Exchange Bias: Interface vs. Bulk Magnetism
Miyeon Cheon Hongtao Shi
Zhongyuan Liu Jorge EspinosaDavid Lederman
Elke Arenholz
Department of Physics
Hendrik OhldagJoachim Stöhr
Optical and Vibrational Spectroscopies Symposium:A Tribute to Manuel Cardona
August 20, 2010
-1.0 -0.5 0.0 0.5 1.0-6
-4
-2
0
2
4
6
m (
10-4em
u)
H (kOe)
HC
MR
Exchange Bias
FM
AF
MR: “Remanent” magnetization- Maximum value of M- Depends on FM
HC: Coercivity- Depends on FM magnetic anisotropy- Represents energy required to reverse magnetic domain
HE: Exchange Bias-Absent in pure FM, results from AF-FM interaction
HE
Application: Magnetic Tunnel Junction /GMR
SensorsFerromagnetic layers~1.0-5.0 nm thick
Insulator/NM Metal ~1.0-2.5 nm
Antiferromagnet~10 - 50 nm
1 - 100 m
cosRRR o
Free magnetic layer (analyzes electron spin)
Pinned magnetic layer (polarizes tunneling electrons)
Pinning Antiferromagnet
Albert Fert & Peter Grünberg2007 Nobel Prize in Physics
“for the discovery of Giant Magnetoresistance”
Key Questions• Given that:
– All EB models require presence of uncompensated magnetization in the antiferromagnet (interface)
– Details of EB behavior (e.g. temperature dependence, magnitude) depend strongly on AF anisotropy (bulk)
• Some key questions are:– Can uncompensated moments in the AF be detected?
– Can the effects of uncompensated moments in the AF be studied systematically?
– Can the magnetic anisotropy be studied systematically?
MF2 Antiferromagnets
FeF2• Rutile structure (a = 0.4704 nm, c = 0.3306 nm) • Antiferromagnetic, TN=78 K• Magnetization along the c-axis
[001]
NiF2 • Rutile structure (a = 0.4651 nm, c = 0.3084 nm)• Antiferromagnetic, TN= 73 K• Weak ferromagnetic • Magnetization lies in the a-b plane
[001]
ZnF2• Rutile structure (a = 0.4711 nm, c = 0.3132 nm) • non-magnetic
weak anisotropy antiferromagnet
[001]
dilute antiferromagnet
So… where does Manuel Cardona fit in?
Naïve graduate student asks: can antiferromagnetic superlattice magnons be observed?
Two-magnon Raman line for 1.3 m FeF2 thin film
• MBE co-deposition of FeF2 (e-beam) and ZnF2, NiF2 (K-cell), Pbase = 7 x 10-10 Torr, Pgrowth < 4 x 10-8 Torr
• TS (AF) = 297 0C, poly-Co @125 0C, poly-MgF2 @RT
• Growth along (110)• Twin sample holder – simultaneous growth of
underlayer, different overlayers• In-situ RHEED, AFM• X-ray diffraction and reflectivity
• Cooling field (HCF = 2 kOe) in the film
plane along the c-axis of FexZn1-xF2
• M vs H via SQUID magnetometer,
horizontal sample rotator
Growth and Characterization
Key Questions
• Can uncompensated moments in the AF be detected?
• Can the effects of uncompensated moments in the AF be studied systematically?
• Can the magnetic anisotropy be studied systematically?
e-e-
Magnetic Dichroism in X-ray Absorption
700 710 720 730
0
2
4
6
Ele
ctro
n Y
ield
(a.
u.)
Photon Energy (eV)
e-e-
Antiferromagnetic Domains
876 879 882
100
200
300
Ele
ctr
on
Yie
ld (
a.u
.)
Photon Energy (eV)
X-ray magnetic circular dichroism
sensitive to FM order.
Fe L3, L2
NiO L2a, L2b
X-ray magnetic linear dichroism
sensitive to AF order.
Element specific technique sensitive to antiferromagnetic as well as ferromagnetic order.
Antiferromagnetic Order of FeF2(110)
Stronger XMLD signal for Co/FeF2(110) compared to bare FeF2(110) indicates an increase in antiferromagnetic order caused by exchange to the FM Co layer.
718 721 724
0
1
2
3Co/FeF
2(110)
E || [001] E || [110]
bare FeF2(110)
E || [001] E || [110]
Ele
ctro
n Y
ield
[a.u
.]
Photon Energy [eV]
Fe Fe
Fe FeFe
F F
F F
Einc || [001]
Einc || [110]
FeF2 L2 absorption edge
Room T: “Free” uncompensated moments follow FM
Low T: Additional “pinned” uncompensated moments antiparallel to easy direction.
Measy
Mpinned
Ferromagnet
Interface-3 -2 -1 0 1 2 3
-10
0
10
Applied Field [kOe]
Co
XM
CD
[%]
-0.5
0.0
0.5
Fe
XM
CD
[%]
-10
0
10
-0.1
0.0
0.1
RT
15K
Interface Coupling and Exchange Bias
MgF2(110) sub.
68 nm FeF2
2.5 nm Co
2 nm Pd cap
Results
0 40 80 120 300
0
200
400
Co XMCD HC
Co XMCD HE
Fie
ld [
Oe]
Temperature [K]
0.0
0.5
1.0
Fe XMCD Fe M XMCD
XM
CD
[%
]
0.0
0.5
1.0 Fe XMLD
XM
LD [
arb.
u.]
• Fe in FeF2/Co interface, despite being non-metallic, has– Unpinned magnetization to RT
– Pinned magnetization to TB
– AF order verified to TN via XMLD
• Co at interface– TB~TN
– HC peak near TB
Ohldag et al., PRL 96, 027203 (2006)
0 20 40 60 80 100 120 300
0.0
0.5
1.0
Temperature [K]
XM
CD
[%
]
0.0
0.5
1.0
XM
LD [
a.u.
]
1.) XMLD and long range AF order vanish at TN.
2.) XMCD is indication of
interfacial magnetic order at RT.
Parallel Interface Coupling and Exchange Bias
Related to enhancement of coercivity for T >> TN
(Grimsditch et al, PRL 2003)
Also, see Roy et al, PRL 2006
Key Questions
• Can uncompensated moments in the AF be detected?– Uncompensated moments exist in AF, not due to “metallization”– Pinned uncompensated moments in AF vanish near TN
– Unpinned uncompensated moments exist up to RT, well above TN
• Can the effects of uncompensated moments in the AF be studied systematically?
• Can the magnetic anisotropy be studied systematically?
Systems
[001][001]
FexNi1-xF2FexZn1-xF2
Dilute antiferromagnet Random anisotropy antiferromagnet
Systematic study of uncompensated M
Effects of Dilution
• Domain state model: dilute AF should make small domain creation easier due to nonmagnetic impurities (Malozemoff model)
• Net magnetization of AF domains should increase effective interface interaction
P. Miltényi, et al., Phys. Rev. Lett., 84, 4224 (2000)
Co1-xMgxO/ CoO (0.4 nm) /Co
Previous Results
Sample Profile
5 nm MgF2 Cap
(110)-MgF2 Sub
5 nm MgF2 Cap
(110)-MgF2 Sub
1.0 nm FeF2
Magnetic interface changes with x in FexZn1-xF2
65 nm (110)FexZn1-xF2 (AF) 65 nm (110)
FexZn1-xF2 (AF)
18 nm Cobalt (F) 18 nm Cobalt (F)
Pure interfacelayer (PIL)
HE, HC Dependence on T
PIL affects HE, HC; no effect on TB
-400
-300
-200
-100
0
0 10 20 30 40
0
100
200
300
400
T (K)
HC (
Oe)
x = 0.34H
CF = 2 kOe
TB
With PIL Without PIL
HE (
Oe)
-500
-400
-300
-200
-100
0
0 10 20 30 40 50 60 70 80 900
100
200
300
T (K)
HC (
Oe)
x = 0.57H
CF = 2 kOe
TB
Without PIL With PIL
HE (
Oe)
HE, HC vs. Temperature for x = 0.75
• HE changes sign as T increases to TB.• HC has two peaks corresponding to HE = 0.• Therefore AF ground state is not unique
Interface Energy Dependence on x
• No large HE enhancement observed• Small AF domains not formed at large x ?
ΔE = -tCo*HE*MS
T = 5K
Key Questions
• Can uncompensated moments in the AF be detected?– Uncompensated moments exist in AF, not due to “metallization”– Pinned uncompensated moments in AF vanish near TN
– Unpinned uncompensated moments exist up to RT, well above TN
• Can the effects of uncompensated moments in the AF be studied systematically?– Uncompensated M does not necessarily lead to HE enhancement;
critical concentration of impurities must be achieved– However, uncompensated M dependent on defect concentration
• Can the magnetic anisotropy be studied systematically?
Systems
[001][001]
FexNi1-xF2FexZn1-xF2
Dilute antiferromagnet Random anisotropy antiferromagnet
Systematic study of AF anisotropy
Magnetic Order
FeF2 Rutile structure (a = 0.4704 nm, c = 0.3306 nm) Antiferromagnet, TN=78 K Magnetization along the c-axis
NiF2 Rutile structure (a = 0.4651 nm, c = 0.3084 nm) Antiferromagnetic, TN= 73 K (80 K in films) Weak ferromagnet Magnetization lies in the a-b plane
[001]
[001]
Growth and measurements
magnetic anisotropy changes with x.
MgF2(110) sub.
50 nm FexNi(1-x)F2
18 nm Co5 nm Al,Pd cap
MBE Growth MgF2 (110) substrate Growth temperature 210 ˚C Fe concentration: 0.0, 0.05, 0.21, 0.49, 0.55 1.0
[001]
x=1.0
[001]
x=0.0
Expectations
For nearest neighbor interactions
cos)1(cos)1(cos 2222FeNiFeNiNiNiNiFeFeFe SSxzxJSxzJSzxJE
)(cos)1(cos 2222 NiNiFeFe SxDxSD
For small , there is a critical Fe concentration xc beyond which spins will lie along the c-axis:
22
2
FeFeNiNi
NiNic SDSD
SDx
For FeF2 and NiF2 xc = 0.14
[001]
FexNi1-xF2
NiF2/CoFeF2/Co
• No exchange bias along c-axis • Exchange bias along c-axis • TB ~ 81 K
0 30 60 90 120 150
-400
-300
-200
-100
0
HE (
Oe
)
T (K)
-2 -1 0 1 2-3
-2
-1
0
1
2
3
m (
10-4
em
u)
H (kOe)
5 K
H┴ c
H || c
-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6-1.0
-0.5
0.0
0.5
1.0
T = 5 K T = 90 K
HCF
= 2 kOe _H, H
CF || NiF
2 [110]
MR/M
S
H (kOe)
49 nm NiF2 / 16 nm Co
H. Shi et al., Phys. Rev. B 69, 214416 (2004).
Fe0.05Ni0.95F2/Co
For T ≤ 45 K• Negative exchange bias along the c-axis• Asymmetric saturation magnetization
For 50 K ≤ T ≤ 70 K• No exchange bias• Wide hysteresis loop
For 75 K ≤ T• No exchange bias
-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5-6
-4
-2
0
2
4
6 5 K 20 K 35 K
m(1
0-4em
u)
H(kOe)-10 -8 -6 -4 -2 0 2 4 6 8 10
-6
-4
-2
0
2
4
6 50 K 80 K
m(1
0-4em
u)H(kOe)
-10 -5 0 5 10-6
-4
-2
0
2
4
6
m(1
0-4
emu)
H (kOe)
50 K 55 K 60 K 65 K 70 K
Large coercivity loops of Fe0.05Ni0.95F2/Co
50 55 60 65 70
-10
-5
0
5
10
H(k
Oe
)
T(K)
-H' +H'
• For 50 K ≤ T ≤ 70 K, large coercivity loops appear for the scanning field range -10 kOe to 10 kOe.
• Negative exchange bias (HE ~ -500 Oe) for T = 50 K and 55 K
Fe0.21Ni0.79F2/Co
• Similar behavior to Fe0.05Ni0.95F2/Co• Negative HE along the c-axis at T≤ 40 K• Asymmetric saturation magnetization
-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5
-6
-4
-2
0
2
4
6 5 K 20 K 35 K
m(1
0-4em
u)
H(kOe)For 45 K ≤ T ≤ 70 K
• No exchange bias effect• Wide hysteresis loop
For 75 K ≤ T• HE = 0
-10 -8 -6 -4 -2 0 2 4 6 8 10
-6
-4
-2
0
2
4
6 50 K 75 K
m(1
0-4em
u)H(kOe)
Large HC loops of Fe0.21Ni0.49F2/Co
-10 -5 0 5 10
-6
-4
-2
0
2
4
6 40 K 50 K 60 K 70 K
m(1
0-4e
mu
)
H(kOe)
40 45 50 55 60 65 70 75-10
-8
-6
-4
-2
0
2
4
6
8
10
H (
kOe)
T (K)
-H' +H' H
E
• For 40 K ≤ T ≤ 70 K, large HC loops appear for the scanning field range ±10 kOe • Negative exchange bias effect (HE ~ - 1000 Oe) for 40 K ≤ T ≤55 K
Fe0.49Ni0.51F2/Co
For T ≤ 15 K• Negative exchange bias • Asymmetric saturation magnetization
For 50 K ≤ T ≤ 65 K• No exchange bias • Wide hysteresis loop
For 70 K ≤ T• No exchange bias
For 25 K ≤ T ≤ 50 K• Positive exchange bias • Asymmetric saturation magnetization
-10 -8 -6 -4 -2 0 2 4 6 8 10
-6
-4
-2
0
2
4
6 55 K 75 K
m(1
0-4em
u)H(kOe)
-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5
-6
-4
-2
0
2
4
6 5 K 10 K 30 K 35 K
m(1
0-4em
u)
H(kOe)
Large HC loops of Fe0.49Ni0.51F2/Co
-60 -40 -20 0 20 40 60
-6
-4
-2
0
2
4
6 5 K 10 K 15 K 20 K
m(1
0-4em
u)
H(kOe)
• For 5 K ≤ T ≤ 55 K, large HC loops appear for H=± 70 kOe • Positive exchange bias effect with HE ≥10 kOe
• For 55 K ≤ T ≤ 70 K, large HC loops appear for H = ±10 kOe
0 10 20 30 40 50 60 70
-40
-20
0
20
40
60
+H' from 10 kOe
H (
kOe)
T(K)
-H' +H' HE
Is it Possible to Control the Sign of HE? Magnetization measurement Exchange bias studies after field cooling with 2000 Oe from 95 K with SQUID Measurement direction: c-axis Measurement sequence: 70 kOe → -70 kOe → 70 kOe, ( ) 70 kOe → -20 kOe → 70 kOe, ( ) -70 kOe, 20 kOe → -70 kOe → 20 kOe ( )
H
M
-70 kOe -20 kOe 20 kOe 70 kOe
Fe0.49Ni0.51F2/Co•Tunable exchange bias (reversal of wide hysteresis loop)
-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0
-6
-4
-2
0
2
4
6 70 kOe, -70 kOe 70 kOe, -20 kOe -60 kOe, 20 kOe
m
(10
-4em
u)
H(kOe)
5 K
Reversible Exchange Bias• MCo favors parallel exchange coupling with Muncompensated
MCo
Muncompensated
M. Cheon, Z. Liu, and D. Lederman, Appl. Phys. Lett. 90, 012511 (2007)
Consistent with micromagnetic modeling
-1.0
-0.5
0.0
0.5
1.0
-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5
-1.0
-0.5
0.0
0.5
1.0
-60 -40 -20 0 20 40 60
-1.0
-0.5
0.0
0.5
1.0
-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5
-1.0
-0.5
0.0
0.5
1.0
(a)
M/M
S
H (kOe)
(b)
M/M
s
H (kOe)
M/M
s
H (kOe)
Summary for FexNi(1-x)F2/Co bilayers
Note sign change of HE correlated with M(same as in FeZnF2 samples)
Note low TB
TN
0.05
0.10
0.15
0.20
0 20 40 60 80 100
-400
-300
-200
-100
0
T(K)
HE(O
e)
M
s/M
s
0.00 0.05 0.21 1.00
-0.10
-0.05
0.00
0.05
0.10
0 20 40 60 80 100-400
-200
0
200
400
0.49HE(O
e)
M
s/M
sT(K)
What about FeZnF2? Can HE be Reversed at Low T?
Fe0.36Zn0.64F2/Co
Fe0.05Ni0.95F2/Co
-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5
-4
-2
0
2
4
m(1
0-4
em
u)
H(kOe)
30 K
Fe0.21Ni0.79F2/Co
-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5-8
-6
-4
-2
0
2
4
6
8
m(1
0-4
em
u)
H(kOe)
30 K
-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5
-8
-6
-4
-2
0
2
4
6
8
m(1
0-4
em
u)
H(kOe)
20 K-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5
-6
-4
-2
0
2
4
6
m(1
0-4
emu)
H(kOe)
20 K
1 nm FeF2
no effect at 5K
Key Questions• Can uncompensated moments in the AF be detected?
– Uncompensated moments exist in AF, not due to “metallization”– Pinned uncompensated moments in AF vanish near TN
– Unpinned uncompensated moments exist up to RT, well above TN
• Can the effects of uncompensated moments in the AF be studied systematically?– Uncompensated M does not necessarily lead to HE enhancement;
critical concentration of impurities must be achieved– However, uncompensated M dependent on defect concentration
• Can the magnetic anisotropy be studied systematically?– Low magnetic anisotropy leads to reversible HE, in addition to low TB,
as a result of reversal of “pinned” uncompensated M in the AF– Low TB ≠ low TN
– Reversible HE requires uncompensated M in the AF– Dilute AF system can also be reversed, but only at higher temperatures
due to coupling of H to uncompensated magnetization
Remaining Questions• How universal is the effect of uncompensated
moments in the AF?– Can it explain, e.g., low TB , in other AFs?– Is it possible to engineer desirable interface exchange
properties by manipulating AF anisotropy?
• What is the size of the AF domains? And does their size really matter?– If they don’t matter, what is the coupling mechanism and
where does the uncompensated magnetization come from?• Strain (piezomagnetism)?• Defects?
– Update: surprisingly, domain size does not seem to matter much – see Fitzsimmons et al., PRB 77, 22406 (2008).
Areas of Interest
-60 -40 -20 0 20 40 60
-6
-4
-2
0
2
4
6 5 K 10 K 15 K 20 K
m(1
0-4em
u)
H(kOe)MgF2(110) sub.
50 nm FexNi(1-x)F2
18 nm Co
5 nm Al,Pd cap
Biomolecular Electronics
Magnetic Nanostructures and Interfaces
Hybrid Multifunctional Heterostructures
Exchange bias GMR in anisotropic structures Self-assembly and surface dynamics
Myoglobin Single Electron Transistor
YMnO3/GaN 3210-1-2
-140
-120
-100
-80
-60
-40
-20
0
20
40
T~5.7K-5.8KMyoglobin
Bia
s V
olta
ge (
mV
)
Gate Voltage (V)
T = 565 °C
Areas of Interest
-60 -40 -20 0 20 40 60
-6
-4
-2
0
2
4
6 5 K 10 K 15 K 20 K
m(1
0-4em
u)
H(kOe)MgF2(110) sub.
50 nm FexNi(1-x)F2
18 nm Co
5 nm Al,Pd cap
Biomolecular Electronics
Magnetic Nanostructures and Interfaces
Hybrid Multifunctional Heterostructures
Exchange bias GMR in anisotropic structures Self-assembly and surface dynamics
Myoglobin Single Electron Transistor
YMnO3/GaN 3210-1-2
-140
-120
-100
-80
-60
-40
-20
0
20
40
T~5.7K-5.8KMyoglobin
Bia
s V
olta
ge (
mV
)
Gate Voltage (V)
T = 565 °C
Uncompensated M, x=0.75
Sign change of HE due to reversal of AF structure
H. Shi and D. Lederman, Phys. Rev. B 66, 094426 (2002)
Measurement Procedure
F
AFJint
HCF
Jint
H
1. Cool in HCF from above T = TN
2. Measure M vs. H at T < TN
-1.0 -0.5 0.0 0.5 1.0-6
-4
-2
0
2
4
6
m (
10-4em
u)
H (kOe)
FjAi SSJE ,,intintConventional view:
Interface exchange interaction sets low T antiferromagnet configuration
Direct Exchange Mechanism• Direct exchange mechanism
(Meiklejohn and Bean, 1956) predicts– a) wrong magnitude (~100 times too large)
– b) no exchange bias in compensated or disordered surfaces
F
AF
Ideal Uncompensated Compensated Roughness
FFE tMaJH 2int / HE = 0
Jint
Random Exchange at Interface
• Due to interface roughness, defects, etc.• Antiferromagnetic domains created with local exchange
satisfied during cooling
FFE taMLJH /2 int
L = domain size in AF
Malozemoff, 1987
AF Domain Wall Formation• AF or F domain walls created during
cool-down procedure
Jint
H H
FFE tMaAKH 2/2
Malozemoff, 1987; Mauri et al. 1987
Correct order of magnitudeaJA FAF /,Exchange stiffness
Magnetic anisotropy energy KLattice parameter a