Nitrides as spintronic materials

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Nitrides as spintronic materials
2003.11.19 Chul-Hwan Choi
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Why Nitride DMS

• Small lattice constant

Large hybridization between the valence orbitals
and the d shells of the magnetic ions
Therefore, the carrier mediated ferromagnetic
coupling is large and can be controlled in nitride

• Light anions

Reduce a destructive influence of the spin-orbit
interaction
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Electron States of Transition metal impurity
ltAntimonides, Arsenidesgt-Proven
Mn atom substituting a trivalent metal
M2 d4(donor) or M4 d5h(acceptor)
if Mn d state in the valance band
Mn acts a an effective mass acceptor(d5h)
Proved by ESR, PE, optical studies as well as
insulator to metal transition in case of low Mn
concentration
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ltGaMnNgt
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Mn forms mid-gap electron trap in GaN
(photoemission)
ESR measurement ? only d5 Mn centers
(compensating donors)
If there is no overwhelming compensation
Zeeman splitting ( the model
of d4 or d5h states)
but
d4 model supported by LSDA is not agree with
LSDAU in case GaMnAs
A large hole binding energy resulted by the
exceptionally strong spin-dependent p-d
hybridization in nitride
Presume that Mn d states reside in the valance
band of GaN
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Spin-Spin interactions and origin of
ferromagnetism of GaMnN

• In general, magnetization of (Ga,Mn)N consists
  of paramagnetic and ferromagnetic
  components ( small antiferromagnetic spin-spin
  interaction).
• Not all the sample show ferromagnetism. however
  both wurzite and cubic structure of GaMnN show
  ferromagnetism.
• Ferromagnetic ordering results in anomalous Hall
  effect.

• Ferromagnetic precipitates. Ex) Mn4N and related
  compounds
• doubt in size, not superparamagnetism but
  ferromagnetism

Question) Why (Ga, Mn)N itself shows rather
antiferromagnetic than ferromagnetic properties?
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Continue
First Scenario

• antiferromagnetism by strong localization of the
  holes within Zener model.

Underestimated Mott-Anderson localization effect

• Ferromagnetism by the double exchange within ab
  initio LSDA theories, even if the holes occupy
  the d4 mid gap states.

Second Scenario

• A phenomenon of self compensation (d5 states).
  ex) InMnAs
• Paramagnetic n-type up Mn concetration(18)

• Double donors. ex) GaMnAs
• reduce Tc in the regime of large Mn concetrations

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Third Scenario
d5 h model
Assuming not too strong compensation

• greater the localization length of the holes
  than the distance between the Mn spins owing to
  an overlap between the hole wave functions.
  Therefore under this condition, Zener model
  becomes approximately valid. Explain high Tc
  without invoking precipitates of other compounds

partial participation of the magnetic constituent

• phase segregation (dont change tetrahedral
  coordination)
• double exchange or superexchange