Two Magnon Bound State Causes the Ultrafast Thermally Induced Switching J. Barker, U. Atxitia, T. A....
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Transcript of Two Magnon Bound State Causes the Ultrafast Thermally Induced Switching J. Barker, U. Atxitia, T. A....
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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
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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?
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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).
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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).
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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.
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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
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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).
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Spinwave modes
FM
AFM
In the disordered ferrimagnet the excitations are mixed
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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!
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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
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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.
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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
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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).
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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