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
• 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)
Localized nature of spin dynamics
Stoner excitations (localized)
0 20
10
20
[T
Hz]
q/kF
D-EF
Stoner continuum
Spin-waves
Localized nature of spin dynamics
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)
• 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
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
Thermal model of ultrafast demagnetization
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
Thermal model of ultrafast demagnetization
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
Thermal model of ultrafast demagnetization
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)
D.A. Garanin, Phys. Rev. B 55, 1997, R. Chantrell, U. Novak, O. Fesenko 2007
Thermal model of ultrafast demagnetization
No spin-scattering
eff
eleeleff
el
eleff Hmmm
TmmTHm
m
TTHm
dt
dm
22
|| )(),(
)()(
“thermal macrospin” M(Tel)
Magnetization dynamics: Landau-Lifshitz-Bloch equation
m, and Heff, are coupled to electron temperature Tel
c
ele
therm TTmTm
mH
2
2
|| )(1~
1
D.A. Garanin, Phys. Rev. B 55, 1997, R. Chantrell, U. Novak, O. Fesenko 2007
m: actual magnetization
me: equilibrium magnetization m(T)
Thermal model of ultrafast demagnetization
eff
eleeleff
el
eleff Hmmm
TmmTHm
m
TTHm
dt
dm
22
|| )(),(
)()(
Magnetization dynamics: Landau-Lifshitz-Bloch equation
m, and Heff, are coupled to electron temperature Tel
c
ele
therm TTmTm
mH
2
2
|| )(1~
1
D.A. Garanin, Phys. Rev. B 55, 1997, R. Chantrell, U. Novak, O. Fesenko 2007
Thermal model of ultrafast demagnetization
• 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
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
Modification of electronic processes: half metals
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
Modification of electronic processes: half metals
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)
Modification of electronic processes: half metals
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
Modification of electronic processes: half metals
• 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)
Modification of electronic processes: half metals
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
Modification of electronic processes: half metals
T. Kubota et al., Appl. Phys. Lett 94, 122504 (2009).
Co2MnSi Co2FeSi
(plus two electrons)
Co2MnAl
• Decreased Gilbert damping in the gap region
Modification of electronic processes: half metals
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.
Modification of electronic processes: half metals
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)
Modification of electronic processes: half metals
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
Modification of electronic processes: half metals
A. Mann, et al. Phys. Rev. X 2, 041008 (2012).
Comparison Co2FeAl/ Co2MnSi/ Co-Fe-B pseudogap
• Vanishing magnon peak (IETS) for negative bias shows half metallicity
magnon peak
Modification of electronic processes: half metals
latelel
latel
spel
EEkEPc
EEEkEncEnE
,,
,,
0,
2
,
,
,
2
,
1
)(1
)()()()(
m
m
mm
mm
mmm
A. Mann, et al. Phys. Rev. X 2, 041008 (2012).
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)
Modification of electronic processes: half metals
A. Mann, et al. Phys. Rev. X 2, 041008 (2012).
1 eV
Modification of electronic processes: half metals
Materials with small gap
Pseudo gap
1 eV, 0.67 P
300 meV
Modification of electronic processes: half metals
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
Modification of electronic processes: half metals
• 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
• 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
Modification of electronic processes: FePt
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)
Modification of electronic processes: FePt
950 K
Occupation
Energy
1600 K
Occupation
Energy
ll
• Low specific heat for FePt
• Large temperatures
•Increase above TC
Modification of electronic processes: FePt
• Low specific heat for FePt
• Large temperatures
• Type I - one time scale
• Type II – second slow
time scale appears
Modification of electronic processes: FePt
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
Modification of electronic processes: FePt
• 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
• 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
Spin-wave propagation in thermal confinements
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)
Spin-wave propagation in thermal confinements
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
B. Lenk et al., Phys. Reports 507, 107 (2011)
Schematic dispersion:
• Spin waves from the
GHz to THz range
Spin-wave propagation in thermal confinements
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
B. Lenk et al., Phys. Reports 507, 107 (2011)
Spin-wave propagation in thermal confinements
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
B. Lenk et al., Phys. Reports 507, 107 (2011)
Spin-wave propagation in thermal confinements
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)
Spin-wave propagation in thermal confinements
(GHz)
x (mm)
T(K)
x (mm)
400K
Spin-wave propagation in thermal confinements
• Simluation with reduced MS(T):
Spin-wave propagation in thermal confinements
• Heating results in a frequency shift
H
k
Spin-wave propagation in thermal confinements
• Heating results in a frequency shift
H
k
E, Heff
No propagation
X
X
Spin-wave propagation in thermal confinements
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
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