All-optical Control of Magnetism I (including pump-probe techniques) - - PowerPoint PPT Presentation

Theo Rasing Radboud University Nijmegen Institute for Molecules and Materials

(including pump-probe techniques)

All-optical Control of Magnetism I

HFML - FELIX (THzFEL)

1

1681: ship to Boston

2

Controlling magnetism by lightning

3

Controlling magnetism by light! How does it work? What about the nanoscale? Can we control magnitude? Can we control direction? Can we switch?

4

Lecture topics:

• 1. Time scales and stimuli in magnetism
• 2. Laser induced effects
• a. Thermal effects

b.Nonthermal opto-magnetic effects

• 3. Experiments
• a. AOS of Ferrimagnets

b.AOS of Ferromagnets

• c. AOS of Dielectrics
• 4. Towards applications
• a. AOS at the nanoscale

b.Neuromorphic applications

• 5. Outlook

5

Lecture topics:

• 1. Introduction: stochastic/deterministic dynamics
• 2. Stroboscopic imaging.
• 3. Magneto-optical setups

a.Faraday/Kerr effects b.XMCD

• 4. Examples
• 5. Outlook

6

Time scales

Time [seconds]

One second

Fundamental Physical/Chemical processes

Electronics Camera flash

10-15 10-12 10-9 100 10-6 10-3 ps ns fs μs ms s

30 fs = 0.00000000000003 seconds (shortest man-made event)

Ultrashort laser pulse

Blink

• f eye

7

How much is 1 fs in magnetism?

1 ns 1 ps 1 fs 100 ps 10 ps 100 fs 10 fs

Stimulus

Thermodynamics Adiabatic approximation

?

phonons

Magnons

electrons

8

1 ns 1 ps 1 fs 100 ps 10 ps 100 fs 10 fs

Stimulus

Thermodynamics Adiabatic approximation

?

Laser Pulse

How much is 1 fs in technology?

9

stochastic/deterministic dynamics

NUCLEATION OF MAGNETIC ORDER

M

H

COHERENT ROTATION OF MAGNETIZATION

10

Leland Stanford

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Edward Muybridge: The Horse in Motion

12

Magnet

All-optical pump-probe technique

100fs pulses 20J/pulse 1kHz rep. rate

Ti:Sapphire laser & amplifier system

Experimental fs-pump-probe technique

13

Revealing ultrafast dynamics with fs flashes

14

15

SHG studies of laser-induced surface melting

• 1.0ps

240fs 3ps C.V. Shank et al, Phys.Rev.Lett.51, 900 (1983)

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• 1. Time-scales in magnetism vs

switching mechanisms

~ 0.01 ps – 0.1 ps Exchange interaction 1 ns 1 ps 1 fs 100 ps 10 ps 100 fs 10 fs Homogeneous spin motion (precession) ~ 1 ps - 1 ns via precessional motion 1 s Domain-wall motion Switching via domain-wall motion

H

?

H H M

17

manipulating spins?

S

𝐟𝐠𝐠

𝒋 𝒋 𝐟𝐠𝐠 eff

Hi

𝑗

Li+JijSj

100 T 10 T 1 T 0.1 T Exchange interaction Spin

• rbit

interaction

Superconducting Magnet

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Oersted/Faraday: electrical current creates magnetic field

Current creates a magnetic field

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• ptical switch for short current pulses:

10 ps rise time 400 ps decay time

short magnetic field pulses!

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magnetic precession

Magnetic fieldpuls

F



Th.Gerrits et al, Nature 418, 509 (2002)

How to switch ?

21

2 .0 1 .5 1 .0 .5 .0

Switching: pulse length=half a precessional period!

CONTROL: SHAPED pulse!

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Th.Gerrits et al, Nature 418, 2002 Switching within 200 ps !

I

Switching by controlling pulse width!

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Magnetism and light

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The Nobel Prize in Physics 1902

"in recognition of the extraordinary service they rendered by their researches into

the influence of magnetism upon radiation phenomena“ (together with Lorentz)

Zeeman effect

Magnetism and light

25

The Nobel Prize in Physics 1902

"in recognition of the extraordinary service they rendered by their researches into

the influence of magnetism upon radiation phenomena“ (together with Lorentz)

Zeeman effect

Magnetism and light

26

dM dM

s(-) s(+)

inverse? Faraday effect

Magnetism and light

27

2.Ultrafast “demagnetization” by light

28

2-Temperature model

Free-electron bath (Te) Lattice (TL=300 K)

Model assumes two heat baths for electrons and phonons:

29

Coupled diffusion equations for Tel and Tph note that Cel << Cph – this is why electrons get so hot!

2-Temperature model

30

2-Temperature model

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Coupled diffusion equations for Tel and Tph note that Cel << Cph – this is why electrons get so hot!

.

• = - 4m ;

Laser-induced spin precession

32

.

• Laser-induced spin precession

33

.

• Laser-induced spin precession

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200 400 600 800 1000 1200

Delay (ps)

250 Oe 500 Oe 750 Oe 1000 Oe 1250 Oe

300 600 900 1200 1 2 3 4

Frequency (GHz) Magnetic field (Oe)

anisotropy field

 

H HA   

thin film of magnetic garnet (~Y3Fe5O12)

~GHz

Laser-induced spin precession

35

Time-resolved Magneto-optical Imaging

• Y. Hashimoto, et al., Review of Scientific Instruments 85, 063702 (2014).

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• 30
• 20
• 10

10 20 30

• 20

20

• 20

20

• 20

20

• 50

50

[100] [010] [110] (b) Rotation Angle [millidegree]

= 40 Oe H = 240 Oe H = 400 Oe H

= 4 ns Δt x [m]

M

• 20

20

• 20

20

• 20

20 10 5 Time delay [ns] y [m] (c) Radium [m]

LMEW TMEW = 15°

Propagation Dynamics of Optically-excited Spin Waves

YIG sample

37

Spin-wave Tomography

2 1 2 1 2 1 2 1

H = 240 Oe H = 400 Oe

(d)

M k

= 15° H = 40 Oe 2000 1000 Amplitude [a. u.]

k [10

4

cm

• 1

]

BVMSW LMEW 100

Frequency [GHz]

TMEW

• Y. Hashimoto et al., Nature Communications 8, 15859 (2017)

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Coherent-energy Transfer from Elastic waves to Spin Waves

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Phonon Spin

Fluence Dependence of Spin-wave Amplitude

400 nm (3.1 eV) 800 nm (1.6 eV)

Linear Square

Optical charge transfer transition near 400 nm (3.1 eV)

• F. Hansteen, et al., Thin Solid Films 455-456, 429 (2004).

800 nm 400 nm Single photon Two photon

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THz excitation of spin waves in TmFeO3

excitation via change in anisotropy

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Faraday rotation (norm.) Delay time (ps)

TmFeO3: antiferromagnetic resonance

Ferromagnetic mode softens at magnetic phase transitions

30 40 50 60 70 80 90 100 150 200 250 0.0 0.1 0.2 0.3 0.8 0.9 Temperature, T (K) Frequency, (THz) d q-FM mode q-AFM mode

24 4 2

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All Optical Switching (AOS) by femtosecond laser pulses:

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Theo Rasing Radboud University Nijmegen Institute for Molecules and Materials

(including pump-probe techniques)

All-optical Control of Magnetism II

HFML - FELIX (THzFEL)

44

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AOS by femtosecond laser pulses:

counterintuitive?

𝑻𝒋

• 𝒋

𝐟𝐠𝐠

• 𝐟𝐠𝐠)

S H

Intuitive estimate:

If 100 fs pulse reverses the magnetization, it should act as an effective magnetic field

• f about 90 Tesla ( =28 GHz/T)!

Simple single spin problem Light acts as a magnetic field, which is either strong (>>1 Tesla)

• r stays long (>>100 fs).

Why?

48

Thermodynamics of laser-matter interaction

Inverse Faraday effect

   

  

*

E E W 

        M

E E M W H              

*

1

     

 

    

*

E E H     

Pitaevskii, Sov. Phys. JETP 12, 1008 (1961). van der Ziel Phys. Rev. Lett. 15, 190 (1965).

s



s



dH



dH



 

            

2

ˆ M

• M

i M i

zz yy xx

     

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(see further lecture on Magneto Optics by Prof. Schaefer)

E H

Magneto-optics

Magneto-optics

E E qF ~Mz Mz Faraday effect

Opto-magnetism

dH dH

s(-)

Inverse Faraday effect

) ( ) ( ) (

*

     E E Heff 

• L. P . Pitaevskii, Sov. Phys. JETP 12, 1008 (1961).
• J. P. van der Ziel et al, Phys. Rev. Lett. 15,190 (1965).

Magneto-optics and Opto-magnetism

both result from spin-orbit interaction!

50

Ultrafast excitation of spins via IFE in DyFeO3

(all-optical spin resonance)

A.V. Kimel, A. Kirilyuk, P.A. Usachev, R.V. Pisarev, A.M. Balbashov, and Th. Rasing, Nature 435, 655-657 (2005) photons L L 0.1 ps 0.1 ps

51

Ultrafast excitation of spins via IFE in DyFeO3

(all-optical spin resonance)

DyFeO T = 95 K

s



s

+

Faraday rotation (deg)

s

s

dH

15 30 45 60 0.0 0.1 0.2

Time delay (ps)

dH

A.V. Kimel, A. Kirilyuk, P.A. Usachev, R.V. Pisarev, A.M. Balbashov, and Th. Rasing, Nature 435, 655-657 (2005) photons L L 0.1 ps 0.1 ps

52

Amplitude of the laser-induced spin-waves

25 50 75 0.0 0.5 1.0

Amplitude (arb. units) Pulse fluence (mJ/cm2)

) ( ) ( ) (

*

     E E H eff 

Inverse Faraday effect

Fields up to 5 T! (even up to 20 T!)

53

Controlling ?

54

Double pump coherent control

20 40 60 80 100 120 140

Faraday rotation Delay time (ps)

20 40 60 80 100 120 140

Delay time (ps) DyFeO3 10 K DyFeO3 10 K

First pump Second pump First pump Second pump

55

AOS of Ferrimagnetic Metals

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Femtosecond laser reversal of magnetization?

GdFeCo

57

Polarization microscope + pulsed laser

Circularly polarized 40fs laser pulses

Magneto-Optical microscope 20 nm GdFeCo film

58

s s

Hext = 0

GdFeCo

40 fs pulses, 1 kHz

Reversal by 40fs laser pulsen!

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Sweeping the pulsed laser beam at high speed across the sample

C.D. Stanciu et al., PRL 99,047601 (2007)

switching of magnetization by single pulse!

60

Femtosecond Magnetic Recording in GdFeCo!

C.D. Stanciu et al., patent #P77323PC00,

PRL 99, 047601 (2007)

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Pulse intensity

All-optical reversal Multidomain state

σ+ σ-

Helicity- independent reversal

Helicity dependent AOS in narrow (~few%) intensity range

+M

50 μm

Pulse profile:

AOS: role of light helicity/intensity

62

Switching No Switching

Size of window is 1.5%. Exactly equal to difference in absorption!

Excitation wavelength: 700nm

63

Gd Fe Gd Fe

Femtosecond laser reversal: role of exchange? JFe-Gd ~ 30-50T

64

2-Temperature model

Gd Fe Fe Free-electron bath (Te) Lattice (TL=300 K)

2-afm coupled sublattices!

65

Temperature dominated: T >> TCurie~100 fs

40 fs

Bloch relaxation

Dynamics scales with magnetic moment

Distinct dynamics of sublattices!

66

Exchange dominated: T<TCurie t~1 ps

S2 S1 ?

Ground state AFM, transient FM! Conservation total angular momentum

67

?

Element specific view: XMCD!

X-rays 400-1400 eV 10-50 ps (FWHM)

BESSY II

fs-Laser pump – X-ray probe

How to probe?

775 780 785 790 795 800 805 L2 L3 Photon energy (eV) TEY (a.u.)

k

Fe

68

(for more details: see lecture Prof. Luning)

Femtosecond-XMCD!

BESSY II

fs-Laser pump – X-ray probe X-rays 400-1400 eV 100 fs (FWHM)

FEMTO-SLICING!

69

Laser heat induced magnetization reversal!

reversal of magnetization driven by exchange!!!

S2 S1

• T. Ostler et al, Nature Comm.3, 666, 2012

J.H. Mentink et al., PRL 057202, 2012

Radu et al, Nature 472, 205-208 (2011)

Fs-XMCD, BESSY

70

10 ps laser (heat) pulse

No hot, spin polarized

• r spin-orbit coupled electrons!

Ultrafast electrical pulse reverses magnetization!

71

AOS of Ferromagnets

72

[Co/Pt]n

AOS of ferromagnetic CoPt (FePt)?

73

What’s the mechanism?

HD-AOS in Co/Pt multilayer

0.1 ps 1.6 ps 2.2 ps Pt2.0 nm Pt 0.7 nm Co0.4 nm Pt5.0 nm Ta5.0 nm Glass Substrate x3

• Y. Tsema et al, APL 2016, R. Medapalli et. al., Phys. Rev. B 96, 224421 (2017).

74

HD-AOS in Co/Pt multilayer

0.1 ps 1.6 ps 2.2 ps Pt2.0 nm Pt 0.7 nm Co0.4 nm Pt5.0 nm Ta5.0 nm Glass Substrate x3

• Y. Tsema et al, APL 2016, R. Medapalli et. al., Phys. Rev. B 96, 224421 (2017).

75

HD-AOS in CoPt: fs single-shot imaging

1) No single shot switching 2) Stochastic + deterministic 3) ~100 pulses

MECHANISM?

76

1 ms 35 μm No pulse 10 30 50 5

Stochastic nucleation & growth!!!

Multi-pulse induced HD-AOS

77

Deterministic displacement of domain walls

2 nm/pulse @ 0.4mJ/cm2 takes many pulses!

• R. Medapalli et. al., arXiv: 1607.02505, PRB 96, 224421 (2017).

78

Magnetic recording in Co/Pt requires multiple pulses. The first pulses form (stochastically) domains with reversed magnetization. The following pulses cause helicity dependent domain wall motion.

HD-AOS in CoPt: mechanism

79

AOS of Dielectrics

80

Photo-magnetism of Co-substituted iron garnet

(Y2CaFe3.9Co0.1GeO12 / GGG (001))

laser CW:

EII[110] EII[1-10]

Light-induced slow (~m/sec) motion of domain wall

A.Chizhik et al. PRB, 57 (1998). A.Stupakiewicz et al. PRB, 64 (2001).

Y3+ Co2

+

Co2

+ Fe3 +

Fe3

+

Co3

+

Co3

+

Recording? Heating? Speed?

81

AOS in iron garnet

Y2CaFe3.9Co0.1GeO12 on GGG (001) thickness d=7.5 μm) 50 fs pulse 50 fs pulse

200×200 m2

82

 single pulse  repeatable switching  zero applied field  room temperature

• A. Stupakewiecz et al,

Nature 542, 71 (2017).

AOS in iron garnet

83

84

All-Optical Switching @ the nanoscale!

Femtosecond magnetic recording Present magnetic recording

But………

10 micron 100 nanometer! 100 x smaller

85

PEEM experiment SLS; L. Le Guyader et al, APL 2012, Nature Comm. 2015

All-Optical Switching @ the nanoscale? All-Optical Switching @ the nanoscale!

86

PEEM experiment SLS; L. Le Guyader et al, APL 2012, Nature Comm. 2015

All-Optical Switching @ the nanoscale? All-Optical Switching @ the nanoscale!

87

Can’t we go smaller? plasmonic antenna!

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Nanoscale switching with plasmonic antennas

(with Bert Hecht, Wuerzburg)

40 nm Switching!!

Tian-Min Liu et al, Nano Letters, 2015

(10 ns/bit) 0,05 $/GB (2 ns/bit) 0,65 $/GB (>pJ/bit) (>nJ/bit) (~fJ/bit) (~ ps/bit) ?? $/GB

Outlook: speed and energy consumption in data storage

(20x20nm)

90

Outlook: Spintronic-Photonic Integrated Circuit

With: Aarhus University, IMEC, CEA SpinTEC, QuantumWise

10-100 times more energy efficient!

91

Laguerre-Gaussian Beams Towards more complex nanostructures:

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19.928 17.337 15.663 12.953 Fluence(mJ/cm2) 34.036 30.968 25.308 22.558 Fluence(mJ/cm2)

Donut Switching with L-G beams

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Towards stable complex nanostructures: Neel skyrmion Bloch skyrmion

94

Single pulse illumination ( = 800 nm) through microscope objective (NA = 0.4) Read-out with Near-field microscopy ( = 532 nm, Resolution 80 nm)

+ =

Sample: Tb22Fe69Co9

Opto-magnetic generation of Skyrmions

Finazzi, et al. Phys. Rev. Lett. 110, 177205 (2013)

95

Low fluence High fluence

FeTb

Opto-magnetic generation of Skyrmions

96

Skyrmion generation model

Minimizing the energy we obtain the Skyrmion radius Single Skyrmion Skyrmionium: Sk+ + Sk- Exchange Energy Anisotropy Energy Dipolar Energy Zeeman Energy

From the model, R0/R1 is sample & fluence independent at Hz = 0: R0/R1 = 1.87 Experimentally R0/R1  2

• M. Finazzi et al, Phys. Rev. Lett. 110, 177205, (2013)

Bloch skyrmion

97

• II. Higher density = too much energy

Create a new paradigm, beyond von Neumann

End of smaller and faster?

0101010 101010

• III. von Neumann bottleneck: transfer information back and forth

I. End of “Moore”: too much heat

98

Supercomputer versus Brain:

10 MW 10 W

Processing and storage Separated and serial and 2D Processing and storage Integrated and parallel and 3D!

Supercomputer versus Brain:

10 MW 10 W

Processing and storage Separated and serial and 2D Processing and storage Integrated and parallel and 3D!

Paradigm shift: to develop materials that “learn”

101

Neuromorphic Computing with Magneto-Optics?

102

see: A. Chakravarty et al, Supervised learning of an opto- magnetic neural network with ultrashort laser pulses,

• Appl. Phys. Lett. 114, 192407 (2019)

To conclude:

• 1. Femtosecond optical excitation:

pump/probe magnetism on timescale of exchange interaction

• 2. Laser induced effects:
• a. Thermal effects

b.Nonthermal opto-magnetic effects

• 3. AOS of Ferrimagnets, Ferromagnets

antiferromagnets, metals, dielectrics

• 4. AOS at the nanoscale, O-MRAM,

Neuromorphic applications

103

s

s

Radboud Nijmegen A.V. Kimel A.Kirilyuk

• F. Hansteen

D.Stanciu A.v.Etteger

• A. Toonen

K.Vahaplar

• M. Savoini (Zurich)
• S. Khorsand (ASML)
• D. Bossini (Dortmund)
• R. Mikhaylovskiy (Lancaster)

with many thanks to:

STW, NWO, EU, NanoNed, UltraMagnetron, FANTOMAS, IFOX Bessy

• I. Radu
• C. Stamm (Zurich)

T.Kachel N.Pontius L.Le Guyader (Hamburg) Stanford Herman Durr (Uppsala) A.Reid C.Graves University of York

• R. Chantrell

T.A. Ostler (Sheffield) J.Barker R.Evans Ioffe Institute R.V. Pisarev

• A. Kalashnikova

Nihon University

• Prof. A. Itoh
• A. Tsukamoto

Kiev

• B. Ivanov
• R. Medapalli

Yassine Quessab Sheena K.K. Patel Eric E. Fullerton UC San Diego

104