EXCHANGE BIAS A comprehensive study Ivan K....
Transcript of EXCHANGE BIAS A comprehensive study Ivan K....
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EXCHANGE BIASA comprehensive study
Ivan K. SchullerIvan K. Schuller
Funded by DOE and AFOSRFunded by DOE and AFOSR
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COLLABORATORSUCSD:R. Morales
Z-P LiI.V. RoshchinO. Petrasic
X. BatlleJ. Eisenmenger
C. LeightonK. Liu
S. K. SinhaS. Roy
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CollaboratorsLANL:
M FitzsimmonsJ.R. Groves
P. ArendtR. Springer
A. HoffmannP.C. Yashar
LLNLJ. KortrightK. Chesnel
U. MinesottaI. Krivotorov
E. D. DahlbergNIST:
C.F. MajkrzakJ.A. Dura
HMI:H. Fritzsche
ILL:Vincent LeinerHans Lauter
U. A. Barcelona:J. Nogues
RWTHA. TillmannsB. BeschotenG. Guntherodt
LBLE. Arenholz
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PLANPLAN• Introduction: Exchange Bias ?• Expectations: Deceptively Simple• Real Life: More Complicated• Key Ingredients: Obvious• Solution: Hard Work• Summary: Understanding• Open Questions
Physical = Structural Length Scale
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Interesting PhenomenaInteresting PhenomenaLength Scales
•Magnetic 1-100 nm
•Superconducting 1-1000 nm
•Elastic 1-10 nm
•Catalysis 1-10 nm
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EVERYTHING YOU WANTED TO KNOW ABOUT EVERYTHING YOU WANTED TO KNOW ABOUT FERROMAGNETISMFERROMAGNETISM………….but .but ……were afraid to askwere afraid to ask
FM
Free FMSmall coercivity HCExchange field HE=0SymmetricUniform
H
M
HC
Reversal.exe
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VERY INTERESTING
50 100 150
-1.0
-0.5
0.0
0.5
1.0
M /
MS
Temperature (K)TN
AF
FM
H HFC=1kOe
Proximity BetweenProximity BetweenDissimilarDissimilar MagnetsMagnets
Z-P Li et al, Phys. Rev. Lett. 96, 137201(2006)
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Exchange BiasExchange Bias
FM
Free FMExchange field HE=0Small coercivity HCSymmetricUniform
FMAF
Pinned FMLarge HELarge HC
HE
2HC
H
M
H
M
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Reversal Mode is the Same in Both Sides of the Hysteresis
Loop
In all materials systems known to man !!
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Why Interesting?Why Interesting?Spin ValvesSpin Valves
Read HeadRead Head Magnetic RAMMagnetic RAM
Magnetic TunnelingMagnetic TunnelingExchange BiasExchange Bias Important DevicesImportant Devices
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Mechanism
HFC
FM
AFM
M
HA
T < T < TCN
W.H. Meiklejohn, C.P. Bean, Phys. Rev., 105, 904(1957).
Cool T<TN
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What to expect
• Maximum Uncompensated Surfaces• Zero for Compensated Surface• Negative• Reversal Symmetric
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Field Cooling(110)MgF2/FeF2(500Å)/Co(38Å)/Al(25Å)
-4000 -3000 -2000 -1000 0 1000 2000 3000 4000
-1.0
-0.5
0.0
0.5
1.0
T=10K
HFC=0.1 kOe
Mag
netiz
atio
n (1
0−4 e
mu)
Magnetic Field (Oe)-4000 -3000 -2000 -1000 0 1000 2000 3000 4000
-1.0
-0.5
0.0
0.5
1.0
T=10K
HFC=0.1 kOe
HFC=30 kOe
Mag
netiz
atio
n (1
0−4 e
mu)
Magnetic Field (Oe)-4000 -3000 -2000 -1000 0 1000 2000 3000 4000
-1.0
-0.5
0.0
0.5
1.0
T=10K
HFC=0.1 kOe
HFC=2 kOe
HFC=30 kOe
Mag
netiz
atio
n (1
0−4 e
mu)
Magnetic Field (Oe)J. Nogues et al, PRL 76, 4624 (1996), I. V. Roshchin et al, EPL 71,297 (2005)
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Polarized Neutron Reflectometry
Spin-flip
cross-sections
• M⊥ H
• vs Q (vs z)
M. Fitzsimmons et alPhys. Rewv. Lett. 84, 3986 (2000)
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0
0.2
0.4
0.6
0.8
1
0.05 0.1 0.15 0.2 0.25
++--SF
Ref
lect
ivity
0
0.2
0.4
0.6
0.8
1
0.05 0.1 0.15 0.2 0.25
++--SF
Ref
lect
ivity
Q [nm-1- ]
-1
-0.5
0
0.5
1
-600 -400 -200 0 200 400 600
M/M
S
H[Oe]
Domain nucleation and wall motion.
Asymmetric reversal
64±4% of M rotated 90°
Q [nm-1- ]
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Moke longitudinal and transverse
s -polH ext p -pol
M LMT
B. Beschoten, G. Guntherodt et al, unpublished
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Kerr EffectHext
ML
s-pol.
p-pol.-0.4 -0.3 -0.2 -0.1 0.0 0.1 0.2
50°-pol
p-pol
s-pol
Ker
r rot
atio
n (a
.u.)
field (a.u.)
Longitudinal
Hext
s-pol.
p-pol.
MT
Transverse
-0.4 -0.3 -0.2 -0.1 0.0 0.1 0.2
45°-pol
s-pol
p-pol
Ker
r rot
atio
n (a
.u.)
field (a.u.)
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Always the SameCoherentRotation
Domains
Asymmetric Magnetization Reversal
-1
-0.5
0
0.5
1
-1000 -500 0 500 1000
M/M
S
H [Oe]
? ? ?
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What to expect
• Maximum Uncompensated Surfaces• Zero for Compensated Surface• Negative• Reversal Symmetric
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Rich Phenomenology
• Spin Orientation: Uncompensated,Compensated• Sign: Negative or Positive• Asymmetry• Domains: F and/or AF• Training
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SYSTEMATIC STUDIES ESSENTIAL
• Quantitative (Structure, )• Systematic (One parameter at a time• Different measurements on same samples• Reevaluate (Even well accepted “facts”)
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Exhaustive System
•XF2 – Ferro X=Fe, Mn, Zn, Co, NiFerro=Fe, Co, Ni, Py
• GrowthHV e-beam evaporation
• Characterizationin-situex-situ
Polycrystaline, twinned, epitaxial
Thank you David Lederman
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ADVANTAGES
Easy growthControlled Crystallography
poly, twinned, single(110), (100), (211)
Different Anisotropies Non Magnetic Control (ZnF2)Low TN (~ 70K)
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PREPARATION
Polished(001) MgO
single crystal
200-300°C
Ag Fe FeF2
MgO
XF
Fe
Ag
2
10nm
90nm
MBEMBE
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Structure-Complicated
MgOFeFFeAl
2
glass
Untwinned TwinnedIn-plane
polycrystalline
FeF2
FeAl
ZnF2
bulk FeF2
MgOFeFFeAg
2
110 FeF110 FeF2 2
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Characterization
• RHEED Electron diffraction epitaxy• HA XRD High angle X-ray growth direction• GIXR X-ray reflectivity interfaces• AFM Scanning probe surface• GID In-plane X-ray in plane epitaxy
Bottom Line:
Poly, twinned, epitaxial fluoridesPolycrystalline Fe
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F Reversal Uniform?
AF
FM
R. Morales et al - In press
Confirmed by Mossbauer, Synchrotron, Neutrons
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Magnetic CharacterizationSQUID MOKE
(Magneto-optical Kerr effect)Depth dependence
(tprobed~ 30 nm)
FM
AF
Whole sample
70 nm30 nmFM
AF
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Magnetic CharacterizationSQUID MOKE
(Magneto-optical Kerr effect)Depth dependence
(tprobed~ 30 nm)
FM
AF
Whole sample
70 nm30 nmFM
AF
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-2000 -1000 0 1000-1.0
-0.5
0.0
0.5
1.0 SQUID MOKE
Py-side FeF2-side M
/ M
s
H (Oe)-200 -100 0 100 200
-1.0
-0.5
0.0
0.5
1.0
M /
Ms
H (Oe)
SQUID and MOKE
• Py/FeF2 bilayerT (150K) > TN (78K) T (10K) < TN
-2000 -1000 0 1000-1.0
-0.5
0.0
0.5
1.0 SQUID MOKE
Py-side FeF2-side M
/ M
s
H (Oe)
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-2000 -1000 0 1000-1.0
-0.5
0.0
0.5
1.0 MOKE loops Py-side FeF2-side
M /
Ms
H (Oe)
Reversal-Py/FeF2
• Incoherent rotation of a spring wall in the whole sample and parallel to the interface.
Py
FeF2
FMAF
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-2000 -1000 0 1000-1.0
-0.5
0.0
0.5
1.0 MOKE loops Py-side FeF2-side M /
Ms
H (Oe)
Reversal- Py/FeF2
• Incoherent rotation of a spring wall in the whole sample and parallel to the interface.
Py
FeF2
FMAF
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AntiFerromagnetAntiFerromagnetSURFACE OR BULK
Py
FeF2
Ni
Confirmed by Mossbauer, Synchrotron, Neutrons
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200 nm
20 40 60 80 100
T (K)
FM/AF/FMParallel
PyFeF2
Ni
PyFeF2
Ni
Anti-parallel
20 40 60 80 100
0
100
200
300
400
HE
B (O
e)
T (K)
Ni
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• Coupled across the interface
• Uncompensated spins at interface
• Uncompensated spins in the bulk
• Grains, inhomogeneity, roughness
• Anisotropy
3-LAYER MODEL
FM
Interface
AF
FM
AF
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KEY INGREDIENTS• Uncompensated Spins (AF)(Vertical Shift, Dichroism, Bulk AF, dAF
• Coupling Across Interfaces(+ & - HE, AF & F Domains, Mossbauer,• Anisotropy (F, AF): Uni, Bi, Four(Crystallography, Angular Dependence,• Inhomogeneity(Crystallography, Dichroism, • 3 Layers: F, Interface, AF(Neutrons, Dichroism,
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OPEN
• Training(4 fold Anisotropy, • Origin of the Uncompensated Spins(Structure, Dichroism, Universal ?????• Magnitude (Fe2O3,• Nanostructures(Competition of Length Scales,
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REFERENCESREFERENCES• “Exchange Bias”J. Nogues and IKS (JMMM 192, 203(1999))
• “The Exchange Bias Manifesto-2002”I.K.Schuller, G.G.Guntherodt,http://ischuller.ucsd.edu
• “Asymmetric reversal”B. Beschoten, G. Guntherodt et al (in press)
• “Length Scales and Nanostructures”I. Roshchin, et al (EPL 71, 297(2005))
• “Depth Profile....”S. Roy, et al (PRL 95,047201(2005)
• “Spontaneous Reversal….”ZhiPan Li et al (PRL 96, 137201(2006))