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 PhysicsUniversity of York

2Instituto de Ciencia de Materiales de Madrid,CSIC, Madrid, Spain.

Intermag, May 2014

How do we switch magnets?

• External fields, Spin injection / Spin transfer torque

We apply a bias to break the symmetry of the system.

Initial state Applying a bias Switched state

Initial State After coolingFemtosecond laser heating

Question 1: How do we do it?

Question 2: Why does it occur?

Element-resolved dynamics of the Fe and Gd magnetic moments measured by time-resolved XMCD with femtosecond time-resolution.

Ultrafast element resolved dynamics

Initial State

Different demagnetization

times

Transient ferromagnetic-like

state

Reversal of the sublattices

Radu et al., Nature 472, 205 (2011).

Atomistic Spin Dynamics

Landau-Lifshitz-Gilbert EquationLangevin Dynamics

thermal forces representedby 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).

Modeling of GdFeCoGdFeCo 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.

Deterministic All Thermal ReversalDiscovered in a systematic investigation using atomistic spin dynamics

T.A. Ostler et al., Nat. Commun. 3, 666 (2012)

No applied field

Two microstructures separated by a

distance (no coupling) under the same laser

spot.

Confirmed experimentally with unpolarized laser light

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).

Spinwave modes

FM

AFM

In the disordered ferrimagnet the excitations are mixed

The dynamic structure factor

Linear Spin Wave Theory(Virtual Crystal Approximation)

Barker et al. Sci. Rep. 2013

So the AFM and FM modes are no longer pure but mixed!

The transfer of energy between sublattices

Non-linear energy transfer between

bands.

Only a single band in the

excited region.

Large band gap precludes efficient energy transfer.

Barker et al. Sci. Rep. 2013

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.

Intermediate Structure Factor

Below switching threshold

No significant change in the ISF

Above switching threshold

How is spin wave amplitude distributed in space?

Excited region during switching

975K1090K FeCo

Gd

M/2

X/2

M/2

X/2

Barker et al. Sci. Rep. 2013

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).

Acknowledgements/references

References

Demagnetization times: Atxitia et al. arXiv:1308.0993 (2013).

Transient ferromagnetic-like state: Radu et al. Nature 472, 205-208 (2011).Atomistic model of GdFeCo: T. Ostler et al., Phys. Rev. B 84, 024407 (2011).

Thermally induced switching: Nat. Commun. 3, 666 (2012).

Switching in heterostructures: R. Evans et al. arXiv:1308.1314 (2013).Switching mechanism: J. Barker et al. Nat. Sci. Rep. (in press) arXiv:1308.1314.

Thank you for your attention