FirstPrinciples Study of Large Magnetoelectric Coupling in Triangular

Magnetoelectrics § Linear Magnetoelectric tensor: § Non-zero a requires T, I symmetry breaking Size

Magnetoelectric Symmetry Requirements Which materials break time-reversal AND space-inversion symmetry? ferroelectric ferromagnets MULTIFERROICS certain

Superexchange § Mn-O-Mn Superexchange θ S 1 § Anderson-Kanamori. Goodenough rules: J(θ=90º)<0 (FM) J(θ=180º)>0

Superexchange-driven Magnetoelectricity § Can occurs in geometrically frustrated AFM o § Route to bulk

Kagomé Lattices E=0 Example Spin Structure E M=0 “Antimagnetoelectric” kdelaney@mrl. ucsb. edu | MRL,

Triangular Lattices in Real Materials § YMn. O 3 Structure: BAS B. VAN AKEN

Breaking Self Compensation: No Vertex Sharing § Break self compensation: One triangle sense per

Calculation Details § Vienna Ab initio Simulation Package (VASP) [1] o o o Density

DFT-LDA Electronic Structure; E=0 Crystal-field splitting and occupations for high-spin Mn 3+ dz 2

Magnetoelectric Coupling § Magnetoelectric Response: E § Compare: Cr 2 O 3 m small

Conclusions § Superexchange-driven Magnetoelectricity: o o Proposed new structure Triangular lattice: § § o

Electric Field Application (Ionic Response) § Force on in applied electric field: where §

Slides: 13

Download presentation

First-Principles Study of Large Magnetoelectric Coupling in Triangular Lattices Kris T. Delaney 1, Maxim Mostovoy 2, Nicola A. Spaldin 3 1. Materials Research Laboratory, University of California, Santa Barbara, USA 2. Zernike Institute for Advanced Materials, University of Groningen, The Netherlands 3. Materials Department, University of California, Santa Barbara, USA kdelaney@mrl. ucsb. edu 03. 13. 2008 Supported by NSF MRSEC Award No. DMR 05 -20415 kdelaney@mrl. ucsb. edu | MRL, UCSB | APS March Meeting 2008 (1)

Magnetoelectrics § Linear Magnetoelectric tensor: § Non-zero a requires T, I symmetry breaking Size limit (in bulk): § M. Fiebig, J. Phys. D: Appl. Phys. 38, R 123 (2005) kdelaney@mrl. ucsb. edu | MRL, UCSB | APS March Meeting 2008 (2)

Magnetoelectric Symmetry Requirements Which materials break time-reversal AND space-inversion symmetry? ferroelectric ferromagnets 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 kdelaney@mrl. ucsb. edu | MRL, UCSB | APS March Meeting 2008 (3)

Superexchange § Mn-O-Mn Superexchange θ S 1 § Anderson-Kanamori. Goodenough rules: J(θ=90º)<0 (FM) J(θ=180º)>0 (AFM) S 2 Superexchange magnetoelectricity: E=0 kdelaney@mrl. ucsb. edu E | MRL, UCSB | E APS March Meeting 2008 (4)

Superexchange-driven Magnetoelectricity § Can occurs in geometrically frustrated AFM o § Route to bulk materials Mechanism: Anderson-Kanamori. Goodenough rules: J(θ=90º)<0 (FM) J(θ=180º)>0 (AFM) kdelaney@mrl. ucsb. edu | MRL, UCSB | APS March Meeting 2008 (5)

Kagomé Lattices E=0 Example Spin Structure E M=0 “Antimagnetoelectric” kdelaney@mrl. ucsb. edu | MRL, UCSB | APS March Meeting 2008 (6)

Triangular Lattices in Real Materials § YMn. O 3 Structure: BAS B. VAN AKEN et al, Nature Materials 3, 164 (2004) kdelaney@mrl. ucsb. edu | MRL, UCSB | APS March Meeting 2008 (7)

Breaking Self Compensation: No Vertex Sharing § Break self compensation: One triangle sense per layer Ca. Al. Mn 3 O 7 kdelaney@mrl. ucsb. edu | MRL, UCSB | APS March Meeting 2008 (8)

Calculation Details § Vienna Ab initio Simulation Package (VASP) [1] o o o Density functional theory (DFT) Plane-wave basis; periodic boundary conditions Local spin density approximation (LSDA) Hubbard U for Mn d electrons (U=5. 5 e. V, J=0. 5 e. V) [3] PAW Potentials [2] Non-collinear Magnetism § § No spin-orbit interaction Finite electric field o o Ionic response only Forces = Z*E § o Z* from Berry Phase [4] Invert force matrix to deduce DR [1] [2] [3] [4] kdelaney@mrl. ucsb. edu G. Kresse and J. Furthmüller, Phys. Rev. B 54, 11169 (1996). G. Kresse and D. Joubert, Phys. Rev. B 59, 1758 (1999). Z. Yang et al, Phys. Rev. B 60, 15674 (1999). R. D. King-Smith and D. Vanderbilt, Phys. Rev. B 47, 1651 (1993). | MRL, UCSB | APS March Meeting 2008 (9)

DFT-LDA Electronic Structure; E=0 Crystal-field splitting and occupations for high-spin Mn 3+ dz 2 3 d dx 2 -y 2 dxy dxz dyz Local moment = 4μB/Mn § Ground-state magnetic structure from LSDA+U Net magnetization = 0 μB kdelaney@mrl. ucsb. edu | MRL, UCSB | APS March Meeting 2008 (10)

Magnetoelectric Coupling § Magnetoelectric Response: E § Compare: Cr 2 O 3 m small effect: E field of 106 V/cm produces M equivalent to reversing 5 out of 106 spins in the AFM lattice kdelaney@mrl. ucsb. edu | MRL, UCSB | APS March Meeting 2008 (11)

Conclusions § Superexchange-driven Magnetoelectricity: o o Proposed new structure Triangular lattice: § § o § uniform orientation in each plane No vertex sharing with triangles of opposite sense Key: avoid self-compensation in periodic systems New materials under investigation kdelaney@mrl. ucsb. edu | MRL, UCSB | APS March Meeting 2008 (12)

Electric Field Application (Ionic Response) § Force on in applied electric field: where § Force-constant Matrix § Equilibrium under applied field (assume linear): kdelaney@mrl. ucsb. edu | MRL, UCSB | APS March Meeting 2008 (13)