Careful study of
Ultrafast Magneto-Optics
[Referenece]
“Ultrafast Magneto-Optics in Nickel: Magnetism or Optics?”
B.Koopmans, M.van Kampen et al. Phys.Rev.Lett. 85,844(2000)
ITOH Lab.
Yoshitaka Sakamoto
(坂本 圭隆)
1
Contents
Introduction
・Background
・Aim of the reference
Main talk
・light, Kerr effect, and TRMOKE
・Measurement configuration
・Predictable signal
・Result and Analysis (in the reference)
Summary
2
Background
Problem:
Clock per second
・storage
1000
(capacity, writing speed)
↓
Solution:
100
10
・spin memory
year
(rapid writing by using light,
1980
2005
large capacity [lamellar magnetic layer])
薄 層 磁 性 膜
⇒TRMOKE (time-resolved MO Kerr effect) is used.
3
MO signal
Aim of the reference
0
Ultrafast demagnetization?
-12
0.5ps(10sec)
delay time
Cu 3nm
2.field
Ni
0~15nm
1.Ni thickness
Cu(111)or(001)
3.temperature
4
TRMOKE measurement
T R M O K E
Time-resolved Magnetic optical
時間分解
磁気光学
Kerr effect
カー効果
pump pulse
amplitude
reflection
polarization
is changed
time
pulse laser
Field H
Kerr effect
5
Polarization
偏
光
: How a electromagnetic wave goes…
x
z
y
<<<<Linearly
Polarization
→
E(t) = Eoexp(-iωt) x^
x
z
y
<<<<Elliptical
Polarization
→
E(t) = E1 exp(-iωt) x^
^
+E2 Eoexp(-iωt) y
6
Elliptical polarization
x
Elliptical Polarization>>>>>
→
^
E(t) = E1 exp(-iωt) x
⇔
+E2 exp(-iωt) y^
→
^
^
E(t) = ½(E1+E2)exp(-iωt) (x+iy)
y
^ ^
+ ½(E1-E2)exp(-iωt) (x-iy)
+
=
7
Magnet-Optic Kerr effect
One of the Magnetic-Optics which contains many
property of the target.
Field H
Field H
Field H
Polar Kerr
Longitudinal Transverse
effect
Kerr effect
極カー効果
縦カー効果
Kerr effect
横カー効果
8
Reflective index
N±=n±+iκ±
N: complex refractive index n: refractive index κ: extinction coefficient
屈折率
複屈折率
Complex reflective index of amplitude
z
E0S
複素振幅反射率
E0P
(Fresnel coefficient)
E1S
ψ0
E1P
ψ1
n0
x
N
消光係数
ψ2
tan(ψ
0－ψ2)
E
1P
^
rP= ― = ―――――
E0P tan(ψ0＋ψ2)
sin(ψ
0－ψ2)
E
1S
^
rS= ―
=
－
―――――
E0S
sin(ψ0＋ψ2)
E2P
E2S
9
Reflective index of amplitude
for Circular Polarized light
^± - n0
N
r^±= ―――
N^±+n0
θK
θ＋ -θ－
= - ―――
2
^
r＋: for right circular light
≡r＋exp(iθ＋)
^
r－: for left circular light
≡r－exp(iθ－)
：Kerr rotation angle
カー回転角
ηK
|r^＋|- |r^－|
= ――――
：Kerr ellipticity
^
^
|r＋|+|r－|
カー楕円率
10
Kerr rotation angle, elliptical index
complex Kerr rotation angle
x
ΦK=θK+iηK
ηK=r /R
r
z
R = R+ + R-
y
R
r = R+ - R①Kerr rotation angle
:difference of phase shift
位相差
②Kerr ellipticity
:difference of reflectivity
反射率の違い
11
Kerr rotation and Magnetization
It is known that
ΦK ∝ M
⇔
θK∝M
ηK∝M
Kerr rotation angle is proportional to
Magnetization. It is called “Magnetic Kerr
effect”.
12
Measurement configuration
Ti:sapph LASER (femto sec. pulse)
photodiode
to amplifier
target
delay stage
PEM polarizer
probe line
pump line
probe pulse
pump pulse
delay time
target
⇒ relaxation process
can be measured
13
In this paper…
complex Kerr rotation
Ψ=Ψ’+ iΨ’’
Ψ’: Kerr rotation angle
Ψ’’: ellipticity
⊿Ψ=Ψ – Ψ0
Ψ0: original Kerr effect value
⊿Ψ’/Ψ’=⊿Ψ’’/Ψ’’～⊿M/M
14
Result A
Comparison of the induced ellipticity (⊿ψ’’/ψ0’’, open
circles) and rotation (⊿ψ’/ψ0’, filled diamonds) as a function
of pump-probe delay time.
It is strange that the changing of the both ratio which
don’t same reaction if it is because of magnetism.
15
Result B
0ps 1ps
0ps
200ps
delay
(a)(b) dependence on the applied field
Instantaneous decrease of ΔΨ’’ doesn’t
relate to applied field.
16
Result C
Pay attention
to the scale of y.
0ps 1ps
0ps
200ps
delay
(c)(d) Temperaure dependence at 4.6nm and
no applied field.
(d) is well explained by a thermal softening
of the effective magnetic potintial.
17
Summary
☆ An instantaneous demagnetization is unlikely.
☆ Rough estimate of the spin relaxation is 0.51ps, and may be explained by a highly efficient
spin-lattice relaxation.
☆ We should pay attention to the Kerr effect
which is not always the reaction of magnetism.
18
19
Measurement method
20
Argument for ultrafast
No H dependence and
only a relatively weak T and dNi dependence.
↓
state filling effects may well account for the
initial response in the TRMOKE experiments.
21
Argument for subnano signal
Surprisingly appeared after 30ps.
strong dependence on applied field.
↓
This can identified the oscillations as a precession
of M.
An intuitive illustration of the process is found by
solving the Landau-Lifshitz-Gilvert equation
in the limit of weak damping.
22
Calculation
N±=n±+iκ±
N: complex refractive index n: refractive index κ: extinction coefficient
屈折率
複屈折率
消光係数
α±=2ωκ±/c
α: absorption coefficient ω: frequency c: speed of light
吸収係数
周波数
ηF=ωΔκ/2c
光速
(=r /R)
ηF: Faraday elliptical index
r
ファラデー楕円率
R = R+ + R-
R
r = R+ - R23
TRMOKE measurement
probe light
(pulse laser)
pump light
(pulse laser)
24
Kerr effect and Magnetization
tanΦK=
εxy cosψ0
= ――――――――――――――― ―――――――――――――――
√εxx(cosψ0＋√εxx cosψ2)√εxx(cosψ2＋√εxx cosψ0)
※polar Kerr effect
  xx  xy 0 


~
    yx  yy 0 
 0

0

zz 

permittivity
誘電率
εij = εij(M) ⇒ change of Kerr effect depends
on magnetization.
25
Result D
(e)(f) Ni thickness dependence at 300K and
2800Oe(e) and 0Oe(f).
With in a couple of picoseconds the excess
energy rapidly diffuses out of the Ni film.
26
☆ On about 100ps time scale, they have observed
optically induced spin movement.
27

Study of Ferromagnetic Process of Magnetic