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Anisotropy and Magnetization Reversal

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Anisotropy and Magnetization Reversal

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    1. Anisotropy and Magnetization Reversal Magnetic anisotropy (a) Magnetic crystalline anisotropy (b) Single ion anisotropy (c) Exchange anisotropy 2. Magnetization reversal (a) H parallel and normal the anisotropy axis, respectively (b) Coherent rotation (Stoner-Wohlfarth model) (c) Micromagnetics: dynamic simulation; solving LLG equation

    2. Magnetocrystalline anisotropy

    5. The Defination of Field Ha A quantitative measure of the strength of the magnetocrystalline anisotropy is the field, Ha, needed to saturate the magnetization in the hard direction. The energy per unit volume needed to saturate a material in a particular direction is given by a generation:

    6. How is L coupled to the lattice ?

    7. Physical Origin of Magnetocrystalline anisotropy

    9. Uniaxial Anisotropy

    11. Single-Ion Model of Magnetic Anisotropy

    15. (1) As for the Fe2+ ion, the sixth electron should occupy the lowest singlet, so that the ground state is degenerate. (2) Co2+ ion has seven electrons, so that the last one should occupy the doublet. In such a case the orbit has the freedom to change its state in plane which is normal to the trigonal axis, so that it has an angular momentum parallel to the trigonal axis. Since this angular momentum is fixed in direction, it tends to align the spin magnetic moment parallel to the trigonal axis through the spin-orbit interaction.

    17. (1) J.J.Rhyne 1972 Magnetic Properties Rare earth matals ed by R.J.elliott p156 (2) Z.S.Shan, D.J.Sellmayer, S.S.Jaswal, Y.J.Wang, and J.X.Shen, Magnetism of rare-earth tansition metal nanoscale multilayers, Phys.Rev.Lett., 63(1989)449; (3) Y. Suzuki and N. Ohta, Single ion model for magneto-striction in rare-earth transition metal amorphous films, J.Appl.Phys., 63(1988)3633; (4) Y.J.Wang and W.Kleemann, Magnetization and perpendicular anisotropy in Tb/Fe multilayer films, Phys.Rev.B, 44 (1991)5132.

    18. Exchange Anisotropy

    21. Mauri et al., (JAP 62(1987)3047) derived an expression for M-H loop of the soft film in the exchange-coupled regime, (tA>tAc)

    22. Oscillation Exchange Coupling

    23. Magnetization Process The magnetization process describes the response of material to applied field. (1) What does an M-H curve look like ? (2) why ?

    24. For uniaxial anisotropy and domain walls are parallel to the easy axis

    26. The other solution fro eq.1 is given by

    27. m = h, ( m = M/Ms ; h = H/Ha )

    29. In summary A purely hard-axis, uniaxial magnetization process involves rotation of the domain magnetization into the field direction. This results in a linear m-h characteristic. An easy-axis magnetization process results in a square m-h loop. It is chracterized in the free-domain-wall limit, Hc=0 and in the single-domain or pinned wall limit by rotational hysterisis, Hc=2Ku/Ms.

    30. Stoner-Wohlfarth Model

    32. From eq.(3) and (4) we obtain

    35. Wall motion coecivity Hc

    37. Micromagnetics-Dynamic Simulation

    40. Micromagnetics-dynamic simulation

    41. Hysterisis Loop Simulation (an example Co/Ru/Co and Co/Ru/Co/Ru/Co Films)

    44. The other fields

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