45th Magnetic Materials Center Seminar
May 25, 2007, 9:00
7th floor seminar room, Sengen
Co-tunneling effect in magnetic double tunnel junctions
H. Sukegawa
In an ultra-small double tunnel junction (DTJ), Coulomb blockade (CB), which suppresses electron tunneling, dominates the electron transport. Within the CB dominant region, TMR enhancement due to a co-tunneling process, in which multiple electron tunneling occurs collectively, is predicted by Takahashi and Maekawa in an ultra-small DTJ [1]. In this study, I have fabricated a DTJ with a ferromagnetic nano-particle layer (hybrid junction) and observed the enhancement of TMR ratio within the CB region. This structure has a 2-dimensional magnetic nano-particle layer, which can be treated as a parallel array of a large number of nano-scale DTJs.
DTJs with the structure of (buffer layer)/IrMn (12)/Co90Fe10 (3)/AlOx (1.4)/Co90Fe10 (1.0)/AlOx (1.4)/Co90Fe10 (3)/IrMn (12) /Ta cap (in nm) were prepared on a thermally oxidized Si wafer using a magnetron sputtering system. The middle Co90Fe10 layer, which is inserted between the two AlOx layers, becomes discontinuous due to the difference in the surface energy of these layers.
At low temperature (< 50 K), a CB is observed in current-voltage (I-V) curves, and TMR ratios increase significantly within the CB region in the hybrid junction. I confined that the TMR enhancement is attributed to the co-tunneling process. This is the first observation of the TMR enhancement in a hybrid junction structure; furthermore, this observation is an important result because it closely connects both a TMR effect and a CB effect.
[1] S. Takahashi and S. Maekawa, Phys. Rev. Lett. 80, 1758 (1998).

Structure, magnetic properties and spin polarization of Co2CrxFe1-xSi bulk alloys and thin films
S.V. Karthik
We have investigated the effect of Cr substitution for Fe on the spin polarization, P, of Co2CrxFe1-xSi bulk alloys using the point contact Andreev reflection (PCAR) method. The alloys with composition 0

T.O. Seki