1. 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)

2. 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

3. 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.

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

5. 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

6. 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!

7. 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!

8. Optical excitation spectra of a cycloidal magnet

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

10. 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

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

12. 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

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

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

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

16. E-field effect on magnetic order

17. 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

18. Magnetic anisotropy in terms of atomic orbitals and their location

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

20. 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

21. E-field control of Antiferromagnetism

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

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M. Allen and R. de Sousa, unpublished

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M. Allen and R. de Sousa, unpublished

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

26. 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.

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