66th Magnetic Materials Center Seminar
February 7, 2008, 9:00-
7th floor small seminar room, Sengen
Ab-initio and micromagnetic studies of strain induced anisotropy in Cobalt ferrite
Z. Gercsi
The extensive magnetic properties make the Cobalt ferrite a promising candidate for spintronics, magnetoelectrics, magnetostrictive and magnetic recording applications. Recently, several papers reported high coercivity up to 12.5kOe in highly strained Co-ferrites in both thin film and mechanical milled powders [1-3]. Although the magnetic properties of these matters also depend on the microstructure, there is a clear correlation between the sign of magnetic anisotropy and strains [4]. On the other hand, Co-ferrite is also a good candidate as a spin filter material in magnetic tunneling junctions (MTJ), however a poor temperature dependence of spin filtering efficiency up to date was reported. Therefore ab-initio calculations on CoFe2O4 are performed to investigate the effect of extensive stress conditions such as hydrostatic compression, tetragonal/rhombohedral deformation and internal strain on the electronic structure, respectively. The GGA theorem was used for the calculations including the spin-orbit interactions (SOI) because it can correctly predict the experimental lattice parameter as compared to the LDA method, which rather underestimates it (V/Vexp?0.95). Moreover, the inverse spinel scenario was applied as it was found experimentally that the Co ions occupy mostly (80%) octahedral sites in the CoFe2O4 [5]. The sign of the calculated anisotropy energies depends on the strain conditions such as compression or tension which turns out to be in good agreement with experimental results. These findings predict that by thoughtful substrate selection the direction (in plane or out of plane) of high magnetic anisotropy could be tailored in Cobalt ferrite thin films. My theoretical investigations found out that rather the magneto-elastic anisotropy than the spin-orbit interaction (magneto-crystalline anisotropy) is responsible for the high coercivity in strained Co-ferrite.
[1] J. Yin, J. Ding, B. Liu, X. Miao, and J. Chen Appl. Phys. Lett. 88, 1622502 (2006)
[2] B. H. Liu, and J. Ding, Appl. Phys. Lett. 88, 042506 (2006)
[3] W. Huang, J. Zhu, H. Z. Zeng, X. H. Wei, Y. Zhang, and Y. R. Li Appl. Phys. Lett. 89, 262506 (2006)
[4] A. Lisfi, C. M. Williams, A. Johnson, L. T. Nguyen, J. C. Lodder, H. Corcoran, P. Chang, and W. Morgan J. Phys.: Condens. Matter 17 (2005) 1399-1204
[5] L. Braicovich, A. Tagliaferri, G. van der Laan, G. Ghiringhelli, and N. B. Brookes Phys. Rev. Lett. 90, 117401 (2003)

Search for highly spin polarized Heusler alloy -what we have done and what to do
K. Hono
In this talk, I will review our work on search for highly spin polarized Heusler alloys using PCAR and discuss what we should do in the next stage.