Terahertz spin transport in magnetic nanostructures - Spin Charge - Uni Mainz
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Terahertz spin transport in
magnetic nanostructures
Spin
Charge
Tobias Kampfrath
Freie Universität Berlin and
Fritz-Haber-Institut/MPG 1
Acknowledgments
FUB/FHI THz group,
for these results:
Samridh Jaiswal
Reza Rouzegar
Gerhard Jakob
Oliver Gückstock
Martin Jourdan
Lukáš Nádvorník
Mathias Kläui
Tom Seifert
Martin Wolf
Marco Battiato Ulrike Martens
Pablo Maldonado Markus Münzenberg
Peter Oppeneer
David Reiß Liane Brandt
Piet Brouwer Georg Woltersdorf
Richard Schlitz
S. Goennenwein
CRC / TRR 227
Ultrafast Spin Dynamics
2
Spintronics: 3 basic operations
1. Turn spins around 2. Transport spins 3. Detect
spin dynamics
↑ Torque ↑ Spin current
Goal: Reach speed of other information carriers: 1 THz = 1012 Hz
Photons in fibers: >10 Tbit/s Electrons in a FET: ~1 THz cut-off
Hillerkuss et al., Nature Phot. (2011) Del Alamo et al., Nature (2011)
3
Spintronics: 3 basic operations
1. Turn spins around 2. Transport spins 3. Detect
spin dynamics
↑ Torque ↑ Spin current
Goal: Reach speed of other information carriers: 1 THz = 1012 Hz
Idea: Use THz electromagnetic pulses
1 ps < (1 THz),1 < 300 µm.c
Spin
THz ∗ spin transport < useful?
Charge
4
Why THz spin transport?
1) Test the speed of spintronic effects, reveal initial steps
E.g. spin-Hall, spin-Seebeck and GMR
2) New physics, new methods 3) Reward for THz photonics
As THz coincides with many E.g. THz sources and modulators
fundamental modes for spectroscopy and imaging
-
- + +
- -
Intraband transport Phonons
Magnons Stantchev et al., Science Adv. (2016)
How to drive spin transport? 5
Important drivers of spin transport
ƒ Electric fields: E.g. spin Hall effect
ƒ Heat: Spin-caloric effects
How to transfer to the THz range?
Spin
THz pulse
Charge
6
Heat-driven: The Seebeck effect
Metal film
Thomas Seebeck (1821):
A temperature gradient drives an electron current
-
- Uchida and Saitoh (2008):
- In ferromagnets, the Seebeck current
is spin-dependent
Hot Cold
Temperature
gradient
7
Spin-dependent Seebeck effect (SDSE)
Fe film ← and → electrons have very different
transport properties
Uchida, Saitoh et al., Nature (2008)
Bauer, Saitoh, Wees, Nature Mat. (2013)
8
Spin-dependent Seebeck effect (SDSE)
Fe film ← and → electrons have very different
transport properties
Uchida, Saitoh et al., Nature (2008)
Bauer, Saitoh, Wees, Nature Mat. (2013)
⇑ Spin-polarized current
Detection with the
inverse spin Hall effect
Hot Cold
Temperature
gradient
9
Inverse spin Hall effect (ISHE)
Heavy
Fe film
metal Spin-orbit coupling
deflects electrons
⇑ Transverse charge current
A
⇑ Spin-to-charge (S2C)
conversion
Saitoh et al., APL (2006)
How can we induce an
Hot Cold
imbalance as fast as possible?
Temperature
gradient
10Inverse spin Hall effect (ISHE)
Heavy Ultrafast spin transport
Fe film Malinowski et al., Nature Phys. (2008)
metal
Melnikov, Bovensiepen et al., PRL (2011)
Rudolf et al., Nat. Comm. (2012)
Eschenlohr et al., Nature Mat. (2013)
A Turgut et al., PRL (2013)
Schellekens et al., Nat. Comm. (2014)
Ganichev et al., Nature (2002)
Goal:
fs pump Measure quasi-electrically
pulse
Technical challenge:
ƒ Electric detection has cutoff at < 50 GHz
ƒ But expect bandwidth > 10 THz
11Inverse spin Hall effect (ISHE)
Heavy
Fe film
metal
Emission of
electromagnetic
pulse (~1 THz)
Kampfrath, Battiato, Münzenberg et al.,
Nature Nanotech. (2013)
fs pump
pulse
⇑ Measure THz emission from photoexcited FM|NM bilayers
Samples: Polycrystalline films (from Kläui, Münzenberg, Woltersdorf labs)
Pump pulses: From Ti:sapphire oscillator (10 fs, 800 nm, 2.5 nJ, 80 MHz)
Quick look into the lab 12Simple THz emission setup in lab
Optical pump beam
Spintronic sample
Si
Probe
Parabolic mirror
beam
Electro-optic crystal for sampling of
the THz electric field
To detection of
probe ellipticity
Typical signals 13Typical THz waveforms from Fe|Pt bilayers
3 3
-20 mT
Pt Fe
2 2
Signal (10-5)
Signal (10-5)
1 1
0 0
-1 -1
-2 -2 Fe Fe Pt
+20 mT ext. field
-3 -3
0 0.5 1 0 0.5 1
Time (ps) Time (ps)
Further findings
Consistent with scenario
ƒ Signal × pump power
spin transfer ∗ ISHE
ƒ THz electric field ] sample magnetization
Need more evidence for the spin Hall scenario 14Ultrafast inverse spin Hall effect
Idea: Ta vs Ir:
Vary nonmagnetic cap layer Opposite spin Hall angles, Ir larger
2
The inverse
Signal (arb. units)
Fe Ir
spin Hall effect is
1 still operative at
Fe Ta THz frequencies
0
Kampfrath, Battiato, Oppeneer,
Freimuth, Mokrousov, Radu,
Wolf, Münzenberg et al.,
-1 Nature Nanotech. (2013)
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
Time t (ps)
Interesting applications:
1) Rapid material characterization regarding spin-to-charge conversion
2) Measurement of spin-current dynamics Compare to state-of-the-art emitter
3) Generation of THz electromagnetic pulses
15Spintronic THz emitter
1
ZnTe
Signal amplitude (arb. units)
4
THz signal (arb. units)
0.8
3 Spintronic emitter,
unoptimized 0.6
2
0.4 Reststrahlen gap
1 Standard: 0.3 mm ZnTe
0.2
0
0
0 1 2 3 4 5 0 4 8 12 16
Time (ps) Frequency (THz)
16Spintronic THz emitter
1
Signal amplitude (arb. units)
4
THz signal (arb. units)
70 samples later: 0.8
3 Spintronic
trilayer emitter 0.6 Spintronic
emitter
2
0.4
1 Standard: 0.3 mm ZnTe
0.2
0
0
0 1 2 3 4 5 0 4 8 12 16
Time (ps) Frequency (THz)
More broadband, efficient and cheaper than standard emitters like ZnTe
Seifert, Radu, Kläui, TK et al., Nature Photon. (2016)
Yang, Wu, Qi et al., Adv. Opt. Mat. (2016)
Wu, Yang et al., Adv. Mat. (2017)
Torosyan, Beigang, Papaioannou et al., Sci. Rep. (2018)
A quick look in the lab again… 17Spintronic THz emitter in the lab Movie by Oliver Gückstock Magnetism and thin metal and Lukáš Nádvorník enable interesting features… 18
More features of spintronic THz emitters
Emitter is insensitive to Cavity pushes pump absorption
pump wavelength from ≈ 50% to ≈ 100%
Papaioannou, Beigang et al., IEEE (2018) Feng et al., Adv. Opt. Mat (2018)
Herapath, Seifert, TK, Hendry et al., APL (2019) Herapath, Seifert, TK, Hendry et al., APL (2019)
Larger emitters are easy and cheap to fabricate 19More features of spintronic THz emitters
Upscaling to ~5 cm diameter Tailor the magnetization
yields 0.3 MV/cm peak field landscape to generate
Seifert, Kläui, TK et al., APL (2017) structured THz beams
Fülöp, Tzortzakis, TK, Adv. Opt. Mat. (2020) Hibberd, Graham et al., APL (2019)
300
Electric field (kV cm )
-1
200
100
Doughnut-
0 like
beam
-100 cross
section
-200
-0.4 0 0.4 0.8
Time t (ps)
The magnetization can also be temporally modulated 20More features of spintronic THz emitters
Polarity modulation of THz field Bendable substrates
up to 20 kHz
Gückstock, Nadvornik, TK et al. (2020) Wu, Yang et al., Adv. Mat. (2016)
Metal films can be nanostructured easily 21More features by nanostructuring
Antenna-coupled spintronic On-chip
emitters THz source
Nandi, Kläui, TK, Preu et al., APL (2019) Weber,
Hoppe, TK,
Woltersdorf
et al. (2020)
Voltage (mV) Time (ps)
Challenge:
How to further increase the THz field strength?
22Challenge: More THz amplitude
fs pump Ferrom. Nonm. THz pulse
(F) (N)
Maximize spin-to-charge-current conversion
ƒ N should have large ISHE and large spin diffusion length
ƒ Interface engineering
Maximize spin current for given pump energy
ƒ Understand how the spin current is induced
What does the driving THz spin current look like? 23Dynamics of the spin current
fs pump Ferrom. Nonm. THz pulse
(F) (N)
Approach: “Calculate back”
THz field ↑ ( )
Seifert, Barker, TK et al.,
Nature Commun. (2018)
)
8 Fe|Pt
6
Spin current (1029
4 ƒ Extremely fast bipolar response
2 ƒ What is the driving force?
0
-2
0.0 0.4 0.8 1.2
Time t (ps)
24Transport vs demagnetization
fs pump Ferrom. Nonm.
(F) (N) What is the origin
of the ultrafast spin transport?
Idea:
Compare to dynamics of another
fundamental process
Ferrom. Ultrafast local magnetization quenching
(F) of single ferromagnetic layers
Beaurepaire et al., PRL (1996)
Is there a correlation with
ultrafast transport in bilayers?
Need to measure both processes in one experiment 25Transport vs demagnetization
fs pump Ferrom. Nonm. THz pulse Spin-to-charge current conversion
(F) (N) ⇑ Electric-dipole (ED) radiation
Seifert et al., Nat. Phot. (2016)
( ) ∝
Ferrom. Ultrafast magnetization quenching
(F) ⇑ Magnetic-dipole (MD) radiation
Beaurepaire et al., APL (2004)
̇( ) ̇ ∝ ̇ = /
How to isolate the weak ̇ signal? 26vs ̇ : Exploit symmetry
Ferrom. Nonm.
(F) (N)
Flip sample about
( )
27vs ̇ : Exploit symmetry
Nonm. Ferrom.
(N) (F)
Flip sample about
ƒ THz field due to
changes sign
− ( ) ƒ Has “odd” symmetry
Beaurepaire et al., Ultrafast Phen. (2004)
Huisman, Kimel et al., PRB (2015)
Ferrom.
(F) Flip sample about
ƒ ̇ emission does not change
ƒ Has “even” symmetry
̇( ) ƒ A potential even signal
is hopefully small
Direct way to measure and ̇ in same setup with ~10 fs resolution 28Sample details
F = CoFe, N = Pt, only 3 nm each:
To minimize pump gradients (quadrupole effects)
FN
Substrate: Cap window: To symmetrize
Diamond Diamond the sample
macroscopically
F and F|N on same substrate
for simple sample change
Pump pulses: From Ti:sapphire oscillator (10 fs, 800 nm, 1 nJ, 80 MHz)
Start with F|N stack 29THz emission from F|N stack
8
FN
6 Sub Cap
THz signal (norm.)
4
2
0
-2
-4 NF
Cap Sub
-6
-8
-0.6 -0.4 -0.2 0 0.2 0.4 0.6
Time (ps)
ƒ ≈ 100% signal reversal ⇑ Even symmetry
ƒ Thus: Emission ( ) ∝ ( ) is dominant
ƒ Use as reference for proper alignment
Let’s go for the single F layer 30THz emission from single F layer
8
F
6
THz signal (arb. units)
Sub Cap
4
2
0
-2
-4 F
Cap Sub
-6
-8
-0.6 -0.4 -0.2 0 0.2 0.4 0.6
Time (ps)
ƒ Signal is ~170 times smaller than from F|N
ƒ Nontrivial signal change upon sample reversal
ƒ Indicates both even ( ̇ ) and odd signals contribute
Separate even and odd contributions 31F monolayer: Odd and even emission
8
THz signal (arb. units)
6
SC
S F C
− C FCS
S 4
= Odd 2
2 0
-2
-4
SC + C FCS
S F C S
= Even -6
2 -8
-0.6 -0.4 -0.2 0 0.2 0.4 0.6
Time (ps)
Odd signal: Considerable
ƒ Origin not fully clear: Due likely to film inhomogeneities along growth direction
Even signal: We assign it to ultrafast demagnetization, ̇ ∝ ̇
ƒ Signal has right polarity and order of magnitude
ƒ Extract ( ) and check temporal dynamics
32F monolayer: ( ) dynamics
0
Magnetization change (%)
( ) Approximate agreement of
-2
ƒ From THz emission ƒ THz-extracted
ƒ From MOKE ƒ ( ) from MOKE
-4
-6 Huisman, Kimel et al., PRB (2015)
Razdolski, Bovensiepen, Melnikov et al.,
-8 J. Phys. D (2017)
-10
-0.4 0 0.4 0.8
Time (ps)
The even signal from the F single layer arises from ultrafast demagnetization
Now compare: Even signal from F ♠ Odd signal from F|N 33Even from F vs odd from F|N
8
6 Even signal from single F layer
THz signal (arb. units)
4
2
0
-2
-4
Rouzegar, Brandt,
-6
Odd signal from F|N stack /170 Reiß, Brouwer,
-8 Woltersdorf, TK
et al. (2020)
-0.6 -0.4 -0.2 0 0.2 0.4 0.6
Time (ps)
Even emission from single F layer ∝ Odd emission from F|N stack
F FN
|
∝
What does this imply? 34Transport vs demagnetization
Ferrom. Nonm. | |
∝ Our observations indicate:
(F) (N)
Demagnetization rate of F
∝ Spin current F↑N in F|N
|
∝ ̇
Confirmed for
ƒ N = Pt and W
Ferrom. ∝ ̇ ƒ F = Co70Fe30 and Ni80Fe20
(F)
1) Ultrafast spin transport and
2) Ultrafast demagnetization
are driven by same force
How to explain? 35Interpretation: Driven by ↑ − ↓
Spin-dependent
Seebeck effect |
∝ ↑ − ↓
Bauer, Saitoh, Wees,
Nature Mat. (2013)
F N
↑ − ↓ ≠0 =0
Rethfeld et al.:
↑ − ↓ drives spin flow
from ↑ to ↓ channel ̇ ∝ ↑ − ↓
PRB 90, 144420 (2014)
Acremann et al.:
Pump-induced heating of Consistent with |
̇ ∝
F layer causes ↑ ≠ ↓ our observations
J. Phys. Cond. Mat. (2017)
Can be extended to nonthermal states
36Summary
Ferrom. Nonm.
(F) (N)
1) Ultrafast spin transport and
2) Ultrafast demagnetization
are driven by the same force
Ferrom. Significance:
(F) ƒ We can apply all knowledge of 2) to 1)
ƒ Will guide us to maximize spin current
from F to N
ƒ Can use N layer as an ultrafast
detector of ↑ − ↓
37Summary
THz spin transport
ƒ Enables interesting applications, e.g. THz wave generation, THz spin torque
ƒ First temporal steps of spintronic effects
New insights
ƒ Ultrafast demagnetization and spin transport are driven by same force
Future directions
ƒ Use THz currents to apply spin torque
THz radiation is a useful tool to probe and control spin dynamics
Spin
Interested in a project in
in THz spintronics?
↑ Contact Tobias Kampfrath
Charge
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