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Exchange Bias from Double Multilayer Structures

Exchange Bias from Double Multilayer Structures. C. H. Marrows , P. Steadman, M. Ali, A. T. Hindmarch, and B. J. Hickey Department of Physics and Astronomy, University of Leeds, Leeds. LS2 9JT S. Langridge, R. Dalgliesh and S. Foster

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Exchange Bias from Double Multilayer Structures

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  1. Exchange Bias from Double Multilayer Structures C. H. Marrows, P. Steadman, M. Ali, A. T. Hindmarch, and B. J. Hickey Department of Physics and Astronomy, University of Leeds, Leeds. LS2 9JT S. Langridge, R. Dalgliesh and S. Foster ISIS Facility, Rutherford Appleton Laboratory, Didcot, Oxon. OX11 0QX

  2. Exchange Bias – AF/F bilayer interaction. Double Multilayer Structures – model system for AF/F studies. Modelling – can predict magnetic structure. Neutron Reflectometry – depth sensitive vector magnetometry. Introduction

  3. Exchange Bias Antiferromagnet Ferromagnet • Problems with Meiklejohn-Bean: • Predicted exchange bias orders of magnitude too large. • Coercivity enhancement is not predicted. • Temperature dependence is not predicted.

  4. Two Models Domain Wall Formation in Antiferromagnet (Mauri) Interface Roughness (Malozemoff) Ferromagnet Antiferromagnet A. P. Malozemoff Phys. Rev. B 35, 3697 (1987). D. Mauri, H. C. Siegmann, P.S. Bagus and E. Kay J. Appl. Phys. 62, 3047 (1987)

  5. Double Multilayer Structure 2/dAF } Ta (75Å) X-ray Reflectivity 2/dF Ferromagnetically Coupled Multilayer {Co (60Å) /Ru (10Å)} 10 dF } {Co (35Å) /Ru (15Å)} 10 Antierromagnetically Coupled Multilayer dAF Ta (75Å) Si (001)

  6. Magnetisation Data Ferromagnet } Double Multilayer } Antiferromagnet

  7. Modelling the spin structure. Energy per unit area = Zeeman + Anisotropy + Coupling Minimise energy by varying moment orientations q as field is swept – trace out hysteresis loop with full magnetic configuration known at each point. Monte-Carlo Algorithm. • Layer index i • Layer moment m • Layer thickness t • Applied field H • Anisotropy constant K • Interlayer Coupling Constant J

  8. Neutron Reflectometry with Polarisation Analysis n kout µn Q kin H µn Non-spin flip scattering µn M Spin flip scattering µn µn M

  9. Neutron Reflectometry Saturation (6 kOe) Spin-flop phase (600 Oe) Exchange Spring (160 Oe) • Spin-flip scattering • AF peak • No spin-flip scattering • No AF peak • Decrease of spin-flip scattering • AF peak kout kin µn 2nd order AF peak 2nd order AF peak

  10. Hysteresis Cycle Spin-flop Phase: 600 Oe Saturation: 6 kOe • Generate spin-structure from calculation. • Pass to PNR simulation code (Polly). • Fit PNR data using simulated annealing (changes <10°). Exchange Spring: 160 Oe

  11. New Double Multilayer • Previous multilayers did not have exchange bias. • Introduce anisotropy into antiferromagnetic layer by adding Pt to magnetic layers. } MOKE Ferromagnetically Coupled Multilayer {Co (56Å) /Ru (5Å)} 10 n=5 n=10 dF } {CoPt (60Å) /Ru (10Å)} n Antiferromagnetically Coupled Multilayer dAF Hex=-46 Oe Hex=-20 Oe Si (001) Sensitive only to upper layers (~200Å)

  12. Polarised Neutron Reflectometry 3.0 kOe Co (60Å) 35 Oe {CoPt (60Å) /Ru (10Å)} 10 Hex=250 Oe 2.5 kOe 950 Oe

  13. A large anisotropy in the antiferromagnet is necessary for exchange bias. A Mauri type exchange spring exists in the exchanged biased multilayers – model system for perfect interface. Planar wall confined to AF layers. Summary

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