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. Introduction 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.

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

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

5. Double Multilayer Structure } } Ferromagnetically Coupled Multilayer Antierromagnetically Coupled Multilayer Ta (75Å) {Co (60Å) /Ru (10Å)}  10 {Co (35Å) /Ru (15Å)}  10 Ta (75Å) Si (001) X-ray Reflectivity dAFdAF dFdF 2  /d AF 2/dF2/dF

6. Magnetisation Data } } Antiferromagnet Ferromagnet Double Multilayer

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

8. Neutron Reflectometry with Polarisation Analysis H µnµn Q k in k ou t Non-spin flip scattering µnµn M Spin flip scattering µnµn M µnµn µnµn nn

9. Neutron Reflectometry Saturation (6 kOe)Spin-flop phase (600 Oe)Exchange Spring (160 Oe) µnµn k in k out No spin-flip scattering No AF peak Spin-flip scattering AF peak Decrease of spin-flip scattering AF peak 2nd order AF peak 2nd order AF peak

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

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

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

13. Summary 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.