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Journal of Magnetism and Magnetic Materials 310 (2007) 2174–2176

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XPS study of ferrimagnetic double perovskite thin films

H. Asano�, N. Koduka, Y. Takahashi, M. Matsui

Nagoya University, Furo-cho, Chikusa-ku, Nagoya, 464-8603, Japan

Available online 28 November 2006

Abstract

This paper reports X-ray photoelectron spectroscopy (XPS) study on thin films of half-metallic materials with an ordered double-

perovskite structure. The XPS core-level spectra were measured for several Sr2FeMoO6 (SFMO) and Sr2CrReO6 (SCRO) films with and

without in situ deposited thin overlayers of MgO and SrTiO3. For a fresh SFMO film surface, the XPS spectrum of the Mo 3d region

showed a multiple feature, which reflects the ferrimagnetic metallic nature. Whereas SFMO films with overlayers of SrTiO3 showed

degraded interface properties, films with overlayers of MgO showed the multiple feature similar to the fresh SFMO surface, indicating

that the electronic properties at the MgO interface were preserved. Very similar tendency was observed for the Re 4f core-level spectra of

the SCRO/overlayer systems.

r 2006 Elsevier B.V. All rights reserved.

PACS: 79.60; 75.70; 73.20-r

Keywords: X-ray photoelectron spectroscopy; Core-level spectrum; Binding energy; Half-metallic ferromagnet; Epitaxial thin film

1. Introduction

The double perovskites of the form A2BB0O6 have

attracted extensive interest both due to their rich physicsand due to their technological importance. Several experi-mental and theoretical studies have demonstrated thatSr2FeMoO6 (SFMO) and Sr2CrReO6 (SCRO) and otherrelated materials [1,2] exhibit a ferrimagnetic half-metallicground state with high Curie temperatures Tc (420K forSFMO and 635K for SCRO), which makes them potentialcandidates for spintronic applications. Since the electricaland magnetic properties of this type of oxides are easilymodulated through surface/interface effects, it is importantto understand the electronic structure of the surface/interface layers. In this paper, we report on the XPScharacterization of SFMO and SCRO thin films with andwithout thin overlayers of MgO and SrTiO3 in order toinvestigate the modification of electronic structures at thesurface and interface.

- see front matter r 2006 Elsevier B.V. All rights reserved.

/j.jmmm.2006.11.118

onding author. Tel.: +8152 789 3568; fax: +81 52 789 3237.

ddress: asano@numse.nagoya-u.ac.jp (H. Asano).

2. Experiment

Epitaxial thin films of SFMO and SCRO were sputterdeposited on the lattice-matched substrates of Ba0.4Sr0.6-TiO3-buffered and bare SrTiO3, respectively. Details ofdeposition procedures and film properties were previouslyreported [3]. X-ray diffraction analysis confirmed that thesefilms were in high phase purity and a high degree of theB-site ordering (the long-range order parameter S40.9).Surface characterization by atomic force microscopydemonstrated that these films had atomically flat andwell-defined surfaces free from any surface precipitates.Thin overlayers of MgO and SrTiO3 of 1 nm in thicknesswere in situ deposited onto SFMO and SCRO films. X-rayphotoelectron spectroscopy (XPS) spectra were acquiredwith MgKa radiation (1254 eV).

3. Results and discussion

At first, the XPS investigations were undertaken on theSFMO and SCRO film surfaces in an as-grown state. It isfound that the spectra of the Sr 3d core-level region forboth films show dominant contribution from those of theSr-containing perovskites with a small amount of surface

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Binding Energy (eV)

Re4f

SrTiO3

SCRO

MgO

as-grown

1 nm Cap/SCRO

a

b

Fig. 2. XPS spectra of the Re 4f core level of SCRO films (a) without an

overlayer and (b) with 1-nm-thick overlayers of MgO and SrTiO3.

H. Asano et al. / Journal of Magnetism and Magnetic Materials 310 (2007) 2174–2176 2175

impurity phase. The SFMO films exhibited a singlespin–orbit doublet with the Fe 2p3/2 peak at 710.5 eV,which corresponds to the oxidized Fe. The SCRO filmsexhibited a single spin–orbit doublet with the Cr 2p3/2 peakat 575.5 eV, which can be assigned to Cr3+. As far as the Sr3d core level and the 2p core level of the magnetic Belements are concerned, the observed spectral behaviors areconsistent with those expected for these materials [1,2].

Since the electronic structures of nonmagnetic B0 ionshave been shown to play an important role in the magneticand electrical properties of these materials, the Mo 3d corelevel of SFMO films and the Re 4f core level of SCRO filmswere systematically examined. Fig. 1 shows XPS spectra ofthe Mo 3d core level of SFMO films (a) without anoverlayer and (b) with 1-nm-thick overlayers of MgO andSrTiO3. For a fresh SFMO film surface, the spectrum ofthe Mo 3d main region showed a multiple feature, whichconsisted of, in addition to the simple Mo 3d5/2 and 3d3/2doublet peaks, the filling in the valley between the Mo 3d5/2and 3d3/2 doublet peaks and the tail at the low bindingenergy side. This multiple feature is an indication of theunusual electronic state of Mo in the metallic ferrimagneticSFMO [4], and is in striking contrast with the sharp Mo3d5/2 and 3d3/2 doublet peaks for insulating SrMoO4 phase,as shown in the dashed line in Fig. 1(a). As shown inFig. 1(b), the Mo 3d spectrum for an SFMO film with anMgO overlayer of 1 nm showed the multiple feature,indicating that the electronic structure of the interfacialSFMO layer was preserved. Interestingly, the multiplefeature of the Mo 3d spectra is strongly modified when theSFMO film is capped with an SrTiO3 overlayer.

Fig. 2 shows XPS spectra of the Re 4f main region ofSCRO films (a) without an overlayer and (b) with 1-nm-thick overlayers of MgO and SrTiO3. Although the largeRe 4f component is mainly responsible for the spectra in

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Binding Energy (eV)

Mo3d5/2Mo3d3/2

SrTiO3

1 nm Cap/SFMO

MgO

as-grown

SFMO

SrMoO4

a

b

Fig. 1. XPS spectra of the Mo 3d core level of Sr2FeMoO6 films

(a) without an overlayer and (b) with 1-nm-thick overlayers of MgO and

SrTiO3. The dashed line in (a) denotes the corresponding spectra of the

insulating SrMoO4.

Fig. 2, small contributions from O 2s and Cr 3p make thequantitative analyses difficult. Even then, the shape and thebinding energy of the Re 4f peak for a SCRO surface arequite different from those of simple Re oxides [5]. Thisbroad feature in the Re 4f peak is possibly due to theunusual electronic state of Re in the metallic ferrimagneticSCRO, as in the case of Mo in the SFMO. As shown inFig. 2(b), the Re 4f spectrum for an SCRO film with anMgO overlayer of 1 nm showed the similar broad feature,suggesting that the electronic structure of the interfacialSCRO layer was preserved. Note that the component at48.5 eV for SCRO film with MgO is ascribed to the Mg 2pcore level. When the SCRO film is capped with a SrTiO3

overlayer, the broad feature of the Re 4f spectra is stronglymodified. Very similar tendency between the SFMO andSCRO cases may reflect the intrinsic electronic andchemical nature of the ferrimagnetic metallic double-perovskite materials. Further studies are necessary toelucidate the origin of the interfacial modification of thedouble-perovskite films with overlayer materials.

4. Conclusions

We systematically investigated the heterointerfaces ofoverlayer/SFMO (SCRO) using the Mo 3d (Re 4f) core-level photoemission. The XPS observation reveals that thequality of heterointerfaces of overlayer/SFMO(SCRO)strongly depends on the overlayer materials. Such informa-tion about the surface/interface characteristics would beuseful for developing multilayer tunneling devices based onthese materials.

Acknowledgment

This work was partially supported by a Grand-in-Aidfor Scientific Research from the Japan Society for thePromotion of Science (No. 1736020).

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