Electric-field control of magnetism and magnons in the room-temperature multiferroic BiFeO3 Image © NPG Rogério de Sousa Dept. of Physics and Astronomy, University of Victoria, B.C., Canada Joint work with: M. Allen (UVic), M. Cazayous (Paris-7) This talk published in R. de Sousa, Physics in Canada, 72, 57 (2016)

E-field control of magnetism in metallic materials: Ferromagnetic thin films and magnetic semiconductors Daalderop et al PRB 1991 Microscopic mechanism: Single ion anisotropy is strongly dependent on the filling of d-orbitals Image © APS Shiota et al, Nat. Mat. 2012 Wang et al, Nat. Mat. 2012 Image © NPG Chiba et al, Nature 2008 Image © NPG

BiFeO3 (BFO): The room temperature multiferroic Ferroelectric order (P), Antiferromagnetic order (L=M1-M2), and weak ferromagnetism (M=M1+M2) P M2 M1 Fe Oxygen Bi TFE = 1093 K ; TNéel= 643 K; Spiral magnetism: Ground state is a cycloid with period of 620 Å.

Antiferromagnetic Cycloid ground state in BFO 620 Å Weak ferromagnetism M = Z X L

E-field control of the cycloid in BFO Lebeugle et al, PRL 2008; Lee et al, PRB 2008 P [111] Bi E q P [1-11] Image © APS

Electrically-switchable magnetic memory? Heron et al, PRL 2011; M. Fiebig Physics 2011; Heron et al Nature 2014 Image © APS Ashraf et al IEDM 2012: Energy dissipation for switching is 5 J/m2 6 X less than the best Spin-transfer torque device!

E-field control of the cycloid in BFO P [1-11] P [111] q E Image © APS Not the whole story! E-field control of magnetism can occur even without switching P!

Optical excitation spectra of a cycloidal magnet

Excitation spectra of a cycloidal multiferroic k/Q ω 3 k/Q -1/2 1/2 ω Cyclons: Extra-cyclons:

High resolution Raman scattering of cycloidal magnons M. Cazayous et al, PRL 2008; R. de Sousa and J.E. Moore, PRB 2008; R.S. Fishman et al, PRB 2013 Image © APS

Magnon excitations as a function of external B field IR spectroscopy: U. Nagel et al PRL 2013 (Theory by R.S. Fishman) Image © APS

Magnon excitations as a function of strain (BFO thin films on different substrates) (LaAlO3)0.3–(Sr2AlTaO6)0.7 SmScO3 Sando et al, Nature Mat. 2013 DyScO3 NdScO3 Same as in Heron et al PRL 2011 GdScO3 PrScO3 Image © NPG

BiFeO3 with a strong E-field (~ 105 V/cm) P. Rovillain et al, Nature Mat. 2010 Image © NPG

E-field shift of magnon spectra E-field shift of magnon spectra! 105 times larger than previous experiments Images © NPG

Symmetry-based origin of the effect Image © NPG  Electric-field induced phase transition to a homogeneous state

E-field effect on magnetic order

Microscopic origin of E-field induced magnetoelectric effect R. de Sousa, M. Allen, M. Cazayous, PRL 2013 Spin-orbit interaction on Bi ions "Lattice mechanism": E-field displaces ions, mixing E-field with spins "Electronic mechanism": E-field induces orbital admixture

Magnetic anisotropy in terms of atomic orbitals and their location

A special kind of linear magnetoelectric effect Explains the origin of the giant shift of magnon spectra!

Spin-current interaction Effect of an external E-field on BFO's ground state? Use continuum field theory Spin-current interaction Exchange E-field effect

E-field control of Antiferromagnetism

E-field control of weak ferromagnetism Apply E=2 X 104 V/cm along one of three special directions <M>≠0

Phase diagram: E//-X, B//Z Homogeneous Planar cycloid M. Allen and R. de Sousa, unpublished

Phase diagram: E//-X, B//X Q E 1st order phase trans. Homogeneous Conical cycloid M. Allen and R. de Sousa, unpublished

BFO's magnon spectra as a function of pressure J. Buhot et al, PRL 2015 Image © APS

Conclusion: A new kind of E-field control of magnetism in insulators E-field control of magnetism without switching P.  Reduced energy dissipation in spin based devices! Microscopic origin: strong spin-orbit coupling (at Bi) combined with the admixture between 3d and 6p orbitals induced by the E-field. An external E-field applied along certain directions is capable of destroying the cycloid, and of controlling the Néel vector and magnetization directions.

References R. de Sousa, Physics in Canada 72, 57 (2016) J. Buhot et al, PRL 115, 267204 (2015) R. de Sousa, M. Allen, M. Cazayous, PRL 110, 267202 (2013) P. Rovillain, R. de Sousa et al, Nature Materials 9, 975 (2010) R. de Sousa and J.E. Moore, PRL 102, 249701 (2009) M. Cazayous, R. de Sousa et al, PRL 101, 037601 (2008) R. de Sousa and J.E. Moore, APL 92, 022514 (2008) R. de Sousa and J.E. Moore, PRB 77, 012406 (2008)