1. 2Instituto de Ciencia de Materiales de Madrid,
Two Magnon Bound State Causes the Ultrafast Thermally Induced Switching
J. Barker, U. Atxitia, T. A. Ostler, O. Hovorka, O. Chubykalo-Fesenko and R. W. Chantrell
1Department of Physics
University of York
2Instituto de Ciencia de Materiales de Madrid,
CSIC, Madrid, Spain.
Intermag, May 2014

2. How do we switch magnets?
We apply a bias to break the symmetry of the system.
External fields, Spin injection / Spin transfer torque
Initial state
Applying a bias
Switched state
Question 1: How do we do it?
Question 2: Why does it occur?
Initial State
Femtosecond laser heating
After cooling

3. Ultrafast element resolved dynamics
Element-resolved dynamics of the Fe and Gd magnetic moments measured by time-resolved XMCD with femtosecond time-resolution.
Radu et al., Nature 472, 205 (2011).
Transient ferromagnetic-like state
Reversal of the sublattices
Different demagnetization times
Initial State

4. Atomistic Spin Dynamics
Langevin Dynamics
Landau-Lifshitz-Gilbert Equation
thermal forces represented
by a stochastic field term - ηi
Fast laser heating is included with a two-temperature model
Kaganov et al., JETP 173 (1957)
Chen et al. International Journal of Heat and Mass Transfer 49, 307 (2006).

5. GdFeCo is an amorphous ferrimagnet.
Modeling of GdFeCo
GdFeCo is an amorphous ferrimagnet.
We approximate this as a random lattice of TM (FeCo) and RE (Gd) spins.
Statistical probability of Gd clusters: they are correlated over some length-scale.

6. Deterministic All Thermal Reversal
Discovered in a systematic investigation using atomistic spin dynamics
T.A. Ostler et al., Nat. Commun. 3, 666 (2012)
No applied field
Confirmed experimentally with unpolarized laser light
Two microstructures separated by a distance (no coupling) under the same laser spot.

7. Deterministic All Thermal Reversal
Question 1: How do we do it? ✓
Question 2: Why does it occur?
No obvious symmetry breaking, to answer that we have to consider microscopic interactions (spinwaves).

8. Spinwave modes FM
AFM
In the disordered ferrimagnet the excitations are mixed

9. The dynamic structure factor
So the AFM and FM modes are no longer pure but mixed!
Linear Spin Wave Theory
(Virtual Crystal Approximation)
Barker et al. Sci. Rep. 2013

10. The transfer of energy between sublattices
Barker et al. Sci. Rep. 2013
Only a single band in the excited region.
Non-linear energy transfer between bands.
Large band gap precludes efficient energy transfer.

11. Excitation of one spin wave modes only causes demagnetization.
Two spin wave modes
Excitation of one spin wave modes only causes demagnetization.
Excitation of two spin wave modes causes transient ferromagnetic like state.
The switching is caused when both branches of the FM and AFM are excited! The part of the BZ that is significant is determined by clustering.

12. Intermediate Structure Factor
How is spin wave amplitude distributed in space?
Below switching threshold
Above switching threshold
No significant change in the ISF
Excited region during switching
1090K
FeCo
975K Gd
Add symmetry points. Talk about amplitude (normalisation). Spin system absorbs energy but we are interested in the distribution.
X/2
X/2
M/2
M/2
Barker et al. Sci. Rep. 2013

13. Conlusions
Magnetization can be switched deterministically with heat alone.
This is due to the excitation of a two spin wave mode.
In GdFeCo the two spin wave mode is determined by the length scale of Gd clusters.
For full details see Nature Scientific Reports, 3, 3262 (2014).

14. Acknowledgements/references
Demagnetization times: Atxitia et al. arXiv: (2013).
Transient ferromagnetic-like state: Radu et al. Nature 472, (2011).
Atomistic model of GdFeCo: T. Ostler et al., Phys. Rev. B 84, (2011).
Thermally induced switching: Nat. Commun. 3, 666 (2012).
Switching in heterostructures: R. Evans et al. arXiv: (2013).
Switching mechanism: J. Barker et al. Nat. Sci. Rep. (in press) arXiv:
Thank you for your attention