Invited Paper MI-WeM9
Controlling Magnetic Anisotropy and Probing Magnetic Structure in Magnetic Nanoparticles and Ferromagnetic/Antiferromagnetic Bilayers

Wednesday, October 17, 2007, 10:40 am, Room 619

Controlling the magnetic orientation and imaging magnetic structure are two of crucial issues in both aspects of fundamental science and application for magnetic nanomaterials. In particular, tuning perpendicular magnetic anisotropy by the more concise and efficient process draws a lot of attentions due to the possible application for perpendicular medium with high storage density. In this work, an enhanced perpendicular magnetic anisotropy of ferromagnetic thin films is demonstrated by introducing an antiferromagnetic (AF) underlayer. A new kind of spin-reorientation transition is also observed with varying thickness of the AF layer. This finding is shown to be related to the strength of the AF coupling of the AF layer. Controlling the magnetic anisotropy can be also important in the magnetic domain imaging with in-plane sensitivity by spin-polarized scanning tunneling microscopy (SP-STM). A simple method by using a ring-shaped magnetically coated wire as the tip of SP-STM is shown to be able to have the spin contrast easily in the in-plane direction of the film. A well-defined magnetization orientation of magnetic tip is achieved due to controlled anisotropy caused by geometrical asymmetry. Finally, magnetic coupling and magnetic structure in magnetic self-aligned Fe particles grown on the single crystalline oxide layer Al₂O₃/NiAl(100)^1,2 will be also discussed. With help of the technique of scanning electron microscopy with polarization analysis (SEMPA) the magnetic domain is imaged, revealing a vortex structure, which is suggested to be attributed to a dipole-dipole interaction. Furthermore, capping the magnetic particles with non-magnetic metallic layer (Cu) can enhance the magnetic coupling, and in turn the Curie temperature of the system.³ This finding can also be confirmed in the enhanced spin contrast observed by SEMPA for magnetic particles with capping layer. The magnetic coupling under magnetic particles is shown to be able to propagate through the Cu layer.

¹ W.C. Lin, C. C. Kuo, M.F. Ro, K. J. Song, and Minn-Tsong Lin, Appl. Phys. Lett. 86, 043105 (2005).
² W. C. Lin, S. S. Wong, P. C. Huang, C. B. Wu, B. R. Xu, C. T. Chiang, H. Y. Yen, and Minn-Tsong Lin, Appl. Phys. Lett. 89, 153111 (2006).
³ W. C. Lin, P. C. Huang, K. J. Song, and Minn-Tsong Lin, Appl. Phys. Lett. 88, 153117 (2006) .