Summary of EPSRC grant: GR/K04972:
S.M. Thompson, University of York.

The magnetic and structural properties of thin films, multilayers and mesomagnetic systems.

1) The effect of the interface on the magnetic and transport properties of exchange coupled multilayers.

Co/Cr/Co is a miscible system with weak interlayer coupling. The effect of competition of the interlayer coupling with in-plane anisotropy has been found to result in stepped Kerr hysteresis loops. Correlation of the interface structure with the magnetic properties and the microscopic domain structure has been made and this interface structure modified by the use of different deposition temperatures.

Uniaxial anisotropy was measured in NiFe/Cr multilayers as a function of temperature and number of bilayers. This was found to be caused by the creation of an antiferromagnetic oxide on the surface of the multilayer resulting in shifted and stepped hysteresis loops controlled by the number of bilayer periods.

Grazing Incidence X-ray Reflectivity, both specular and diffuse has been used to study the substrate, interface, surface and correlation of roughness in giant magnetoresistive NiFe/Cu multilayers prepared by both UHV evaporation and sputtering. The X-ray wavelength was tuned to the absorption edges of each of the three constituent elements in order to achieve sufficient electron density difference at the Daresbury synchrotron source. The sputtered samplesexhibited a greater degree of roughness which was primarily uncorrelated in contrast to the UHV evaporated samples.

2) Giant magnetoresistance in granular systems

The importance of the spin diffusion length on the giant magnetoresistance in granular systems has been both determined experimentally in a study on epitaxial Co-Ag films and considered theoretically by the development of the "Jitterbug" mechanism. This describes the spin depolarisation as a result of the precession of the electron spin as it enters a magnetic particle with its spin axis at an angle to the moment axis in the particle. The angle of the spin axis as it emerges form the particle will be determined by the exchange energy in the particle and its diameter thus mixing the spin channels and deteriorating the giant magnetoresistance. This effect is particularly important at the approach to saturation where the relative angles between the particles are small, but a substantial amount of depolarisation is still possible.

3) Mesomagnetic structures

Magnetic nanostructures in the form of both wires (80nm diameter) and dots (200nm diameter) have been successfully prepared using the techniques of X-ray and electron beam lithography respectively. In the former case a mask plus resist technique was used and in the latter the electron beam was used to "flash" expose the resist enabling a variety of patterns to be traced onto the resist. A study to investigate the feasibility of using the step-substrate technique to deposit directly magnetic nanowires has been instigated. In this method traditional photolithography techniques are used to pattern the substrate prior to evaporation at a high incidence angle.

Titles of papers and internal reports etc. arising from the research

1. K.O'Grady, S.J. Greaves and S.M. Thompson Exchange Anisotropy Effects in NiFe/Cr Multilayers Proceedings of MML 1995, J. Magn. Magn. Mat. 156 253(1996)
2. A. Azzizi, L. El Chahal, K. Ounadjela, J.P. Deville, S.M. Thompson and J.F. Gregg Structural Characterisation of epitaxial Co-Ag Proceedings of MMM 1995, J.Appl. Phys. 79(8) 6247 (1996)
3. J.F. Gregg, W. Allen., S.M. Thompson, M.L. Watson and G.A. Gehring Jitterbug spin channel mixing in heterogeneous GMR material Proceedings of MMM 1995, J.Appl. Phys. 79(8) 5593(1996)
4. J.F. Gregg, W. Allen, K. Ounadjela, M. Viret, M. Hehn, S.M. Thompson and JMD Coey Giant Magnetoresistance in a single element magnetic thin film Phys. Rev. Lett. 77(8) 1580
5. K. Ounadjela, S.M. Thompson, J.F. Gregg, A.Azzizi, M. Gester and J.P. Deville Correlation between the structural and transport properties of as grown epitaxial phase separated CoAg granular films Phys. Rev. B 54(17) 1252 (1996)
6. D.S. Daniel, J.F. Gregg, S.M. Thompson and K. Ounadjela Spin information transfer lengths in giant thermal and electrical magnetoresistance to be submitted to Phys. Rev. B. 1997

Conference Presentations (unpublished):-

CMMP'94

1. M. Jackson, S. Jordan, J.S.S. Whiting, S.M. Thompson and A. Chambers Comparison of MOKE and FMR studies of Co/Cr trilayers
2. CMMP 1994: S.M. Thompson, J.F. Gregg and K. Ounadjela Correlation between the structural and magnetotransport properties of granular CoAg systems prepared by Molecular beam Epitaxy
3. CMMP 1994: M.L. Watson, S.M. Thompson, J.F. Gregg, G.A. Gehring and D.S. Daniel The role of "Jitterbug" spin depolarisation and spin accumulation in granular GMR

1. T.P.A. Hase, I. Pape, B.K. Tanner, S.M. Thompson, M. Jackson, J.S.S. Whiting, H. Niedoba, B. Mirescki, A. Hernando, C. Prados and A.C. Daykin Magnetic and Structural Effects of Annealing Co-Cr-Co Trilayers
2. J.S.S. Whiting, S.M. Thompson, S. Jordan, M. Jackson, A. Chambers, H. Niedoba, A. Hernando and C. Prados Ambiguities in the determination of weak interlayer exchange coupling by different analytical techniques.
3. G. S. Green, S.M. Thompson and M.L. Watson X-ray near edge reflectivity of NiFe/Cu magnetic multilayers
4. D.S. Daniel, J.F. Gregg, S.M. Thompson and K. Ounadjela Spin information transfer lengths in giant thermal and electrical magnetoresistance
5. W. Allen, D.S. Daniel, J.F. Gregg, M. Viret, J.M.D. Coey, S.M. Thompson and K. Ounadjela Spin depolarisation at domain walls and interfaces in ferromagnets and GMR material
6. W. Allen, J.F. Gregg, S.M. Thompson, M.L. Watson and G.A. Gehring Jitterbug spin channel mixing in granular GMR material
7. A.N. Farley, S.M. Thompson, S. Hoon, D.B. Lambrick, J.F. Gregg, M. Gester, K. Ounadjela and M.S. Valera Observation of domain structure in CoAg magnetoresistive granular metals by magnetic force microscopy

1. T.P.A. Hase, I.Pape, B.K. Tanner, S.M. Thompson, S.M. Jordan and J-J. Freijo The effect of the growth temperature on the structure of Co-Cr-Co trilayers
2. S.M. Thompson Magnetoresistance from conventional to colossal

1. T.P.A. Hase, I.Pape, B.K. Tanner, S.M. Thompson, S.M. Jordan and J-J. Freijo The effect of the growth temperature on the structure of Co-Cr-Co trilayers
2. J.F. Gregg, W. Allen, K. Ounadjela, M. Viret, M. Hehn, S.M. Thompson and JMD Coey Giant Magnetoresistance in a single element magnetic thin film

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