Ultrafast spintronics with

terahertz waves

Tobias Kampfrath

Terahertz Physics Group

Freie Universität Berlin

PhD students: organize your own symposium!

2

http://www.dpg-physik.de/dpg/gliederung/junge/profil/ateam/wissenschaftlich/tagungen/berlin18/phd-symposium/announcement.html

Marco Battiato

Pablo Maldonado

Peter Oppeneer

Acknowledgments

Ulrike Martens

Markus Münzenberg

Ilie Radu

Michael Gensch

Dmitry Turchinovich

FHI THz group

Lukas Braun

Sebastian Mährlein

Tom Seifert

Martin Wolf

Frank Freimuth

Stefan Blügel

Yuriy Mokrousov

S. Jaiswal, G. Jakob,

M. Jourdan, J. Henrizi,

J. Cramer, M. Kläui

SpinCaT

3

1. Turn spins around

2. Transport spins through space

Goal: reach speed of other information carriers, i.e. THz bandwidth

Two elementary spin operations

4

Light in fibers: >10 Tbit/s Electrons in a FET: ~1 THz cut-off

How to manipulate magnetic order ultrafast?

Spintronics and femtomagnetism

5

Spintronics: voltages in circuits Femtomagnetism: fs light pulses

100 THz 1000 THz 0.01 THz DC

Bandwidth <10 GHz

Force/torque applied field

Magnetism Roadmap 2017

Frequency ~400 THz

Force/torque light intensity

Kirilyuk, Kimel, Rasing, RevModPhys 2010

~

Terahertz gap of magnetism

1…30 THz, 4…120 meV

THz + magnetism = useful?

Why THz magnetism?

6

1) Push spintronics to THz range

Spin Hall effect, spin Seebeck effect etc.: still operative at THz frequencies?

2) New physics, new methods

THz coincides with many

fundamental modes

1 THz ≙ 4 meV

- -

+

-

+

-

Magnons Phonons

Transport

Hillenbrand et al., Nano Lett. (2008)

3) Reward for THz technology

E.g. THz sources and modulators

for spectroscopy and imaging

Need THz radiation…

not straightforward

And more:

Excitons

Cooper pairs

0.3

Duration down to 50 fs

Peak fields up to ~30 MV/cm (~10 T)

Detection of full transient field,

threshold down to 1 V/m

Ultrashort THz pulses

7 How to control/probe magnetic order by THz fields?

Tunable center frequency

0.5…50 THz, i.e. 2…200 meV

But: gaps between 5 and 15 THz

THz radiation can be generated and detected with femtosecond laser pulses

How to control spins as fast as possible?

8

Most natural stimulus: magnetic field 𝑩 𝑡

𝑩int

𝑻 = ℏ−1𝑔𝜇B𝑺 × 𝑩 𝑺

𝑬(𝑡)

THz

pulse 𝑩(𝑡)

Zeeman torque

Most natural stimulus: magnetic field 𝑩 𝑡

Most efficient coupling on resonance

Larmor frequency ℏωL = 𝑔𝜇B|𝑩int|

Ferromagnets: <10 GHz, antiferromagnets: ~1 THz

Why are the precession frequencies so different?

How to control spins as fast as possible?

9

𝑩int

𝑺

B

Ferromagnet Antiferromagnet

B B=0 B=0

Spins remain parallel

No work against exchange

interaction

Spins not antiparallel any more

Exchange interaction causes

additional repulsion

ωL ∝ 𝐵ani

Ferromagnets vs antiferromagnets

10

ωL/2𝜋 ∼ 1 THz ωL/2𝜋 ≪ 1 THz

ωL ∝ 𝐵XC𝐵ani

Conduct a THz-pump---magnetooptic-probe experiment

THz pump – magnetooptic probe

11

Sample: antiferromagnetic NiO

Neel temperature 523 K

Magnon (𝒒 = 0) at 1 THz

2𝜋/|𝒒|

∝ 𝒌 ⋅ 𝑴 𝑡

Detect Faraday rotation

Alternative: detect re-emitted

magnetic-dipole THz radiation

Yamaguchi, Suemoto et al., PRL (2010)

Arikawa, Kono et al., PRB (2011)

In the lab…

Simple THz setup in the lab

THz emitter

Pump beam: generates the THz beam

Parabolic mirror

Sample

Probe

beam

To detection of

Faraday rotation

Si

THz-induced magnon oscillation

13

Incident

magnetic pulse

THz-induced magnon oscillation

14

Incident

magnetic pulse

Faraday response

Signature of 𝒒 = 0 magnon at 1 THz Oscillation at 1 THz, decay time ~40 ps

Driven by electric or magnetic field component?

The magnon is driven by the magnetic field

15

Driving force is magnetic

(not electric) field

Induced magnetization

driving field

NiO is centrosymmetric C = 0

No linear magnetoelectric effect

in centrosymmetric NiO

M = CE

How to model microscopically?

Microscopic description

16

==

=2

1

2

1

22

21 )(i

i

i

iyyixx tSDSDJH SBSS

Two-spin Hamiltonian: Hutchings et al., PRB (1972)

Derive/solve LLG-type

equations of motion

S1 S2

Compare to experiment

Exchange

interaction

Crystal-field and

dipolar anisotropy

Zeeman coupling

to THz pulse

Measured signal 𝜃 = 𝑺1 + 𝑺2 ⋅ 𝒌𝑑 ⋅ MO constant

Pisarev et al., JETP (1972)

𝑧

𝑩(𝑡) 𝑺2

𝑺1

Experiment and theory

17

incident

magnetic pulse

Faraday response

THz magnetic pulse drives 𝒒 = 0 magnon

at 1 THz, induced spin deflection is 0.4

Experiment and theory

18

Quantitative agreement

incident

magnetic pulse

Faraday response

Long decay time permits THz coherent spin control

Coherent spin control with THz pulse pairs

19

Second pulse

after 6 cycles:

amplifies magnon

Second pulse

after 6.5 cycles:

switches magnon off

1 2

1 2

Summary and current work

20

Kampfrath, Sell, Wolf, Huber et al., Nature Phot. (2011)

Baierl, Huber et al., PRL (2016)

THz spin control is feasible by straightforward Zeeman torque of THz magnetic pulses

Application: use magnons as probe, e.g

of 𝐵XC𝐵ani for characterization of

antiferromagnets:

not easy with non-optical methods

with ultrafast time resolution

Review:

„Antiferromagnetic opto-spintronics“

Nemec, Fiebig, Kampfrath, Kimel, ArXiv 2017

Stronger THz fields offer many more opportunities…

Nishitani, Hangyo et al., APL 2010, PRB 2012

Kanda, Kuwata-Gonokami et al., Nature Comm 2012

Nonlinear THz response

21

Electron

orbits

Ionic

lattice

Electron spins

3 MV/cm

1 T

Interesting application: reveal elementary spin couplings

Spin-electron coupling

Baierl, Mikhailovsky et al., Nature Phot. 2016

Bonetti, Dürr et al., PRL 2016

Magnon-magnon

coupling

Mukai, Hirori, Tanaka

et al., New J. Phys 2016

Lu, Suemoto, Nelson

et al., PRL 2017

Spin-phonon coupling

Kubacka, Johnson, Staub et al., Science 2014

Nova, Cavalleri et al., Nature Phys. 2016

Maehrlein et al., ArXiV 2017

What about transport?

1. Turn spins around

2. Transport of spin angular momentum

Two elementary spin operations

22

Conduction-electron

spin current

Conduction-electron

spin current

Spin wave: also possible in insulators

How to launch spin currents ultrafast?

Idea: make use of spin-Seebeck effects

The Seebeck effect

23

Metal film

- -

Thomas Seebeck (1821):

A temperature gradient drives an electron current

Ken-ichi Uchida (2008):

In ferromagnets, the Seebeck current

is spin-dependent -

Temperature

gradient

Hot Cold

Spin-dependent Seebeck effect (SDSE)

24

Uchida et al., Nature (2008)

Bauer, Saitoh, Wees, Nature Mat (2013)

and electrons have very different

transport properties

Fe film

Spin-dependent Seebeck effect (SDSE)

25

How can we induce a thermal

imbalance as fast as possible?

Uchida et al., Nature (2008)

Bauer, Saitoh, Wees, Nature Mat (2013)

Spin-polarized current

and electrons have very different

transport properties

Fe film

Temperature

gradient

Hot Cold

fs pump

pulse

Photoinduced ultrafast spin transport

26

Pump pulse excites and electrons

: d sp bands become fast

Battiato et al., PRL (2010)

Fe film

Pump launches a spin-polarized current

fs pump

pulse

Pump pulse excites and electrons

: d sp bands become fast

: d d bands stay slow

Battiato et al., PRL (2010)

How to detect the ultrafast spin current?

Could be considered as

Ultrafast SDSE Melnikov et al., PRL (2011)

Rudolf et al., NatComm (2012)

Example of spin-photogalvanic effect Ganichev et al., Nature (2002)

Photoinduced ultrafast spin transport

27

Fe film

A

Inverse spin Hall effect (ISHE)

28

Spin-orbit coupling

deflects electrons

transverse charge current

Saitoh et al., APL (2006)

fs pump

pulse

Challenge:

Electric detection has cutoff at <50 GHz

But expect bandwidth >10 THz

Heavy

metal Fe film

Inverse spin Hall effect (ISHE)

29

Emission of

electromagnetic

pulse (~1 THz)

Measure THz emission from photoexcited FM/NM bilayers

fs pump

pulse

Heavy

metal Fe film

Samples: (poly)crystalline, from labs of M. Kläui and M. Münzenberg

Pump pulses: 10 fs, 800 nm, 2.5 nJ from Ti:sapphire oscillator

Typical THz waveforms from bilayers

Consistent with scenario

SDSE + ISHE

+20 mT

-20 mT

Sig

nal (1

0-5

)

Time (ps) 0 0.5 1

-3

-2

-1

0

1

2

3

Fe Pt

Sig

nal (1

0-5

)

Time (ps) 0 0.5 1

-3

-2

-1

0

1

2

3

Fe Pt Fe

Further findings

Signal pump power

THz electric field sample magnetization

Need more evidence for the spin-Hall scenario

30

Idea:

vary nonmagnetic cap layer

Ta vs Ir:

opposite spin Hall angles, Ir larger

Ultrafast inverse spin Hall effect

2

1

0

-1

0.7 0.6 0.5 0.4 0.3 0.2 0.1 0

Time t (ps)

Sig

nal (

arb

. units) The inverse

spin Hall effect is

still operative at

THz frequencies

Applications?

Kampfrath, Battiato, Oppeneer,

Freimuth, Mokrousov,

Radu, Wolf, Münzenberg et al.,

Nature Nanotech. (2013)

Fe Ir

Fe Ta

31

Estimate strength of spin Hall effect

Calculations:

F. Freimuth, S. Blügel,

Y. Mokrousov

e.g. PRL (2010)

Simple, contact-free method to obtain relative estimates of spin-Hall conductivity

4

3

2

1

0

-1

-2

NM material

1

0.8

0.6

0.4

0.2

0

-0.2

-0.4

Cr Pd Ta W Ir Pt Pt38Mn62

TH

z a

mplit

ude

(norm

aliz

ed

)

Calc

. spin

Hall c

onductiv

ity (1

05 S

m-1)

Fe NM

Idea: optimize Fe/Pt geometry to obtain a useful THz emitter

Need: simple model of THz emission process

32

Model of THz emission

33

Attenuation length 𝜆rel,

reflection at interfaces

FM

Fs

pump NM

THz

pulse

𝜃NM

𝑗s

𝑗c

𝐸(𝑡)

𝐼 s 𝜔 = d𝑧 𝑗 s(𝑧, 𝜔) =

Pump absorption density

(𝐴~0.5 for our films)

Ohm‘s law:

𝐸 𝜔 = 𝑍 𝜔 ⋅ 𝑒𝜃𝐼 s(𝜔)

1

𝑍 𝜔= d𝑧 𝜎 (𝑧, 𝜔)

𝑑

0

+𝑛substr + 𝑛air

𝑍0

Parallel connection of resistors

Compare to experiment

Model suggests:

Reduce film thickness 𝑍 and 𝐴/𝑑 increase 𝐸 increases

𝜆rel

FM

𝜆rel tanh𝑑NM

2𝜆rel⋅𝐴

𝑑

Optimization of the spintronic emitter

Fe Pt

Pump

1

0.8

0.6

0.4

0.2

0

TH

z a

mp

litu

de (

no

rma

lize

d)

25 20 15 10 5 0

Total metal thickness (nm)

Trilayers

Bilayers

Mainz (sputter) Mainz (epitaxial) Greifswald Model

THz

Simple modeling shows:

THz current flows in a thin Pt sheet of thickness 𝜆rel = 1 nm

Thinner films enhance the Fabry-Perot effect for pump and THz wave

A trilayer is even better. How does it work?

5 nm

34

THz emitter: from bilayer to trilayer

Idea: take advantage of electrons running forward and backward

W

<0

Spin

current

Pt

>0

CoFeB

Charge

current

Constructive superposition of the two spin Hall currents

70 samples later: evaluate emitter performance

Seifert, Turchinovich, Freimuth, Mokrousov, Wolf, Münzenberg, Kläui, Kampfrath et al., Nature Photon. (2016)

35

5 4 3 2 1 0

Time (ps)

4

3

2

1

0 TH

z s

ignal (

arb

. units)

Sig

nal am

plit

ude

(arb

. units) 1

0.8

0.6

0.4

0.2

0

16 12 8 4 0

Frequency (THz)

ZnTe

Spintronic

emitter Spintronic

trilayer emitter

Standard emitter: ZnTe

Reststrahlen

gap

Broadband spintronic THz emitter

More broadband, efficient, cheaper, scalable than standard THz emitters

Spintronics is useful for THz photonics

Idea: upscale the emitter size

Seifert, Turchinovich, Freimuth, Mokrousov, Wolf, Münzenberg, Kläui, Kampfrath et al., Nature Photon. (2016)

36

Large-area spintronic THz emitter

Trilayer emitter

W(1.8 nm) | CoFeB (2 nm) | Pt(1.8 nm)

On fused silica (thickness 500 µm)

Pump beam

800 nm, 40 fs, 5.5 mJ, 1 kHz

Diameter on emitter: 4.8 cm (FWHIM)

One parabolic mirror

Diameter 7.6 cm

Focal length 5.1 cm

Result: intense THz pulses of 5 nJ energy, 0.3 MV cm-1 peak field Seifert et al.,

APL (2017)

What does the THz source current look like?

Outlook: ultrafast charge/spin transport

Ultrafast spin Ampere-meter

~10 fs time resolution

Contact-free

CoFeB/Pt

Rise in

~30 fs

Backflow

In metals:

Extremely fast (~30 fs) onset

of the photoinduced spin current

Magnetic insulators:

Understanding of ultrafast response Seifert et al., arXiv 1709.00768 (2017)

Braun, Münzenberg, Kampfrath et al.,

Nature Comm. (2016)

Pt Magnet

Jc

Js

10 fs

2 nJ,

1.6 eV

Curr

ent

in P

t la

yer

(arb

. untis)

Time (ps)

-1 0 1 2 -1

0

38

THz radiation is a useful tool

to reveal and control spin dynamics

Charge

Spin

THz fields can access elementary spin couplings

Spin Hall and spin Seebeck effects are operative up to 10s of THz

Studying ultrafast regime permits new insights into spin physics

and new applications in THz photonics

Summary

39 THz pulse

For more THz spin transport:

See poster of Oliver Gückstock