first-principles study of large magnetoelectric coupling in triangular lattices
DESCRIPTION
First-Principles Study of Large Magnetoelectric Coupling in Triangular Lattices. Kris T. Delaney 1 , Maxim Mostovoy 2 , Nicola A. Spaldin 3. Materials Research Laboratory, University of California, Santa Barbara, USA - PowerPoint PPT PresentationTRANSCRIPT
(1)[email protected] | MRL, UCSB | APS March Meeting 2008
1. Materials Research Laboratory, University of California, Santa Barbara, USA2. Zernike Institute for Advanced Materials, University of Groningen, The
Netherlands3. Materials Department, University of California, Santa Barbara, USA
First-Principles Study of Large Magnetoelectric Coupling in
Triangular Lattices
Supported by NSF MRSEC Award No. DMR05-20415
Kris T. Delaney1, Maxim Mostovoy2, Nicola A. Spaldin3
03.13.2008
(2)[email protected] | MRL, UCSB | APS March Meeting 2008
Magnetoelectrics
Linear Magnetoelectric tensor:
Non-zero a requires T,I symmetry breaking Size limit (in bulk):
iijj
jiji
EM
HP
jjiiij 2
M. Fiebig, J. Phys. D: Appl. Phys. 38, R123 (2005)
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Magnetoelectric Symmetry Requirements
ferroelectricferromagnets
MULTIFERROICS
certain anti-ferromagnets
OR
+ Many materials- Weak - relies on S.O.
+ Large ε, μ potentially large α- Few materials at room T NA Hill, JPCB 104, 6694 (2000)
Our route: superexchange-driven magnetoelectric coupling
Which materials break time-reversal AND space-inversion symmetry?
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θ
Superexchange
S1 S2
21 SSJH ex
Anderson-Kanamori-Goodenough rules:J(θ=90º)<0 (FM)
J(θ=180º)>0 (AFM)
E=0 E E
Mn-O-Mn Superexchange
Superexchange magnetoelectricity:
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Superexchange-driven Magnetoelectricity
Can occurs in geometrically frustrated AFMo Route to bulk materials
Mechanism:
Anderson-Kanamori-Goodenough rules:J(θ=90º)<0 (FM)
J(θ=180º)>0 (AFM)
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Kagomé Lattices
E=0Example Spin Structure
E M=0
“Antimagnetoelectric”
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Triangular Lattices in Real Materials YMnO3 Structure:
BAS B. VAN AKEN et al, Nature Materials 3, 164 (2004)
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Break self compensation: One triangle sense per layer
Breaking Self Compensation: No Vertex Sharing
CaAlMn3O7
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Calculation Details Vienna Ab initio Simulation Package (VASP) [1]
o Density functional theory (DFT)o Plane-wave basis; periodic boundary conditionso Local spin density approximation (LSDA)o Hubbard U for Mn d electrons (U=5.5 eV, J=0.5 eV)
[3]o PAW Potentials [2]o Non-collinear Magnetism
No spin-orbit interaction
Finite electric fieldo Ionic response onlyo Forces = Z*E
Z* from Berry Phase [4]o Invert force matrix to deduce R[1] G. Kresse and J. Furthmüller, Phys. Rev. B 54, 11169 (1996).
[2] G. Kresse and D. Joubert, Phys. Rev. B 59, 1758 (1999).[3] Z. Yang et al, Phys. Rev. B 60, 15674 (1999).[4] R. D. King-Smith and D. Vanderbilt, Phys. Rev. B 47, 1651 (1993).
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DFT-LDA Electronic Structure; E=0
Ground-state magnetic structure from LSDA+U
Net magnetization = 0 μB
Crystal-field splitting and occupations for high-spin Mn3+
Local moment = 4μB/Mn
3d
dxz dyz
dx2-y2 dxy
dz2
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Magnetoelectric Coupling Magnetoelectric Response:
Compare: Cr2O3
E
m
small effect: E field of 106 V/cm produces M equivalent to reversing 5 out of 106 spins in the AFM lattice
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Conclusions
Superexchange-driven Magnetoelectricity:o Proposed new structureo Triangular lattice:
uniform orientation in each plane No vertex sharing with triangles of opposite sense
o Key: avoid self-compensation in periodic systems
New materials under investigation
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Electric Field Application (Ionic Response)
Force on ion in applied electric field:
where
Force-constant Matrix
Equilibrium under applied field (assume linear):
i
iijj ZF *
j
iij R
PZ
*
j
iij R
FC
i
iijj FCR 1