Magnetic Field-Induced Metaelectric Critcal End Point

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Magnetic Field-Induced Metaelectric Critcal End
Point in BiMn2O5
Jung Hoon Han (SungKyunKwan U, Korea)
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• arXiv0810.1907 cond-mat.mtrl-sci - Experiment
  
• Observation of a multiferroic critical end
  point
• arXiv0810.2332 cond-mat.mtrl-sci - Theory
• Theory of magnetic field-induced
  metaelectric
• critical end point in BiMn2O5
• Theory Gun Sang Jeon (SNU)
• Jin Hong Park (SKKU)
• Piers Coleman (Rutgers)
• Experiment Kee Hoon Kim, Jae Wook Kim (SNU)
• SW Cheong (Rutgers)

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Multiferroics Zoo
RMn2O5
(RY,Tb,Dy,Ho,Bi)
RMnO3 (RY,Lu,Er,Ho,Tb,Dy)
chain-like
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RMnO3
RMnO3 hexagonal RY,Er,Lu,Ho
RMnO3 orthorhombic RTb,Dy
YMnO3 Ferroelectric at high temperature
Ferroelectric moment increases further
upon magnetic ordering TN Mn
positions move at TN Evidence
of huge phonon-paramagnon coupling above TN
Seongsu Lee et al, Nature 451, 805 (2008)
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RMnO3
RMnO3 hexagonal RY,Er,Lu,Ho
RMnO3 orthorhombic RTb,Dy
TbMnO3 Collinear spin order at TC
Non-collinear spin order at TNC Onset
of ferroelectricity at TNC FE
is believed to be due to vector spin chirality
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RMn2O5
RMn2O5 Orthorhombic RY,Tb,Dy,Ho,Bi
Collinear spin order at TC
Ferroelectricity in the collinear phase
Switching of polarization direction by magnetic
field Q(qx,qy,qz) show commensurate/incomm
ensurate behavior FE is
believed to be due to exchange-striction
Hur et al, Nature 429, 392 (2004)
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Magnetic Structure of RMn2O5
Mn3 Tetrahedrally coordinated (pink) Mn4
Octahedrahedrally coordinated (green)
BiMn2O5
YMn2O5
Vecchini et al. PRB 77, 134434 (2008)
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• Mn3 (S2, dark) surrounded by oxygen tetrahedron
• Mn4 (S3/2, bright) surrounded by oxygen
  octahedron
• 4 Mn3 and 4 Mn4 per unit cell

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• c-axis exchange interaction is weak and
  unfrustrated
• Effective spin exchange pathway is 2D
• Stronger intra-chain, weaker inter-chain exchange

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• AFM along each chain results in every other
• inter-chain bond being frustrated
• Moving spins for one chain shifts the frustrated
  bonds
• Sub-extensive degeneracy 2L, Lchain number

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• Degeneracy lifted through exchange-striction
• Mn3 dimer as a rigid object that moves up or
  down
• Movement along a-direction cancels out
• A uniaxial ferroelectric along b-direction

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Existing Theories of RMn2O5
Exchange-striction mechanism for ferroelectricy
proposed along with the experiment,
Chapon et al. PRL 96, 097601 (2006) More
elaborate version including dynamics developed in
Harris, Aharony, Entin-Wohlman PRL 100, 217202 (2008) Group theory
Fang Hu EPL 82, 57005 (2008) Effective model
Sushkov et al J Phys C 20, 1 (2008) Electromagnon exp. reviewtheory
None of the theories address the field-induced
critical end point in RMn2O5
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Putting on a strong magnetic field BiMn2O5

• Ferroelectric polarization, initially along b,
  decreases then reverses with field along a-axis

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Putting on a strong magnetic field BiMn2O5

• Ferroelectric polarization, initially along b,
  decreases then reverses with field along a-axis
• Dielectric constant for b-axis show peaks at P0

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Putting on a strong magnetic field BiMn2O5

• Ferroelectric polarization, initially along b,
  decreases then reverses with field along a-axis
• Dielectric constant for b-axis show peaks at P0
• Magnetic susceptibility also shows peaks at P0

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Putting on a strong magnetic field BiMn2O5

• P near HHc show scaling
• Dielectric constant near HHc show scaling

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Hamiltonian Inter- Intra-Chain Exchange
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Hamiltonian Spin-Lattice Coupling
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Hamiltonian Single-Ion Anisotropy
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Hamiltonian Zeeman field
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Phase Diagram - qualitative
Strong SL
Weak SL
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Polarization vs. H
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Susceptibilities vs. H
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Critical Exponents
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Summary

• Extension of exchange-striction mechanism for
  ferroelectricity
• in RMn2O5 appears to explain high-field
  behavior well
• BiMn2O5 seems to be an example of critical end
  point,
• with ferroelectric order parameter, tunable
  with magnetic field
• We havent developed theory of dynamics will
  be important
• for addressing issue of QCP?
• Classical/quantum nature of critical end point
  is unclear