Nonlinear Magneto-Optical Studies in Magnetic Superlattices and Magnetic Nano Structures

1
Nonlinear Magneto-Optical Studies in Magnetic
Superlattices and Magnetic Nano Structures
1st International Conference on Quantum Photonic
Science

• K. Sato, A. Kodama, M. Miyamoto, M. Tsuruga, T.
  Matsumoto, T. Ishibashi Y. Morishita, Department
  of Applied Physics, Tokyo University of
  Agriculture and Technology, Koganei, Tokyo, Japan
• K. Takanashi, S. Mitani, Institute of Materials
  Science, Tohoku University, Sendai, Miyagi, Japan

TUAT COE Project Future Nano Materials
2
Nonlinear Magneto-optics

• What is the nonlinear magneto-optical effect?
• Magnetization-induced nonlinear optics
• What is the nonlinear Kerr rotation?
• When P-polarized primary light is incident both
  P- and S-polarized SH light emits which leads to
  rotation of E vector from the plane of incidence.
• In centrosymmetric materials such as Fe and Au no
  SHG occurs due to cancellation of P and P.

3
Azimuthal angle dependence of SHG from Si and
GaAs wafer
Si wafer (001) centrosymmetric
GaAs wafer (001) Non-centrosymmetric
4
Theoretical prediction and experimental
verification

• Nonlinear magneto-optical Kerr rotation larger
  than linear rotaion was theoretically
  predicted1), and was experimentally proved2,3).
• 1) W. Hübner and K.-H. Bennemann Phys. Rev. B40,
  5973 (1989)
• 2) Th. Rasing et al. J. Appl. Phys. 79, 6181
  (1996)
• 3) Th. Rasing J. Mag. Soc. Japan 20 (Suppl. S1),
  13 (1996)

5
Surface and interface sensitivity of MSHG

• Application of MSHG
• Sensitive to Evaluation of
• the break of Multilayers
• symmetry at surface Imaging of domains
• This effect cannot be expected to be applied to
  some practical memory devices but is thought to
  be useful for characterization of surfaces and
  interfaces of materials.

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Nonlinear magneto-optical effect
linear response
For weak incident laser field E(w)
Nonlinear response
For strong incident laser field E(w)
Third rank tensor is not allowed in
centrosymmetric materials.
Nonlinear polarization P(2) for incident field
of EE0sinwt
Second harmonic generation (SHG)
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Nonlinear polarization of 2nd order
parametric process
SHG process
Light rectification
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Definition of nonlinear susceptibility
Centrosymmetric materials all the ?ijk(2)
components vanish. (from symmetry
operations) Surfaces and interfaces symmetry
breaks, leading to appreciable amount of
nonlinear magneto-optical effect even in the
centrosymmetric materials
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Wave equation of linear magneto-optical effect
YKfKihK (???????)
c1(1)eyz, c0(1)exx-1N2-1
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Wave equation of nonlinear magneto-optics.
Source term does not depend on optical constants
of materials, leading to special solution
associated with the second order susceptibility.
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Nonlinear Kerr roation
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Nonlinear Kerr rotaion
Different from the linear case ?(2)odd/?(2)even
contributes? This term is zero in centrosymmetric
materials And takes a finite value at
surfaces Surface sensitivity useful for
surface magnetism studies!
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Difference between linear and nonlinear Kerr
rotation
Linear factor reduces the
magnitude Also ?xy is order of magnitude smaller
that ?xx Nonlinearno such factor exists Also
?odd and ?even are of the same order ?
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Microscopic origin of MSHG
3 photon proicess
SHG
Here
k q//l gt?kq//lgt kq//lgt?k2q//l" gt
klgt ?k2q//lgt
w
w
2w
Intermediate state
Ground state
Excited state
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Illustration of microscopic process of MSHG
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Nonlinear Kerr rotation of Fe
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Nonlinear Kerr rotation of Fe/Cu
Nonlinaer Kerr effect
nolinear
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Cu cover layer-thickness dependence of Co/Cu
SH??
Cu???
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Superlattices ?Fe(xML)/Au(xML)?N
Integer x1, 2, 3, 4, 5, 6, 8, 10, 15
Non-integer x1.25, 1.5, 1.75, 2.25, 2.5, 2.75,
3.25, 3.5, 3.75
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Fe(1ML)/Au(1ML) superlattice
4.054Å
bcc-Fe (001)
Au
Fe
2.867Å
Schematic structure for the Fe/Au superlattice
L10
fcc-Au (001)
4.079Å
Atomic arrangement in a unit cell of Fe-Au with a
L10 suructure.
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MSHG Measureing System
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Laboratory

• Experimental setup for MSHG measurement

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Optical Setups(Longitudinal Kerr)
Sample
????
Sample stage
w (810nm)
P-polarized or S-polarized light
Pole piece
45
Rotating analyzer
w (810nm)
Analyzer
Filter
2w (405nm)
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Nonlinear Kerr Rotation and Kerr Ellipticity
Result
The curves show a shift for two opposite
directions of magnetic field
Analyzer angle-dependence for Fe(3.5ML)/Au(3.5ML)
(Sin)
Nonlinear Kerr rotation ellipticity ?K(2) 17.2
? hK(2)3
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Largest nonlinear Kerr rotationobserved in the
Fe/Au series
Df 31.1
Fe(1.75ML)/Au(1.75ML) Sin
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Azimuthal Angle Dependence
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
nonlinear
linear
45?
SHG intensity (counts/10sec.)
SHG intensity (counts/10sec.)
(a) Linear (810nm)
(b) SHG (405nm)
Azimthal angle-dependence of MSHG intensity for
Fe(3.75ML)/Au(3.75ML) superlattice. (Pin Pout)
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Pin-Pout
Azimuthal Angle Dependence
Sin-Sout
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The equation of the azimuthal angle-dependence by
the theoretical analysis
Discussion
A Surface nonmagnetic term The
electric dipole origin give rise to isotropic
signal.
B Bulk nonmagnetic term The quadrupole
origin causes an anisotropic contribution for
four rank tensor.
C Surface magnetic term The time reversal
symmetry is lifted by magnetization Mirror
symmetry operations should be supplemented with
an additional reversion of M.
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Calculated azimuthal angle dependence of SHG and
MSHG signals
Kerr rotation calculated from parameters Axx,B,C
Sin
Pin
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Surface non-magnetic term
SHG response causes an isotropic contribution
only.
Bulk non-magnetic term
For crystallographic contribution the electric
quadrupole should be introduced to get four rank
tensor.
SHG response causes an anisotropic contribution
(parameter B).
Surface magnetization induced term
The surface magnetic response comes from the
electric dipole term expanded by magnetization
and contributes to the parameter C.
31
Calculated polar patterns of the azimuthal
angle-dependence (Sin-Pout)
Without nonlocality
The equation of the azimuthal angle-dependence by
theoretical analysis
(a) A5, B0, C0.85 For B much smaller than
C, the polar pattern shows 45? rotation for the
magnetization reversal.
With nonlocality
(b) A5, B0.85, C0.85 ?For B comparable C,
the polar pattern undergo a smaller rotation.
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Azimuthal angle-dependence of MSHG for a
Fe(3.5ML)/Au(3.5ML) superlattice(Sin-Pout,
Sin-Sout configuration)
The equation of the azimuthal angle-dependence by
theoretical analysis
Sin-Pout
Sin-Sout
ASP(surface nonmagnetic term) 460 ASS(surface
nonmagnetic term) 100 B(bulk nonmagnetic
term) 26 C(surface magnetic term) -88
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Calculated and experimental patterns x3.5
Dotsexp. Solid curvecalc.
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Calculated and experimental pattern of Nonlinear
Kerr rotation and ellipticity
(a) Experimental pattern (Sin)
The azimuthal angle-dependences of nonlinear
Kerr rotation angle and ellipticity in
Fe(3.75ML)Au(3.75ML)
(b) Calculated pattern (Sin)
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Experimental and calculated patterns of Kerr
rotation angle
(b) Calculation
(a) Experiment
(b) Calculation
(a) Experiment
Nonlinear Kerr rotation (deg.)
Fe(2.75ML)/Au(2.75ML)
Sin configuration (a) Experimental data,
(b) Calculated
using parameters determined by fitting to the
azimuth patterns
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Nonlinear Kerr rotation angle of
Fe(xML)/Au(xML) (1.25?x?3.75) superlattices
(Sin)
Calculation and experimental result
Calculated nonlinear Kerr rotation angle ?K(2)
using the fitting parameter ASP, ASS, B, C of the
azimuthal pattern (The maximum ?K(2) was
selected for azimuth angle)
? The experimental maximum ?K(2) for x1.75
superlattice was 31.1?. The calculated ?K(2)
reproduced the muximum ?K(2) for x1.75
superlattice.
The nonlinear Kerr rotation was explained by
theoretical analysis.
Fig. Nonlinear Kerr rotation angle of
Fe(xML)/Au(xML) (1.25?x?3.75) superlattices
(a)Calculation, (b)Experiment
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Summary MSHG of Fe/Au superlattice
The four-fold pattern clearly reflects the
symmetry of the MgO(100) substrate. This suggests
that the Fe/Au superlattice is perfectly
epitactic to the substrate.
The azimuthal angle dependence was analyzed in
terms of nonlinear electrical susceptibility
tensor taking into account the magnetic symmetry
of the superlattice.
?The azimuthal pattern was explained by symmetry
analysis, taking into account the surface
non-magnetic A, bulk non-magnetic B and surface
magnetic C contributions.
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Summary (contd)
MSHG was shown to lead to a nonlinear Kerr
rotation ?(2)K that can be orders of magnitude
larger than its linear equivalent (0.2?), e.g.,
?(2)K for x1.75 was 31.1?

• We observed azimuthal angle-dependence of the
  nonlinear Kerr rotation for the first time.

The azimuthal angle-dependence of the nonlinear
Kerr rotation were explained using parameters
determined from azimuthal patterns of MSHG
response

• Modulation period dependence of parameters
• A (Surface nonmagnetic) is large for short period
• B (Bulk nonmagnetic) is nearly constant
• C (Surface magnetic) becomes larger with
  modulation Period.

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Fabrication of permalloy nanostructure by
Damascene technique

• ?Preparation of substrate Spin-coating of ZEP
  resist with high etching resistance
• ?EB-exposition Write patterns by EB
• ?Development Formation of mask-pattern by
  development
• ?EtchingBy dry-etching process mask-pattern is
  transferred to the substrate
• ?Deposition of magnetic film Deposition of
  magnetic films by sputter or evaporation
• ?Polishing Obtain flat buried structure using
  chemical-mechanical polishing
• Process is simplified by abbreviation of lift-off
  and repeated spin-coating

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EB-patterning process
Spin coating of resist
EB exposure
Development
Si substrate

• ?1?Dot size
• 100nm300nm rectangular dot with 300nm-spacing
• 100nm square dot with 300nm-spacing
• ?2?Patterned area 3mm3mm
• ?3?EB-resist thickness 300 nm
• by spin-coating with 5000 rpm rotation
• ?4?Baking 160? 20min

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Clean Room Laboratory

• Electron beam lithography

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Dry etching process
?1?Etching gas CF4 ?2?Vacuum 3.010-3Pa ?3?Gas
pressure 9.2Pa ?4?RF power 400W ?5?Etching
rate 0.1µm/min
Silicon surface after etching
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Dry-etching
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Laboratory

• EB deposition

RF magnetron sputtering
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Embedding of permalloy
?1?material permalloy(Ni80Fe20) ?2?Vacuum
3.010-6Torr ?3?Accelerating voltage 4kV
?4?Deposition rate 1.0Å/sec
Embedding of permalloy film by electron beam
deposition
Chemical mechanical polishing
?1?Polishing chemicals Si wafer
grain-size20nm ?2?pH 11 ?3?polishing rate
60nm/min
flatting
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Observation

• AFM/MFM

FE-SEM
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SEM observation
300nm100nmsquare dot, 300 nm space
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Cross sectional SEM observation
Dot depth?
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Cross section SEM image of Line and space pattern
(width 100nm)
0.3µm
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MFM observation of a permalloy film
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AFM and MFM observation
1µm
AFM Line scan Surface roughness10nm MFM image
magnetization axis along the longer side
direction
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Comparison between two scans after magnetization
in opposite direction
5kOe
5kOe
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MFM-image for different scanning direction
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Scan-direction dependence
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Pattern variation with scan direction
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VSM measurement
Perpendicular
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100nm circular dotswith 300 nm spacing
SEM
AFM
Surface roughness 10nm
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VSM measurement of circular dot array
Parallel to the plane
Perpendicular to the plane
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MFM measurement of circular dots
Magnetic field applied Perpendicular to the plane
Demagnetized
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Influence of stray field from the MFM probe tip
MFM measurenment
AFM sensing (2-3nmlevitation) MFM probe
80nm
A
B
A
B
Magnetization
Magnetization
Reading by second scan
Recording by first scan
61
Models to explain MFM images
MFM image
Magnetization
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1?m square dot array
MFM
AFM
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MFM image of 300nm x 100nm dot with a low-moment
probe tip
MFM
AFM
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300nm x 100nm dot (wide scan)with a low-moment
probe tip
AFM
MFM
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Simulation by Nakatani
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MSHG studies of Nano-structured magnetic patterns
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Azimuthal angle dependence of SHG from
unpatterned permalloy film
PinPout
Longitudinal
(counts/10sec)
Unstructured permalloy film H2.5kOe
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Azimuthal angle dependence of SHG from
unpatterned Si wafer
PinPout
(counts/10sec)
H2.5kOe
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Azimuthal angle dependence of SHG from GaAs wafer
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Azimuthal angle dependence of MSHG from 1?m
square dot array
PinPout
(counts/10sec)
Longitudinal Kerr configuration
H4kOe
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Azimuthal angle dependence of MSHG from 300nm x
100nm rectangular dot array (Longitudinal)
PinPout
SinPout
(counts/10sec)
(counts/10sec)
Longitudinal configuration
H4kOe
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Azimuthal angle dependence of MSHG from 300nm x
100nm rectangular dot array (Polar)
PinPout
(counts/10sec)
polar Kerr configuration
H6kOe
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?????
PinSout
PinPout
(counts/10sec)
(counts/10sec)
SinPout
SinSout
(counts/10sec)
(counts/10sec)
????(100nm)?????????4kOe
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???????? 6.00