Beating the superparamagnetic limit with exchange bias


Interest in magnetic nanoparticles has increased in the past few years by virtue of their potential for applications in fields such as ultrahigh-density recording and medicine1,2,3,4. Most applications rely on the magnetic order of the nanoparticles being stable with time. However, with decreasing particle size the magnetic anisotropy energy per particle responsible for holding the magnetic moment along certain directions becomes comparable to the thermal energy. When this happens, the thermal fluctuations induce random flipping of the magnetic moment with time, and the nanoparticles lose their stable magnetic order and become superparamagnetic5. Thus, the demand for further miniaturization comes into conflict with the superparamagnetism caused by the reduction of the anisotropy energy per particle: this constitutes the so-called ‘superparamagnetic limit’6,7 in recording media. Here we show that magnetic exchange coupling induced at the interface between ferromagnetic and antiferromagnetic systems8,9 can provide an extra source of anisotropy, leading to magnetization stability. We demonstrate this principle for ferromagnetic cobalt nanoparticles of about 4 nm in diameter that are embedded in either a paramagnetic or an antiferromagnetic matrix. Whereas the cobalt cores lose their magnetic moment at 10 K in the first system, they remain ferromagnetic up to about 290 K in the second. This behaviour is ascribed to the specific way ferromagnetic nanoparticles couple to an antiferromagnetic matrix.

Access options

Rent or Buy article

Get time limited or full article access on ReadCube.


All prices are NET prices.

Figure 1: TEM micrographs and electron diffraction of CocoreCoOshell particles.
Figure 2: Magnetic moments of 4-nm CocoreCoOshell particles.
Figure 3: Hysteresis loops at 4.2 K of 4-nm CocoreCoOshell particles embedded in different matrices.
Figure 4: Coercivity and remanence of 4-nm CocoreCoOshell particles.


  1. 1

    Kodama, R. H. Magnetic nanoparticles. J. Magn. Magn. Mater. 200, 359–372 (1999)

    ADS  CAS  Article  Google Scholar 

  2. 2

    Martín, J. I. et al. Ordered magnetic nanostructures: Fabrication and properties. J. Magn. Magn. Mater. 256, 449–501 (2003)

    ADS  Article  Google Scholar 

  3. 3

    Sun, S. H. et al. Monodisperse FePt nanoparticles and ferromagnetic FePt nanocrystal superlattices. Science 287, 1989–1992 (2000)

    ADS  CAS  Article  Google Scholar 

  4. 4

    Häfeli, U., Schütt, W., Teller, J. & Zborowski, M. (eds) Scientific and Clinical Applications of Magnetic Materials (Plenum, New York, 1997)

  5. 5

    Chikazumi, S. Physics of Ferromagnetism (Oxford Univ. Press, New York, 1997)

    Google Scholar 

  6. 6

    Weller, D. & Moser, A. Thermal effect limits in ultrahigh-density magnetic recording. IEEE Trans. Magn. 35, 4423–4439 (1999)

    ADS  CAS  Article  Google Scholar 

  7. 7

    Thompson, D. A. & Best, J. S. The future of data storage technology. IBM J. Res. Dev. 44, 311–322 (2000)

    CAS  Article  Google Scholar 

  8. 8

    Meiklejohn, W. H. & Bean, C. P. New magnetic anisotropy. Phys. Rev. 102, 1413–1414 (1956)

    ADS  Article  Google Scholar 

  9. 9

    Nogués, J. & Schuller, I. K. Exchange bias. J. Magn. Magn. Mater. 192, 203–232 (1999)

    ADS  Article  Google Scholar 

  10. 10

    Huberland, H. et al. Thin film growth from energetic cluster impact: A feasibility study. J. Vac. Sci. Technol. A 10, 3266–3271 (1992)

    ADS  Article  Google Scholar 

  11. 11

    Gangopadhyay, S. et al. Exchange anisotropy in oxide passivated Co fine particles. J. Appl. Phys. 73, 6964–6966 (1993)

    ADS  CAS  Article  Google Scholar 

  12. 12

    Luis, F. et al. Magnetic relaxation of interacting Co clusters: Crossover from two- to three-dimensional lattices. Phys. Rev. Lett. 88, 217205 (2002)

    ADS  CAS  Article  Google Scholar 

  13. 13

    García-Otero, J. et al. Influence of dipolar interaction on magnetic properties of ultrafine ferromagnetic particles. Phys. Rev. Lett. 84, 167–170 (2000)

    ADS  Article  Google Scholar 

  14. 14

    Gu, E. et al. Two-dimensional paramagnetic-ferromagnetic phase transition and magnetic anisotropy in Co(110) epitaxial nanoparticle arrays. Phys. Rev. B 60, 4092–4095 (1999)

    ADS  CAS  Article  Google Scholar 

  15. 15

    Parent, F. et al. Giant magnetoresistance in Co-Ag granular films prepared by low-energy cluster beam deposition. Phys. Rev. B 55, 3683–3687 (1997)

    ADS  CAS  Article  Google Scholar 

  16. 16

    Held, G. A. et al. Competing interactions in dispersions of superparamagnetic nanoparticles. Phys. Rev. B 64, 012408 (2001)

    ADS  Article  Google Scholar 

  17. 17

    Woods, S. I. et al. Direct investigation of superparamagnetism in Co nanoparticle films. Phys. Rev. Lett. 87, 137205 (2001)

    ADS  CAS  Article  Google Scholar 

  18. 18

    Murray, C. B. et al. Monodisperse 3d transition-metal (Co,Ni,Fe) nanoparticles and their assembly into nanoparticle superlattices. Mater. Res. Soc. Bull. 26, 985–991 (2001)

    ADS  CAS  Article  Google Scholar 

  19. 19

    Sako, S., Osshima, K. & Saki, M. Magnetic property of oxide passivated Co nanosized particles dispersed in two dimensional plane. J. Phys. Soc. Jpn 70, 2134–2138 (2001)

    ADS  CAS  Article  Google Scholar 

  20. 20

    Lund, M. S. et al. Effect of anisotropy on the critical antiferromagnetic thickness in exchange biased bilayers. Phys. Rev. B 66, 054422 (2002)

    ADS  Article  Google Scholar 

  21. 21

    Peng, D. L. et al. Magnetic properties of monodispersed Co/CoO clusters. Phys. Rev. B 61, 3103–3109 (2000)

    ADS  CAS  Article  Google Scholar 

  22. 22

    Sort, J. et al. Room temperature coercivity enhancement in mechanically alloyed antiferromagnetic-ferromagnetic powders. Appl. Phys. Lett. 75, 3177–3179 (1999)

    ADS  CAS  Article  Google Scholar 

  23. 23

    Liu, K. et al. Fabrication and thermal stability of arrays of Fe nanodots. Appl. Phys. Lett. 81, 4434–4436 (2002)

    ADS  CAS  Article  Google Scholar 

  24. 24

    Jensen, P. J. Magnetic recording medium with improved temporal stability. Appl. Phys. Lett. 78, 2190–2192 (2001)

    ADS  CAS  Article  Google Scholar 

  25. 25

    Schulthess, T. C. & Butler, W. H. Consequences of spin-flop coupling in exchange biased films. Phys. Rev. Lett. 81, 4516–4519 (1998)

    ADS  CAS  Article  Google Scholar 

  26. 26

    Stamps, R. L. Mechanisms for exchange bias. J. Phys. D 33, R247–R268 (2000)

    ADS  CAS  Article  Google Scholar 

  27. 27

    Kanamori, J. Theory of the magnetic properties of ferrous and cobaltous oxides. Prog. Theor. Phys. 17, 177–196 (1957)

    ADS  CAS  Article  Google Scholar 

  28. 28

    Takano, K. et al. Interfacial uncompensated antiferromagnetic spins: Role of unidirectional anisotropy in polycrystalline Ni81Fe19/CoO bilayers. Phys. Rev. Lett. 79, 1130–1133 (1997)

    ADS  CAS  Article  Google Scholar 

  29. 29

    Kodama, R. H., Makhlouf, S. H. & Berkowitz, A. E. Finite size effects in antiferromagnetic NiO nanoparticles. Phys. Rev. Lett. 79, 1393–1396 (1997)

    ADS  CAS  Article  Google Scholar 

  30. 30

    Coey, J. M. D. Noncollinear spin arrangement in ultrafine ferrimagnetic crystallites. Phys. Rev. Lett. 27, 1140–1143 (1971)

    ADS  CAS  Article  Google Scholar 

Download references


We thank N. Dempsey for critical reading of the manuscript, and D. Weller for discussions. This work was partly supported by the US National Science Foundation, Seagate Technology, the Catalan Direcció General de Recerca, and the Spanish Comisión Interministerial de Ciencia y Tecnología.

Author information



Corresponding author

Correspondence to Vassil Skumryev.

Ethics declarations

Competing interests

The authors declare that they have no competing financial interests.

Rights and permissions

Reprints and Permissions

About this article

Cite this article

Skumryev, V., Stoyanov, S., Zhang, Y. et al. Beating the superparamagnetic limit with exchange bias. Nature 423, 850–853 (2003).

Download citation

Further reading


By submitting a comment you agree to abide by our Terms and Community Guidelines. If you find something abusive or that does not comply with our terms or guidelines please flag it as inappropriate.


Quick links

Nature Briefing

Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily.

Get the most important science stories of the day, free in your inbox. Sign up for Nature Briefing