NAN ZHENG

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Dilute Magnetic Semiconductors(DMS)

• NAN ZHENG
• COURSE SOLID STATE II
• INSTRUCTOR ELBIO DAGOTTO
• SEMESTER SPRING 2008
• DEPARTMENT OF PHYSICS AND ASTRONOMY
• THE UNIVERSITY OF TENNESSEE KNOXVILLE

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Outline

• Introduction spintronics and DMS
• DMS materials
• (Ga,Mn)As
• (Ga,Mn)N
• Transitional metal doped oxide
• Magnetic mechanism studied by the Mean Field
  Approach
• Summary

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Introduction Spintronics and DMS

• Spintronics Spin-based electronics
• Idea a combination of microelectronics and
  magnetic storage technique.
• Searching for Materials??

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Introduction Spintronics and DMS

• Diluted Magnetic Semiconductor (DMS)
• Traditional semiconductors doped with
  transition metals
• Why Dilute? Small doping
  concentration (a few )
• Why Magnetic? Display
  ferromagnetisation
• Why Semiconductor?
• While preserving the
  semiconducting properties

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Introduction Spintronics and DMS

• Criteria of ideal materials for spintronics
• Room temperature ferromagnetisation
• Fit into current electronic technique

Theoretical predictions by Dietl, Ohno et al.
Various DMS displays room temperature
ferromagnetism!
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DMS materials I (Ga,Mn)As

• First DMS material, discovered in 1996 by Ohno et
  al using molecular beam epitaxy (MBE), a
  breakthrough in experiment.
• Curie temperature K at optimal
  doping

Ohno et al., APL 69, 363 (1996)
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DMS materials I (Ga,Mn)As

• Metal to Insulator Transition (MIT)

Resistance measurements on samples with different
Mn concentrations Metal ? R ? as T
? Insulator ? R ? as T ? ?MIT happens at TC for
intermediate Mn concentrations (0.0350.053)
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DMS materials I (Ga,Mn)As

• Annealing Effect (observed in other DMSs as well)

• Resistance ? decreases with annealing time, up to
  2 hrs, and then increases again

• Two regimes at annealing time
• Below 2h, T ?, FM ?, metallicity ?, lattice
  constant ?

WHY?? Origin related to
defects, details unknown
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DMS materials II (Ga,Mn)N

• First room temperature DMS discovered in 2001
  using metal organic chemical vapor deposition
  (MOCVD) method.
• High curie temperature
• Experiment up to K
• Theory up to K

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DMS materials III Transition metal doped oxide

• Room temperature ferromagnetism discovered in Mn
  doped ZnO through reactive magnetron
  co-sputtering and fast annealing in 2001.
• Material
• Mn doped ZnO
• Co doped TiO
• Reported up to 400K

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Magnetic Mechanism and Physical Properties

• Carrier-mediated mechanism

Interaction between hole spin and Mn local moment
is AFM, giving rise to an effective FM coupling
between Mn spins
Dietl et al., PRB 55, R3347(1997)
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Magnetic Mechanism and Physical Properties

• Two basic approaches to understand magnetism in
  DMS
• Mean Field Theory based on Zener model
• Clusters formed by magnetic atoms are responsible
  for ferromagnetism

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Magnetic Mechanism and Physical Properties

• MF approach further explained
• (A) High carrier density
• Carrier (electrons or holes, depending on
  doping) mediated interaction leads to
  ferromagnetism.
• (B) Low carrier density Percolation network is
  formed, carriers hop from site to site freely,
  aligning Mn moments within the cluster network.

Pearton et al, Mat. Sci. Eng. R 40 (2003)
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Magnetic Mechanism and Physical Properties

• How good is Mean Field Theory?
• Its reliability is case dependent.

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Summary and Outlook

• Room temperature DMS already realized, while
  explanation on the origin of ferromagnetism still
  under refinement.
• Further development on mean field approach in
  DMS
• Monte Carlo simulations on local moment (eg. Mn)
  distribution
• Incorporation of defect structures (implied by
  annealing effect)
• Correlation effects in the hole sub-system