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

11A, NE1B/2003G010

Magnetic and Electronic States of a Monatomic Fe(001) Layer Facing
an Epitaxial MgO(001) Tunnel Barrier Studied by Using
XAS and XMCD Measurements
Toshiaki SAITO1*, Kouya MIYOKAWA1, Shinji SAITO1, Tomoyuki KAMINO1, Koji Hanashima1,Toshikazu KATAYAMA2, Yoshishige SUZUKI2,3, Kazutoshi Mamiya4, Ttsuneharu KOIDE4,and Shinji YUASA2,31

2

Department of Physics, Toho Univ., Funabashi, Chiba 274-8510, Japan
National Institute of Advanced Industrial Science and Technology, Tsukuba, 305-8568, Japan
3CREST-JST, 4-1-8, Honcyo, Kawaguchi, Saitama 332-0012, Japan
4
KEK Photon Factory, Tsukuba, Ibaraki 305-0801, JapanIntroduction
Magnetic tunnel junctions (MTJs) consisting of two
ferromagnetic layers separated by an insulating layer(tunnel barrier) exhibit the tunnel magnetoresistance(TMR) effect, which has application to magnetoresistive
random-access-memory (MRAM) and magnetic sensors.
Fully epitaxial Fe(001)/MgO(001)/Fe(001) is one ofpromising candidates for novel high-MR MTJs because
an extremely high MR ratio has been predicted byabinitio calculations [1]. In fact, in our study [2], a large
MR ratio over 180% at RT (250% at 20K) wasexperimentally achieved. On the other hand, there has
also been pointed out that partially oxidized FeOx could
be formed at the Fe(001)/MgO(001) interface, which
considerably reduces the MR ratio. In this report, wepresented the electronic and magnetic states of 1ML-Fe
layer adjacent to MgO(001) single crystalline layer byusing x-ray absorption spectra (XAS) and x-ray magnetic
circular dichroism (XMCD) at the Fe L2,3 edges [3].
Experimental
We prepared fully epitaxial multilayer with structure ofsample (a) bcc-Co(001) (6ML) / bcc-Fe(001) (1ML)/MgO(001)(20Å)/Al-O(11 Å) with a buffer layer ofMgO(001)substrate/MgO(001)(200Å)/Fe(001)(500Å)/Cr(001) (500 Å) by using molecular beam epitaxy (MBE).
The single crystalline MgO(20 Å) layer was grown at RT
using electron beam evaporation of MgO source material.
For a reference of Fe oxide, sample (b) Co(001) (6ML)/FeOx(1ML)/a-Al-O(25Å) with a buffer layer ofMgO(001)/Cr(001)(500 Å )/Au(001)(2000 Å) was also
prepared. The XAS and XMCD measurements wereperformed at KEK Photon Factory BL11A. The XAS ofFeOx was measured at the KEK PF-AR NE1B.
Results and Discussion
XAS and XMCD spectra of Co(001)(6 ML)/Fe(001)(1ML)/MgO(001) at the Fe L2,3 edges are shown in Fig.1.
The XAS of FeOx(1 ML) is shown in Fig.2, in which
extra shoulders (denoted by arrows) due to the multiplet
effect typical to FeOx are observed. In contrast, suchshoulders are not seen in Fig.1. This result indicates thatthe Fe 1 ML facing the MgO(001) layer is not oxidized.
We evaluated the magnetic moments of the Fe 1 ML byusing the sum rules from the energy integration of theXAS and XMCD spectra; spin magnetic moment is 2.41 ±148

Users' Report

0.1 µB; orbital moment is 0.13 ± 0.1 µB and the totalmagnetic moment of Fe 1 ML is 2.54 ± 0.1 µB which isconsiderably higher than that of bulk Fe (2.07 µB). We
also obtained almost the same value for Fe 2ML sample[3]. On the other hand, the magnetic moment of FeOx
(0.27 ± 0.1µB) is an order of magnitude smaller than the Femoments. These results also suggest no oxidation of theFe layer facing a single crystalline MgO(001).

Fig. 1 XAS, XMCD and XMCD integration for samples withinterfaces of 1ML-Fe/MgO

Fig. 2 XAS spectra at Fe L2,3 edges for FeOx/a-Al-O.

References[1] W. H. Butler et al., Phys. Rev. B 63, 54416 (2001).
[2] S.Yuasa, et al., Nature Materials 3, 968 (2004).
[3] K. Miyokawa et al., Jpn. J. Appl. Phys. 44, L9 (2005).

* saito@ph.sci.toho-u.ac.jp