Sinha - NIST · Title: Microsoft PowerPoint - Sinha.ppt Author: jcopley Created Date: 9/20/2005...
Transcript of Sinha - NIST · Title: Microsoft PowerPoint - Sinha.ppt Author: jcopley Created Date: 9/20/2005...
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Combined X-Ray and Neutron Scattering Studies of Magnetic
NanostructuresSunil K. SinhaUCSD/LANL
Rowe/Rush Symposium Sept.,2005
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Importance of Interfaces
• Interface Roughness can affect many properties: coercive field, magnetoresistance, spin injection, etc.
• Magnetic profiles across interfaces (e.g. AF/F, etc.)
• Magnetic domain structure.
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ki
kfQ Qz
Qx
Scattering Geometry & Notation
Wave-Vector: Q = Q = kkff –– kkii
Reflectivity:Qx= Qy = 0Qz= (4π/λπ/λπ/λπ/λ)sinααααi
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Specular Reflectivity
• Measures laterally averaged density profile normal to surface
• PNR with polarization analysis can measure vector magnetization profile
• Resonant X-Ray reflectivity at L- or M-edges using circularly polarized X-Rays can measure element-specific magnetization along k0
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NEUTRONS:
R+ +( Qz) - R- - (Qz) ~ M xy,�� (Qz) n (Qz)
R + - (Qz) = R - + (Qz) ~ � M xy,⊥ (Qz) �2
X-RAYS:
R+ ( Qz) - R- (Qz) ~ M �� (Qz) n (Qz)
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��������������� ������������������ � ������ ������� �����
The scattering factor in the dipole approximation is given by,
]2)[.)(.(][)(83
][83
).( 11
11
10
*11
11
11
11 −−− −−+−⋅×+
���
��� −+= FFFFFiZrFFf ise mememeeee i
*si
*s λ
πλ
πfC
fm
� Note that, I+ - I- ∝ fC . fm
S. A. Stepanov and S. K. Sinha, Phys. Rev. B 61, 15302 (2000)
D.R. Lee, S.K. Sinha, D.Haskel, Y. Choi, J.C. Lang, S.A. Stepanov and G. Srajer, Phys. Rev. B 68, 224409 (2003) ;
D.R. Lee, S.K. Sinha, C.S. Nelson, J.C. Lang, C.T. Venkataraman, G. Srajer and R.M. Osgood III, Phys. Rev. B 68, 224410 (2003)
�Parrat recursive formula for calculating reflectivity generalized to include magnetism
�Magnetism introduced by calculating the off diagonal terms of dielecric tensor
�Separate structural and magnetic roughness terms introduced
�Reflectivity and diffuse scattering calculated using full theory in the context of Distorted Wave Born Approximation
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Resonant Magnetic X-Ray Scattering
fαβ=Aδαβ -iB�γεαβγMα+CMαMβ
A = f0 + 3λ/8π (F11 + F1-1)
B = 3λ/8π (F11 - F1-1)
C = 3λ/8π ( 2 F10 - F11 - F1-1)
χαβ(r) = (4π/k02)nm(r)fαβ (r)
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For smooth surfaces, can be solved exactly using BoundaryConditions and iterative Matrix Method a la Born & Wolff or Parratt. Matrices here are 4 X 4 because of 2 ingoing and2 outgoing waves with 2 polarizations each. Can be expressedin terms of 2 X 2 Matrices. (Stepanov and Sinha, Phys. Rev. B 61, 15302 (2000) )
Website:http://sergey.gmca.aps.anl.gov/MAG_sl.html
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Numerical Results for Specular Reflectivity of GdFe multilayer
Gd LIII Edge
15[Gd(50Å)Fe(35Å)]
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How to include both Chemical and Magnetic Roughness?
• Imagine for every interface there are two interfaces: chemical and magnetic.
• They may or may not be separated.
• They may or may not be correlated.
Osgood et al. J. Magn. Magnetic Materials, 198-199, 698 (1999)D.R.Lee et al., Phys. Rev. B 68, 22409 (2003)
D.R.Lee et al., Phys. Rev. B 68, 22410 (2003)
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Simulation of Reflectivity Expt. On Co film at Co LIII Edge
C.C.Kao et al. Phys. Rev. B 50, 9599 ( 1994)
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For each interface, 3 correlation functions
Css(R)→σss, ξss, hss
Cmm(R)→σmm, ξmm, hmm
Cms(R)→σms, ξms, hms
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� ����������� ��������� ����
W.H. Meiklejohn, C.P. Bean, Phys. Rev. 105, 904 (1957).J. Nogués, Ivan K. Schuller, JMMM 192, 203 (1999).A.E. Berkowitz, K. Takano, JMMM 200, 552 (1999).
� EB vanishes above TN ; Must be related to the AF
M
H
FM
0
M
HFM
AFM
0
HE
FMFM
FMAFE tM
SJSH −=
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� �� ����������������������
M
H
TN < T < TC
FC
T < TN
FM
AFMAssume AF coupling at FM/AF interface
Non-vanishing AF moment
FMFM
FMAFE tM
JH
SS−=
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� ������ ��������������������������
FMFM
FMAFE tM
JH
SS−=
Gives bias values 2-3 order higher
Does not explain why exchange bias is observed for compensated AF surfaces like Fe/FeF2
� Does not realistically represent the FM/AF interfacial environment
Various models proposed:
� Random Field Model (Malozemoff, Phys. Rev. B 35 (1987) 3679)
� Domain Wall Model ( Mauri et al, J. Appl. Physics 62 (1987) 3047)
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Random-field, domain state, etc., models
HCF
+’ve HE
Frustrated super exchange (AF-coupling)
+1
-’ve HE
Super exchange (AF-coupling)
-1
U. Nowak et. al., J. Magn. Magn. and Mater., 240, 243 (2002).A.P. Malozemoff, J. Appl. Phys., 63, 3874 (1988).
HCF10nm
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� Interface properties could be very different from bulk
� Roughness could be a source of AF uncompensated moments
� Compelling need to determine spin structure across F/AF interface (e.g. domain structure, magnetic roughness, etc.)
Measuring Reflectivity using resonance X-ray scattering technique
� Can quantitatively determine depth dependent magnetic density
�Interface sensitive
� Element Specific
�Diffuse scattering – lateral structures like domains, magnetic roughness etc.
� Need interface sensitive experimental technique
!����������� ���� �����������"����������������
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� ��������� ��������������#���������$����������� ���
Co
ko
θθθθ
Qz
Applied Field Detector
k1Circularly Polarized Beam
FeF2
Resonant X-ray scattering measurements performed on Beamline 4.0.2 at the ALS using Kortright endstation
Field of 1 T applied at 300 K along (001) direction
Sample field cooled through TN to 20 K
Three types of measurements:(1). Hysteresis loops by switching incident beam polarization at L3 edge of Co and Fe
(2). Reflectivity measurements as function of QZ by switching applied field
Info about both pinned and rotating moments
(3). Measurement of diffuse scattering
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� ��� �� ���
FeF2 grown epitaxially on MgF2, Co is polycrystalline
(001) – Easy axis of FeF2, TN = 78 K
Exhibits positive exchange bias
10-7
10-6
10-5
10-4
10-3
10-2
10-1
100
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4
obs
fitted
X-r
ay re
flect
ivity
Q [Å-1]
42 Å
FeF2
Co
Al
365 ÅMgF2
FeF2σ = 4 Å
σ = 5 Å
σ = 9 Å
� The thicknesses and roughness were constrained to be same for subsequent resonant reflectivity fits
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Polarized Neutron Reflectivity(ASTERIX/LANSCE)
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-0.003 -0.002 -0.001 0.000 0.001 0.002 0.003-0.1
0.0
0.1
0.2
0.3
0.4
0.5 Co Fe
I+ -
I- (a
.u.)
qx (�-1)
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Diffuse Scattering at an Applied Field of 1 T
� Off specular scattering show peaks due to domains.
�Domains in the FM and AF are oppositely aligned
� Domains correlated with structural features (roughness or defect)
-0.004 -0.002 0.000 0.002 0.004-0.2
0.0
0.2
0.4
I+ -
I-
Qx (Å-1)
Co Fe
H = 0.4 T
R. M. Osgood III, S. K. Sinha, J. W. Freeland, Y. U. Idzerda and S. D. Bader, JMMM 198-199, 698 (1999)
-0.2 0.0 0.2 0.4 0.6-0.12
-0.06
0.00
0.06
0.12
I+ -
I-
Applied Field (T)
Co Fe x15
HE = 0.18 T
Hk0
k1
QZ
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-0.004 -0.002 0.000 0.002 0.004
0.0
0.2
0.4
Inte
nsity
QX (�-1)
Co-rot Fe-rot
-0.004 -0.002 0.000 0.002 0.004
0.0
0.2
0.4 Co-pin Fe-pin
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�����������
� Resonant X-Ray scattering combined with polarized neutron reflectivity is a powerful tool to determine in an element sensitive way the depth profileand direction of magnetism in a magnetic thin film structure
� For Co/FeF2 system we found that
� interface coupling is antiferromagnetic
�existence of pinned and rotating moments for Fe
� interface mostly contains rotating moments while the bulkcontains pinned moments (from neutron scattering results)
� exchange bias is due to exchange interaction between Fe pinned and Fe rotating moments
� Diffuse scattering indicates formation of domains
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Acknowledgements
• Sujoy Roy, Michelle Dorn UCSD• Ivan Schuller, O.Petracic, Zhipan Li, Igor
Roshchin UCSD• Jeff Kortright, Karine Chesnel LBL• Mike Fitzsimmons, Sungkyun Park LANL• DOE/BES