1
Exchange Bias in Antiferromagnetic-Ferromagnetic Bilayers
Kannan M. Krishnan
Department of Materials Science and EngineeringUniversity of Washington, Seattle
DoE/BES
Condensed Matter Colloquium, Physics Department, UW (11/01)
Broad Research Themes and Projects
Growth and Properties of Thin Film Structures
Nanoscience and Nanotechnology
Advanced Characterization with Electron and Photon Probes
Deposition by Ion Beam Sputtering, Sol-Gel ProcessingExchange Interactions in AFM/FM bilayersGrowth mode of complex oxidesMagnetically actuated shape memory alloy films
Synthesis by chemical, metallurgical and lithography routes (Group)Achieving theoretical coercivity and role of dilution in Fe-Nd-B alloys Self-Assembled Magnetic Nanocrystals (Mike Beerman , Yuping Bao)Patterned Media to overcome the superparamagnetic limit (J.D. Wright, G. Kusinski)
Measurements of Interface roughness and correlation with GMR (M.E. Gomez)Quantitative measurements of elemental segregation in Co-Cr media (Dr. W. Grogger)Detection/Resolution limits of energy-filtered imaging (Dr. W. Grogger, G. Kusinski)Magnetic imaging with electrons and photons - complementarity (G. Kusinski)
UW, UCB/LBNL
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Magnetic Thin Films and NanostructuresMagnetic Thin Films and Nanostructures
☛ Physical phenomena (magnetoresistance, hysteresis etc.) - fundamental studies☛ Microstructure - unique characterization tools and atomic level control☛ Technology (Magnetic Recording, MRAM, hard magnets, MEMS, NDE ….. )
Vertically integrated ProgramVertically integrated Program
• UHV Ion-beam deposition • Chemical synthesis• Pulsed Laser Deposition (IML)
Synthesis & Processing Properties & Phenomena
Characterization NCEM - Electron microscopy
Mg Fe Mn
- 2
-1.5
- 1
-0.5
0
0.5
1
-4
-3
-2
-1
0
1
2
690 700 710 720 730
MC
D
MC
D Integration
Energy (eV)
MCDMCD Integr.
b0
2
4
6
690 700 710 720 730
Ab
sorp
tion
Energy (eV)
+3T/LC-3T/LC
a
ALS - X-ray magnetic circular dichroism
EELS
Mn-L Sr-M
O-K
• Magnetic• Transport
0
0.05
0.1
0.15
0.2
0.25
100 150 200 250 300ρρρρ
(((( ΩΩΩΩ
))))
T (K)
cm
H = 0
H = 1T
-0.0015
0
0.0015
-150 0 150
M (
emu)
H (Oe)
Spin Valve - Hysteresis Loop
Systems of Reduced Dimensions3D 2D 1D 0D
DoS
EnergyEF
d
DoS
EnergyEF
f(d)
For 2D behaviorOnly ONE level within∆∆∆∆E = + kT (0.026 eV) of EF
1st estimate, set energyof lowest level ~ kT ~ p2/2m
de Broglie wavelengthλλλλ ~ h/p ~ 8 nm (slabthickness)
Ultrathin filmsMultilayersInterfaces
ChemicalSynthesis
+Self
Assembly
And
Lithography
3
Growth & Novel Properties of 2D Structures
Ultrathin FilmsMultilayersInterfaces
UHV Ion-beam DepositionAFM FM
Frustration & Proximity Effects
M/M
s
Field (Oe)
-1.2
0
1.2
-400 0 400
(A) 200 °C(B) 250 °C(C) 300 °C
He
Exchange Bias
Cheng, Ahn and Krishnan, J. Appl. Phys., 89, 6597 (2001).
Perpendicular Interface Anisotropy
Ean = Keff Sin2θ
Keff = -2πMs2 + 2Ks/tm + KvCho, Krishnan, Lucas and Farrow, J. Appl. Phys. , 72, 5799 (1992).
Giant Magnetoresistance
H
R
∆R/R ~ 20-80%
Cyrille, Kim, Gomez, Santamaria, Leighton, Krishnan and Schuller, Phys. Rev. B. 62, 15079-15083 (2000)
H
200 nm
50 nm
Nanoscale materialsby Self-assembly
Chemical method --Size-selective precipitationSurfactant controls shapeBottoms-up approachMagnetism in systems of reduced dimensions
Puntes, Krishnan & Alivisatos,Science, 291, 2115 (2001)
Small Particles
Metallurgical ApproachAchieving theoretical coercivity limits & understanding mechanisms
Girt, Krishnan, Thomas and Girt, J. Mag. Mag. Mat., 231, 219 (2001)
4 µm
“Patterned” Media
Ion-implantation throughStencil Masks; Top-down approachOvercoming the super -paramagnetic limit
Kusinski, Krishnan, Denbeaux, Thomas, Weller, Rettner. Terris Appl. Phys. Lett. 79, 2211(2001)
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Eij = -2 Jij Si Sj = Jij |S|2 θθθθ 2 for small θθθθ
• No fundamental limit on strength• Isotropic, Amorphous magnets• Determines magnetic orderJij =
3 kB Θc2 Z S(S+1)
θ
Jij
Mn
Fe
Co
Nirabrd
Bethe- Slater Curve
Factors affecting magnetic behaviour: Exchange Interactions
The spin-orbit-lattice interaction
sin2 sin4 Emcuniaxial = Ko + Ku1 θ + Ku2 θ + ..Cobalt Ku1 = 4.1x10
5 J/m3 Ku2 = 1.0x105 J/m3
Factors affecting magnetic behaviour: Anisotropy
Determines Magnetic Orientation
5
Magnetic Behavior of Ferromagnets & Antiferromagnets
IronΘc ~ 790°C (1063 K)Strong magnetic Order
Weak Orientation
Superparamagnetic limit
KV
~25kBT
Rc ~ kBT
K
13
AFM + FM heterostructures
Exchange Bias : Unique Behavior not Observed in Bulk FM
ΘN
χ
T
Weak Magnetic OrderStrong Orientation
Antiferromagnet
Angle (Theta °)
He(B)= -111Oe [cos(ϑ )-0.15cos(3ϑ )-0.01cos(5ϑ )+0.01cos(7ϑ )+...]Hc(B)= 44Oe [1+0.82cos(2ϑ )+0.24cos(4ϑ )+0.05cos(6ϑ )+...]
He(A)= -26Oe [cos(ϑ )-0.34cos(3ϑ )+0.13cos(5ϑ)-0.07cos(7ϑ )+...]Hc(A)= 80Oe [1+0.46cos(2ϑ )-0.04cos(4ϑ )+0.01cos(6ϑ)+...]
-100
-50
0
50
100
0 100 200 300
H (
Oe)
He(B)
Hc(B)
Hc(Fe)
Hθ
M/M
s
Field (Oe)
-1.2
0
1.2
-400 0 400
(A) 200 °C(B) 250 °C(C) 300 °C
Unidirectional CouplingUnidirectional CouplingSample
HysteresisHysteresis
Exchange Bias in AFM/FM Bilayers
6
Frustration , Interface and Proximity Effects
• Phenomenon poorly understood• Interface effects dominate• Need element-specific probes• Growth of ideal structures - epitaxy• Technology
H
R0 RH
∆∆∆∆R/R ~ 20 %
R
H
“Spin”Valves
AFMFM
20nm 20nm
MnFe
Free Energy:
AFM
FMJint
θ
F = H MFM tFM Cos θ - Jint Cos θ + KFM Sin2θ
Solution in terms of an effective field
H’ = H - Jint / (MFM tFM ) = H - HE
HE = Jint / (MFM tFM )
If typical values for JFM are usedthen observed HE are too small
Exchange Bias: Basics
Easy Direction
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Uncompensated
Compensated
Exchange Bias: Basics ….
Mauri (1987) Planar Domain Wall
Malozemoff (1987, 1988) Random Field Model
AFM Domain
Koon (1997) Spin Flop Coupling Model
Frustration Spin-Flop
a-axis normal
3.58Å
4.07Å
4.07Å
Spin Uncompensated
3.58Å
4.07Å
4.07Å
c-axis normalSpin Compensated
Pal et al, Jour. Appl. Phys. 39, 538. 1968
Crystal and Spin Structure of AFM Mn50Pd50
8
Ion-beam Deposition
Vacuum System
ThicknessMonitor
Substrate Heater (1000°C)
KaufmanIon Gun
Multiple Target Holder
Ar
Cathode
Anode
GlowDischarge
Magnets
2.5 cm diameter 1-2 keV20-50 mAmps0.5 Å/sec sputtering
UHV Reactive Ion-beam Deposition SystemUHV Reactive Ion-beam Deposition System
ComputerCotrolled Process &Diagnostics
Portable Platform
Accurate control of gas flowrates (O2, N2, Ar)
Load-lock Gate Valve
Sample-transferChamber
Transfer Rod
Kauffman-type ionsource
Target(4)rotation
Targetblock
Shutter
Substrate heater feed through (1000°C)Thermocouple
Thickness Monitor
Residual gas analyser
Versatile, Portable (ALS)
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Epitaxial growth and exchange biasing of FM/AFM bilayers
Normal Structure
PdMn (~280Å)
Fe (~60Å)MgO(001)
10 20 30 40 50 60 70 80
inte
nsity
(a.u
.)1
theta
a-growth (002)MgO
(200)MnPd
(004)Fe-0.8
0
0.8
-500 0 500
mom
ent(
mem
u)
H(Oe)
He= 9 Oe
theta
inte
nsity
(a.u
.)
10 20 30 40 50 60 70 80
(001)MnPd
(002)MnPd
(002)MgO
(004)Fe
c-growth
-0.5
0
0.5
-500 0 500
mom
ent(
mem
u)
H(Oe)
He = 30 Oe
a-axis normal
3.58Å4.07Å
4.07Å
Spin Uncompensated
3.58Å
4.07Å
4.07Å
c-axis normal
Spin Compensated
X-ray TEM
GIXRD
Plan View
X-sectionθθθθ−−−−2222θθθθ
Scattering Geometries:
Complementary information for thin films
Electron Diffraction in a TEM
10
SAD[100]a-axis growthannealed
Chemical ordering of a-axis grown PdMn by annealing
a -ordereda -disordered
SAD[100]a-axis growth as-grown
10 20 30 40 50 60 70 80
inte
nsity
(a.u
.)
2theta (degree)
(002)MgO(200)
Fe( 002)
He ~ 0 He ~ 8-10 Oe
Cheng, Ahn and Krishnan, JAP (2001)
PdMn/Fe/MgO : Summary of growth & magnetic properties
-10
0
10
20
30
40
0 100 200 300 400 500
He
(Oe)
Growth Temperature (°C)
Annealed
As-grown
a-axis growth c-axis growth
Disordered
Ordered
11
Epitaxial growth and exchange biasing of FM/AFM bilayers
-1
0
1
-500 0 500
Mom
ent(
mem
u)
H(Oe)
He ~ 010 20 30 40 50 60 70 80
inte
nsity
(a.u
.)
theta
(002)MgO
(200)MnPd
(004)Fe3.58Å4.07Å
4.07Å
a-axis normal
-1
0
1
-500 0 500
Mom
ent (
mem
u)
H(Oe)
He = 72 Oe10 20 30 40 50 60 70 80
inte
nsity
(a.u
.)
theta
(002)MgO
(002)MnPd
(001)MnPd
(004)Fe
3.58Å
4.07Å
4.07Å
c-axis normal
Inverse Structure
PdMn (~280Å)
Fe (~60Å)
MgO(001)
Phillips 200/FEG with Image Filter
d band
2p3/22p1/2
1s
e-
Diplole Selection Rule ∆l = + 1 apply
High Resolution EELS Studies in a TEMElement-specific spectroscopy and imaging at high spatial resolution
550 600 650 700 750 800
100K
200K
300K
400K
500K
600K
700K
Energy Loss (eV)
12 nm
Cr L 3,2 Fe L 3,2
Element-resolvedlocal electronic structure
Spatially-resolved measurements
12
Energy-filtered Imaging in a TEM: Detection Limits
Phillips 200/FEG with Image Filter
d band
2p3/22p1/2
1s
e-
Diplole Selection Rule ∆l = + 1 apply
Spatially-resolved measurements
Si
PdMn
Mn
PdMn
PdMn
PdMn
PdMn
PdMn
PdMn
Mn
Mn
Mn
Mn
Mn
Mn
10 nm
arb.
uni
tsnm
0
1
2
3
4
5
6
arb.
uni
ts0 1 0 2 0 3 0 4 0 5 0
nm
0.4 0.9 1.3 1.7 2.0 2.4 nm1.1
4 Å
8 Å
12 Å
16 Å
20 Å
24 Å
c)
20 nm
-1
0
1
- σσσσ
+ σσσσ
Simple Roughness Measurements Using Energy-filtered Images
MgO
NiMnTEM Bright Field Image
Fe-L2,3 jump ratio image
Trace or Mean Position
13
Fe Layer Roughness in Fe/MnPd bilayers
2/12
1
2 ))1
1()( ( >
14
x0
ξξξξ////////
xL
σσσσL(x0)z(x)
x
h(x)
h
Interface Profile h(x)Average Value or hHeight Deviation z(x) = h(x) - h
Interface width ~ rms roughness σ(L) ∼ < σ L(x0) >x where the averaging is done over all points x within L and
σL(x0) = < |z(x) - zav(L)|2>L1/2 zav(L) is z(x) averaged over L
Correlation length, ξξξξ || is extractedby fitting
σσσσ((((L)))) ∼∼∼∼ σσσσsat [ 1- exp (-L/ξξξξ ||)]
Definitions: Rough interface with various characteristic length scales
Xray Magnetic Circular Dichroism
d band
2p3/22p1/2
1s
EE
( or )
L3
∆l = + 1XAS & EELS
L2
XMCD∆l = + 1
∆mj = + 1
left
right
spinorbital Sum Rules
- 2
-1.5
- 1
-0.5
0
0.5
1
-4
-3
-2
-1
0
1
2
690 700 710 720 730
MC
D
MC
D Integration
Energy (eV)
MCDMCD Integr.
b
0
2
4
6
Ab
sorp
tion
+3T/LC-3T/LC
a
morb / mspin = 0.03
morbmspin
= 2q
(9 p - 6 q)
Element Specific, Spin/Orbital Resolved Magnetic Measurements
15
circularlypolarizedx-ray photon
silicon window
silicon nitridemembrane
NiMnFe(Co)
Au cap
Magnetic structure of AFM-FM bilayer interfaces
690 710 730 750
Absorption
680 700 720 740 760
Dichroism
Cheng, Krishnan, Farrow, Young, Girt , .JAP (in press)
M/M
s
Field (Oe)
-1.2
0
1.2
-400 0 400
(A) 200 °C(B) 250 °C(C) 300 °C
Fe L3,2
660 680 700 720 740 760 780
C
B
A
He
He ( B > C > A ) MFe (A > C > B)
• Iron Moment Correlates inversely with Exchange field• The films show (111) texture• (111) is spin compensated• Iron diffuses into MnNi and orders antiferromagnetically• The decrease in iron moment suggests ~ 4MLs of diffusion• Fe-Mn is a well known disordered AFM• Need to investigate chemical and structural roughness
MicrospectroscopyMicrospectroscopy & & SpectromicroscopySpectromicroscopy with Photons (ALS) with Photons (ALS)
PhotoEmission Electron Microscope
16
3, RT 18, 170 C 27, 220 C
39, 280 C 48, 3.9, 300 C 63, BKRT
PEEM : Temperature Dependence
HB
Cheng, Ohldag, Krishnan (2001)
Interface is key to understanding such anisotropy.
K = - 2 ππππ Ms+ + K 2 Kt
s
MV
2
effE = K sin ΘΘΘΘan eff
2
Perpendicular Interface Anisotropy
GaAs (001)
Ag
[110] Pt, Ag
GaAs (001)
Ag
[001] Pt, Ag
GaAs (111)
Ag
[111] Pt, Ag
Co
Kef
f tm
CoxPd18
FexPd18
x(Å)4 8
Recent discoveries in artificially structured magnetic films
17
Exchange Bias in Systems with Out-of-Plane Coupling ?
3.58Å
4.07Å
4.07Å
MnPd
FePdFePd
Malozemoff (1987, 1988)
Random Field Model
AFM Domain
MgOFe
MnPd
Lithographically Patterned Antiferroamgnetic Elements ?
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Acknowledgements
Ning Cheng - Growth & Magnetic measurementsDr. J.-P. Ahn - Ion Beam DepositionDr. Werner Grogger - EFTEM Imaging & AnalysisChris Nelson - Philips CM200 OperationDr. H. Ohldag - PEEM Beamline ScientistDr. Tony Young - XMCD measurements
DoE/BESMax Kade Foundation