. Panicker, M. Knaster, and F. A. Kroger, J. Electrochem. Soc., 125, 566

1978 .24. F. A. Kroger, J. Electrochem. Soc., 125, 2029 1978 .25. T. P. Moffat, J. Electrochem. Soc., 141, 3059 1994 .26. A. Budniok, J. Electroanal. Chem., 123, 365 1981 .

27. J. Horkans, I.-C. H. Chang, P. C. Andricacos, and H. Deligianni, J. Electrochem.Soc., 142, 2244 1995 .

28. D. Lu and K. Tanaka, J. Electroanal. Chem., 430, 69 1997 .29. D. Lu, M. Ichihara, and K. Tanaka, Electrochim. Acta, 43, 2325 1998 .30. J.-F. Huang and I.-W. Sun, J. Electrochem. Soc., 151, C8 2004 .31. G. R. Stafford and C. L. Hussey, Advances in Electrochemical Science and Engi-

neering, p. 277, Wiley-VCH, Weinheim, Germany 2002 .32. R. Kessel, J. R. Beckett, and E. M. Stolper, Am. Mineral., 86, 1003 2001 .33. K. Barmak, J. Kim, S. Shell, E. B. Svedberg, and J. K. Howard, Appl. Phys. Lett.,

80, 4268 2002 .34. R. Aogaki, K. Kitazawa, Y. Kose, and K. Fueki, Electrochim. Acta, 25, 965 1980 .35. S. Gadad and T. M. Harris, J. Electrochem. Soc., 145, 3699 1998 .36. J. L. Stickney, Advances in Electrochemical Science and Engineering, p. 1, Wiley-

VCH, Weinheim, Germany 2002 .

Figure 5. The experimentally determined composition of FePt alloys, ne-

glecting the oxygen content, compared to prediction based on thermochemi-cal data Eq. 4 .

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