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XAFS and CEMS study of dilute magneto-optical semiconductor,Fe doped TiO2 films

Kiyoshi Nomura a,⁎, Hiromi Eba b, Kenji Sakurai b, Alexandre Rykov a,c, Tetsuya Hasegawa d

a School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japanb National Institute for Material Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan

c Siberian Synchrotron Radiation Center, Lavrent'eva 11, Novosibirsk, 630090, Russiad School of Science, The University of Tokyo,7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan

Available online 6 April 2007

Abstract

TiO2 films doped with 6% Fe were prepared by pulsed laser deposition (PLD) under different oxygen pressures, and characterized by X-rayabsorption fine spectra (XAFS) and conversion electron Mössbauer spectra (CEMS). The edge energy and spectrum profiles of Fe– and Ti KX-ray absorption showed only Fe3+ and Ti4+ states for rutile TiO2 films prepared under 10−1 Torr, the metallic Fe and Ti4+ for rutile TiO2 filmsprepared in 10− 6 Torr, and the metallic Fe and the average valance of less than “4+” for Ti in TinO2n−x films prepared by the PLD under10−8 Torr. The metallic Fe clusters are also found in the TEM images of TinO2n−x film. Magnetic property of Fe doped TiO2 films prepared byPLD at high vacuum (10− 6 and 10−8 Torr) is considered to originate mainly from the magnetic metal iron clusters.© 2007 Elsevier B.V. All rights reserved.

Keywords: Fe doped TiO2 films; Dilute magneto-optical semiconductor (DMS); Ti K X-ray; Fe K X-ray; EXAFS; XANES; Mössbauer spectra

1. Introduction

It has been found recently that TiO2 films doped with a smallamount of Co showdilutemagneto-optical semiconductor (DMS)properties at room temperature [1]. DMSmaterial are prospectiveto develop new spintronics devices such as spin field effecttransistor. The ferromagnetic properties of Ti1−x

57 FexO2 increasewith the decrease of doping 57Fe content [2]. Fe doped rutileTiO2−δ films show the ferromagnetism at room temperature. Theresistivity shows nearly metallic behavior at room temperaturebut semiconducting at low temperature [3]. Fe doped TiO2 filmsprepared on γ-Al2O3 substrate at the temperatures from 600 to675 °C by pulsed laser deposition (PLD) under oxygen pressureof PO2

=10−6 Torr also show the ferromagnetism and Kerr effectstrongly [4]. Magnetic domain structures were observed in Fedoped TiO2 films prepared in 10−6–10−8 Torr, suggesting thepresence of long range ordering of magnetic moment induced byFe doping in these thin films. These films were characterized by57Fe conversion electron Mössbauer spectroscopy (CEMS) in

order to clarify the chemical states of iron doped into TiO2 films[5]. We found three kinds of iron species in the TiO2 films. Adoublet of paramagnetic Fe3+ (isomer shift, δ=0.37(2) mm/s andquadrupole splitting, Δ=0.9(2) mm/s) was observed for trans-parent TiO2 films prepared under the low-vacuum condition of10− 1 Torr. Two sextets with the larger magnetic field(Bhf=33.0 T) and the smaller magnetic field (Bhf=29.5 T) wereobserved for TiO2 films prepared under high-vacuum pressure of10−6 and 10−8 Torr.

The purpose of this paper is to clarify more the chemicalstates of Fe-doping species and host Ti ions in these oxides.Therefore, we measured Fe- and Ti- K X-ray absorption finestructure (XANES and EXAFS) of these films as prepared andannealed, using synchrotron radiation. Fe clusters in theseoxides were also observed by transmisson electron microscopes(TEM). These observations consist with the results of CEMS.

2. Experimental details

Samples No. 1, No. 2, and No.3 were as prepared on γ-Al2O3

substrate heated at 650 °C by PLD (KrF laser wave length:248 nm, output power: 5 J/cm2 pulse, pulse interval:6 ns, pulse

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⁎ Corresponding author. Fax: +81 3 5841 6017.E-mail address: k-nomura@t-adm.t.u-tokyo.ac.jp (K. Nomura).

0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.tsf.2007.03.177

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frequency:2 Hz) under the three different oxygen atmospheres of10−1, 10−6 and 10−8 Torr, respectively [5]. Some samples werepost-annealed to study the different valence states. Sample No.2'was obtained by annealing as prepared sample No.2 at 300 °C inair for 20 h and at 400 °C in oxygen atmosphere for 16 h andfurther at 900 °C for 4 h in Ar+5%H2 atmosphere. Sample No.3'was obtained by annealing sample No.3 at low temperature of300 °C for 2 h in air.

CEMS can characterize a thin surface layer within ca. 100 nmin thickness non-destructively and selectively. CEMS spectraweremeasured using a homemade back-scattered type ofHe+5%CH4 gas counter [6]. The velocity was calibrated using a metallicFe foil.

TEM images of a cross section of the films prepared at10−8 Torr were taken by an electron microscope (JEM-2010F),applied at 200 kV, and some spots in the films were analyzed byenergy dispersed X-ray (EDX).

X-ray absorption spectra of XANES and EXAFS weremeasured at a station of BL-4A (bending magnet, XRF) in SRring of KEK-PF (2.5 GeV, ∼70 mA (single bunch)) and BL-9C(XAFS line, 2.5 GeV, ∼450 mA at the end of 2006), using a

monochromator of Si(111) double crystal (ΔE/E ∼10−4) withthe slit of 2 mm (V)×7 mm (H) and a detector of Si(Li) solid-state detector when fluorescence yield was used or an ionchamber when transmission measurement was used.

3. Results and discussion

Fe K-edge XANES and EXAFS spectra of titanium oxidefilms doped with 57Fe, measured by a fluorescence yield modeare shown in Fig. 1a) and b) respectively together with those ofreference materials, measured by a transmission mode. Theedge energy of X-ray absorption increases typically with theincrease of the oxidation number of the species under study.

In first sample No.1, it is reasonable that we observed the FeK-edge XANES spectrum, which showed the Fe3+ state,although the Fe–EXAFS was different from that of Fe2O3. TiK-edge XANES and EXAFS spectra measured by a fluores-cence yield mode are shown in Fig. 2 together with those ofreference materials, alsomeasured by a fluorescence yieldmode.The Ti–XANES spectrum was a little different from that of purerutile TiO2 although the chemical state was similar to Ti4+. Fromthese results, it is considered that 6% doped Fe3+ areincorporated into TiO2 to form a rutile structure.

Fe K-edge XANES of samples No.3 and No.3' showedalmost all metallic states before and after annealing at 300 °C

Fig. 1. Fe K-edge XANES and EXAFS spectra of Fe doped TiO2 films preparedby pulsed laser deposition (PLD). Pressure of oxygen and annealing condition;No.1: as prepared by PLD under 10−1 Torr, No.2: as prepared by PLD under10−6 Torr. No.2': sample No.2 annealed at 300 °C for 20 h, oxidized at 400 °Cfor 6 h in oxygen atmosphere, and further annealed at 900 °C for 4 h in Ar+5%H2 atmosphere. No.3: as prepared by PLD under 10−8 Torr, No.3': No.3annealed at 300 °C for 2 h in air.

Fig. 2. Ti K-edge XANES and EXAFS spectra of Fe doped TiO2 films preparedby pulsed laser deposition. Samples used are the same in Fig. 1.

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for 2 h in air, whereas Ti K-edge XANES showed the inter-mediate valance of 3+ and 4+ and Ti–EXAFS showed adifferent atomic arrangement from TiO2. Fe K-edge XANESshowed no Fe4+, and the possibility of the Fe4+ state producedonly under high pressure is ruled out. Two magnetic sextets anda small amount of paramagnetic peaks were observed for thefilms prepared under 10−8 Torr although the structure is amagneli type of TinO2n−x [5]. TEM images also did not show theperfect atomic arrangement of TiO2 films as shown in Fig. 3.Fe cluster was found because Fe enriched area with 50% of TiX-ray intensity was observed at point A by EDX analysis underTEM images, whereas the slightly diluted Fe with 2% of Fewith respect to 100% Ti K X-ray intensity was observed at pointB in the film. A large number of defects are observed at theinterface between substrate γ-Al2O3 and titanium oxide filmbecause of crystallographic mismatch.

In CEMS data [5], the chemical state of sample No. 3displayed not so much change when the sample was annealed at300 °C for 2 h in air to make sample No. 3'. In Fe XANES ofsamples No.3 and No. 3’ (Fig.1), the pre-edge peak intensitiesare similar. Comparing the EXAFS profiles in the samples No.3and No.3’ with that for bulk Fe foil, we find No. 3' a little closerto Fe than No. 3. Since the contents of Fe in the whole films areonly 6%, the Fe clusters are unlikely to agglomerate into largerclusters by annealing at low temperature. The segregation of Feassociated with the oxidation of TinO2n−x to TiO2 is consideredto occur more preferably by annealing the film in air.

XANES spectrum of as prepared sample No.2 is similar tothat of sample No.3, but a little far from XANES profile of bulkFe. In the CEMS spectrum, the relative intensity of the Fe sextetcomponent with 29 T was a little larger in sample No. 2 (29%)than in No.3 (17%). The Fe component with 29 T did notdisappear after annealing at 300 °C in air for 20 h and further inO2 atmosphere for 6 h although the metallic Fe with 33 T wasoxidized a little into paramagnetic Fe3+. Fe component with29 T may be due to metallic Fe incorporated with a Ti or Oatom, which is located at the surface of metallic Fe clusters. TheKerr effect of sample No.2 is larger than that of sample No.3 [5].

From the XANES results of sample No.2', it is found that theedge energy of peak profile is shifted a little from that of Fe2+O

reference, whereas Ti K-edge XANES shows the same oxidationstate as the one in pure TiO2. CEMS of sample No.2' showed twoparamagnetic species of Fe2+ (δ=1.06mm/s,Δ=0.60mm/s, AreaIntensity; 86%, and δ=1.01 mm/s, Δ=1.96 mm/s, Area intensity;14%), which are different from the parameters of FeO(δ=0.93 mm/s, Δ=0.8 mm/s). The former species of Fe2+ isclose to Fe2+ in FeTiO3, which shows δ=1.08 mm/s andΔ=0.66 mm/s [8]. These results suggest that Fe2+O is notproduced in the TiO2 films. The Fe component with largeΔ=1.96 mm/s may be due to inverse spinel Fe2TiO4.

Inaba et al. [4] detected only Fe3+ at the film surface becauseonly the surface was observed by XPS. This corresponds to theparamagnetic Fe3+ component observed in CEMS [5]. But, infact, metallic Fe existed mostly in the deep layers of TiO2 filmsbecause annealing at 300 °C could not easily oxidize metallicFe. Lee et al. [2] reported that the gradual increase of Feconcentration in Fe doped anatase TiO2 powder suppressesrapidly the ferromagnetic properties rather than enhances them.And two species of the ferromagnetic and the paramagneticphases of Ti0.995Fe0.005O2 are found to show the isomershifts δ=0.31 mm/s and δ=0.26 mm/s, i.e. the δ valuesappropriate of high spin Fe3+. The metal Co nanoclusters arefound in epitaxial Ti0.96Co0.04O2 thin films prepared by PLD[7]. The origin of “DMS-like" behavior of Fe doped TiO2

film by PLD may be also, therefore mainly due to ferromagneticmetal Fe clusters. The magnetic component with 29 T isassigned to the Fe component located on the surface layer of Feclusters in the TiO2 matrix. This component contains more

Fig. 3. TEM images of Fe doped TiO2 film on γ-Al2O3 heated at 650 °C, prepared by PLD under 10−8 Torr. The right picture is an expanded picture of point A in theleft picture. Point A: Fe enriched area; EDX intensity ratio of Fe/Ti=0.5, Point B: Fe diluted area; EDX intensity ratio of Fe/Ti=0.02 when EDX analysis was done inthe spot with 1.5 nm in diameter in the vicinity of Points A and B.

Table 1Fe and Ti states of Fe doped TiO2 films prepared by PLD and annealed

Sample Fe states Ti states

No.1 (10−1 Torr) Fe(III) in TiO2 IV, TiO2 (rutile)No.2 (10−6 Torr) Fe(0), metallic Fe IV, TiO2 (rutile)No.2' (anneal) a Fe(II) in FeTiO3, IV, FeTiO3

No.3 (10−8 Torr) Fe(0), metallic Fe III–IV (magneli)No.3' (anneal) b Fe(0), metallic Fe III–IV (magneli)a After annealing sample No.2 at 300 °C in air and 400 °C in O2 atmosphere,

and further at 900 °C in 5% H2+Ar atmosphere for 4 h.b After annealing as prepared sample under 10−8 Torr at 300 °C for 2 h.

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abundant in TiO2 film prepared under 10−6 Torr than 10−8 Torr,and it is thus considered to contribute to DMS properties as wellas metallic Fe cluster itself.

The above results are summarized in Table 1.

4. Conclusions

Fe doped titanium oxide films were prepared by PLD andpost-annealed in various atmospheres. XAFS results weresupported by CEMS results. It is concluded that paramagneticFe3+ species is incorporated in TiO2 film prepared by PLD under10−1 Torr (not ferromagnetic at RT), and metallic Fe clusters areproduced in the TiO2 films prepared by PLD at high vacuumconditions of 10−6 and 10−8 Torr (ferromagnetic at RT). Thechemical state of titanium in rutile TiO2 film prepared under10−6 Torr is Ti4+, whereas the chemical state of titanium is closeto the intermediate state of Ti3+ and Ti4+ rather than Ti4+ forTiO2 film prepared under 10−8 Torr. The combination of bothXANES and CEMS is very effective for characterization ofdilute magnetic semiconductor films doped with Fe.

Acknowledgements

Authors expressed to thank Dr. K. Inaba, who was a graduatestudent in Tokyo Institute of Technology, for initial sample

preparation, Mr. T. Ito, and Mr. K. Ibe, School of Engineering,The University of Tokyo, for taking TEM pictures and Dr. CesarBarrero, a visiting scientist from University of Antioquio,Columbia, for discussion. This study was partially supported byAsahi Glass Foundation. XAFS experiment was performed withthe approval of the Photon Factory Program Advisory Com-mittee (Proposal No. 2002S-003).

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