ultrafast magnetization dynamics and their signatures in ...corpes13.xfel.eu/files/weinelt.pdf ·...

CORPES Hamburg 2013 1

Ultrafast magnetization dynamics and their signatures in the transient band structure

Martin Weinelt

Fachbereich Physik, Freie Universität Berlin


Robert Carley

Martin Teichmann Björn Frietsch

Kristian Döbrich

Cornelius Gahl Jan Wolter

Olaf Schwarzkopf

Philippe Wernet

Acknowledgement:

Funding:

AG Weinelt FU-Berlin

WE2037/4-1

John Bowlan


Magnetic Switching

Conventional switching: Domain-wall nucleation and propagation > 1 ns

Coherent rotation of magnetization: „precessional switching“ > 10 ps Laser-induced magnetic switching !

Time

> 1 ns

> 10 ps

> 100 fs

I. Radu et al., Nature 472 (2011) 205 T.A. Ostler et al., Nature Communications 3 (2012) 666


A. Vaterlaus et al., Phys. Rev. B 46 (1992) 5280

Spin–lattice relaxation time τSL in iron

30 ps

τSL > 30 ps

20 ns

Spin polarization


Three Temperature Model

Lattice Tp

Electrons Te

Spins TS

optical excitation

spin and orbital momentum

~ 2 ps

> 30 ps, Gps

S.I. Anisimov et al., Sov. Phys. JETP 39 (1974) 375 A. Vaterlaus et al., Phys. Rev. Lett. 67 (1991) 3314

ns-pulse ps-pulse

Heat


Spin–lattice relaxation

5/2

7/2

𝜏𝑆𝑆 ~ 1 / (Eanisotropy)2

Gd: 0.7 meV, 48 ps

W. Hübner and K. H. Bennemann, Phys. Rev. B 53 (1996) 3422

Tb: 10 meV, < 1ps ?


20 nm Ni film 60 fs, 7 mJ/cm2

E. Beaurepaire, J.-C. Merle, A. Daunois, and J.-Y. Bigot., Phys. Rev. Lett. 76 (1996) 4250

MOKE

τ < 1 ps

Femtosecond magnetization dynamics



Lattice Tp

Electrons Te

Spins TS

optical excitation

spin and orbital momentum

~ 2 ps

> 30 ps, Gps

< 1 ps, Ges

S.I. Anisimov et al., Sov. Phys. JETP 39, (1974) 375 A. Vaterlaus et al., Phys. Rev. Lett. 67 (1991) 3314 E. Beaurepaire et al.; Phys. Rev. Lett. 76 (1996) 4250


Einstein- de-Haas effect

Ultrafast transfer of angular momentum to lattice and / or spin transport within sample ?

Change of spin angular momentum compensated by macroscopic rotation

Conservation of angular momentum

Vorführender

Präsentationsnotizen

Einstein de-Haas effect: if you place a ferromagnetic rod in a coil and drive a current through the latter, the spins in the rod are aligned and the rod starts to rotate. The underlying mechanims is that a spin corresponds to an angular momentum. Since the total angular momentum is conserved the change in spin orientation is compensated by a macroscopic rotation of the sample. Thus the time for this transfer of angular momentum to the lattice denotes the ultimate limit in magnetic switching speed. In the itinerant ferromagnets the total magnetic moment is carried by the valence electrons and therefore optical excitation and spin dynamics are intimately intermixed. Magnetic field realigns microscopic spin moments Related change of angular momentum is compensated by macroscopic rotation


Microscopic processes

Photon-spin interaction G.P. Zhang and W. Hübner, Phys. Rev. Lett. 85 (2000) 3025 J.-Y. Bigot et al., Nature Phys. 5 (2009) 515

only at high exitation density !

Ni substrate

Electron-phonon spin-flip scattering Koopmans et al., Nature Mat. 9 (2010) 259. Electron-magnon, electron-electron E. Carpene et al., Phys. Rev. B 78 (2008) 174422 M. Krauß et al., Phys. Rev. B 80 (2009) 180407

Superdiffusive spin transport Battiato et al., Phys. Rev. Lett. 105 (2010) 027203. Rudolf et al., Nature Comm. 3 (2012) 1037.


Ultrafast Magnetization Dynamics

© Bert Koopmans

e-ph spin-flip scattering !

Superdiffusive transport !

Laser field - induced !

Mechanism ? spin-lattice coupling


Outline

• Introduction: Ultrafast magnetization dynamics

• Time-resolved photoemission with higher harmonic radiation • Gd (5d6s)3 valence-electron vs. 4f7 spin-systems

• Gd vs. Tb

• Temperature dependence of the dynamics of the exchange splitting

Goal

Follow the signatures of „phase transitions“ in the transient electronic band structure


Introduction: Thermal phase transition

Gd / W(110)

∆2 majority spin bulk-band

∆2 minority spin bulk-band

exchange splitting ∆ex

0 100 200 300 400 Temperature (K) C. Schüßler-Langeheine, PhD thesis, FU Berlin, 1999

Bin

ding

ene

rgy

(eV

) ∆ e

x (e

V)

1.2

2.2

1.7

1.0

0.5

0


Time-resolved bulk photoemission


High order harmonic generation in gas cell

Laser field

Ar Gas Cell

P. Corkum. Phys. Rev. Lett. 71, 1994 (1993) P. Agostini and L. DiMauro Rep. Prog. Phys. 67, 813 (2004) (cartoon)


VUV photoemission beamline

Rev. Sci. Instrum. 84 (2013) 075106

Vorführender

Präsentationsnotizen

Weight Mirror 6-axis holders


Experiment

Red Dragon KML

Rev. Sci. Instrum. 84 (2013) 075106


XUV sensitive silicon photodiode (AXUV100)

0.5 eV

0.2 eV

∆EM = 210 meV

Higher harmonic spectrum of Argon

XUV photons / pulse

7x104

0

1

2

3

4

5

6


Pulse tilt

large IR focal length: 600 mm small divergence of VUV beam: 4 mrad slit - grating distance: 330 mm 200 lines / mm

~ 90 fs

Pulse broadening c∆t

decrease in bandwith:

Group-velocity dispersion: ~ 8 fs

Energy - chirp: ~ 30 meV

Pulse broadening (35 nm):

O.E. Martinez, Opt. Comm. 59 (1986) 229


Harmonic order

Tem

pora

l bro

aden

ing

(fs)

Harmonic order

Ene

rgy

reso

lutio

n (m

eV)

Olaf Schwarzkopf, Helmholtz Zentrum Berlin REFLEC CODE, F. Schäfers, Technical Report 201, BESSY, (1996)

Ray tracing

Rev. Sci. Instrum. 84 (2013) 075106

confirmed by experiment


Transient band structure of Gadolinium


Magnetic coupling in Gadolinium

4f

RKKY interaction

5d6s

Gd: [Xe] 4f7 5d1 6s2

• 4f shell half-filled, Sf = 7/2, Lf = 0

• localized 4f moments 7µB

• polarized valence electrons 0.55 µB


Magnetic coupling in Gadolinium

4f 5d6s

• 4f shell half-filled

• localized 4f moments 7µB

• polarized valence electrons 0.55 µB

• TC = 293 K

• exchange coupling J5d,4f ~ 88 meV

R. Ahuja et al., Phys. Rev. B 50 (1994) 5147

RKKY interaction


Structure of Gd and Tb on W(110)

hν = 35.6 eV c = 5.78 Å ΓΑ ~ 0.54 Å-1

ΓΜ ~ 0.5 Å-1 a,b = 3.64 Å

k = 0.511 ℎν Å-1

Γ

Γ

Γ

Γ

Α

Α

Α

Α

Μ

Μ Μ


Band structure of Gd(0001) on W(110)

hν = 35.6 eV (h23)

100 K 300 K

P. Kurz, G. Bihlmayer, S. Blügel; J. Phys.: Condens. Matter 14 (2002) 6353.


Photoemission measurement

260 meV energy resolution, 0.5 counts / XUV pulse, 5 min. recording time


Energy- and time-resolved spectra

hot electrons

correct for shifts


Optically-driven demagnetization

dz2 - surface state

minority bulk band

majority bulk band

( 1.2 mJ / cm2)


dz2 - surface state

minority bulk band

majority bulk band

40 meV

Optically-driven demagnetization

( 1.2 mJ / cm2)

Shift identical to P. Loukakos et al., Phys. Rev. Lett. 98 (2007) 097401.


Remagnetization in equilibrium

minority band

majority band

thermal remagnetization

( 1.2 mJ / cm2)

Data from CSL, 1999

τSL~40 ps


Non-equilibrium magnetization dynamics

minority band

majority band

∆Eex ∆Eex


Hysteresis - Gadolinium (1.2 mJ/cm²)

electronic demagnetization thermal recovery

2.10

2.00

2.20

EB m

ajority component (eV

)

1.60

1.50

1.40

EB m

inor

ity c

ompo

nent

(eV

)

0.75 0.70 0.65 0.60 0.55 0.50

Exchange splitting (eV)


Conclusion

5d6s valence-electron spins and 4f corel-level spins are out of equilibrium at least in the first 2 ps

Phys. Rev. Lett. 109 (2012) 057401.


Photoemission vs. MOKE vs. XMCD

τ = 0.7 – 1.0 ps

τ = 0.8 ps

Transient ∆ex is a measure of magnetization dynamics !

Fpump ~ 1 mJ / cm2 Fpump ~ 4 mJ / cm2

Fpump ~ 1 mJ / cm2

Phys. Rev. Lett. 106 (2011) 127401


XMCD and MLD

Magnetic linear dichroism (MLD) in time-resolved photoemission

X-ray magnetic circular dichroism (XMCD) in time-resolved transmission

probing occupied 4f-levels

probing unoccupied 4f-levels


Magnetic linear dichroism in 4f photoemission

r k e

r M

r E EUV

r k e

r M

r E EUV

MLD =I↑ − I↓

I↑ + I↓

4f

32 Kinetic energy (eV)

28 24

Inte

nsity


Temperature dependence of MLD

Phys. Rev. Lett. 100 (2008) 107202 H.E. Nigh et al., Phys. Rev. 132 (1963) 1092

MLD follows spontaneous magnetization



Lattice Tp

Electrons Te

optical excitation

~ 2 ps

> 10 ps, Gps

< 1 ps, Ges

4f spins

J5d,4f

5d↓ 5d

↓


Non-equilibrium dynamics (1.2 mJ / cm2)

1 ps

minority bulk band

majority bulk band


Conclusion

Transient band-structure mapping by TR-ARPES

Complex response of valence bands

Spin transport, spin-flip scattering, `and molecular field 5d6s and 4f spin systems are out of equilibrium !!!

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