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rial in applications such as photo-catalyst[1] and gate .... brookite, are present. The average grain size d of. TiO2 can be estimated from the XRD profiles using.
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Chinese Journal of Aeronautics 20(2007) 162-167

Journal of Aeronautics

www.elsevier.com/locate/cja

Magnetic and Optical Properties of the TiO2-Co-TiO2 Composite Films Grown by Magnetron Sputtering LIU Fa-min*, DING Peng, SHI Wei-mei, WANG Tian-min Department of Physics, School of Sciences, Beijing University of Aeronautics and Astronautics, Beijing 100083, China Received 10 August 2006; accepted 3 November 2006

Abstract The TiO2-Co-TiO2 sandwich films were successfully grown on glass and silicon substrata making alternate use of radio frequency reactive magnetron sputtering and direct current magnetron sputtering. The structures and properties of these films were identified with X-ray diffraction (XRD), Raman spectra and X-ray photoemission spectra (XPS). It is shown that the sandwich film consists of two anatase TiO2 films with an embedded Co nano-film. The fact that, when the Co nano-film thickens, varied red shifts appear in optical absorption spectra may well be explained by the quantum confinement and tunnel effects. As for magnetic properties, the saturation magnetization, remnant magnetic induction and coercivity vary with the thickness of the Co nano-films. Moreover, the Co nano-film has a critical thickness of about 8.6 nm, which makes the coercivity of the composite film reach the maximum of about 1413 Oe. Keywords: TiO2-Co-TiO2 sandwich films; surface structure and properties; optical and magnetic properties

1 Introduction* Titanium dioxide (TiO2) is an attractive material in applications such as photo-catalyst[1] and gate oxides in MOS transistors[2]. Since the discovery of Matsumoto et al[3], Co/TiO2 thin films have drawn many researchers’ attention due to their very high Curie temperature (TC). While much work[4-10] has been done on the ferromagnetic properties of Co/TiO2 thin films grown by various methods, a few papers have been published on the magnetic properties of other 3d metals[11] doped with TiO2. Also have been made public a lot of researches as to other dilute magnetic semiconductors[12-16], which are of considerable use to spin injectors in spintronic devices. However, so far very few studies over optical and magnetic properties of the TiO2*Corresponding author. Tel.: +86-10-82317935. E-mail address: [email protected] Foundation items: Aeronautical Science Foundation of China (03G51069); Items of Engineering Research Institute, Peking University (ERIPKU-204031)

Co-TiO2 sandwich films have been performed. Recently, it is found that TiO2-Co-TiO2 sandwich films are able to be grown on glass and silicon substrata making alternate use of radio-frequency reactive (RFR) magnetron sputtering and direct current (DC) magnetron sputtering. This paper presents microstructures, optical and magnetic properties of these films, obtained by means of X-ray diffraction, X-ray photoelectron spectroscopy, Raman spectroscopy, UV-visible spectrophotometer and vibrating sample magnetometer. Moreover, a discussion is carried out about the influences of the Co nano-film and the substrate temperature on the optical and magnetic properties of the TiO2-Co-TiO2 sandwich films.

2 Experimental Procedure The TiO2-Co-TiO2 sandwich films were deposited making alternate use of radio frequency reactive magnetic sputtering and direct current magnetron sputtering. Glass and silicon substrata in size of

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20 mm×20 mm were ultrasonically washed successively in acetone, alcohol, and de-ionized water to achieve a clean surface before being placed in a vacuum chamber. Every substrate was further inversely sputtered for 5 min before the high film deposited. The target was separated from the substrate 5 cm. First, the high anatase TiO2 was deposited by radio frequency reactive magnetron sputtering. When the chamber was evacuated to a pressure of 5×10-5 Pa, a mixed stream of high purity (99.9999%) argon and oxygen (99.999 %) gases at a ratio of 50/5.0 sccm was introduced. A round high purity (99.999%) Ti plate Ø60 mm×3 mm thick was used as a target. The output voltage of the radio frequency sputter gun was 1 020 V. During sputtering, the chamber pressure was maintained 2 Pa. Then, the film was heated in situ at various temperatures (100-700 ℃) to ensure high-quality TiO2 films. And thus a Co nano-film was grown onto TiO2 layer by direct current magnetron sputtering. A high purity (99.98 %) Co plate Ø60 mm×3 mm thick was used as a target. When the chamber was again evacuated to a pressure of 5×10-5 Pa, only a stream of high purity (99.9999%) argon gas was introduced. The direct current sputtering power was about 26 W. In order to produce a TiO2-Co-TiO2 sandwich film, the second anatase TiO2 is formed by the same route the first one has been made. The film thickness d was determined by surface profilometry with a DEKTAK 3 α-step instrument. The deposition rate was obtained by dividing d by sputtering time. In our experiment, the sputtering rate of TiO2 was as large as ~2.5 nm/min, and that of Co about 0.28 nm/s. A computer-controlled sputtering system was used. The TiO2-Co-TiO2 sandwich films were identified by X-ray diffraction (XRD), Raman scattering and X-ray photoemission spectra (XPS). X-ray diffraction (XRD) studies were carried out on a rotating anode of D/max-rB type. The X-ray diffractometer was operated at a voltage of 40 kV and a current of 150 mA; CuKα1 radiation (λ=0.154 nm) monochromated with a graphite sample. X-ray photoemission spectra data were recorded with a VG ESCALAB MK II spectrometer using MgKα

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source (1 253.6 eV) radiation of 12 kV and 20 mA. All samples measured were calibrated with respect to the C1s peak at 284.6 eV. Raman scattering spectra were measured using a SPEX-1403 laser Raman spectrometer with a typical resolution of 1 cm-1 in the measured frequency region. All spectra reported here were measured in backscattering geometry using the 514.5 nm line of Ar+ laser excitation at the room temperature. An output laser power of 200 mW was needed to avoid sample overheated. Optical absorption spectra without consideration of reflection loss were measured with a Perkin-Elmer λ9 double beam spectrophotometer in the range from 300 to 1 000 nm at the room temperature. The magnetic properties were studied on an LDJ-model 9600 vibrating sample magnetometer at the room temperature.

3 Results and Discussion In our experiments, the thickness of about 80 nm of TiO2 films of the TiO2-Co-TiO2 sandwich film was kept constant, while the thickness of Co film varied from 3 nm to 20 nm. Fig.1 is the X-ray diffraction patterns pertaining to a TiO2-Co-TiO2 sandwich film and an anatase TiO2 film grown on a glass substrate making alternate use of radio frequency reactive magnetron sputtering under the condition that the power equal to 250 W, Ar/O2 ratio 50/5.0 sccm and substrate temperature 500 ℃. As for the anatase TiO2, can be observed seven diffracted lines labeled by (101), (004), (200), (211), (204), (220) and (215). The XRD data accord with those of anatase TiO2 in Refs.[17, 18]. The cell dimensions calculated from the diffracted lines are a = 0.378 nm and c=0.951 nm. No diffracted lines characterized by other polymorphs of TiO2, rutile or brookite, are present. The average grain size d of TiO2 can be estimated from the XRD profiles using Scherrer’s equation[18]: d = kλ / β cosθ where k is a constant (shape factor, about 0.9), λ is X-ray wavelength, β is full-width at half-maximum (FWHM) of the diffraction line and θ is a diffraction angle. In TiO2 film, the mean particle size of 13.2

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nm is estimated. Some feature diffraction peaks of anatase TiO2 and a Co (111) could be noticed in the TiO2-Co-TiO2 sandwich film.

Fig.1

level of the anatase TiO2 and the TiO2-Co-TiO2 sandwich film. From it, the core levels of Ti 2p1/2 and Ti 2p3/2 are approximately at 464.1 and 458.4 eV, respectively, which, in line with Ref.[20], are ascribed to the Ti4+ (TiO2), and has a peak separation at 5.7 eV between them.

X-ray diffraction patterns of the TiO2-Co-TiO2 sandwich film and the anatase TiO2 film.

Weak XRD peak signals can be sourced back to very thin films (about 150 nm). The XRD pattern of TiO2-Co-TiO2 sandwich films reveals that the film is a mixture of the anatase TiO2 and the cubic cobalt. Raman scattering spectra of the anatase TiO2 film and the TiO2-Co-TiO2 sandwich film are shown in Fig.2. From it, can be seen four Raman scattering peaks of the anatase TiO2. The two of them are approximately 144 cm-1 and 400 cm-1 which are attributable to the B1g modes, and the others 515 cm-1 and 640 cm-1 to the Eg modes. These results are in good agreement with Ref.[19]. In addition, there is another Raman peak of around 1 106 cm-1 attributable to the vibration mode of Ti-O-SiO2, which indicates a strong interaction at the interface between the TiO2 film and the glass substrate.

Fig.3

Compared with Refs.[21, 22], the core levels of Ti 2p1/2 and Ti 2p3/2 have a small shift of 0.1 eV. Fig.4 shows XPS O 1s core level of (a) the anatase TiO2 and (b) the TiO2-Co-TiO2 sandwich film. From it, in approximate accordance with the main peak at 529.9 eV of TiO2[21], the core level of O 1s is at line of 529.7 eV, which is assigned to the titanium dioxide at the surface.

Fig.4

Fig.2

Raman scattering spectra of the anatase TiO2 film and the TiO2-Co-TiO2 sandwich film.

Fig.3 shows the comparison of XPS Ti 2p core

XPS Ti 2p core level of (a) anatase TiO2; (b) the TiO2-Co-TiO2 sandwich film.

XPS O 1s core level of (a) anatase TiO2; (b) the TiO2-Co-TiO2 sandwich film.

Fig.5 shows XPS Co 2p core level of the TiO2Co-TiO2 sandwich film. The core levels of Co 2p1/2 and Co 2p3/2 are at 797.1 and 782.2 eV respectively, having a peak separation of 14.9 eV between them. Compared to Ref.[21], it is obvious Co in a metal state in the anatase TiO2. However, the core levels of Co 2p3/2 and Co 2p1/2 in the anatase TiO2 have chemical shifts of 0.42 eV and 0.43 eV, respectively. This demonstrates an interaction be-

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tween Co and TiO2. The optical absorption spectra of the TiO2-Co- TiO2 sandwich films with different thicknesses of Co nano-layer are shown in Fig.6.

Fig.5

presented in Fig.7. It is shown that the threshold α of the fundamental absorption of the TiO2-Co-TiO2 sandwich film may be described by the expression: α = A ( E – Eg)m (1) where E is the optical band gap of the TiO2-Co-TiO2 sandwich film, Eg is the optical band gap of the pure anatase TiO2, and A is a constant. The value of m (m = 2) is a characteristic value for the indirect allowed transition, which dominates over the optical absorption[24,25].

XPS Co 2p core level of the TiO2-Co-TiO2 sandwich film.

Fig.7

Fig.6

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Optical absorption of the TiO2-Co-TiO2 sandwich films with different thicknesses of Co nano-film.

For nanocrystalline anatase TiO2 film with zero thickness of Co nano-layer, the optical absorption edge blue shifted to 320.9 nm (3.86 eV) in comparison with that of the bulk anatase TiO2 (3.20 eV)[23], which may well be explained by the quantum confinement effects. However, in case of a Co nano-film embedded between two-anatase TiO2 films, Co atoms will diffuse into the matrix of TiO2 and change the band gap of the TiO2-Co-TiO2 sandwich films. From Fig.6, the optical absorption of the TiO2-Co-TiO2 sandwich films red shift to 321.4, 330.8, 333.5 and 345.2 nm, which correspond to the thickness of the Co nano-film of 3, 9, 12 and 48 nm, respectively. This indicates that the band gap of the anatase TiO2 narrows when the Co nano-film thickens. The variation of band gap with the thickness d of the Co nano-film in the TiO2-Co-TiO2 sandwich films is

Variation of band gap vs the thickness of Co-nanofilm in TiO2-Co-TiO2 sandwich films.

The optical absorption spectra of the TiO2Co-TiO2 sandwich films deposited at different substrate temperatures under the condition that the power be 250 W, the Ar/O2 ratio 50/5.0 sccm, are shown in Fig.8.

Fig.8

Optical absorption spectra of the TiO2-Co-TiO2 sandwich film deposited at different substrate temperatures.

It is observed that as the substrate temperature increases, the optical absorption edges see varied red shifts. The measured results of these optical absorption edges are about 339.7, 349.8 and 400.6 nm, which correspond to the substrate temperatures of 300, 350 and 400 ℃ respectively. These can be in-

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terpreted by the decrease of the band gaps of TiO2-Co-TiO2 sandwich films with an ever-increasing number of Co atoms diffusing into the matrix of TiO2 caused by increased substrate temperature. Fig.9 shows a hysteresis loop of a TiO2Co-TiO2 sandwich film in magnetic field, which is parallel to the film plane. This film, composed of two 80 nm TiO2 layers and an embedded Co film 12 nm thick, has the saturation magnetization of about 4.35 emu/g, the remnant magnetic induction of 1.02 emu/g and the coercivity of 569.6 Oe. It demonstrates ferromagnetic properties possessed by the TiO2-Co-TiO2 sandwich film.

bedded film is higher than that of the 12 nm Co-embedded one. Fig.11 shows the coercivity of the TiO2-Co-TiO2 sandwich films versus varied thicknesses of Co nano-films, where circle points present the experimental data, and the solid line the Gaussian fit.

Fig.11

The coercivity of the TiO2-Co-TiO2 sandwich films vs. the thicknesses of the Co films.

From Fig.11, a critical thickness of about 8.6 nm of the Co nano-film can be noticed in this kind of composite films which have the maximum coercivity of about 1 413 Oe. Fig.9

A hysteresis loop of a TiO2-Co-TiO2 sandwich film in magnetic field parallel to the film plane.

Fig.10 illustrates a hysteresis loop of another TiO2-Co-TiO2 sandwich film in magnetic field. The film has an embedded Co film about 9 nm thick and coercivity of about 1 399.2 Oe at the room temperature. Its loop is parallel to the film plane as well.

Fig.10

A hysteresis loop of another TiO2-Co-TiO2 sandwich film in magnetic field.

It is clear that the coercivity of 9 nm Co-em-

4 Conclusions The TiO2-Co-TiO2 sandwich films were successfully grown on glass and silicon substrates making alternate use of radio frequency reactive magnetic sputtering and direct current magnetron sputtering. The structures and the properties of these films were investigated with XRD, Raman spectra and X-ray photoemission spectra (XPS). The sandwich films comprise two anatase TiO2 films and an embedded Co nano-film. Transmission absorption spectra see varied red shifts as the Co nano-film thickens, which originated from the quantum confinement effects. The films show ferromagnetic properties at the room temperature. The saturation magnetization, remnant magnetic induction and coercivity vary with the thickness of Co nano-films. The Co nano-film has a critical thickness of about 8.6 nm, which makes the coercivity of the composite film reach the maximum value of about 1 413 Oe. This may well be accredited to the quantum interface effects.

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Biography:

films grown on LaAlO3 and SrTiO3 substrates by laser ablation.

LIU Fa-min Born in 1961, male, Pro-

Appl Phys Lett 2004; 84(15): 2850-2852.

fessor. His main research interests are in

Ando K, Saito H, Jin Z, et al. Large magneto-optical effect in an

dilute magnetic semiconductor oxides and

oxide diluted magnetic semiconductor Zn1–xCoxO. Appl Phys Lett

functional thin film.

2001; 78(18): 2700-2702.

lished more than 60 papers in various pe-

Ueda K, Tabata H, Kawai T. Magnetic and electric properties of

riodicals.

transition-metal-doped ZnO films. Appl Phys Lett 2001; 79(7):

E-mail: [email protected]

Now he has pub-

988-990. [14]

Kimura H, Fukumura T, Kawasaki M, et al. Rutile-type oxide-diluted magnetic semiconductor: Mn-doped SnO2. Appl Phys Lett 2002; 80(1): 94-96.

[15]

Kuang A L, Liu C Y, Liu X C, et al. Room-temperature ferromag-

DING Peng

Born in 1983, male, graduate student. He will

get his Master degree in 2007. He is doing research work on functional thin films. E-mail: [email protected]