Nonlinear Magneto-Optical Studies in Magnetic Superlattices and Magnetic Nano StructuresK. 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, JapanK. Takanashi, S. Mitani, Institute of Materials Science, Tohoku University, Sendai, Miyagi, Japan1st International Conference on Quantum Photonic Science TUAT COE Project Future Nano Materials

Nonlinear Magneto-opticsWhat is the nonlinear magneto-optical effect?Magnetization-induced nonlinear opticsWhat 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.

Azimuthal angle dependence of SHG from Si and GaAs waferSi wafer (001)centrosymmetricGaAs wafer (001)Non-centrosymmetric

Theoretical prediction and experimental verificationNonlinear magneto-optical Kerr rotation larger than linear rotaion was theoretically predicted1), and was experimentally proved2,3).1) W. Hbner 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)

Surface and interface sensitivity of MSHGApplication of MSHG Sensitive toEvaluation of the break ofMultilayerssymmetry at surfaceImaging of domainsThis 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.

Nonlinear magneto-optical effect For strong incident laser field E(w) :Second harmonic generation (SHG) Nonlinear polarization P(2) for incident field of E=E0sinwt Third rank tensor is not allowed in centrosymmetric materials. For weak incident laser field E(w) :linearresponseNonlinearresponse

Nonlinear polarization of 2nd orderSHG processLight rectificationparametric process

Definition of nonlinear susceptibilityCentrosymmetric 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

Wave equation of linear magneto-optical effectc1(1)=eyz, c0(1)=exx-1=N2-1YK=fK+ihK

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.

Nonlinear Kerr roation

Nonlinear Kerr rotaionDifferent from the linear case(2)odd/(2)even contributesThis term is zero in centrosymmetric materialsAnd takes a finite value at surfacesSurface sensitivityuseful for surfacemagnetism studies

Difference between linear and nonlinear Kerr rotationLinear

factor reduces the magnitudeAlso xy is order of magnitude smaller that xxNonlinearno such factor existsAlso odd and even are of the same order

Microscopic origin of MSHGSHG3 photon proicessHere|k q//l >|k+q//l>|k+q//l>|k+2q//l" >|kl> |k+2q//l>ww2wIntermediate stateGround stateExcited state

Illustration of microscopic process of MSHG

Nonlinear Kerr rotation of Fe

Nonlinear Kerr rotation of Fe/CuNonlinaer Kerr effectnolinear

Cu cover layer-thickness dependence of Co/CuCuSH

Superlattices Fe(xML)/Au(xML)NNon-integer: x=1.25, 1.5, 1.75, 2.25, 2.5, 2.75, 3.25, 3.5, 3.75Integer: x=1, 2, 3, 4, 5, 6, 8, 10, 15

Fe(1ML)/Au(1ML) superlatticeAtomic arrangement in a unit cell of Fe-Au with a L10 suructure.4.079fcc-Au (001)2.8674.054bcc-Fe (001)AuFeL10Schematic structure for the Fe/Au superlattice

MSHG Measureing System

LaboratoryExperimental setup for MSHG measurement

Optical Setups(Longitudinal Kerr)P-polarized or S-polarized lightw (810nm)2w (405nm)AnalyzerFilterw (810nm)Pole pieceRotatinganalyzerSample stage45Sample

Nonlinear Kerr Rotation and Kerr EllipticityResultAnalyzer angle-dependence for [Fe(3.5ML)/Au(3.5ML)] (Sin)Nonlinear Kerr rotation & ellipticityK(2)= 17.2 hK(2)=The curves show a shift for two opposite directions of magnetic field

Largest nonlinear Kerr rotationobserved in the Fe/Au seriesFe(1.75ML)/Au(1.75ML)SinDf = 31.1

Azimuthal Angle DependenceAzimthal angle-dependence of MSHG intensity for [Fe(3.75ML)/Au(3.75ML)] superlattice. Pin Pout(a)Linear (810nm)(b)SHG (405nm)Linear optical response(=810nm)The isotropic response for the azimuthal angleNonlinear optical response (=405nm)The 4-fold symmetry patternAzimuthal pattern show 45-rotation by reversing the magnetic fieldSHG intensity (counts/10sec.)SHG intensity (counts/10sec.)45linearnonlinear

Azimuthal Angle DependencePin-PoutSin-Sout

The equation of the azimuthal angle-dependence by the theoretical analysisDiscussion 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.

Calculated azimuthal angle dependence of SHG and MSHG signalsKerr rotation calculated from parameters Axx,B,CSinPin

input-output polarization

surface non-magnetic

bulk non-mangetic

surface magnetization-induced

sum

Sin-Sout

0

|Bsin4|2

|AssCcos4|2

|Ass+Bsin4Csin4|2

Sin-Pout

|Asp|2

|AspBcos4|2

|Csin4|2

|AspBcos4Csin4|2

Pin-Sout

0

|Bsin4|2

|Aps

Ccos4|2

|ApsBsin4 +Ccos4|2

Pin-Pout

|App|2

|App+Bcos4|2

|

Csin4|2

|App+Bcos4

Csin4|2

_1018291396.unknown

Surface non-magnetic termSHG response causes an isotropic contribution only.For crystallographic contribution the electric quadrupole should be introduced to get four rank tensor.SHG response causes an anisotropic contribution (parameter B).Bulk non-magnetic termSurface magnetization induced termThe surface magnetic response comes from the electric dipole term expanded by magnetization and contributes to the parameter C.

Calculated polar patterns of the azimuthal angle-dependence (Sin-Pout)(a) A=5, B=0, C=0.85For B much smaller than C, the polar pattern shows 45 rotation for the magnetization reversal.(b) A=5, B=0.85, C=0.85For B comparable C, the polar pattern undergo a smaller rotation.The equation of the azimuthal angle-dependence by theoretical analysisWithout nonlocalityWith nonlocality

Azimuthal angle-dependence of MSHG for a [Fe(3.5ML)/Au(3.5ML)] superlattice(Sin-Pout, Sin-Sout configuration)Sin-SoutASP(surface nonmagnetic term)=460ASS(surface nonmagnetic term)=100B(bulk nonmagnetic term)=26C(surface magnetic term)=-88The equation of the azimuthal angle-dependence by theoretical analysisSin-Pout

Calculated and experimental patterns :x=3.5Dotsexp.Solid curvecalc.

Calculated and experimental pattern of Nonlinear Kerr rotation and ellipticity(a) Experimental pattern (Sin)(b) Calculated pattern (Sin)The azimuthal angle-dependences of nonlinear Kerr rotation angle and ellipticity in [Fe(3.75ML)Au(3.75ML)]

Fe(2.75ML)/Au(2.75ML)Nonlinear Kerr rotation (deg.)Experimental and calculated patterns of Kerr rotation angle Sin configuration: (a) Experimental data, (b) Calculated usingparameters determined by fitting to the azimuth patterns(a) Experiment(b) Calculation(a) Experiment(b) Calculation

Nonlinear Kerr rotation angle of [Fe(xML)/Au(xML)] (1.25x3.75) superlattices (Sin)Calculation and experimental result The experimental maximum K(2) for x=1.75 superlattice was 31.1. The calculated K(2) reproduced the muximum K(2) for x=1.75 superlattice. The nonlinear Kerr rotation was explained by theoretical analysis.Fig. Nonlinear Kerr rotation angle of [Fe(xML)/Au(xML)] (1.25x3.75) superlattices [(a)Calculation, (b)Experiment]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)

Summary: MSHG of Fe/Au superlatticeThe 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.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.

Summary (contd)The azimuthal angle-dependence of the nonlinear Kerr rotation were explained using parameters determined from azimuthal patterns of MSHG responseMSHG 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 x=1.75 was 31.1We observed azimuthal angle-dependence of the nonlinear Kerr rotation for the first time.Modulation period dependence of parameters: A (Surface nonmagnetic) is large for short periodB (Bulk nonmagnetic) is nearly constantC (Surface magnetic) becomes larger with modulation Period.

Fabrication of permalloy nanostructure by Damascene techniquePreparation of substrate: Spin-coating of ZEP resist with high etching resistanceEB-exposition: Write patterns by EBDevelopment: Formation of mask-pattern by developmentEtchingBy dry-etching process mask-pattern is transferred to the substrateDeposition of magnetic film: Deposition of magnetic films by sputter or evaporationPolishing: Obtain flat buried structure using chemical-mechanical polishing Process is simplified by abbreviation of lift-off and repeated spin-coating

EB-patterning processDot size100nm300nm rectangular dot with 300nm-spacing100nm square dot with 300nm-spacingPatterned area: 3mm3mmEB-resist thickness: 300 nmby spin-coating with 5000 rpm rotationBaking16020minSpin coating of resistEB exposureSi substrateDevelopment

Clean Room LaboratoryElectron beam lithography

Etching gas:CF4Vacuum3.010-3PaGas pressure 9.2PaRF power:400WEtching rate: 0.1m/minSilicon surface after etchingDry etching process

Dry-etching

LaboratoryEB depositionRF magnetron sputtering

Embedding of permalloyEmbedding of permalloy film by electron beam depositionmaterial:permalloyNi80Fe20Vacuum3.010-6TorrAccelerating voltage 4kV Deposition rate 1.0/secPolishing chemicals:Si wafergrain-size20nmpH11polishing rate:60nm/minflattingChemical mechanical polishing

Observation/MFMFE-SEM

SEM observation 300nm100nmsquare dot, 300 nm space

Dot depth?Cross sectional SEM observation

Cross section SEM image of Line and space pattern (width =100nm)m

MFM observation of a permalloy film

AFM and MFM observation1mAFM Line scan Surface roughness~10nmMFM image magnetization axis along the longer side direction

Comparison between two scans after magnetization in opposite direction5kOe5kOe

MFM-image for different scanning direction

Scan-direction dependence

Pattern variation with scan direction

VSM measurementPerpendicular

100nm circular dotswith 300 nm spacingSurface roughness ~10nmAFMSEM

VSM measurement of circular dot arrayParallel to the plane Perpendicular to the plane

MFM measurement of circular dotsDemagnetizedMagnetic field applied Perpendicular to the plane

Influence of stray field from the MFM probe tipMFM measurenmentMagnetizationABAFM sensing (2-3nmlevitation)MFM probeMagnetizationAB80nmRecording by first scanReading by second scan

Models to explain MFM imagesMagnetizationMFM image

1m square dot arrayAFMMFM

MFM image of 300nm x 100nm dot with a low-moment probe tipAFMMFM

300nm x 100nm dot wide scanwith a low-moment probe tipAFMMFM

Simulation by Nakatani

MSHG studies of Nano-structured magnetic patterns

Azimuthal angle dependence of SHG from unpatterned permalloy filmPinPoutUnstructured permalloy film: H=2.5kOeLongitudinal(counts/10sec)

Azimuthal angle dependence of SHG from unpatterned Si waferPinPout(counts/10sec)H=2.5kOe

Azimuthal angle dependence of SHG from GaAs wafer

Azimuthal angle dependence of MSHG from 1m square dot arrayPinPoutLongitudinal Kerr configuration(counts/10sec)H=4kOe

Azimuthal angle dependence of MSHG from 300nm x 100nm rectangular dot array (Longitudinal)PinPoutSinPoutLongitudinal configuration(counts/10sec)H=4kOe(counts/10sec)

Azimuthal angle dependence of MSHG from 300nm x 100nm rectangular dot array (Polar)polar Kerr configurationPinPout(counts/10sec)H=6kOe

PinPoutSinPoutSinSoutPinSout(counts/10sec)(counts/10sec)(counts/10sec)(counts/10sec)(100nm)4kOe

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