atomistic modelling of ultrafast magnetization switching
DESCRIPTION
Atomistic Modelling of Ultrafast Magnetization Switching. J. Barker 1 , T. Ostler 1 , O. Hovorka 1 , U. Atxitia 1,2 , O. Chubykalo-Fesenko 2 and R. W. Chantrell 1 1 Dept. of Physics, The University of York, York, United Kingdom. - PowerPoint PPT PresentationTRANSCRIPT
Atomistic Modelling of Ultrafast Magnetization Switching
Ultrafast Conference on Magnetism
J. Barker1, T. Ostler1, O. Hovorka1, U. Atxitia1,2, O. Chubykalo-Fesenko2 and R. W. Chantrell1
1Dept. of Physics, The University of York, York, United Kingdom.2Instituto de Ciencia de Materiales de Madrid, CSIC, Madrid, Spain.
Overview
Thermal switching observed
• No good explanation.
Can we develop a theory/framework?
• Can we predict something?
• Better/new materials.
Is it predictive?
• Can it explain all observed behaviour?
• Verification.
Deterministic all-thermal switching
Predicted using atomistic spin dynamics.
No applied field required.
Verified experimentally.
Ostler et al. Nat. Commun., 3, 666 (2012).
Single shot.
Linear polarised light. No IFE.
Element-resolved dynamics.
Initial State
Different demagnetization
times
Transient ferromagnetic-like
state
Reversal of the sublattices
Important features of the dynamics
Radu et al. Nature, 472, 205-208 (2011).
Different demagnetisation timesI. Radu et al., Nature 472, 205 (2011)
U. Atxitia et al, arXiv:1308.0993.
Transient ferromagnetic like stateI. Radu et al., Nature 472, 205 (2011)
Deterministic reversal without fieldT.A. Ostler et al., Nat. Commun. 3, 666 (2012)
Difference in magnetic moment (mostly, see talk by O. Chubykalo-Fesenko)
?
?
What we know/unanswered questions
Understanding the mechanism driving this process is crucial for finding new materials.
The atomistic model of GdFeCo
Amorphous nature
Random lattice model
Exchange Interactions: Heisenberg Hamiltonian
Dynamics
T. Ostler et al., Phys. Rev. B 84, 024407 (2011)
Femtosecond heating
Chen et al. Int. Journ. Heat and Mass Transfer. 49, 307-316 (2006)
Beyond magnetization
How can we explain the observed effects in GdFeCo?
Large demagnetization.
Deterministic switching.
Suggests something is occurring on microscopic
level
Below switching threshold
No significant change in the ISF
Above switching threshold
Excited region during switching2 bands excited
975K
M/2
X/2
1090K FeCoGd
M/2
X/2
Intermediate structure factor (ISF)
ISF distribution of modes even out of equilibrium.
J. Barker, T. Ostler et al. Nature Scientific Reports, in press. arXiv:1308.1314
Relative Band Amplitude
Dynamic structure factor (DSF)
To calculate the spinwave dispersion from the atomistic model we calculate the DSF.
The point (in k-space) at which both bands are excited corresponds to the spinwave excitation (ISF).
1090K FeCoGd
M/2
X/2
Frequency gap
By knowing at which point in k-space the excitation occurs, we can determine a frequency (energy) gap.
This can help us understand why we do not get switching at certain concentrations of Gd.
Overlapping bands allows for efficient transfer of
energy.
Large band gap precludes
efficient energy
transfer.
What is the significance of the excitation of both bands?
Excitation of only one band leads to demagnetization.
Excitation of both bands simultaneously leads to the transient ferromagnetic-like state.
Can we predict where in k-space both bands will be excited?
Effects of clustering
Randomly populating lattice
Recall overlap in spectrum.
Length-scale corresponds to physical clusters.
The point at which we have band overlap in the spinwave spectrum and the cluster size are correlated.
Clustering
Linear Spin Wave Theory
Virtual Crystal Approximation
Bogolioubov Transform
Spinwave dispersion
From linear spinwave theory (LSWT) we can derive the magnon dispersion relation.
Use cluster analysis to determine which part of spectrum to consider gap.
No Switching
Switc
hing
Laser Fluence
High
Low
By combining the analytic treatments:
Predicting the switching window
We can predict the energy gap required to excite modes in both bands at significant |k|.
Theoretical Prediction Simulation Result
VCA ClusteringMFALSWT
Different demagnetisation timesI. Radu et al., Nature 472, 205 (2011)
U. Atxitia et al, arXiv:1308.0993.
Transient ferromagnetic like
stateI. Radu et al., Nature
472, 205 (2011)
Deterministic reversal without field
T.A. Ostler et al., Nat. Commun. 3, 666 (2012)
Difference in magnetic moment (mostly, see talk by O. Chubykalo-
Fesenko)
Can we now explain the observed effects?
• transient state arising from two magnon excitation • cooling ~ps means excitation decays
Summary
Our aim was to explain observed dynamics.
Distribution of modes showed excitation at finite k-vector.
Transient state arises from two-magnon excitation.
Energy of two-magnon excitation predicts composition dependent switching.
Conclusions/outlook
Understanding this mechanism we can engineer other anti-ferromagnetically coupled materials/structures[1].
Key ingredients
Two bands arising from two (or more) species
AFM coupled
Stimulus with sufficient energy
to excite both bands
Stimulus must be faster than the
timescale of the decay of the
modes
The species that reverses first
must form stable sublattice
[1] R. Evans et al., arXiv: (2013)
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
Only A is fitted to account for finite size lattice, pc and ν are
universal exponents.
The spin wave spectrum and physical clustering are correlated.
Hoshen-Kopelman method to calculate typical correlation
length for a given Gd concentration.
Clustering effects
Linear Spin Wave Theory
Virtual Crystal Approximation
Bogolioubov Transform
Linear Spin Wave TheoryVirtual Crystal Approximation
Bogolioubov Transform
Prediction Switching observed in simulations
VCA PercolationMFALSWT
No Switching
Switc
hing
Laser Fluence
High
Low
Predicting switching
Non-linear energy transfer between
bands.
Only a single band in the excited region.
Large band gap precludes efficient
energy transfer.
The transfer of energy between sublattices
Element-resolved dynamics.
Initial State
Different demagnetization
times
Transient ferromagnetic-like
state
Reversal of the sublattices
Important features of the dynamics
Radu et al. Nature, 472, 205-208 (2011).