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

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

Application: Magnetic Tunnel Junction /GMR Sensors Ferromagnetic layers ~1.0-5.0 nm thick Insulator/NM Metal ~1.0-2.5 nm Antiferromagnet ~10 - 50 nm 1 - 100 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”

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

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?

weak anisotropy antiferromagnet dilute antiferromagnet MF2 Antiferromagnets 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] weak anisotropy antiferromagnet [001] FeF2 Rutile structure (a = 0.4704 nm, c = 0.3306 nm) Antiferromagnetic, TN=78 K Magnetization along the c-axis dilute antiferromagnet [001] ZnF2 Rutile structure (a = 0.4711 nm, c = 0.3132 nm) non-magnetic

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

Growth and Characterization 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

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?

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.

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.

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.

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, 027203 (2006)

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

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 FexNi1-xF2 FexZn1-xF2 Random anisotropy antiferromagnet [001] [001] 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

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

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

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

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

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

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

Net AF Magnetization

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 FexNi1-xF2 FexZn1-xF2 Random anisotropy antiferromagnet [001] [001] 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 [001] 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]

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, 0.55 1.0 magnetic anisotropy changes with x. x=1.0 x=0.0 [001] [001]

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

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, 214416 (2004).

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

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

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

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 ~ - 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 25 K ≤ T ≤ 50 K Positive exchange bias Asymmetric saturation magnetization For 70 K ≤ T No exchange bias

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

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)

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, 012511 (2007)

Summary for FexNi(1-x)F2/Co bilayers TN 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)

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

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).

Group

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

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

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 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

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

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

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