Exchange Bias in Antiferromagnetic- Ferromagnetic Bilayers. Broad Research Themes and Projects

Transcription

1 Exchange Bias in Antiferromagnetic- Ferromagnetic Bilayers Kannan M. Krishnan Department of Materials Science and Engineering University of Washington, Seattle DoE/BES Condensed Matter Colloquium, Physics Department, UW (/) Broad Research Themes and Projects Growth and Properties of Thin Film Structures Deposition by Ion Beam Sputtering, Sol-Gel Processing Exchange Interactions in AFM/FM bilayers Growth mode of complex oxides Magnetically actuated shape memory alloy films Nanoscience and Nanotechnology 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) Advanced Characterization with Electron and Photon Probes 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

2 cm T (K) H = H = T Spin Valve - Hysteresis Loop -5 5 H (Oe) Magnetic Thin Films and Nanostructures Synthesis & Processing Properties & Phenomena Characterization NCEM - Electron microscopy O-K Mn-L Sr-M EELS Mg Fe Mn UHV Ion-beam deposition Chemical synthesis Pulsed Laser Deposition (IML) ρ (Ω ) Magnetic Transport 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 Program M (emu) ALS - X-ray magnetic circular dichroism Absorption T/LC -3T/LC Energy (ev) a MCD MCD.5 MCD Integr b Energy (ev) MCD Integration Systems of Reduced Dimensions 3D D D D DoS E F Energy For D behavior Only ONE level within E = + kt (.6 ev) of E F st estimate, set energy of lowest level ~ kt ~ p /m de Broglie wavelength λ ~ h/p ~ 8 nm (slab thickness) DoS d f(d) E F Energy Chemical Synthesis + Self Assembly And Ultrathin films Multilayers Interfaces Lithography

3 Growth & Novel Properties of D Structures Giant Magnetoresistance H Frustration & Proximity Effects Exchange Bias. R R/R ~ -8% Cyrille, Kim, Gomez, Santamaria, Leighton, Krishnan and Schuller, Phys. Rev. B. 6, () H UHV Ion-beam Deposition Ultrathin Films Multilayers Interfaces AFM FM M/M s (A) C (B) 5 C -. (C) 3 C -4 H e 4 Field (Oe) Cheng, Ahn and Krishnan, J. Appl. Phys., 89, 6597 (). Perpendicular Interface Anisotropy E an = K eff Sin θ K eff = -πm s + K s /t m + K v Cho, Krishnan, Lucas and Farrow, J. Appl. Phys., 7, 5799 (99). Small Particles Nanoscale materials by Self-assembly Patterned Media nm 5 nm 4 µm Metallurgical Approach Achieving theoretical coercivity limits & understanding mechanisms Girt, Krishnan, Thomas and Girt, J. Mag. Mag. Mat., 3, 9 () Chemical method -- Size-selective precipitation Surfactant controls shape Bottoms-up approach Magnetism in systems of reduced dimensions Puntes, Krishnan & Alivisatos, Science, 9, 5 () Ion-implantation through Stencil Masks; Top-down approach Overcoming the super -paramagnetic limit Kusinski, Krishnan, Denbeaux, Thomas, Weller, Rettner. Terris Appl. Phys. Lett. 79, () 3

4 Factors affecting magnetic behaviour: Exchange Interactions θ Bethe- Slater Curve E ij = - J ij S i S j J ij Fe Mn Co Ni r ab r d = J ij S θ for small θ J ij = 3 k B Θ c Z S(S+) No fundamental limit on strength Isotropic, Amorphous magnets Determines magnetic order Factors affecting magnetic behaviour: Anisotropy The spin-orbit-lattice interaction uniaxial E mc = K sin sin 4 o + K u θ + K u θ +.. Cobalt K u = 4.x 5 J/m 3 K u =.x 5 J/m 3 Determines Magnetic Orientation 4

5 Magnetic Behavior of Ferromagnets & Antiferromagnets Iron Θ c ~ 79 C (63 K) Strong magnetic Order Weak Orientation Superparamagnetic limit ~5k B T Rc ~ k BT 3 K KV Antiferromagnet χ Θ N T Weak Magnetic Order Strong Orientation AFM + FM heterostructures Exchange Bias : Unique Behavior not Observed in Bulk FM Exchange Bias in AFM/FM Bilayers. Hysteresis Sample Unidirectional Coupling 5 H c (Fe) M/M s H (Oe) H c (B) -. Field (Oe) (A) C (B) 5 C (C) 3 C H θ Angle (Theta ) H e (B) - 3 H e (B)= -Oe [cos(ϑ)-.5cos(3ϑ)-.cos(5ϑ)+.cos(7ϑ)+...] H c (B)= 44Oe [+.8cos(ϑ)+.4cos(4ϑ)+.5cos(6ϑ)+...] H e (A)= -6Oe [cos(ϑ)-.34cos(3ϑ)+.3cos(5ϑ)-.7cos(7ϑ)+...] H c (A)= 8Oe [+.46cos(ϑ)-.4cos(4ϑ)+.cos(6ϑ)+...] 5

6 Frustration, Interface and Proximity Effects nm nm AFM FM Spin Valves H Fe Mn R R R H R/R ~ % H Phenomenon poorly understood Interface effects dominate Need element-specific probes Growth of ideal structures - epitaxy Technology Exchange Bias: Basics J int FM AFM Easy Direction θ Free Energy: F = H M FM t FM Cos θ - J int Cos θ + K FM Sin θ Solution in terms of an effective field H = H - J int / (M FM t FM ) = H - H E H E = J int / (M FM t FM ) If typical values for J FM are used then observed H E are too small 6

7 Exchange Bias: Basics. Uncompensated Mauri (987) Planar Domain Wall Malozemoff (987, 988) Random Field Model Compensated AFM Domain Koon (997) Frustration Spin Flop Coupling Model Spin-Flop Crystal and Spin Structure of AFM Mn 5 Pd 5 a-axis normal Spin Uncompensated c-axis normal Spin Compensated 4.7Å 3.58Å 4.7Å 3.58Å 4.7Å 4.7Å Pal et al, Jour. Appl. Phys. 39,

8 Ion-beam Deposition Ar Thickness Monitor Substrate Heater ( C) Magnets Cathode Glow Discharge Anode Kaufman Ion Gun.5 cm diameter - kev -5 mamps.5 Å/sec sputtering Vacuum System Multiple Target Holder UHV Reactive Ion-beam Deposition System Thermocouple Substrate heater feed through ( C) Kauffmantype ion source Target(4) rotation Load-lock Gate Valve Residual gas analyser Sample-transfer Chamber Transfer Rod Thickness Monitor Computer Cotrolled Process & Diagnostics Shutter Portable Platform Target block Accurate control of gas flow rates (O, N, Ar) Versatile, Portable (ALS) 8

9 Epitaxial growth and exchange biasing of FM/AFM bilayers a-axis normal 4.7Å 3.58Å 4.7Å Spin Uncompensated c-axis normal 3.58Å 4.7Å 4.7Å Spin Compensated Normal Structure intensity(a.u.) intensity(a.u.) a-growth PdMn (~8Å) Fe (~6Å) MgO() () MgO () MnPd theta c-growth () MnPd theta () MgO.8 moment(memu) H e = 9 Oe -.8 (4) -5 5 Fe H(Oe) () MnPd (4) Fe.5 moment(memu) H e = 3 Oe H(Oe) Electron Diffraction in a TEM Scattering Geometries: X-ray TEM GIXRD Plan View θ θ X-section Complementary information for thin films 9

10 Chemical ordering of a-axis grown PdMn by annealing He ~ He ~ 8- Oe ()MgO () intensity(a.u.) Fe( ) theta (degree) SAD[]a-axis growth as-grown SAD[]a-axis growth annealed a -disordered a -ordered Cheng, Ahn and Krishnan, JAP () PdMn/Fe/MgO : Summary of growth & magnetic properties 4 3 a-axis growth c-axis growth He (Oe) Ordered Annealed As-grown Disordered Growth Temperature ( C)

11 Epitaxial growth and exchange biasing of FM/AFM bilayers Inverse Structure Fe (~6Å) PdMn (~8Å) MgO() a-axis normal 4.7Å 3.58Å 4.7Å c-axis normal 3.58Å 4.7Å 4.7Å intensity(a.u.) () MgO () MnPd (4) Fe theta intensity(a.u.) () MnPd () MgO () MnPd (4) Fe theta Moment(memu) H e ~ H(Oe) Moment (memu) H e = 7 Oe H(Oe) High Resolution EELS Studies in a TEM Element-specific spectroscopy and imaging at high spatial resolution d band Element-resolved local electronic structure Spatially-resolved measurements p 3/ p / s e - 7K 6K 5K 4K 3K K K Cr L 3, Fe L 3, Energy Loss (ev) Phillips /FEG with Image Filter Diplole Selection Rule l = + apply nm

12 Energy-filtered Imaging in a TEM: Detection Limits d band Mn Spatially-resolved measurements PdMn e - p 3/ p / 4 Å PdMn 8 Å Mn Mn s PdMn Å Mn PdMn nm 6 Å Mn Å PdMn Mn nm PdMn 5 4 Å Mn arb. units 4 3 PdMn Phillips /FEG with Image Filter Diplole Selection Rule l = + apply Si nm Simple Roughness Measurements Using Energy-filtered Images NiMn TEM Bright Field Image MgO nm c) Fe-L,3 jump ratio image Trace or Mean Position + σ - - σ

13 Fe Layer Roughness in Fe/MnPd bilayers 3nm PdMn Fe MgO Fe EFTEM image y i Trace of Layer Roughness : σ=. ±.4nm σ = N ( y) = ( ( y < y > N i i= ) ) / Layer Roughness for a-axis, c-axis normal and c-axis inverted structures 3nm 3nm PdMn Fe MgO Layer Roughness: a-axis normal:.5 ±.4nm c-axis normal:.574 ±.4nm c-axis inverted:.33 ±.4nm 3

14 Definitions: Rough interface with various characteristic length scales h(x) Interface Profile h(x) Average Value <h(x)> or h Height Deviation z(x) = h(x) - h h σ L (x ) z(x) ξ // Interface width ~ rms roughness σ(l) < σ L (x ) > x where the averaging is done over all points x within L and x L x x σ L (x ) = < z(x) - z av (L) > L / z av (L) is z(x) averaged over L Correlation length, ξ is extracted by fitting σ(l) σ sat [ - exp (-L/ξ )] Xray Magnetic Circular Dichroism Element Specific, Spin/Orbital Resolved Magnetic Measurements L 3 XAS & EELS L l = + Absorption T/LC -3T/LC E d band p 3/ p / s E left right XMCD l = + m j = + spin orbital Sum Rules ( or ) MCD MCD MCD Integr. -.5 b Energy (ev) m orb m spin = a q (9 p - 6 q) m orb / m spin = MCD Integration 4

15 Magnetic structure of AFM-FM bilayer interfaces circularly polarized x-ray photon silicon window Absorption Au cap Fe(Co) M/M s. NiMn (A) C (B) 5 C -. (C) 3 C -4 H e 4 Field (Oe) silicon nitride membrane Fe L 3, He ( B > C > A ) M Fe (A > C > B) C B A Dichroism Iron Moment Correlates inversely with Exchange field The films show () texture () 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 Cheng, Krishnan, Farrow, Young, Girt,.JAP (in press) Microspectroscopy & Spectromicroscopy with Photons (ALS) PhotoEmission Electron Microscope 5

16 PEEM : Temperature Dependence H B 3, RT 8, 7 C 7, C 39, 8 C 48, 3.9, 3 C 63, BKRT Cheng, Ohldag, Krishnan () Recent discoveries in artificially structured magnetic films Perpendicular Interface Anisotropy E = K sin Θ an eff K = - π M s + eff K t M s + K V [] Pt, Ag [] Pt, Ag [] Pt, Ag Co Ag GaAs () Ag GaAs () Ag GaAs () K eff t m Co x Pd x(å) Fe x Pd 8 Interface is key to understanding such anisotropy. 6

17 Exchange Bias in Systems with Out-of-Plane Coupling? Pd Fe Pd Fe 4.7Å MnPd 4.7Å 3.58Å Lithographically Patterned Antiferroamgnetic Elements? Malozemoff (987, 988) Random Field Model MnPd Fe MgO AFM Domain 7

18 Acknowledgements Ning Cheng - Growth & Magnetic measurements Dr. J.-P. Ahn - Ion Beam Deposition Dr. Werner Grogger - EFTEM Imaging & Analysis Chris Nelson - Philips CM Operation Dr. H. Ohldag - PEEM Beamline Scientist Dr. Tony Young - XMCD measurements DoE/BES Max Kade Foundation 8