Structural and magnetic properties of 2d layered cobaltates

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Structural and magnetic properties of 2-d layered
cobaltates A.Podlesnyak
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• Outline
• Introduction
• What are cobalt perovskites?
• Why are they interesting?
• Experiments
• Oxygen nonstoichiometry in RBaCo2Ox, 5ltxlt6
• Magnetic phase separation in RBaCo2O5.5
• Discussion
• Base from crystal field theory
• Possible scenario
• Summary

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• Laboratory for Neutron Scattering
• S. Streule, J. Mesot, M. Medarde, D. Cheptiakov,
  
• E. Pomjakushina, K. Conder
• Inst. of Solid State Semiconductor Physics,
  Minsk
• S. Barilo, D. Khalyavin, S.
  Ustinovich, A. Soldatov, A. Shestak
• Institute of Solid State Chemistry,
  Ekaterinburg
• V. Kozhevnikov, I. Leonidov, E.
  Mitberg

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What are cobalt perovskites?
Perovskite - an Inorganic Chameleon CaTiO3
dielectricBaTiO3 ferroelectricPb(Zr1-xTix)O3
piezoelectric(Ba1-xLax)TiO3
semiconductor(Y1/3Ba2/3)CuO3-x
superconductorSrCeO3 - H - protonic
conductorLaMnO3-x - Giant magneto- resistance
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3D-perovskite structure solid solution of R and
M R1-xMxCoO3-y (Rrare earth MSr, Ba
0ltxlt1 0ltylt1)
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Why are they interesting? Scientific points

• rich phase diagram

• metal-insulator transitions
• (both as a function of T and doping)

• large magneto-resistance effects

• low- to high-spin state transitions
• -gt unusual magnetic behavior

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• Why are they interesting?
• Technological aspect
• large ionic conduction at high temperatures -gt
  potential for
• applications as gas sensors or electrode
  materials for fuel cells.

• Benefits
• high electrical efficiency
• broad power spectrum
• low pollutant emission
• low operating costs
• quiet operation

Solid Oxide Fuel Cell
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Effect of oxygen nonstoichiometry on structural
and magnetic properties of RBaCo2Ox
perovskites, R Pr, Gd, Dy 5 lt x lt 6

• Motivation
• Wide variety of challenging phenomena which
  depend strongly
• on oxygen content x
• - spin-state transitions
• - charge ordering
• - metal-insulator transition
• - giant magnetoresistance.
• Contradictory reports on the evolution of the
  structural and
• magnetic properties.
• Unknown phase diagram.

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Instrumentation
Double-Axis Multi-Counter Powder Diffractometer
DMC
the multidetector diffractometer HRPT
the triple-axis-spectrometer TASP
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Oxygen nonstoichiometry in PrBaCo2Ox
(200)/(020)
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Oxygen nonstoichiometry in PrBaCo2Ox
(200)/(020)
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Oxygen nonstoichiometry in GdBaCo2Ox
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Oxygen nonstoichiometry in RBaCo2Ox summary
RBaCo2O5 1x1x2
RBaCo2O5.5 1x2x2
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Oxygen nonstoichiometry in RBaCo2Ox summary
1. The direct relation between oxygen content x
and cell parameters has been established
FM
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MI transition in RBaCo2Ox
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Magnetic properties of DyBaCo2O5.5
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Magnetic properties of DyBaCo2O5.5
300K
270K
10K
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Magnetic properties of DyBaCo2O5.5 summary

• Crystal structure Pmmm

• 240 lt Tlt 280 K G-type AFM structure with µCo
  0.9µB

• T lt 240 K additional magnetic phase,
  spin-state ordered with µCo 1.9µB

• Evidence for a field-induced FM transition under
  external magnetic field.

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Crystal field theory
In a free ion (no ligands), d-orbitals are
degenerate ie. they have the same energy
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Crystal field theory

• The difference in energy between the eg and the
  t2g energy levels is the
• crystal field splitting parameter, Do.

• The Pairing Energy P is the energy required to
  pair two electrons

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Crystal field theory
High spin state
eg
?
Do
t2g
D is small Do lt P
electrons occupy eg and t2g orbitals singly
before pairing
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Possible scenario Low spin vs. High spin state
Ground state electron configuration
Ar.3d7.4s2
Intermediate spin state
High spin state
Low spin state
Co2 (3d7)
S1/2
S3/2
S1/2
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Possible scenario superexchange
In order for a material to be magnetically
ordered, the spins on one atom must couple with
the spins on neighboring atoms. The most common
mechanism for this coupling is through the
superexchange interaction. The spin information
is transferred through covalent interactions with
the oxygen.
O 2p s
O 2p s
Co4 HS dx2-y2 ½ Filled
Co4 LS dx2-y2 Empty
Co4 HS dx2-y2 ½ Filled
Co4 HS dx2-y2 ½ Filled
Here the oxygen based electron will spend some
time on Co4 and due to Hunds rule polarize the
t2g e- leading to ferromagnetic coupling.
The covalent interaction through the O 2p orbital
stabilizes antiferromagnetic coupling.
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Possible scenario double-exchange
Localized electrons polarize itinerant
(delocalized) electrons. Magnetism and
conductivity are correlated.
Ferromagnetic Delocalized transport of tp
electrons allowed.
Antiferromagnetic Delocalized transport violates
Hunds Rule.
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Possible scenario
RBaCo2O5.5 100 Co3
RBaCo2O5 50 Co3 50 Co2
RBaCo2O6 50 Co3 50 Co4
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Possible scenario Jahn-Teller Distortion
eg
eg
t2g
t2g
Cubic Structure No Jahn-Teller Distortion All Co
atoms equivalent Localized t2g electrons Delocaliz
ed eg electrons Metallic
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Possible scenario Jahn-Teller Distortion
The Jahn-Teller theorem tells us there should be
a distortion when the eg orbitals octahedral
complex are partially occupied.
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Possible scenario Jahn-Teller Distortion
Symmetric MnO6
Jahn-Teller Distortion
Orthorhombic Structure Pronounced Jahn-Teller
Distortion Localized t2g eg electrons Semicondu
ctor
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possible scenario (I)
Y. Moritomo et al., Phys. Rev. B 61 (2000)
13325. TgtTMI HS state of Co3 ions (in the
both Co sites) T-gtTMI both the Co sites shift
by 0.2 A from basal plane toward the apical
oxygen --gt the spin state transition
from HS to IS states --gt orbital
degree of freedom couples with the JT
instability TTMI JT distortion and a
type orbital order TltTMI
double-exchange mechanism between the
local t2g spins and the eg electrons
--gt ferromagnetic behavior With
further decrease of temperature, the
inherent AFM superexchange interaction
overwhelms the suppressed double-exchange
interaction due to the carrier
localization --gt AFM transition.
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possible scenario (II)
C. Frontera et al., Phys. Rev. B 65 (2002) 180405.
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Summary The exceptional role of cobalt among the
3d transition elements can be emphasized. Low
spin, intermediate spin and high spin electronic
configurations can be stabilized for Co ions
depending on the intra-atomic exchange (paring)
energies and crystal-field effects. Moreover,
the competition between superexchange and
double-exchange interactions, as well as the
Jahn-Teller effect leads to spin, charge and
orbital ordering. The electronic and magnetic
properties of these compounds can be tuned by
oxygen nonstoichiometry x.