Coexistence of magnetism and ferroelectricity in Pr,CaMnO3 near half doping: a first principles inve

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Coexistence of magnetism and ferroelectricity in
(Pr,Ca)MnO3 near half doping a first principles
investigation G. Colizzi, A. Filippetti and
V. Fiorentini CNR-INFM SLACS and Department of
Physics University of Cagliari
FOxE - Functional Oxides for Electronics Villa
Angelina, Massa Lubrense, Sorrento (NA)? 25-27
Marzo 2009
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PCMO phase diagram
After B.Raveau et al. j. mater. Chem. 2001, 11
29-36
Our Regions of interest
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A debated question CE-ordering or Zener
Polaron?
CE-ordering
Zener-polaron
After A. Doud-Aladine et al. PRL. 89 , 2002
After A. Trokiner et al. PRB. 74 , 2006
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An even more exotic phenomenology is predicted
!!!!

Mn3
Mn4
0.4 doping ZP -ord.
0.5 dopingCE-ordering
0.45 dopingFE !!!
D. Efremov et al. Nat. Mat. 3, 853 (2004)?
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Pr0.5Ca0.nO3 crystal(high-T)?

• Computational details
• Large supercell 80 atoms(necessary for
  simulating AF-CE )?
• Structural and magnetic point group C2h
  (centrosymmetric)
• Ab initio density functional theory (DFT)? VASP
  code and GGAU8 (Anisimov et al. PRB 1997)
  Polarization within the Berry phase formalism
• All atoms relaxed !!!!
• Anisimov results well reproduced with some
  exceptionInsulating, OO and (small) CO ok but
  the Magn.-Mom.disproportion of Mn present only
  after relaxation otherwise all Mn atoms Are
  equivalent !

• Orthorhombic crystal space group Pbnm (Z. Jirak
  et al. 1985)20 atoms primitive cell
• Rotated and tilted almost undistorted MnO6 oxygen
  octahedra

• Zener polarons never realized together with AF-CE
  magnetic phase even when starting from a inizial
  structure made of Mn-O-Mn ZP trimers

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Pbnm structure relaxed Ferroelectric!!!

• Atoms which are struct.-elect. magn.-ly
  equivalent show similar displacements but ...
  because of the OO in Mn atoms no exactly equal
  !!! gt A small polarization appears!!!
• Because of the Orbital Ordering The Polarization
  is not exactly parallel to a crystal axis (b in
  our picture) as predicted by Efremov et al.
• This polarization grows dramatically to 0.77
  mC/m2 if just a system with a slightly less
  omogeneous Ca and Pr distribution is considered

Ptot 0.89mC/m2 (Pion0.55mC/m2,Pel0.34mC/m2)
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Conclusions

• According to DFT Pr0.5Ca0.5MnO3 Pbnm crystal

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Ackowledgments

• Special thanks to Pietro Delugas now
• at IMM-CNR Catania for all his precious
  suggestion
• Alessandra Lehmann and her group who introduced
  us to PCMO physics
• All the organizers of FoxE
• All of you for your attention

FOxE - Functional Oxides for Electronics Villa
Angelina, Massa Lubrense, Sorrento (NA)? 25-27
Marzo 2009
9
Pbnm structure relaxed

• Atoms which are struct.-elect. magn.-ly
  equivalent show similar displacements
• Intrachain O's mainly move parallel to the
  Mn-O-Mn bond approaching their Mn-corner atom
• O-Mn-O AF trimers are formed strong Mn-O
  hybrydization and Mn atoms deeply hybridized!!!!
• Interchain O's move less and mainly
  perpendicularly
• to the Mn-O-Mn bond
• Charge-Ordering(CO) all Mn's have very similar
  electric charge, ?q0.05e- more relevant the
  difference for Mn's magnetic moment ?m0.30uB a
  similar CO also for O's where ?q0.03e- and also
  played by the
• MnO6 octahedra of two types distorted(side-chain
  centred) and undistorted (corner-chain centred)
• Ab initio density functional theory (DFT)? VASP
  code and GGAU8 (Anisimov et al. PRB 1997)?
• All atoms relaxed !!!!
• Anisimov results well reproduced with some
  exceptionInsulating, OO and (small) CO ok but
  the Magn.-Mom.disproportion of Mn present only
  after relaxation otherwise all Mn atoms Are
  equivalent !