120
121
or Δ5 symmetries are attenuated by the filtering effect, so that Δ1 symmetry electrons governs the tunneling process. Furthermore, the SDRT effect through the QW states confined in the ultrathin Fe(001) layer indicates that the spin, symmetry, and wave vector of electrons are conserved through the coherent tunneling process. It is noted that the appearance of a strong peak at 0 V shows that the dominant Fe thickness of the MTJ is five MLs, ~0.7 nm, while a relatively weak peak at around −0.6 V indicates that the local existence of four or six MLs in the MTJ area. It was concluded that the PMA at the Fe/MgO interface is likely to little influence the Δ1 electrons’ transport. The present results may give an insight into the understanding of the coherent and the resonant tunneling under a perpendicular magnetization configuration.
3) The magnetic anisotropy property of ultrathin Fe(0.7 nm) layers covered with the MgAl2O4 layers having three different crystallographic structures, i.e., monocrystalline, polycrystalline, and amorphous, were demonstrated. Regardless of the crystalline structure of the MgAl2O4 layer, we obtained the perpendicularly magnetized Fe layers (Keff > 0), after annealing process ranging from 300 to 450°C. The enhanced interface PMA and increased magnetization with respect to the annealing temperature suggested that abundant oxygen atoms in Fe layer were absorbed to the MgAl2O4 layers.
A large Keff of ~0.4 MJ/m3 was obtained for the Fe/mono-MgAl2O4 structure which is comparable to that of CoFeB/MgO structures. The Fe/poly- and a-MgAl2O4 structures exhibited Ki twice as large as that for the Fe/mono-MgAl2O4 structure, after annealing at 450°C.
These values for Ki are much larger than that of CoFeB/MgO, or Co2FeAl/MgO structures.
The magnetic anisotropy property of ultrathin Fe(0.7 nm) layers covered with the Al2O3 layers were demonstrated. In addition, the epitaxial growth of γ-Al2O3 on the ultrathin bcc-Fe(001) layer through the plasma oxidation process was demonstrated. Moreover, we obtained the perpendicularly magnetized Fe layers (Keff > 0) covered with the plasma oxidized γ-Al2O3 layer, after annealing process ranging from 300 to 350°C. The enhanced interface PMA and increased magnetization with respect to the annealing temperature suggested that abundant oxygen atoms in Fe layer were absorbed to the Al2O3 layers. A large Keff of ~0.42 MJ/m3 and Ki of ~0.89 mJ/m2 were obtained for the Fe/Al2O3 structure which is comparable to that of CoFeB/MgO structures.
The magnetic anisotropy property, electronic, and magnetic structures of ultrathin Fe(0.7 nm) layers covered with the C60 layers were demonstrated. When the samples are annealed at 250°C, the magnetization for each sample was turned out to be smaller than that of the bulk Fe. The magnetization was 40% smaller than that of the bulk magnetization which corresponds to the deadlayer thickness of ≈ 0.3 nm. However, the magnetization was not recovered even after the annealing at 450°C.
Furthermore, the depth-resolved The XAS and XMCD measurements were performed to investigate the electronic and magnetic structures of Fe(0.7 nm)/C60 structures. It was estimated that 1.8 ~ 2.0 electrons
122
transferred from Fe surface to C60 layer. XMCD spectra of Fe(0.7 nm)/C60 structures indicated that the large orbital magnetic moments are enhanced at the interface, in the comparison with the orbital magnetic moments in the bulk region. In the case of C K-edge XAS spectra, we found a peak that was located near 285 eV which could not observed from the bulk region or thicker C60 layer. It indicated that an extra local maximum of DOS in the valence band, due to the hybridization between Fe and C60 layer.
This work showed the possibility of the ultrathin Fe layer to be an electrode material in the p-MTJ structures for realizing the ultrahigh density MRAM. Moreover, it showed the superiority of ultrathin Fe layers over other complex compound electrode materials from the perspective of easiness of understanding the fundamental physics underlying the ultrathin magnetic bilayer systems, such as 1) effect of the interface oxidation condition on the interface PMA, 2) the correlation between the interface PMA and the perpendicular TMR effect, 3) the spin-dependent resonant tunneling in the perpendicular magnetized system, 4) the experimental evidence of the direct relationship between the interface PMA and the anisotropic orbital magnetic moment, and 5) the interface electronic and magnetic structure at the interface between Fe and organic molecule, etc.
123
Publication
Papers related to this thesis
1) J. W. Koo, S. Mitani, T. T. Sasaki, H. Sukegawa, Z. C. Wen, T. Ohkubo, T. Niizeki, K. Inomata and K. Hono
“Large perpendicular magnetic anisotropy at Fe/MgO interfaces”
Appl. Phys. Lett. 103, 192401 (2013).
2) J. W. Koo, H. Sukegawa, S. Kasai, Z. C. Wen, S. Mitani
“Magnetotransport properties in perpendicularly magnetized tunnel junctions using an ultrathin Fe electrode”
J. Phys. D: Appl. Phys. (Fast Track Commun.) 47 322001, 2014.
3) J. W. Koo, H. Sukegawa and S. Mitani
“Interface perpendicular magnetic anisotropy in Fe/MgAlO bilayer structures with different crystallographic characteristics”
Phys. Status Solidi RRL. doi: 10.1002/pssr.201409340 (2014).
4) Z. C. Wen, H. Sukegawa, T. Furubayashi, J. W. Koo, K. Inomata, S. Mitani, J. P. Hadorn, T.
Ohkubo and K. Hono
A 4-fold symmetry hexagonal ruthenium for magnetic heterostructures exhibiting enhanced perpendicular magnetic anisotropy tunnel magnetoresistance
Adv. Mater., doi: 10.1002/adma.201401959 (2014).
5) J. Okabayashi, J.W. Koo, H. Sukegawa, S. Mitani, Y. Takagi, and T. Yokoyama,
Perpendicular magnetic anisotropy at the interface between ultrathin Fe film and MgO studied by angular-dependent X-ray magnetic circular dichroism
Appl. Phys. Lett., 105, 122408 (2014)
Presentations related to this thesis
I. Domestic
1) J. W. Koo, S. Mitani, H. Sukegawa, Z. C. Wen, T. Niizeki, S. Kasai, K. Inomata.
“Perpendicular magnetic anisotropy in Fe/MgAl2OX structures”
The JSAP Sping Meeting, Waseda University, Japan, 2012, 3 (16a-B4-3) (Oral presentation) 2) J. W. Koo, S. Mitani, H. Sukegawa, Z. C. Wen, T. Niizeki, S. Kasai, K. Inomata.
Characterization of interface perpendicular magnetic anisotropy in Fe/MgAl2OX thin films The Physical Society of Japan Annual Meeting, Kwansei Gakuin University, 2012. 3 (24aPS-20) 3) S. Mitani, J. W. Koo, H. Sukegawa.
“Structural phase transition in the spinel (MgAl2OX) tunnel barrier.”
The JSAP Sping Meeting, Ehime University, Japan, 2012, 9 (11p-H6-10) 4) J. W. Koo, S. Mitani, H. Sukegawa, Z. C. Wen.
“Perpendicular magnetic anisotropy of Fe/MgO interfaces for magnetic tunnel junctions with