Switching with Ultrafast Magnetic
Field Pulses
Ioan Tudosa
Outline
• Motivation
• Experiments with in-plane samples
– Damping
– Anisotropy (induced by electric field)
• Future ideas
– Terahertz radiation switching
• Wish List
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Motivation: Understanding Ultrafast Physics
Basic Question: Is there any new physics to be found in exploring the
fundamental limits of fast magnetization dynamics?
Current work :
Extremely strong
electromagnetic field
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Experiment principle
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Experimental Set Up
Long and Short Pulse Field Strengths
Short
WE HAVE PEAK FIELD
VALUES OF
60 TESLA AND 20 GV/m !
We use two pulse lengths:
Long pulse
τ = 2.3*10-12 sec
Long
Short pulse
τ = 70*10-15 sec
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Comparison of Field Magnitudes
60 T Magnetic Field!
• Hard disk write head: 1-2 T
• Superconducting magnet: 15-20 T
• SLAC experiment: 60 T
20 GV/m Electric Field!
• AlGaAs/GaAs quantum wells : 106 V/m
• Vacuum breakdown (millitorr): 107 V/m
• SLAC experiment: 2*1010 V/m
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Beamline setup
manipulator
chamber
Electron bunches
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1.Sample Holder
Wire scanners
Samples
Sample holder
1cm
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Precession Torques
Sample is uniformly magnetized initially
Maximum torque
Minimum Torque
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Lines of constant torque
T ~ MxH ~ sin(M,H)
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Fe/GaAs Thin Films
Au 10 layers
Fe 10 or 15 layers
GaAs
 Grown using MBE
 Uniaxial in-plane anisotropy
 Imaged with SEMPA
15 ML Fe
10 ML Fe
M0
100 mm
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Precessional Magnetization Reversal
3 Step Process
Field Pulse Kick
Rotations Around Hdemag
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Final Alignment
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Dynamics of Magnetization
Damping dissipates the energy pumped into the system  circle widths
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Damping of Magnetic Energy
Experiment
Calculation
(LLG+magnons)
FMR damping
Inset: Reduction of magnetization
due to magnon scattering
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Experiment with ultrastrong fields
Sample Composition:
MgO/30nm Cr80Mo20/10nm Co70Fe30/1.5 nm Pt
Imaging: SEM with Polarization Analysis
Magnetism and topography
electric field strength is up to 20 GV / m (2 V / Angstrom)
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Magneto-electronic anisotropy is strong 1000
~E
times stronger
B-field torque
E-field torque
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Manipulating Magnetic Anisotropy
Method 1: Move Atoms
●New ATOMIC
arrangement gives new axis
●True magnetocrystalline
anisotropy alteration
Method 2: Move Electrons
● New ELECTRONIC
arrangement gives new axis
● New magneto-electric
anisotropy alteration
● Need ~ fs to move electrons
● Uses spin-orbit coupling
Gamble S. J. et al. - PRL, vol 102, 217201, (2009)
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Experiment vs simulation
Simulation
Takes into account:
Increased damping near the center
Additional E-field anisotropy
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Topographic contrast data
2.3 Picosecond Exposure
•Sample has heated to at
least Tc = 1200 K in a 100 μm radius
•Sample is visibly damaged
at the point of beam impact
70 Femtosecond Exposure
• Sample shows no evidence
of heating or ablation
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Energy loss transfer
What do we know?
• The pulse will excite the electron gas
• The electron gas will equilibrate with the phonon system in ~1 picosecond
This means some energy from the excited electron gas will
reach the phonon system DURING the picosecond pulse!
What do we need to know?
• How does all the other energy get out?
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Transition Radiation
Transition Radiation  a charged particle crosses a boundary ε1| ε2
Coherent Transition Radiation  is emitted for λ>lbunch
Bunch Electric Field
Sample Response
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Half cycle terahertz radiation
– Coherent transition radiation at λ  bunch length.
• Highly compressed bunches: λ  10 to 100 µm
• Corresponding frequencies: 3 to 30 THz
• Intense pulses with the time structure of the electron beam
– When focused:
• Electric fields > 1 GV/m = 0.1 V/Å
• Magnetic fields > 3 T
• Well above other THz sources
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E-202
• Explore the electric field effect
• Get the time scale of beam damage
• Ferromagnetic and ferroelectric samples
• Expose to THz radiation outside the e-beam
• Use magnetic media with Hc > 9T
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Wish List
 Beamtime during day for better support
 Predictable schedule
 Better access for changing samples




Bunch length diagnostics
Less radiation background (for electronics)
Tighter focus
One shot or 0.1Hz mode
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Conclusion
 SLAC still a useful, powerful EM pulse source
 Electric field influences magnetization dynamics
 Potential to direct the EM pulse and focus it
 Applications to magnetic recording??
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