Unusual features in magnetism and magnetoresistance of double perovskite oxides

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Unusual features in magnetism and
magnetoresistance of double perovskite oxides
D.D. Sarma Centre for Advanced Materials Indian
Association for the Cultivation of
Science Kolkata mlsdds_at_iacs.res.in
D.D. Sarma Solid State and Structural Chemistry
Unit Indian Institute of Science Bangalore sarma_at_
sscu.iisc.ernt.in http//sscu.iisc.ernet.in/DDSarm
a/

• Phys. Rev. B 73, 094419 (2006).
• Phys. Rev. Lett. 96, 087205 (2006)
• Phys. Rev. Lett. 95, 117201 (2005).
• Phys. Rev. Lett. 87, 097204 (2001).
• Nano Lett. 2, 605 (2002).
• Phys. Rev. Lett. 85, 2549 (2000).
• Solid State Communications 114, 465 (2000).

and Unpublished results
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MnxGa1-xAs and Tl2Mn2O7

• H.R. Krishnamurthy
• Chandan Dasgupta
• E. V. Sampathkumaran
• T.N. Guru Row

• Sugata Ray
• Dinesh Topwal
• Prabuddha Sanyal
• Rajesh Karan
• Priya Mahadevan
• Tanusri Saha-Dasgupta

• Nicola Spaldin
• A. Fujimori
• Y. Tokura
• M. Takano

Department of Science and Technology, Government
of India
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Fe Mo
Crystal structure of ordered double perovskite
A2BBO6
Sr Fe Mo O
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• Several advantages
• Large MR at higher T and lower H.
• High ferromagnetic Tc.
• Half-metallic ferromagnet (100 spin
  polarization of charge carriers).
• Simple crystal structure. Possibly, no serious
  complication arising from J-T type distortions.
• Therefore, attractive both in terms of
  theoretical investigations and possible
  technological applications.

Magnetoresistance in polycrystalline Sr2FeMoO6
On the basis of the structural data, however,
ferromagnetism appears to be a rather unexpected
ground state for this system.
Sarma et al., Solid State Comm. 114, 465 (2000).
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TC 420 K
What is the origin of the strong ferromagnetic
interaction between Fe ions in this compound,
implicit in such an unusually high TC?
Or double-exchange?
Must be a different mechanism from the manganites
that polarises the delocalised, nominally Mo
derived, conduction electrons.
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Spin polarized fat-band representation of the
band dispersions in Sr2FeMoO6
Sarma et al., PRL 85, 2549 (2000).
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Origin of the robust ferromagnetism in double
perovskite, Sr2FeMoO6
Sarma et al., PRL 85, 2549 (2000).
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R. Viswanatha et al., PRB, 72 (2005), 045333.
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Now, to bring in magnetism
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DOS as a function of ? for U 4 eV
? -3.0 eV
-2.5 eV
0.5
1.0
eV
-2.0 eV
2.0
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Comparison of E(Q) and J(Q)
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Convergence check!
J(Q) Fit to TB energy E(Q) at various nearest
neighbor
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Sarma et al., unpublished and PRL 2000
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Origin of the robust ferromagnetism in double
perovskite, Sr2FeMoO6
Intra-atomic, Ieff 1-1.5 eV, Inter-atomic, J
18 meV
Why Sr2FeWO6 is an antiferromagnet?
Sarma et al., PRL 85, 2549 (2000).
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Hopping induced antiferromagnetic coupling of
itinerant and localized spins ? Ferromagnetic
coupling of the localized moments.
Half-metallic or not depends on the strength of
hopping and other details.
EF
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Low doping
Can be localized by disorder, when the doping
level is low, but will remain ferromagnetic!
What about insulating ferromagnetic state in
presence of a high carrier density?
Can be localized (structural distortions/electron
correlations), but will remain ferromagnetic!
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Effect of anti-site disorder in Sr2FeMoO6
What is the Fe/Mo antisite disorder?
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How do we check ordering?
Why does the magnetisation change so drastically?
Fe ?? or Mo d depolarisation?
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Fe Mo
Crystal structure of ordered double perovskite
A2BBO6
Sr Fe Mo O
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X-ray diffraction patterns of Sr2Fe1xMo1-xO6
Intensity (arb. units)
Topwal et al., Phys. Rev. B 73, 94419 (2006).
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Magnetoresistance in polycrystalline Sr2FeMoO6
Sarma et al., Solid State Comm. 114, 465 (2000).
But, why does magnetoresistance change so
drastically? Inter- or intra-grain mechanism?
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Sample 1
Sample 2
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• Small grain samples have much larger MR
  compared to the large grain ones ? Inter-grain
  mechanism.
• Within the small grain samples, samples with
  higher ordering and therefore, lower anti-site
  defects show larger MR ? Effect of anti-site
  (intra-grain) boundaries, if any, is dominated
  over by other (inter-grain and magnetisation)
  effects.
• Inter-grain tunnelling MR should have some
  tell-tale signatures, such as MR M2 and
  consistent hysteresis.

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Calculated results for MR
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Mn in GaAs MnxGa1-xAs
Is RKKY relevant?
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Ferromagnetism in Mn-doped GaAs
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Origin of the ferromagnetism in Mn-doped GaAs
GaAs p
Mn d
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Origin of Ferromagnetism and its Pressure and
Doping Dependence in Tl2Mn2O7
Saha-Dasgupta, De Raychaudhury and Sarma, Phys.
Rev. Lett. 96, 087205 (2006).
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Mn-doped GaAs Bulk and cluster
Issues in diluted magnetic semiconductors (DMS)
nanoparticles.
Sapra, Sarma, Sanvito and Hill, Nano Lett. 2
(2002) 605.
Mn-doped GaAs is ferromagnetic in the bulk
What happens if one reduces the size of Mn-GaAs?
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• Suggestive of
• Valence transition
• Electronic transition
• Magnetic transition

Sapra et al., Nano Lett. 2 (2002) 605 Also see
Sarma et al., Phys. Rev. Lett. 85 (2000) 2549.
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Thank you for your attention
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Sr2FeMoxW1-xO6
Therefore, Sr2FeMoxW1-xO6 assures magnetic and
metal-insulator transition over the composition
range.
What drives this MIT?
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A valence transition model
Fe3,(Mo/W)5 Fe2,(W/Mo)6
Nakagawa et al., J. Phys. Soc. Jap. 27, 880
(1969).
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Second model Phase separation Kobayashi et al.,
JMMM 218, 17 (2000)
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M vs. H
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How does the Mo and W change with doping?
Ray et al., unpublished
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What happens to Fe?
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57Fe Mössbauer measurements on the series at 5 K
Phase segregation???
Ray et al., unpublished
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Ray et al., unpublished
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T-x Phase diagram of Sr2FeMoxW1-xO6
Ray et al., unpublished
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• Conclusions
• Sr2FeMoO6 has a new mechanism to stabilise the
  ferromagnetic ground state with an unusually high
  Tc.
• The magnetoresistance of SFMO is driven by the
  magnetism of the grain boundary material via a
  magnetically triggered nearly-resonant condition
  for tunnelling.
• Magnetic interactions in Mn-doped GaAs cannot be
  described within the RKKY mechanism. It is in
  agreement with the mechanism that we have
  proposed for Sr2FeMoO6. (Also compare Tl2Mn2O7).
• Mn-doped GaAs nanocrystals are expected to
  undergo a valence state transition for very small
  (nm) size systems, indicating a transport and
  magnetic transitions.
• A rich phase diagram for Sr2FeMoxW1-xO6

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• Conclusions
• Sr2FeMoO6 has a new mechanism to stabilise the
  ferromagnetic ground state with an unusually high
  Tc.
• The magnetoresistance of SFMO is driven by the
  magnetism of the grain boundary material via a
  magnetically triggered nearly-resonant condition
  for tunnelling.
• Magnetic interactions in Mn-doped GaAs cannot be
  described within the RKKY mechanism. It is in
  agreement with the mechanism that we have
  proposed for Sr2FeMoO6. (Also compare Tl2Mn2O7).
• A study of Sr2Fe1-xMo1xO6 helps us to
  understand the effect of anti-site defects on
  magnetism of this class of compounds in terms of
  antiferromagnetically coupled Fe-Fe pair and a
  depolarisation of the nominally Mo 4d band.
• A rich phase diagram for Sr2FeMoxW1-xO6

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Thank you for your attention
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We consider an Anderson Hasegawa Hamiltonian (JAF
? 8, where JAF is the antiferromagnetic exchange
coupling between Iron and Molybdenum spins) as a
model for transport by mobile Mo electrons moving
in the background of iron spins modelled as
classical spins (S ? 8).
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Properties of Sr2Fe1xMo1-xO6
or
FexSr2Mo2-xO6 as in MnxGa1-xAs
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MnGaAs data from S. J. Potashnik et al., J.
Appl. Phys. 93, 6784 (2003)
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A tale of two compounds Sr2FeMoO6 and MnxGa1-xAs

• Sugata Ray
• Dinesh Topwal
• Priya Mahadevan
• Tanusri Saha-Dasgupta
• Prabuddha Sanyal
• R. Karan

• H.R. Krishnamurthy
• Chandan Dasgupta
• T.N. Guru Row
• E.V. Sampathkumaran
• Alex Zunger
• A. Fujimori
• Y. Tokura

Contributed by
Department of Science and Technology, Government
of India
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Charge density plots
PRB 2003