Recent Advances in Magneto-Optics

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Recent Advances in Magneto-Optics

• Katsuaki Sato
• Department of Applied Physics
• Tokyo University of Agriculture Technology

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CONTENTS

• Introduction
• Fundamentals of Magneto-Optics
• Magneto-Optical Spectra
• Experiments and theory
• Recent Advances in Magneto-Optics
• Magneto-optics in nano-structures
• Nonlinear magneto-optical effect
• Scanning near-field magneto-optical microscope
• Current Status in Magneto-Optical Devices
• Magneto-optical disk storages
• Magneto-optical isolators for optical
  communication
• Other applications
• Summary

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1. Introduction

• Magneto-Optical EffectDiscovered by Faraday on
  1845
• PhenomenonChange of Linear Polarization to
  Elliptically Polarized Light Accompanied by
  Rotation of Principal Axis
• CauseDifference of Optical Response between LCP
  and RCP
• Application
• Magneto-Optical Disk
• Optical Isolator
• Current Sensors
• Observation Technique

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2.Fundamentals of Magneto-Optics

• MO Effect in Wide Meaning
• Any change of optical response induced by
  magnetization
• MO Effect in Narrow Meaning
• Change of intensity or polarization induced by
  magentization
• Faraday effect
• MOKE(Magneto-optical Kerr effect)
• Cotton-Mouton effect

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2.1 Faraday Effect

• (a) Faraday Configuration
• Magnetization // Light Vector
• (b)Voigt Configuration
• Magnetization ? Light Vector

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Faraday Effect

• MO effect for optical transmission
• Magnetic rotation(Faraday rotation)?F
• Magnetic Circular Dichroism(Faraday
  Ellipticity)??F
• Comparison to Natural Optical Rotation
• Faraday Effect is Nonreciprocal (Double rotation
  for round trip)
• Natural rotation is Reciprocal (Zero for round
  trip)
• Verdet Constant
• ?FVlH (For paramagnetic and diamagnetic
  materials)

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Illustration of Faraday Effect
Rotation of Principal axis

• For linearly polarized light incidence,
• Elliptically polarized light goes out (MCD)
• With the principal axis rotated (Magnetic
  rotation)

Elliptically Polarized light
Linearly polarized light
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Faraday rotation of magnetic materials
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2.2 Magneto-Optical Kerr Effect

• Three kinds of MO Kerr effects
• Polar Kerr(Magnetization is oriented
  perpendicular to the suraface)
• Longitudinal Kerr(Magnetization is in plane and
  is parallel to the plane of incidence)
• Transverse Kerr (Magnetization is in plane and is
  perpendicular to the plane of incidence)

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Magneto-optical Kerr effect
M
M
M

• Polar Longitudinal Transverse

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MO Kerr rotation of magnetic materials
"a-" means "amorphous".
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2.3 Electromagnetism and Magnetooptics

• Light is the electromagnetic wave.
• Transmission of EM waveMaxwell equation
• Medium is regareded as continuum?dielectric
  permeability tensor
• Effect of Magnetic field?mainly to off-diagonal
  element
• Eigenequation
• ?Complex refractive indextwo eigenvalues
• eigenfunctionsright and left circularpolarizatio
  n
• Phase difference between RCP and LCP?rotation
• Amplitude difference ?circular dichroism

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Dielectric tensor
Isotromic mediaM//z Invariant C4 for 90rotation
around z-axis
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MO Equations (1)
Maxwell Equation
Eigenequation
Eigenvalue
EigenfunctionLCP and RCP
Without off-diagonal termsNo difference between
LCP RCP
No magnetooptical effect
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MO Equations (2)
Both diagonal and off-diagonal terms contribute
to Magneto-optical effect
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Phenomenology of MO effect
Linearly polarized light can be decomposed to LCP
and RCP
Difference in phase causes rotation of the
direction of Linear polarization
Difference in amplitudes makes Elliptically
polarized light
In general, elliptically polarized light With the
principal axis rotated
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2.4 Electronic theory of Magneto-Optics

• Magnetization?Splitting of spin-states
• No direct cause of difference of optical response
  between LCP and RCP
• Spin-orbit interaction?Splitting of orbital
  states
• Absorption of circular polarization?Induction of
  circular motion of electrons
• Condition for large magneto-optical response
• Presence of strong (allowed) transitions
• Involving elements with large spin-orbit
  interaction
• Not directly related with Magnetization

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Dielectric functions derived from Kubo formula
where
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Microscopic concepts of electronic polarization
Expansion by unperturbed orbitals
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Orbital angular momentum-selection rules and
circular dichroism
py-orbital
px-orbital
ppxipy
Lz1
Lz-1
p-px-ipy
Lz0
s-like
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Role of Spin-Orbit Interaction
Jz-3/2
Jz-1/2
L1
Jz1/2
LZ1,0,-1
Jz3/2
Jz-1/2
L0
Jz1/2
LZ0
Exchange spin-orbit
Exchange splitting
Without magnetization
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MO lineshapes (1)
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MO lineshapes (2)
2.Paramagnetic lineshape
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3. Magneto-Optical Spectra

• Measurement technique
• Magnetic garnets
• Metallic ferromagnetFe, Co, Ni
• Intermetallic compounds and alloysPtMnSb etc.
• Magnetic semiconductorCdMnTe etc.
• SuperlatticesPt/Co, Fe/Au etc.
• AmorphousTbFeCo, GdFeCo etc.

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Measurement of magneto-optical spectra using
retardation modulation technique
Light source
chopper
filter
ellipsoidal mirror
monochromator
polarizer
eletromagnet
sample
sample
analyzer
detector
computer
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Magnetic garnets

• One of the most intensively investigated
  magneto-optical materials
• Three different cation sites octahedral,
  tetrahedral and dodecahedral sites
• Ferrimagnetic
• Large magneto-optical effect due to strong
  charge-transfer transition
• Enhancement of magneto-optical effect by
  Bi-substitution at the dodecahedral site

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Electronic level diagram of Fe3 in magnetic
garnets
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Experimental and calculated magneto-optical
spectra of Y3Fe5O12
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Electronic states and optical transitions of Co2
and Co3 in Y3Fe5O12
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Theoretical and experimental magneto-optical
spectra of Co-doped Y3Fe5O12
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Theoretical and experimental MO spectra of bcc Fe
Katayama
Krinchik
theory
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MO spectra of PtMnSb
Magneto-optical Kerr rotation ?K and ellipticity
?K
Off-diagonal Dielectric function
Diagonal dielectric functions
(a)
(b)
(c)
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Comparison of theoretical and experimental
spectraof half-metallic PtMnSb
After Oppeneer
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Magneto-optical spectra of CdMnTe
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Pt/Co superlattices
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MO spectra in RE-TM (1)
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MO spectra in R-Co
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MO spectra of Fe/Au superlattice
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Calculated MO spectra of Fe/Au superlattice
By M.Yamaguchi et al.
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Au/Fe/Au sandwich structure
By Y.Suzuki et al.
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4. Recent Advances in Magneto-Optics

• Nonlinear magneto-optics
• Scanning near-field magneto-optical microscope
  (MO-SNOM)
• X-ray magneto-optical Imaging

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NOMOKE(Nonlinear magneto-optical Kerr effect)

• Why SHG is sensitive to surfaces?
• Large nonlinear magneto-optical effect
• Experimental results on Fe/Au superlattice
• Theoretical analysis
• Future perspective

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MSHG Measurement System
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Optical arrangements
Sample
????
Sample stage
w (810nm)
Pole piece
P-polarized or S-polarized light
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Rotating analyzer
w (810nm)
Analyzer
Filter
2w (405nm)
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Azimuthal dependence of
Linear optical response (?810nm) The
isotropic response for the azimuthal angle
Nonlinear optical response (?405nm) The
4-fold symmetry pattern Azimuthal pattern show
45?-rotation by reversing the magnetic field
MSHG
linear
45?
SHG intensity (counts/10sec.)
SHG intensity (counts/10sec.)
(a) Linear (810nm)
(b) SHG (405nm)
Fe(3.75ML)/Au(3.75ML) ???? (Pin
Pout)??????????????????
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Calculated and experimental patterns x3.5
Dotsexp. Solid curvecalc.
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Nonlinear Kerr Effect
Df 31.1
The curves show a shift for two opposite
directions of magnetic field
Fe(1.75ML)/Au(1.75ML) Sin
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Nonlinear Magneto-optical Microscope
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MO-SNOM(Scanning near-field magneto-optical
microscope)

• Near-field optics
• Optical fiber probe
• Optical retardation modulation technique
• Stokes parameter of fiber probe
• Observation of recorded bits on MO disk

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Near-field
Scattered wave by a small sphere placed in the
evanescent field produced by another sphere
Total reflection and near field
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Levitation control methods
Shear force type
Canti-lever type
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Collection mode(a) and illumination mode(b)
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SNOM/AFM System
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Recorded marks on MO diskobserved by MO-SNOM
MO image
topography
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MO-SNOM image of 0.2?m recorded marks on Pt/Co MO
disk
Resolution ?
MO image
Line profile
Topographic image
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Reflection type SNOM
P. Fumagalli, A. Rosenberger, G. Eggers, A.
Münnemann, N. Held, G. Güntherodt Appl. Phys.
Lett. 72, 2803 (1998)
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XMCD (X-ray magnetic circular dichroism)
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Magnetic circular dichroism of L-edge

(b)
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Domain image of MO media observed using XMCD of
Fe L3-edge
SiN(70nm)/ TbFeCo(50nm)/SiN(20nm)/ Al(30nm)/SiN(20
nm) MO ??
  N. Takagi, H. Ishida, A. Yamaguchi, H. Noguchi,
M. Kume, S. Tsunashima, M. Kumazawa, and P.
Fischer Digest Joint MORIS/APDSC2000, Nagoya,
October 30-November 2, 2000, WeG-05, p.114.
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Spin dynamics in nanoscale region
GaAs high speed optical switch
Th. Gerrits, H. van den Berg, O. Gielkens, K.J.
Veenstra and Th. Rasing Digest Joint
MORIS/APDSC2000, Nagoya, October 30-November 2,
2000, TuC-05, p.24.
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Further Prospects-For wider range of researches-

• Time (t)Ultra-short pulse?Spectroscopy using ps,
  fs-lasers, Pump-probe technique
• Frequency (?)Broad band width, Synchrotron
  radiation
• Wavevector (k)Diffraction, scattering,
  magneto-optical diffraction
• Length (x)Observation of nanoscale magetism,
  Appertureless SNOM, Spin-polarized STM, Xray
  microscope
• Phase (?)Sagnac interferrometer

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5. Magneto-optical Application

• Magneto-optical disk for high density storage
• Optical isolators for optical communication
• Other applications

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Magneto-optical (MO) Recording

• RecordingThermomagnetic recording
• Magnetic recording using laser irradiation
• Reading out Magneto-optical effect
• Magnetically induced polarization state
• MO disk, MD(Minidisk)
• High rewritabilitymore than 107 times
• Complex polarization optics
• New magnetic concepts MSR, MAMMOS

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History of MO recording

• 1962 Conger,Tomlinson Proposal for MO memory
• 1967 Mee Fan Proposal of beam-addressable MO
  recording
• 1971 Argard (Honeywel) MO disk using MnBi films
• 1972 Suits(IBM) MO disk using EuO films
• 1973 Chaudhari(IBM) Compensation point recording
  to a-GdCo film
• 1976 Sakurai(Osaka U) Curie point recording on
  a-TbFe films1980 Imamura(KDD) Code-file MO memory
  using a-TbFe films
• 1981 Togami(NHK) TV picture recording using
  a-GdCo MO disk
• 1988 Commercial appearance of 5MO disk (650MB)
• 1889 Commercial appearance of 3.5 MO
  disk(128MB)
• 1991 Aratani(Sony) MSR
• 1992 Sony MD
• 1997 Sanyo ASMO(5 6GBL/G, MFM/MSR) standard
• 1998 Fujitsu GIGAMO(3.5 1.3GB)
• 2000 Sanyo, Maxell iD-Photo(5cmf730MB)

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Structure of MO disk media

• MO disk structure

Polycarbonate substrate
SiNx layer for protection and MO-enhancement
Al reflection layer
MO-recording layer (amorphous TbFeCo)
Land
Groove
Resin
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MO recording How to record(1)

• Temperature increase by focused laser beam
• Magnetization is reduced when T exceeds Tc
• Record bits by external field when cooling

M Tc
Temp
Tc
Laser spot
MO media
External field
Coil
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MO recording How to record(2)

• Use of compensation point
• writing
• Amorphous TbFeCo
• Ferrimagnet with Tcomp
• HC takes maximum at Tcomp
• Stability of small recorded marks

Hc
M
Tb
FeCo
Mtotal
Fe,Co
T
Tcomp
Tc
Tb
RT
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??????TbFeCo??
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Two recording modes

• Light intensity modulation (LIM) present MO
• Laser light is modulated by electrical signal
• Constant magnetic field
• Elliptical marks

• Magnetic field modulation (MFM)MD, ASMO
• Field modulation by electrical signal
• Constant laser intensity
• Crescent-shaped marks

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Shape of Recorded Marks
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MO recording How to read

• Magneto-optical conversion of magnetic signal to
  electric signal

D1
-
LD
D2
Differential detection
Polarized Beam Splitter
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Structure of MO Head
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Advances in MO recording

1. Super resolution
2. MSR
3. MAMMOS/DWDD
4. Use of Blue Lasers
5. Near field
6. SIL
7. Super-RENS (AgOx)

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MSR(Magnetically induced super-resolution)

• Resolution is determined by diffraction limit
• d0.6?/NA, where NAn sin a
• Marks smaller than wavelength cannot
• be resolved
• Separation of recording and reading layers
• Light intensity distribution is utilized
• Magnetization is transferred only at the heated
  region

d
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Illustration of 3 kinds of MSR
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AS-MO standard
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iD-Photo specification
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MAMMOS(magnetic amplification MO system)
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Super-RENSsuper-resolution near-field system

• AgOx filmdecomposition and precipitation of Ag
• Scattering center?near field
• Ag plasmon?enhancement
• reversible
• Applicable to both phase-change and MO recording

??????
?????
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To shorter wavelengths

• DVD-ROM Using 405nm laser, successful play back
  of marks was attained with track pitch
  0.26?m?mark length 213?m (capacity 25GB) using
  NA0.85 lens i?
• i M. Katsumura, et al. Digest ISOM2000, Sept.
  5-9, 2000, Chitose, p. 18.
• DVD-RW Using 405nm laser, read / write of
  recorded marks of track pitch0.34?m and mark
  length0.29?m in 35?m two-layered
  disk(capacity27GB) was succeeded using NA0.65
  lens, achieving 33Mbps transfer rate ii ?ii
  T. Akiyama, M. Uno, H. Kitaura, K. Narumi, K.
  Nishiuchi and N. Yamada Digest ISOM2000, Sept.
  5-9, 2000, Chitose, p. 116.

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Read/Write using Blue-violet LD and SIL (solid
immersion lens)
NA1.5 405nm 80nm mark 40GB
SILhead
405nm LD
I. Ichimura et. al. (Sony), ISOM2000 FrM01
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SIL (solid immersion lens)
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Optical recording using SIL
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Hybrid Recording
405nm LD
Recording head (SIL)
Readout MR head
Achieved 60Gbit/in2
H. Saga et al. Digest MORIS/APDSC2000, TuE-05,
p.92.
TbFeCo disk
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Optical elements for fiber communication

• Necessity of optical isolators
• Principles of optical isolators
• Structure of optical isolators
• Polarization-independent type
• Polarization-dependent type
• Optical multiplexing and needs of optical
  isolators

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Optical circuit elements proposed by Dillon
87
Optical isolator for Laser diode module
88
Optical fiber amplifier and optical isolator
89
Optical Circulator
90
Optical add-drop and circulator
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Polarization dependent isolator
92
Polarization independent isolator
93
Magneto-optical circulator
94
Optical absorption in YIG
95
Waveguide type isolators
96
Mach-Zehnder type isolator
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Current-field sensor
98
Current sensors used by power engineers
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Field sensor using optical fibers
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SUMMARY

• Basic concepts of magneto-optics are described.
• Macroscopic and microscopic origins of
  magneto-optics are described.
• Some of the recent development of magneto-optics
  are also given.
• Some of the recent application are summarized.