1. Exchange Bias: Interface vs. Bulk Magnetism
Hendrik Ohldag
Joachim Stöhr
Miyeon Cheon Hongtao Shi
Zhongyuan Liu
Jorge Espinosa
David Lederman
Elke Arenholz
Department of Physics
Optical and Vibrational Spectroscopies Symposium:
A Tribute to Manuel Cardona
August 20, 2010

2. Exchange Bias MR HC HE FM AF MR: “Remanent” magnetization
- Maximum value of M
- Depends on FM MR
HC: Coercivity
- Depends on FM magnetic anisotropy
- Represents energy required to reverse magnetic domain HC HE
HE: Exchange Bias
-Absent in pure FM, results from AF-FM interaction FM AF

3. Application: Magnetic Tunnel Junction /GMR Sensors
Ferromagnetic layers
~ nm thick
Insulator/NM Metal
~ nm
Antiferromagnet
~ nm mm
Free magnetic layer (analyzes electron spin)
Pinned magnetic layer (polarizes tunneling electrons)
Pinning Antiferromagnet q
Albert Fert & Peter Grünberg
2007 Nobel Prize in Physics
“for the discovery of Giant Magnetoresistance”

4. How does the pinning of bottom FM layer work?(

5. Key Questions Given that: Some key questions are:
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?

6. weak anisotropy antiferromagnet dilute antiferromagnet
MF2 Antiferromagnets
NiF2
Rutile structure (a = nm, c = nm)
Antiferromagnetic, TN= 73 K
Weak ferromagnetic
Magnetization lies in the a-b plane
[001]
weak anisotropy antiferromagnet
[001]
FeF2
Rutile structure (a = nm, c = nm)
Antiferromagnetic, TN=78 K
Magnetization along the c-axis
dilute antiferromagnet
[001]
ZnF2
Rutile structure (a = nm, c = nm)
non-magnetic

7. So… where does Manuel Cardona fit in?
Naïve graduate student asks: can antiferromagnetic superlattice magnons be observed?
Two-magnon Raman line for 1.3 mm FeF2 thin film

8. Growth and Characterization
MBE co-deposition of FeF2 (e-beam) and ZnF2, NiF2 (K-cell), Pbase = 7 x Torr, Pgrowth < 4 x 10-8 Torr
TS (AF) = 297 0C, 0C,
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

9. 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?

10. Magnetic Dichroism in X-ray Absorption
X-ray magnetic circular dichroism
 sensitive to FM order.
Fe L3, L2 e- e-
Antiferromagnetic Domains
NiO L2a, L2b
X-ray magnetic linear dichroism
 sensitive to AF order.
Element specific technique sensitive to antiferromagnetic as well as ferromagnetic order.

11. Antiferromagnetic Order of FeF2(110)
FeF2 L2 absorption edge
Einc || [001]
Einc || [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.

12. Interface Coupling and Exchange Bias
Measy
Mpinned
MgF2(110) sub.
68 nm FeF2
2.5 nm Co
2 nm Pd cap RT
Ferromagnet
15K
Interface
Room T: “Free” uncompensated moments follow FM
Low T: Additional “pinned” uncompensated moments antiparallel to easy direction.

13. Results 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, (2006)

14. Parallel Interface Coupling and Exchange Bias
2.) XMCD is indication of interfacial magnetic order at RT.
1.) XMLD and long range AF order vanish at TN.
Related to enhancement of coercivity for T >> TN
(Grimsditch et al, PRL 2003)
Also, see Roy et al, PRL 2006

15. 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?

16. Systems FexNi1-xF2 FexZn1-xF2 Random anisotropy antiferromagnet
[001]
[001]
Dilute antiferromagnet
Random anisotropy antiferromagnet
Systematic study of uncompensated M

17. 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

18. Previous Results Co1-xMgxO/ CoO (0.4 nm) /Co
P. Miltényi, et al., Phys. Rev. Lett., 84, 4224 (2000)

19. Sample Profile Magnetic interface changes with x in FexZn1-xF2
5 nm MgF2 Cap
(110)-MgF2 Sub
5 nm MgF2 Cap
(110)-MgF2 Sub
18 nm Cobalt (F)
18 nm Cobalt (F)
Pure interface
layer (PIL)
65 nm (110)
FexZn1-xF2 (AF)
1.0 nm FeF2
65 nm (110)
FexZn1-xF2 (AF)
Magnetic interface changes with x in FexZn1-xF2

20. HE, HC Dependence on T
PIL affects HE, HC; no effect on TB

21. 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

22. TB vs. x in FexZn1-xF2
TB agrees reasonably well with bulk TN data

23. Interface Energy Dependence on x
T = 5K
ΔE = -tCo*HE*MS
No large HE enhancement observed
Small AF domains not formed at large x ?

24. Net AF Magnetization

25. 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?

26. Systems FexNi1-xF2 FexZn1-xF2 Random anisotropy antiferromagnet
[001]
[001]
Dilute antiferromagnet
Random anisotropy antiferromagnet
Systematic study of AF anisotropy

27. Magnetic Order
FeF2
Rutile structure (a = nm, c = nm)
Antiferromagnet, TN=78 K
Magnetization along the c-axis
[001]
NiF2
Rutile structure (a = nm, c = nm)
Antiferromagnetic, TN= 73 K (80 K in films)
Weak ferromagnet
Magnetization lies in the a-b plane
[001]

28. Growth and measurements
MgF2(110) sub.
50 nm FexNi(1-x)F2
18 nm Co
5 nm Al,Pd cap
MBE Growth
MgF2 (110) substrate
Growth temperature 210 ˚C
Fe concentration: 0.0, 0.05, 0.21, 0.49,
magnetic anisotropy changes with x.
x=1.0
x=0.0
[001]
[001]

29. Expectations FexNi1-xF2 For nearest neighbor interactions
[001]
For nearest neighbor interactions
For small f, there is a critical Fe concentration xc beyond which spins will lie along the c-axis: q
q+f
For FeF2 and NiF2 xc = 0.14

30. FeF2/Co NiF2/Co 49 nm NiF2 / 16 nm Co H┴ c H || c
5 K
49 nm NiF2 / 16 nm Co
H┴ c
H || c
Exchange bias along c-axis
TB ~ 81 K
No exchange bias along c-axis
H. Shi et al., Phys. Rev. B 69, (2004).

31. Fe0.05Ni0.95F2/Co For 50 K ≤ T ≤ 70 K For T ≤ 45 K No exchange bias
Wide hysteresis loop
For T ≤ 45 K
Negative exchange bias along the c-axis
Asymmetric saturation magnetization
For 75 K ≤ T
No exchange bias

32. Large coercivity loops of Fe0.05Ni0.95F2/Co
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

33. Fe0.21Ni0.79F2/Co
Similar behavior to Fe0.05Ni0.95F2/Co
Negative HE along the c-axis at T≤ 40 K
Asymmetric saturation magnetization
For 45 K ≤ T ≤ 70 K
No exchange bias effect
Wide hysteresis loop
For 75 K ≤ T
HE = 0

34. Large HC loops of Fe0.21Ni0.49F2/Co
For 40 K ≤ T ≤ 70 K, large HC loops appear for the scanning field range ±10 kOe
Negative exchange bias effect (HE ~ Oe) for 40 K ≤ T ≤55 K

35. 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 25 K ≤ T ≤ 50 K
Positive exchange bias
Asymmetric saturation magnetization
For 70 K ≤ T
No exchange bias

36. Large HC loops of Fe0.49Ni0.51F2/Co
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

37. 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

38. Fe0.49Ni0.51F2/Co
Tunable exchange bias (reversal of wide hysteresis loop)

39. Reversible Exchange Bias
MCo favors parallel exchange coupling with Muncompensated
MCo
Muncompensated
Consistent with micromagnetic modeling
M. Cheon, Z. Liu, and D. Lederman, Appl. Phys. Lett. 90, (2007)

40. Summary for FexNi(1-x)F2/Co bilayersTN
Note low TB
Note sign change of HE correlated with DM
(same as in FeZnF2 samples)
Uncompensated magnetization
Exchange bias and coercive field
(note low TB)

41. What about FeZnF2? Can HE be Reversed at Low T?
Fe0.05Ni0.95F2/Co
30 K
Fe0.21Ni0.79F2/Co
30 K
20 K
Fe0.36Zn0.64F2/Co
20 K
1 nm FeF2
no effect at 5K

42. 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

43. 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, (2008).

44. Group

45. Areas of Interest Magnetic Nanostructures and Interfaces
MgF2(110) sub.
50 nm FexNi(1-x)F2
18 nm Co
5 nm Al,Pd cap
T = 565 °C
Exchange bias
GMR in anisotropic structures
Self-assembly and surface dynamics
Magnetic Nanostructures and Interfaces
YMnO3/GaN
Myoglobin Single Electron Transistor
Hybrid Multifunctional Heterostructures
Biomolecular Electronics

46. Areas of Interest Magnetic Nanostructures and Interfaces
MgF2(110) sub.
50 nm FexNi(1-x)F2
18 nm Co
5 nm Al,Pd cap
T = 565 °C
Exchange bias
GMR in anisotropic structures
Self-assembly and surface dynamics
Magnetic Nanostructures and Interfaces
YMnO3/GaN
Myoglobin Single Electron Transistor
Hybrid Multifunctional Heterostructures
Biomolecular Electronics

47. Uncompensated M, x=0.75
Sign change of HE due to reversal of AF structure
H. Shi and D. Lederman, Phys. Rev. B 66, (2002)

48. Measurement Procedure
1. Cool in HCF from above T = TN
2. Measure M vs. H at T < TN
Conventional view:
Interface exchange interaction sets low T antiferromagnet configuration F AF
Jint
HCF
Jint H

49. 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
HE = 0 F
Jint AF
Ideal Uncompensated
Compensated
Roughness

50. Random Exchange at Interface
Due to interface roughness, defects, etc.
Antiferromagnetic domains created with local exchange satisfied during cooling
L = domain size in AF
Malozemoff, 1987

51. AF Domain Wall Formation
AF or F domain walls created during cool-down procedure H H
Jint
Exchange stiffness
Correct order of magnitude
Magnetic anisotropy energy K
Lattice parameter a
Malozemoff, 1987; Mauri et al. 1987