Spin-wave and spin-current dynamics in ultrafast demagnetization ...

Markus Münzenberg

Georg-August-Universität Göttingen, Germany

Spin-wave and spin-current dynamics in ultrafast

demagnetization experiments

Magnonics

Review: B. Lenk et al., Phys. Reports 507, 107 (2011)

200 mm

• Spin-waves exited by

femtosecond laser pulses

Terahertz spintronics

T. Kampfrath al., Nature Nanotech. 9, 256 (2013)

• Femtosecond spin(-wave) injection

•Localized and delocalized nature of spin dynamics

•Thermal model of ultrafast demagnetization

• Modification of electronic processes

• Half metals

• FePt “noble metal”

• Spin-wave propagation in thermal confinements

•Summary

Outline

DEex

Spin flip on fs time scales

(laser induced)

?

Localized nature of spin dynamics

Tim

e (

~10 f

s)

Length (~nm)


Stoner excitations (localized)

0 20

10

20

[T

Hz]

q/kF

D-EF

Stoner continuum

Spin-waves


Electronic equivalent

DEex

E

+ - V

Metal

Electron -

Hole +

)(En

Fermi energy dE

Edf )(

Delocalized nature of spin dynamics

M. Battiato, et al. Phys. Rev. Lett. (2010)

A. Melnikov, et al., Phys. Rev. Lett. (2011)

- + m - + m

)(En

E

)(En )(En

)(En

Fermi energy

EFerromagnet,

Metal

Ferromagnet,

Half metal

Delocalized nature of spin dynamics

M. Walter, et al., Nature Mater. (2011),

T. Kampfrath al., Nature Nanotech. (2013)





• Half metals



•Summary

Outline

Easy axis z

x y

1600 K

Occupation

Energy

l

Ele

ctr

onic

vie

w

Spin

ensem

ble

Thermal model of ultrafast demagnetization

Easy axis z

x y

Ele

ctr

onic

vie

w

Spin

ensem

ble


1600 K

Occupation

Energy

l

effHmdt

dm

Magnetization dynamics: Landau-Lifshitz-Gilbert equation (LLG)

No spin-scattering

tHstsi iidtd ,

Single spin (Quantum mechanics QM) “Macroscopic” ensemble


effiieleffii Hmm

mTHm

dt

dm

2),(

l

Include stochastic fluctuations of the spin system by a Fokker-Planck equation

D.A. Garanin, Phys. Rev. B 55, 1997, R. Chantrell, U. Novak, O. Fesenko 2007

Magnetization dynamics with magnetic fluctuations


No spin-scattering

)()(

)()(

)(22

||

eleffel

eleff

el

eleff THmmm

TmTHm

m

TTHm

dt

dm

Magnetization dynamics: Landau-Lifshitz-Bloch equation

m, and Heff, are coupled to electron temperature Tel

)( cTT Gilbert damping

“thermal macrospin” M(Tel)



No spin-scattering

eff

eleeleff

el

eleff Hmmm

TmmTHm

m

TTHm

dt

dm

22

|| )(),(

)()(

“thermal macrospin” M(Tel)



c

ele

therm TTmTm

mH

2

2

|| )(1~

1


m: actual magnetization

me: equilibrium magnetization m(T)


eff

eleeleff

el

eleff Hmmm

TmmTHm

m

TTHm

dt

dm

22

|| )(),(

)()(



c

ele

therm TTmTm

mH

2

2

|| )(1~

1







• Half metals



•Summary

Outline

Elliott-Yafet process

Modification of electronic processes: half metals

Spin-orbit coupling :

•Mixes spin-up and spin-down states U. Atxitia, Phys. Rev. B 81, 174401 (2010).

M. Münzenberg, New’s View’s (2010)

l

Spin-orbit coupling :

•Mixes spin-up and spin-down states

Intraband scattering with a phonon q, DE:

nk ,

nk ',

SLl~

• No spin conservation

nk ',

nk ,

nk ',

qph, DE

k

E

E. Yafet PR 96 (1954), Monod, Beneu, PRB 18 (1978), Das Sarma PRL 81 (1998)

Elliott- process and Gilbert damping


R. J. Elliott, Phys. Rev. 96 (1954).

G. M. Müller et al., Nature Mater. 8, 56 (2009) Fs pump-probe:

n

el

spelPc

1

12

0,

0,el

Spin polarization P dependent

relaxation

Momentum

scattering

Supression factor

,1P

W

)(En )(En

E

m

)()(~ 2

FhFe EncEnW

exchSO Ec D~ Spin-orbit interaction

Spin mixing:

11~

nP

0~1

spelrate


Elliott process: Ultrafast time scales

0 5 10 15

Ni

Co2MnSi

D

Kerr[a

rb.

un

its]

CrO2

LSMO

D[ps]

Fe3O

4

• Spin-flip processes are prohibited

• Electron and spin system are

isolated

Three temperature model: half metal

spins (Ts)

latsp

spel

lattice (Tl)

latel

electrons (Te)

e-

G. M. Müller et al., Nature Mater. 8, 56 (2009)


6.0log m

Half metal

8.0P

0.2 0.4 0.6 0.8 1.00.1

1

10

100

1000

0.01

0.1

1

10

E [m

eV

]

m [p

s]

|P|

Additional materials:

• Chalcospinels

• Manganites

• Ruthenates,

Double perovskites

M. Cinchetti Phys. Rev. Lett 97 (2006), A. Melnikov Phys. Rev. Lett. 100, (2008), T. Ogasawara Phys. Rev. Lett. 94 (2005), A. I.

Rev. B 63 (2001), Y. H. Ren J. Appl. Phys. 91 (2002), T. Kise Phys. Rev. Lett. 85 (2000), R. D. Averitt Phys. Rev. Lett. 87 (2001).

Fast:

Ni, Fe, Py,

Heusler Co2MnSi

Slow:

Manganite LSMO,

Magnetite; CrO2

Heusler Co2MnSi


• HGST, San Jose

• Department of Physics, Bielefeld

University

Isoelectronic Heusler films, spin-

transport (TMR)

A.Thomas, U Bielefeld

S. Maat, HGST

Pseudogap high polarized materials, spin

transport (GMR)


D. Steil, et al Phys. Rev. Lett. 105, 217202 (2010).

Co2MnSi (CMS) Co2FeSi (CFS)

(plus one electron) Demagnetization by hole channels

• Spin-flips are determined by Fermi level and gap width


T. Kubota et al., Appl. Phys. Lett 94, 122504 (2009).

Co2MnSi Co2FeSi

(plus two electrons)

Co2MnAl

• Decreased Gilbert damping in the gap region


Material [ps] Polarization P Method

Ni 0.160 (10) [10] 0.45 [37] Mes. Tedr.

Py 0.175 (5) [47] 0.48 [18] Mes. Tedr.

CoFe 0.172 (5) 0.55 [38] Mes. Tedr.

Co0.25Fe0.55B0.2 0.185 (6) 0.65 (am.) [39] Andreev refl.

Co2MnSi on MgO(001) 0.226 (7) 0.59 [43] TMR (*)

Co2MnGe 0.270 (4) 0.60 [] (~0.70 [42]) Andreev refl., GMR

(*)

Co2MnSi on Si 0.297 (5) 0.66 [40] TMR (*)

(CoFe)0.72Ge0.28 0.310 (4) 0.78 [41] GMR (*)

Co2FeAl 0.383 (5) 0.86 [43] TMR (*)

CrO2 83.6 (5.0) 0.99 [45] Mes. Tedr. (*)

(CoMnSb) (18.4) 1.00 (theor. value[44]) Theory

(*) Measured on the same/ similar set of films. For tunneling magnetoresistance (TMR) measurements P is

measured with CoFe counter electrode of know polarization as reference using the Jullière formula, giant

magnetoresistance (GMR) was analyzed by the Valet-Fert model, more details see section B.


Material [ps] Polarization P Method

Ni 0.160 (10) [10] 0.45 [37] Mes. Tedr.

Py 0.175 (5) [47] 0.48 [18] Mes. Tedr.

CoFe 0.172 (5) 0.55 [38] Mes. Tedr.

Co0.25Fe0.55B0.2 0.185 (6) 0.65 (am.) [39] Andreev refl.

Co2MnSi on MgO(001) 0.226 (7) 0.59 [43] TMR (*)

Co2MnGe 0.270 (4) 0.60 [] (~0.70 [42]) Andreev refl., GMR

(*)

Co2MnSi on Si 0.297 (5) 0.66 [40] TMR (*)

(CoFe)0.72Ge0.28 0.310 (4) 0.78 [41] GMR (*)

Co2FeAl 0.383 (5) 0.86 [43] TMR (*)

CrO2 83.6 (5.0) 0.99 [45] Mes. Tedr. (*)

(CoMnSb) (18.4) 1.00 (theor. value[44]) Theory

(*) Measured on the same/ similar set of films. For tunneling magnetoresistance (TMR) measurements P is

measured with CoFe counter electrode of know polarization as reference using the Jullière formula, giant

magnetoresistance (GMR) was analyzed by the Valet-Fert model, more details see section B.

Isolelectronic Heusler

Co2FeAl, Co2MnSi, Co2MnGe

(same number of electrons: 29)


A. Mann, et al. Phys. Rev. X 2, 041008 (2012).

Growth optimization Co2FeAl/ MgO(100)

• P=86% (RT) from spin transport experiments

B2 order spin dynamics spin transport



Comparison Co2FeAl/ Co2MnSi/ Co-Fe-B pseudogap

• Vanishing magnon peak (IETS) for negative bias shows half metallicity

magnon peak


latelel

latel

spel

EEkEPc

EEEkEncEnE

,,

,,

0,

2

,

,

,

2

,

1

)(1

)()()()(

m

m

mm

mm

mmm


For a one band model Energy dependent polarization

1 eV T=600 K

• Start with generalized

Kambersky multiband

approach

• Transitions in terms of

band polarization P(E)n,m

P(E)



1 eV


Materials with small gap

Pseudo gap

1 eV, 0.67 P

300 meV


0.1

1

10

100

t m,

fit (

ps)

1.00.90.80.70.60.50.4

spin polarization P

Ni [0

.45, 0

.160]

Ni8

1 Fe

19 [0

.48, 0

.175]

Co-F

e [0

.55, 0

.172]

Co

2 MnS

i (MgO

) [0.6

1, 0

.226]

Co

2 MnS

i (Vn/S

i) [0.6

6, 0

.297]

Co

2 MnG

e [0

.7, 0

.270]

(CoF

e)72 G

e28 [0

.78, 0

.310]

Co

2 FeA

l [0.8

6, 0

.383]

CrO2 [0.99, 83.61]

CoMnSb [0.99, 18.35]Co

25 F

e55 B

20 [0

.65, 0

.185]

Material [P, tm (ps)]

•Co-Fe-Ge, Co2FeAl: increase compared to by 3-4 0,el






• Half metals



•Summary

Outline

• CSIC, Madrid

Oksana Chubykalo-Fesenko

Thermal macrospin modelling

Pablo Nieves, Oksana Chubykalo-Fesenko

• HGST Western Digital

Simone Pisana, Tiffany Santos

Tiffany Santos

FePt storage media

Modification of electronic processes: FePt

Fe (bcc) FePt L10 “noble metal“

Pt

Energy

Density of states Density of states

Energy

Occupation

Energy

1600 K

Occupation

Energy

latsp

spel

latel

950 K

Small electron

specific heat

Tc ee


latsp

spel

latel

0 1 2 3

0.5

1.0

0 1 2 3

0.5

1.0

600

1200

1800

600

1200

1800

DM

Delay time (ps)

Te (

K)

e=110 J/ m3K2 e=1700 J/ m3K2

J. Mendil et al., arXiv:1306.3112 (2013)


950 K

Occupation

Energy

1600 K

Occupation

Energy

ll

• Low specific heat for FePt

• Large temperatures

•Increase above TC


• Low specific heat for FePt

• Large temperatures

• Type I - one time scale

• Type II – second slow

time scale appears


J. Mendil et al., arXiv:1306.3112 (2013)

• Type I - one time scale

• Type II – second slow time

scale appears at 25 mJ/cm2

Electron temperature

determines the spin dynamics

This is a new knob to design

new materials for ultrafast spin

dynamics






• Half metals



•Summary

Outline

• Simulation of the laser pulse excitation – spin-wave on the

picosecond and nanometer time scale

5 nm

M. Djordjevic, Phys. Rev. B 75 (2007)

Spin-wave propagation in thermal confinements

0.0 0.2 0.4 0.61E-12

1E-7

0.01

1000

1E8

1E13

t=16.7 ps

t=0.7 ps

t=125 ps

t=6.7 ps

Po

we

r F

FT

(Mx,y

,z /

M0)/

Po

we

r F

FT

(25

0p

s)

kz/2 [nm

-1]

t=0 ps

Wave vector (1

m)

106

107

108

109

Freq

uenc

y (G

Hz)

0

10

20

30

k [m-1]

[G

Hz]

50 nm Ni

Dipolar modes

Exchange modes

Kittel mode

LLB U. Atxitia, Phys. Rev. B 81 (2010), M3TM B. Koopmans, Nature Mater. (2010)

• also: microscopic model for ultrafast demagnetization


Schematic dispersion:

• Spin waves from the

GHz to THz range

/THz

Exchange interactions

k║M

k/nm-1

~k2

~(1-cos kr)

ql 2

B. Lenk et al., Phys. Reports 507, 107 (2011)


k

eff, HM

/THz

Exchange interactions Dipolar interactions

k/mm-1

k ┴M

k║M

/10GHz

k/nm-1

~k2

~(1-cos kr)

0H

k

eff, HM


Schematic dispersion:

• Spin waves from the

GHz to THz range


E, Heff

Magnetic film

/THz

k k

Exchange interactions Dipolar interactions

k /10-100nm /mm / 10 mm

~k2

k ┴M

k║M

k ┴M

k║M

/10GHz /10GHz



Magnonics

0 400 800

Kerr

rota

tion (

a.u

.)

T ime delay (ps)0 400 800

Time delay (ps)0 5 10 15 20

0

2

4

6

8

10

Fouri

er

Pow

er

(a.u

.)

Frequency (GHz)

m0H = 150mT

m0H = 0mT

20 nm Ni



5

10

15

20

25

Fre

qu

en

cy (

GH

z)

0 30 60 90 120 1500

5

10

15

20

25

External field (mT)

Fre

qu

en

cy (

GH

z)

H

k

D = 0.7 µm; a = 3.5 µm

5

10

15

20

25

Fre

qu

en

cy (

GH

z)

0 30 60 90 120 1500

5

10

15

20

25

External field (mT)

Fre

qu

en

cy (

GH

z)

a

k

/

μm)5(99.0 -1

DE wave vector

is imprinted

H k45

H. Ulrichs et al., Appl. Phys. Lett. 97, 092506 (2010)


(GHz)

x (mm)

T(K)

x (mm)

400K


• Simluation with reduced MS(T):


• Heating results in a frequency shift

H

k

E, Heff

No propagation

X

X


x (mm)

• Potential application: Thermal spin-wave traps (~1 GHz/ 50 mm)

People who do the work

Priority program

SpinCaT

Thesis 1: Difficulty lies in the complexity, not in the nature of

the process

Thesis 2: Models have to take into account localized and

delocalized character of magnetism

Thesis 3: Electronic structure is modified and interacts with

dynamics (spin-flip rate, electron temperature,

exchange splitting etc.)

Thesis 4: Critical behavior plays a crucial role

Thesis 5: As a consequence of spin-orbit interaction,

momentum conservation does not give insights for

ferromagnets (different for two sublattice spin

systems)

Five bullet points – ultrafast dynamics

Spin-wave and spin-current dynamics in ultrafast demagnetization ...

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