Journal of Materials Science

, Volume 41, Issue 23, pp 7843–7852 | Cite as

The formation mechanism of aluminium oxide tunnel barriers

  • A. CerezoEmail author
  • A. K. Petford-Long
  • D. J. Larson
  • S. Pinitsoontorn
  • E. W. Singleton


The functional properties of magnetic tunnel junctions are critically dependant on the nanoscale morphology of the insulating barrier (usually only a few atomic layers thick) that separates the two ferromagnetic layers. Three-dimensional atom probe analysis has been used to study the chemistry of a magnetic tunnel junction structure comprising an aluminium oxide barrier formed by in situ oxidation, both in the under-oxidised and fully oxidised states and before and after annealing. Low oxidation times result in discrete oxide islands. Further oxidation leads to a more continuous, but still non-stoichiometric, barrier with evidence that oxidation proceeds along the top of grain boundaries in the underlying CoFe layer. Post-deposition annealing leads to an increase in the barrier area, but only in the case of the fully oxidised and annealed structure is a continuous planar layer formed, which is close to the stoichiometric Al:O ratio of 2:3. These results are surprising, in that the planar layers are usually considered unstable with respect to breaking up into separate islands. Analysis of the various driving forces suggests that the formation of a continuous layer requires a combination of factors, including the strain energy resulting from the expansion of the oxide during internal oxidation on annealing.


Interface Energy CoFe Continuous Layer Tunnel Barrier Ferromagnetic Layer 



The authors are grateful to Prof. G.D.W. Smith FRS for provision of laboratory facilities and for helpful discussions during the preparation of this paper. We would also like to thank Xiaowang Zhou, University of Virginia, for valuable contributions on the mechanisms of oxide growth. Laser-pulsed 3DAP experiments were performed at Oxford nanoScience Limited, Milton Keynes, UK and we are grateful to Dr. Peter Clifton for his assistance in the collection of the data. This work was supported by funding from the Engineering and Physical Sciences Research Council. AC is also grateful for support from Oxford nanoScience Limited during the writing of this paper. Argonne National Laboratory is supported by the U.S. Department of Energy, Basic Energy Sciences – Materials Sciences, under contract (W-31-109-ENG-38.


  1. 1.
    Barthélémy A, Fert A, Contour J-P, Bowen M, Cros V, de Teresa JM, Hamzic A, Faini JC, George JM, Grollier J, Montaigne F, Pailloux F, Petroff F, Vouille C (2002) J Mag Mag Mat 242–245:68CrossRefGoogle Scholar
  2. 2.
    Jullière M (1975) Phys Lett 54A:225CrossRefGoogle Scholar
  3. 3.
    Moodera JS, Kinder LR, Wong TM, Meservey R (1995) Phys Rev Lett 74:3273CrossRefGoogle Scholar
  4. 4.
    Moodera JS, Kinder LR, Nowak J, Leclair P, Meservey R (1996) Appl Phys Lett 69:708CrossRefGoogle Scholar
  5. 5.
    Moodera JS, Kinder LR (1996) J Appl Phys 79:4724CrossRefGoogle Scholar
  6. 6.
    Park BG, Lee TD, Lee TH, Kim CG, Kim CO (2003) J Appl Phys 93:6423CrossRefGoogle Scholar
  7. 7.
    Bae JS, Shin KH, Lee HM (2002) J Appl Phys 91:7947CrossRefGoogle Scholar
  8. 8.
    Tsymbal EY, Mryasov ON, Le Clair PR (2003) J Phys Condens Matt 15:R109CrossRefGoogle Scholar
  9. 9.
    Tsymbal EY, Pettifor DG (1998) Phys Rev B 58:432CrossRefGoogle Scholar
  10. 10.
    Da Costa V, Tiusan C, Dimopoulos T, Ounadjela K (2000) Phys Rev Lett 85:876CrossRefGoogle Scholar
  11. 11.
    Rabson DA, Jönsson-Åkerman BJ, Romero AH, Escudero R, Leighton C, Kim S, Schuller IK (2001) J Appl Phys 89:2786CrossRefGoogle Scholar
  12. 12.
    Shang P, Petford-Long AK, Nickel JH, Sharma M, Anthony TC (2001) J Appl Phys 89:6874CrossRefGoogle Scholar
  13. 13.
    Ozkaya D, Mcbride W, Cockayne DJH (2004) Interface Sci 12:321CrossRefGoogle Scholar
  14. 14.
    Ozkaya D, Dunin-Borkowski RE, Petford-Long AK, Wong PK, Blamire MG (2000) J Appl Phys 87:5200CrossRefGoogle Scholar
  15. 15.
    Stobiecki T, Kanak J, Wrona J, Czapkiewicz M, Kim CG, Kim CO, Tsunoda M, Takahashi M (2004) Phys Stat Sol A 201:1621CrossRefGoogle Scholar
  16. 16.
    Zhu W, Hirschmugl CJ, Laine AD, Sinkovic B, Parkin SSP (2001) Appl Phys Lett 78:3103CrossRefGoogle Scholar
  17. 17.
    Miller MK, Cerezo A, Hetherington MG, Smith GDW (1996) Atom probe field ion microscopy. Oxford University Press, OxfordGoogle Scholar
  18. 18.
    Cerezo A, Larson DJ, Smith GDW (2001) MRS Bull 26:102CrossRefGoogle Scholar
  19. 19.
    Petford-Long AK, Ma YQ, Cerezo A, Larson DJ, Singleton EW, Carr BW (2005) J Appl Phys 98:124904CrossRefGoogle Scholar
  20. 20.
    Larson DJ, Wissman BD, Martens RL, Viellieux RJ, Kelly TF, Gribb TT, Erskine HF, Tabat N (2001) Micro Microanal 7:24Google Scholar
  21. 21.
    Larson DJ, Petford-Long AK, Ma YQ, Cerezo A (2004) Acta Mater 52:2847CrossRefGoogle Scholar
  22. 22.
    Cerezo A, Godfrey TJ, Sibrandij SJ, Smith GDW, Warren PJ (1998) Rev Sci Instrum 69:49CrossRefGoogle Scholar
  23. 23.
    Lehnert T, Billon D, Grassl C, Grundlach KH (1992) J Appl Phys 72:3165CrossRefGoogle Scholar
  24. 24.
    Zhou XW, Wadley HNG (2005) Phys Rev B 71:054418CrossRefGoogle Scholar
  25. 25.
    Zhou XW, Wadley HNG, private communicationGoogle Scholar
  26. 26.
    Larson DJ, Cerezo A, Clifton PH, Petford-Long AK, Martens RL, Kelly TF, Tabat N (2001) J Appl Phys 89:7517CrossRefGoogle Scholar
  27. 27.
    Gas P, Bergman C, Lábár JL, Barna PB, D’heurle FM (2004) Appl Phys Lett 84:2421CrossRefGoogle Scholar
  28. 28.
    Buchanan JDR, Hase TPA, Tanner BK, Chen PJ, Gan L, Powell CJ, Egelhoff WF Jr (2003) J Appl Phys 93:8044CrossRefGoogle Scholar
  29. 29.
    Raynor GV, Rivlin VG (1988) Phase equilibria in iron ternary alloys. Institute of Materials, London, p 76Google Scholar
  30. 30.
    Macallister AJ (1990) In: Massalski TB, Okamoto H, Subramaniam PR, Kacprzak L (eds) Binary alloy phase diagrams, 2nd edn. ASM International, OH, p 136Google Scholar
  31. 31.
    Levin EM, Mcmurdie HF (1975) Phase diagrams for ceramicists, 1975 supplement. American Ceramic Society, Columbus, OH, pp 236Google Scholar
  32. 32.
    Murr LE (1975) Interfacial phenomena in metals and alloys. Addison-Wesley, LondonGoogle Scholar
  33. 33.
    Mchale JM, Auroux A, Perotta AJ, Navrotsky A (1997) Science 277:788CrossRefGoogle Scholar
  34. 34.
    Levi G, Kaplan WD (1997) Acta Mater 51:788Google Scholar
  35. 35.
    Ozkaya D, Mcbride W, Cockayne DJH (2004) Interface Sci 12:321CrossRefGoogle Scholar
  36. 36.
    Fei GT, Barnes JP, Petford-Long AK, Doole RC, Serna R, Gonzalo J (2002) J Phys D Appl Phys 35:916CrossRefGoogle Scholar
  37. 37.
    Weast RC (1983) CRC handbook of chemistry and physics, 63rd edn. CRC Press, Boca Raton, FLGoogle Scholar
  38. 38.
    Zhou XW, Wadley HNG, Wang DX, submitted to Comput Mater SciGoogle Scholar
  39. 39.
    Nabarro FRN (1940) Proc Roy Soc A 175:519CrossRefGoogle Scholar
  40. 40.
    Christian JW (2002) The theory of transformations in metals and alloys, Part I. 2nd edn. Elsevier, pp 464Google Scholar

Copyright information

© Springer Science+Business Media, LLC 2006

Authors and Affiliations

  • A. Cerezo
    • 1
    Email author
  • A. K. Petford-Long
    • 1
    • 3
  • D. J. Larson
    • 1
    • 4
  • S. Pinitsoontorn
    • 1
  • E. W. Singleton
    • 2
  1. 1.Department of MaterialsUniversity of OxfordOxfordUK
  2. 2.Recording Head OperationsSeagate TechnologyMinneapolisUSA
  3. 3.Argonne National LaboratoryArgonneUSA
  4. 4.Imago Scientific InstrumentsMadisonUSA

Personalised recommendations