Magnetic Nanoclusters

1
Magnetic Nanoclusters

• By Adam Krause
• 2/27/07
• Physics 672

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Nanocluster Quick Introduction

• From a few atoms to several thousand atoms
• High fraction of atoms on the surface
• Different elements form different bonds and
  different nanocluster structures

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A Few Types of Nanoclusters
Van der Waals Nanoclusters

• Binding energy lt 0.3 eV / atom
• Balance between induced dipole force and quantum
  closed shell interaction
• Noble gases form icosahedral Van der Waals
  clusters

Figure above from Alonso, J. A., Structure and
Properties of Atomic Nanoclusters, 2005
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A Few Types of Nanoclusters
Van der Waals Nanoclusters

• The drops at 148 and 309 atoms correspond to
  completed icosahedra.

Figure above from Echt, O., et al., J. Chem.
Soc. Faraday Trans., 86 (1990) 2411
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A Few Types of Nanoclusters
Ionic Nanoclusters

• Bond Strength 2-4 eV / atom
• Tend to form boxes

NaCl Cluster
Graph above from Martin, T. P., Physics Reports,
273 (1996) 199
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A Few Types of Nanoclusters
Metal Nanoclusters

• Metal clusters have complicated bonding that
  varies from metal to metal
• Due to this variation the bond strength varies
  from around 0.5 eV to 3 eV per atom

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Laser Vaporization
Metal Nanoclusters Produced By Laser Vaporization
Figure above from Billas et al., J. Magn. Magn.
Mater. 168 (1997) 64
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Stern-Gerlach Apparatus
Figure above from Billas et al., J. Magn. Magn.
Mater. 168 (1997) 64
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Description of magnetic particles
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Band Structure Evolution
Increasing Coordination Number
Figure above from Billas et al., J. Magn. Magn.
Mater. 168 (1997) 64
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Magnetic Moment vs. Cluster Size
Figure above from Billas et al., J. Magn. Magn.
Mater. 168 (1997) 64
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Closed Shell Cluster Size vs. Magnetic Moment
Minima.
Table above from Jensen, P. J., and K. H.
Bennemann, Z. Phys. D. 35 (1995) 273
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Magnetic Shell Model
(1)
(2)
Graphs from Jensen, P. J., and K. H. Bennemann,
Z. Phys. D. 35 (1995) 273
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Magnetic Moment vs. Temperature
Co
Ni
Fe
Graphs from Billas, M. L., A. Chatelain, and W.
A. de Heer, Science 265 (1994) 1682
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Monte Carlo Simulation of Magnetization vs. 1/Temp
Graph from Binder, K., et al., J. Phys. Chem.
Solids, 31 (1970) 391
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Superparamagnetism
Magnetization Loops of Fe Nanoclusters
Graph from Jackson, T. J., et al., J. Phys.
Condens. Matter, 12 (2000) 1399
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Summary

• Metal nanoclusters of an element behave
  differently than bulk matter of the same element.
• d-orbital overlap reduces magnetic moment per
  atom.
• Metal nanoclusters exhibit magnetic shell
  phenomenon
• Metal nanoclusters do not lose their
  magnetization as quickly above the Curie temp.
• Metal nanoclusters exhibit superparamagnetic
  behavior.
• Superparamagnetism provides a theoretical minimum
  size per bit in magnetic moment based memory
  systems.

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References

1. Alonso, J. A., Structure and Properties of Atomic
   Nanoclusters (Imperial College Press, London,
   2005).
2. Echt, O., et al., J. Chem. Soc. Faraday Trans.,
   86 (1990) 2411
3. Martin, T. P., Physics Reports, 273 (1996) 199
4. Dietz, T. G., et al., J. Chem. Phys., 74 (1981)
   6511
5. Bondybey, V. E., and J. H. English, J. Chem.
   Phys., 76 (1982) 2165
6. Billas, M. L., A. Chatelain, and W. A. de Heer,
   J. Magn. Magn. Mater. 168 (1997) 64
7. Cox, D. M., et al, Phys. Rev. B., 32 (1985) 7291
8. Billas, M. L., A. Chatelain, and W. A. de Heer,
   Science 265 (1994) 1682
9. Jensen, P. J., and K. H. Bennemann, Z. Phys. D.
   35 (1995) 273
10. Billas, M. L., et al., Phys. Rev. Lett., 71
    (1993) 4067
11. Binder, K., et al., J. Phys. Chem. Solids, 31
    (1970) 391
12. Jackson, T. J., et al., J. Phys. Condens.
    Matter, 12 (2000) 1399