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Exchange Bias: Interface vs. Bulk Magnetism Miyeon Cheon Hongtao Shi Zhongyuan Liu Jorge Espinosa David Lederman Elke Arenholz Department of Physics Hendrik.

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Presentation on theme: "Exchange Bias: Interface vs. Bulk Magnetism Miyeon Cheon Hongtao Shi Zhongyuan Liu Jorge Espinosa David Lederman Elke Arenholz Department of Physics Hendrik."— Presentation transcript:

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

2 HCHC MRMR Exchange Bias FM AF M R : “Remanent” magnetization - Maximum value of M - Depends on FM H C : Coercivity - Depends on FM magnetic anisotropy - Represents energy required to reverse magnetic domain H E : Exchange Bias -Absent in pure FM, results from AF-FM interaction HEHE

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

4 (www.research.ibm.com) How does the pinning of bottom FM layer work?

5 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?

6 MF 2 Antiferromagnets FeF 2 Rutile structure (a = nm, c = nm) Antiferromagnetic, T N =78 K Magnetization along the c-axis [001] NiF 2 Rutile structure (a = nm, c = nm) Antiferromagnetic, T N = 73 K Weak ferromagnetic Magnetization lies in the a-b plane [001] ZnF 2 Rutile structure (a = nm, c = nm) non-magnetic weak anisotropy antiferromagnet [001] dilute antiferromagnet

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  m FeF 2 thin film

8 MBE co-deposition of FeF 2 (e-beam) and ZnF 2, NiF 2 (K- cell), P base = 7 x Torr, P growth < 4 x Torr T S (AF) = C, 0 C, poly-MgF Growth along (110) Twin sample holder – simultaneous growth of underlayer, different overlayers In-situ RHEED, AFM X-ray diffraction and reflectivity Cooling field (H CF = 2 kOe) in the film plane along the c-axis of Fe x Zn 1-x F 2 M vs H via SQUID magnetometer, horizontal sample rotator Growth and Characterization

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 e-e- e-e- Magnetic Dichroism in X-ray Absorption e-e- e-e- Antiferromagnetic Domains X-ray magnetic circular dichroism  sensitive to FM order. Fe L 3, L 2 NiO L 2a, L 2b X-ray magnetic linear dichroism  sensitive to AF order. Element specific technique sensitive to antiferromagnetic as well as ferromagnetic order.

11 Antiferromagnetic Order of FeF 2 (110) Stronger XMLD signal for Co/FeF 2 (110) compared to bare FeF 2 (110) indicates an increase in antiferromagnetic order caused by exchange to the FM Co layer. E inc || [001] E inc || [110] FeF 2 L 2 absorption edge

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

13 Results Fe in FeF 2 /Co interface, despite being non-metallic, has –Unpinned magnetization to RT –Pinned magnetization to T B –AF order verified to T N via XMLD Co at interface –T B ~T N –H C peak near T B Ohldag et al., PRL 96, (2006)

14 1.) XMLD and long range AF order vanish at T N. 2.) XMCD is indication of interfacial magnetic order at RT. Parallel Interface Coupling and Exchange Bias Related to enhancement of coercivity for T >> T N (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 T N –Unpinned uncompensated moments exist up to RT, well above T N Can the effects of uncompensated moments in the AF be studied systematically? Can the magnetic anisotropy be studied systematically?

16 Systems [001] Fe x Ni 1-x F 2 Fe x Zn 1-x F 2 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 P. Miltényi, et al., Phys. Rev. Lett., 84, 4224 (2000) Co 1-x Mg x O/ CoO (0.4 nm) /Co Previous Results

19 Sample Profile 5 nm MgF 2 Cap (110)- MgF 2 Sub 5 nm MgF 2 Cap (110)- MgF 2 Sub 1.0 nm FeF 2 Magnetic interface changes with x in Fe x Zn 1-x F 2 65 nm (110) Fe x Zn 1-x F 2 (AF) 65 nm (110) Fe x Zn 1-x F 2 (AF) 18 nm Cobalt (F) Pure interface layer (PIL)

20 H E, H C Dependence on T PIL affects H E, H C ; no effect on T B

21 H E, H C vs. Temperature for x = 0.75 H E changes sign as T increases to T B. H C has two peaks corresponding to H E = 0. Therefore AF ground state is not unique

22 T B vs. x in Fe x Zn 1-x F 2 T B agrees reasonably well with bulk T N data

23 Interface Energy Dependence on x No large H E enhancement observed Small AF domains not formed at large x ? ΔE = -t Co *H E *M S T = 5K

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 T N –Unpinned uncompensated moments exist up to RT, well above T N Can the effects of uncompensated moments in the AF be studied systematically? –Uncompensated M does not necessarily lead to H E enhancement; critical concentration of impurities must be achieved –However, uncompensated M dependent on defect concentration Can the magnetic anisotropy be studied systematically?

26 Systems [001] Fe x Ni 1-x F 2 Fe x Zn 1-x F 2 Dilute antiferromagnet Random anisotropy antiferromagnet Systematic study of AF anisotropy

27 Magnetic Order FeF 2  Rutile structure (a = nm, c = nm)  Antiferromagnet, T N =78 K  Magnetization along the c-axis NiF 2  Rutile structure (a = nm, c = nm)  Antiferromagnetic, T N = 73 K (80 K in films)  Weak ferromagnet  Magnetization lies in the a-b plane [001]

28 Growth and measurements magnetic anisotropy changes with x. MgF 2 (110) sub. 50 nm Fe x Ni (1-x) F 2 18 nm Co 5 nm Al,Pd cap MBE Growth  MgF 2 (110) substrate  Growth temperature 210 ˚C  Fe concentration: 0.0, 0.05, 0.21, 0.49, [001] x=1.0 [001] x=0.0

29 Expectations For nearest neighbor interactions   For small , there is a critical Fe concentration x c beyond which spins will lie along the c-axis: For FeF 2 and NiF 2 x c = 0.14 [001] Fe x Ni 1-x F 2

30 NiF 2 /Co FeF 2 /Co No exchange bias along c-axis Exchange bias along c-axis T B ~ 81 K 5 K H┴ c H || c 49 nm NiF 2 / 16 nm Co H. Shi et al., Phys. Rev. B 69, (2004).

31 Fe 0.05 Ni 0.95 F 2 /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

32 Large coercivity loops of Fe 0.05 Ni 0.95 F 2 /Co For 50 K ≤ T ≤ 70 K, large coercivity loops appear for the scanning field range -10 kOe to 10 kOe. Negative exchange bias (H E ~ -500 Oe) for T = 50 K and 55 K

33 Fe 0.21 Ni 0.79 F 2 /Co Similar behavior to Fe 0.05 Ni 0.95 F 2 /Co Negative H E 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 H E = 0

34 Large H C loops of Fe 0.21 Ni 0.49 F 2 /Co For 40 K ≤ T ≤ 70 K, large H C loops appear for the scanning field range ±10 kOe Negative exchange bias effect (H E ~ Oe) for 40 K ≤ T ≤55 K

35 Fe 0.49 Ni 0.51 F 2 /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

36 Large H C loops of Fe 0.49 Ni 0.51 F 2 /Co For 5 K ≤ T ≤ 55 K, large H C loops appear for H=± 70 kOe Positive exchange bias effect with H E ≥10 kOe For 55 K ≤ T ≤ 70 K, large H C loops appear for H = ± 10 kOe

37 Is it Possible to Control the Sign of H E ? 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 Fe 0.49 Ni 0.51 F 2 /Co Tunable exchange bias (reversal of wide hysteresis loop)

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

40 Summary for Fe x Ni (1-x) F 2 /Co bilayers Note sign change of H E correlated with  M (same as in FeZnF 2 samples) Exchange bias and coercive field (note low T B ) Uncompensated magnetization Note low T B TNTN

41 What about FeZnF 2 ? Can H E be Reversed at Low T? Fe 0.36 Zn 0.64 F 2 /Co Fe 0.05 Ni 0.95 F 2 /Co 30 K Fe 0.21 Ni 0.79 F 2 /Co 30 K 20 K 1 nm FeF 2 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 T N –Unpinned uncompensated moments exist up to RT, well above T N Can the effects of uncompensated moments in the AF be studied systematically? –Uncompensated M does not necessarily lead to H E 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 H E, in addition to low T B, as a result of reversal of “pinned” uncompensated M in the AF –Low T B ≠ low T N –Reversible H E 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 T B, 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 MgF 2 (110) sub. 50 nm Fe x Ni (1-x) F 2 18 nm Co 5 nm Al,Pd cap Biomolecular Electronics Magnetic Nanostructures and Interfaces Hybrid Multifunctional Heterostructures Exchange biasGMR in anisotropic structures Self-assembly and surface dynamics Myoglobin Single Electron Transistor YMnO 3 /GaN T = 565 °C

46 Areas of Interest MgF 2 (110) sub. 50 nm Fe x Ni (1-x) F 2 18 nm Co 5 nm Al,Pd cap Biomolecular Electronics Magnetic Nanostructures and Interfaces Hybrid Multifunctional Heterostructures Exchange biasGMR in anisotropic structures Self-assembly and surface dynamics Myoglobin Single Electron Transistor YMnO 3 /GaN T = 565 °C

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

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

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 F AF Ideal UncompensatedCompensatedRoughness H E = 0 J int

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 J int H H Malozemoff, 1987; Mauri et al Correct order of magnitude Exchange stiffness Magnetic anisotropy energy K Lattice parameter a


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