Study of Ferromagnetic Process of Magnetic

Study of Ferromagnetism of
Diluted Magnetic Semiconductor
Zn1-xCrxTe with MCD
[Referenece]
H.Saito,V.Zayets,S.Yamagata,and K.Ando Phys.Rev.Lett. 90,207202(2003)
ITOH Lab.
Yoshitaka Sakamoto
(坂本圭隆)
Contents
Introduction
・Abstract
・Background of reference
・How MCD is used in the reference
Main talk
・What is MCD
・MCD spectroscopy
・Method ,result ,analysis (in the reference)
Summary
Abstract
I’ll explain about…
Magnetic Circular Dichroism
磁気
円
二色性
Reference source: “Zn1-xCrxTe” made in AIST
(産業技術総合研究所)
Background of the reference
希釈磁性半導体
DMS=Diluted Magnetic Semiconductor
⇒
Ga1-xMnxAs,In1-xMnxAs etc…
[Ref.1]
H.Saito,V.Zayets,S.Yamagata,and
K.Ando Phys.Rev.Lett.
90,207202(2003)
MCD use in the article
Magnetism
I can’t wait any more…
Electronics
Optics
problem of DMS
・need magnetism in room
temperature → Zn1-xCrxTe
・to know what the magnetism
causes → MCD is useful
[Ref.2] http://www.nanoelectronics.jp
The problem of Zn1-xCrxTe
強磁性
Where does the Ferromagnetism come from?
CrTe
ZnTe
Zn1-xCrxTe
CrTe
sp-d exchange interaction
CrTe
direct interaction
(CrTe cluster)
comparing MCD spectra
between ZnTe and CrTe and ZnCrTe
Measurement method in the reference
SQUID(=Superconducting Quantum interference device :超伝導量子干渉計)
It can determine the magnetization of a substance by searching
the changes of the magnetic bunch which through the substance.
H-M curve can be made. But we cannot get the information of
electron states.
MCD(=Magnetic Circular dichroism :磁気円二色性)
We can see the difference of absorption between right circular
polarization and left circular polarization. And we can
determine the energy of Zeeman’s splitting, permittivity,
magnetization and so on. We are able to observe the excitation
of electrons.
What is MCD?
Difference of transmission between two Circular
円
Polarizations
偏光
σ＋
Field
I＋
I0
T+
σ－
I0
Field
I－
T－
I0：input light intensity
σ±: right (left) circular light
T±: transmissivity
I±: penetrating intensity
How’s the light?
= electromagnetic wave, which result in the vibration
of a lot of oscillators
x
z
<<<<Linearly
y
Polarization
→
E(t) = Eoexp(-iωt) x^
x
z
y
<<<<Elliptical
Polarization
→
E(t) = E1 exp(-iωt) x^
+E2 Eoexp(-iωt) y^
Circular polarized light is…
x
Linearly Polarization>>>>
→
E(t) = Eoexp(-iωt) x^
⇔
→
^ ^
E(t) = ½Eoexp(-iωt) (x+iy)
y
^ ^
+ ½Eoexp(-iωt) (x-iy)
Elliptical Polarization>>>>>
→
E(t) = E1 exp(-iωt) x^
⇔
+E2 exp(-iωt) y^
→
^ ^
E(t) = ½(E1+E2)exp(-iωt) (x+iy)
^ ^
+ ½(E1-E2)exp(-iωt) (x-iy)
x
y
Angular momentum
The two (right and left) circular light that one has the
same energy as the other differs the angular momentum
each other that influences to electrons (or atoms).
Right Circular Polarization (右円偏光) ：σ+ = 1
Left Circular Polarization (左円偏光) ：σ- = -1
So…
J=3/2
for example:
RCP is able to be absorbed
LCP is not absorbed
J=1/2
Absorption difference on ZnTe
J=1/2
Conduction band
Conservation law of
Angular momentum
σ+ ：+1 σ－：- 1
J= - 1/2
σσ+
E-=Eg-μBH
E+=Eg+μBH
J=3/2
Valence band
J= -3/2
How to measure
Polarizer(b)
PEM
light source
(not parallel to (a))
monochrometer
(Photo Elastic Modulator)
(分光器)
target
photo multiplier
Polarizer(a)
(光電子増倍管)
Lock-in amp.
Predictable signal
Absorption coefficient
α+
α-
I+=(1-α+ζ)Io+
I-=(1-α-ζ)Io-
E-
E+
Energy
α-－α+
E+
EEnergy
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-
R
Valuable graph
ηF= tanηθ
ηθ： angle of elliptical coefficient
楕円率角
almost all the cases ηθ<<1, so,
ηF≒ηθ
RCP is less absorbed than LCP
ηθ(deg/cm)
0
Energy (eV)
LCP is less absorbed than RCP
Result and Analysis
(a) CrTe film at room temp.
(b) ZnTe film
(c) Zn1-xCrxTe(x=0.20)
80nm film at 20K
(d) Znexchange
1-xCrxTe(x=0.20)
“sp-d
400nm film atmay
20K causes
interaction”
the ferromagnetism☆
L-CP ： symmetrical point
in brillouin zone
Γ-CP: center of k-space
Summary
・As far as we see MCD spectra, CrTe cluster isn’t a factor of the
ferromagnetism of ZnCrTe and the spectrum of ZnCrTe similar
to that of ZnTe. It implies that the ferromagnetism comes from
sp-d exchange interaction.
・I think the analysis becomes more sophisticated when the
permittivity and magnetization are calculated.
・We cannot do the pico or femto second time-resolved measurement
but by using LIGHT. It’ll disclose the excitation state more clearly.
Such MCD spectroscopy is the plan for my reserch.
Physical point of view
Zeeman’s splitting(ゼーマン分裂)
・Magnetic semiconductor：
RKKY interaction
(RKKY相互作用)
Field H
or
・Ferromagnet：
Domain wall(磁区)
2gμBH
permittivity(誘電率)
and
magnetization(磁化率)
Application
nonvolatile memory
(不揮発性メモリ)
with very large storage!
domain wall
sp-d interaction
RKKY interaction
H
localized electron
doped by Cr
Itinerant electron
(遍歴電子)
Not same as Ferromagnet!!
localized electron
doped by Cr
?Diluted :x<=0.20 多すぎるとFerromagnet を形成。
AIST National Institute of Advanced Industrial Science and Technology.
③ZnCrTeの結晶構造はどうなるか。
Zn：
30番4周期12族d電子持ち[3d(10)4s(2)]、反磁性体
六法細密構造、135pm
Te：
52番5周期16族p電子持ち[4d(10)5s(2)5p(4)]、非磁性体
六方晶構造、140pm
Cr：
24番4周期6族d電子持ち[3d(5)4s(1)]、強磁性体
体心立方格子、140pm
アクセプターとして働く。
ZnTe：
バンドギャップ＝2.39eV at 0K(約520nm(GaAsは830nm,InAsは3000nm))
格子定数＝6.103Å
赤みがかった透明。赤褐色
XRD(=X-ray diffraction :X線回折)
It is used to check the crystal structure. And we can know the
local gradient of it.
RHEED(=reflection high-energy electron diffraction :反射高速電子回折)
We are able to get the crystal structure and direction of growth
by analyzing the diffraction pattern. And can get the information
how the substance surface is flat from the spot shape.

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