Photon Factory Activity Report 2004 #22 Part B (2005)
Materials Science

11A/2003G010

X-ray Absorption and X-ray Magnetic Circular Dichroism Studies ofa Monatomic bcc-Co(001) Layer Facing an Amorphous Al-O Tunnel Barrier
Toshiaki SAITO1*, Toshikazu KATAYAMA2, Shinji SAITO1, Yoshinari KUROSAKI2,Kouya MIYOKAWA1, Tomoyuki KAMINO1, Kie KOBAYASHI1, Yoshishige SUZUKI2,3,Shinji YUASA2,3, Hirotaka MANAKA4 and Ttsuneharu KOIDE412

Department of Physics, Toho Univ., Funabashi, Chiba 274-8510, JapanNational Institute of Advanced Industrial Science and Technology, Tsukuba, 305-8568, Japan
3CREST-JST, 4-1-8, Honcho, Kawaguchi, Saitama 332-0012, Japan
4
KEK Photon Factory, Tsukuba, Ibaraki 305-0801, JapanIntroduction
The magnetic tunnel junctions (MTJs) with anamorphous Al-O tunnel barrier show a large tunnel
magnetoresistance (TMR) ratio up to 70%, and the MTJs
with crystalline MgO(001) tunnel barrier show up to230% at room temperature [1], which attracts muchattention because they are applicable to magnetoresistive
random-access-memory and reading heads for magnetic
recording. Experimental results of TMR have been
usually interpreted by Julliere’s expression with spinpolarization of the density of states at the Fermi level ofboth ferromagnetic electrodes. However, the amplitude ofTMR and even its sign cannot be fully explained by thetheory. Oxidization of the ferromagnetic electrode at theinterface could be an origin of the problem. In order toexamine the oxidization state of the interface, weevaluated magnetic moments of the one monolayer (1ML) Co(001) facing Al-O barrier by using x-ray magnetic
circular dichroism (XMCD) measurements.

magnetic moments of the 1-ML Co by using the sum
rules from the energy integration of the XAS and XMCDspectra; spin magnetic moment is 1.58 ± 0.2 µ B; orbital
moment is 0.18 ± 0.2 µ B and the total magnetic moment of
1-ML Co is 1.76 ± 0.2 µ B which is almost the same value
as bulk hcp-Co (1.774 µ B). On the other hand, the totalmagnetic moment 0.51± 0.1µ B is obtained for Co-O/Al-O.
It also clearly indicates that the 1 ML-Co adjacent to AlO barrier is not oxidized.

Experimental
1-ML Co was fabricated on the Fe(001)(6 ML) witha buffer layer of MgO(001)/Cr(001)/Au(001) by using
molecular beam epitaxy (MBE) method. After the growthof Co, Al (1 nm) was deposited only on half of the Cosurface. Then, the whole surface was naturally oxidized(20 min, in situ) in O2 atmosphere (100 Torr) in order toprepare Al-O and CoO layers. Finally, Al-O (3.5 nm) wasdeposited by using a reactive deposition in O2 atmosphereof 5x10-6 Torr. Thus, two samples with interfaces of (a)Co/Al-O and (b) CoOx/Al-O (intentionally oxidized Co)
were prepared. XAS and XMCD of these samples weremeasured from 635 to 855 eV at the KEK Photon Factory(BL11A). A magnetic field (B) of ±3T was applied for
XMCD measurement using an ultrahigh vacuum
superconducting magnet.
Results and Discussion
Extra shoulders of Co L2,3 in XAS due to multiplet
effects are not observed [2] for Fe(001)/Co(1 ML)/Al-O
contrasting with Fe(001)/CoOx(1 ML)/Al-O as shown in
Figs.1(a) and (b), indicating that the 1 ML-Co is notoxidized. XMCD at Co L2,3 for the samples (a) and (b)are also shown in the figure (XMCD were normalized by
the peak height of Co L3 XAS). We evaluated theFig. 1 XAS, XMCD and XMCD integration for samples withinterfaces of (a) 1ML-Co/Al-O and (b) CoOx/Al-O.

References[1] S.Yuasa, et al., Nature Materials 3, 968 (2004).
[2] T. Saito et al., J. Magn. Magn. Mat. 272-276, e1489(2004).
* saito@ph.sci.toho-u.ac.jp

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