CEMS study on diluted magneto titanium oxide films prepared by pulsed laser deposition

Transcription

1 Hyperfine Interact (2006) 168: DOI /s CEMS study on diluted magneto titanium oxide films prepared by pulsed laser deposition K. Nomura & K. Inaba & S. Iio & T. Hitosugi & T. Hasegawa & Y. Hirose & Z. Homonnay Published online: 5 December 2006 # Springer Science + Business Media B.V Abstract 6% 57 Fe doped titanium oxide films, prepared by pulsed laser deposition (PLD) on sapphire substrate at 650 C under various vacuum conditions, were characterized mainly by conversion electron Mössbauer spectrometry (CEMS). Two magnetic sextets with hyperfine fields 33 and 29 T, and one doublet were observed in the CEMS spectra of TiO 2 films prepared under PO 2 =10 6 and 10 8 torr, which showed ferromagnetism at room temperature, whereas only the doublet of paramagnetic Fe 3+ species was observed for the film prepared under PO 2 =10 1 torr. Key words Conversion Electron Mössbauer Spectroscopy (CEMS). Dilute Magnetic Semiconductor (DMS). Ti oxide doped with Fe. pulsed laser deposition 1 Introduction Dilute magneto-semiconductors (DMS) are new materials with both semiconductor and magnetic properties [1]. They are prospected as materials for spin electronics. DMS of K. Nomura (*) : S. Iio School of Engineering, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo , Japan k-nomura@t-adm.t.u-tokyo.ac.jp K. Inaba Materials and Structures Laboratory, Tokyo Institute of Technology, Yokohama , Japan T. Hitosugi : T. Hasegawa School of Science, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo , Japan T. Hasegawa: Y. Hirose Kanagawa Academy of Science and Technology, Sakato, Takastu-ku, Kawasaki , Japan Z. Homonnay Research Group for Nuclear Methods in Structural Chemistry, HAS and Department of Nuclear Chemistry ELTE, 1117 Budapest Pázmány P. s. 1/A, Budapest, Hungary

2 1066 K. Nomura, K. Inaba, et al. Figure 1 Relation between Kerr rotation angles and light wavelengths for 6% Fe doped TiO 2 films prepared at different oxygen pressures indicated. Kerr rotation angle [deg] 0,2 0,15 P=10-6 torr 0,1 0,05 P=10-8 torr 0-0,05 P=10-1 torr -0,1-0,15-0, Wavelength [nm] PO 2 =10-1 Torr Intensity [a.u.] TiO 2 (101) θ [deg] PO 2 =10-6 Torr 100 PO 2 =10-8 Torr Intensity [a.u.] TiO 2 (101) Intensity [a.u.] θ [deg] θ [deg] Figure 2 XRD patterns of TiO 2 films prepared under 10 1,10 6 and 10 8 torr by pulsed laser deposition. Three strong peaks are due to the substrate of γ Al 2 O 3.TiO 2 peaks were observed together with three large peaks of sapphire substrate. GaAs doped with Mn show ferromagnetism at low Curie temperatures (Tc < 250 K) [2]. It has been found recently that TiO 2 films doped with Co show transparent and ferromagnetic semiconducting properties at room temperature [3]. Wang et al. [4] showed that rutile type TiO 2 films doped with Fe show p-type semiconductivity, whereas anatase type TiO 2 films doped with Co are n-type semiconductors. H. M. Lee et al. [5] and Kim et al. [6] suggested, on the basis of transmission Mössbauer spectra of Fe doped TiO 2 powders that the ferromagnetism may be due to either electron carriers induced by substitution in the TiO 2 lattice or to the formation of granular magnetite. Inaba et al. [7] reported that Fe doped TiO 2 films prepared by PLD under PO 2 =10 6 torr and at the substrate temperature ranges of 600 to 675 C show the ferromagnetism and Kerr effect. In this paper, in order to make the chemical states of iron doped in TiO 2 films clear, these films were characterized by 57 Fe CEMS.

3 CEMS study on diluted magneto titanium oxide films prepared by pulsed laser deposition 1067 Figure 3 Hysteresis of the Kerr rotation angle on magnetic field and at various wavelengths before and after post-annealing at 300 C for 20 h in air and for 6 h in oxygen atmosphere. 2 Experimental Titanium dioxide films with ca. 100 nm in thickness were prepared on sapphire substrates kept at 650 C under 10 1 to 10 8 torr by pulsed laser deposition (PLD) (KrF laser; wavelength: 248 nm, power: 5 J/cm 2, pulse: 6 ns, frequency: 2 Hz). The mixed pellet of 94 w/w%tio 2 (of 99.9% purity) and 6 w/w% 57 Fe 2 O 3 (of 99.99% purity, 57 Fe: > 95%) was prepared by thermal treating at 1,200 C for 12 h in air, and was used as the target for PLD. The as-prepared and post-annealed films were characterized by X-ray diffraction, scanning SQUID microscopy, magneto-optical measurements and Mössbauer spectrometry. Conversion electron Mössbauer spectroscopy (CEMS) can provide us the characterization of a thin surface layer within ca.100 nm in thickness non-destructively and selectively. CEMS spectra of the TiO 2 films doped with 6% 57 Fe 2 O 3 were measured by using a homemade He + 5%CH 4 gas flow counter [8] and a gamma source of 57 Co(Cr) with the activity of 1.3 GBq. The Doppler velocity was calibrated using metallic iron foil. 3 Results and discussion From the micro images by atomic force microscopy, the surface roughness over 25 nm 2 area increases from 0.47 to 1.9 nm with decreasing PO 2 from torr to torr. The ferromagnetic phases in the films prepared under 10 1, 10 6 and 10 8 torr were observed by a scanning SQUID microscope. The scanning area was μm. The films prepared under PO 2 =10 6 torr and at substrate temperature T s = 650 C show strong Kerr effect as shown in Figure 1. Magnetic domain structures were clearly observed in 57 Fe doped TiO 2 films prepared in 10 6 torr, suggesting the presence of long range ordering of magnetic moment induced by Fe doping in these thin films. It was confirmed by XRD as shown in Figure 2 that epitaxial (101) films of the rutile polymorph of TiO 2 were obtained for the samples prepared under 10 1 and 10 6 torr. On the other hand, other diffraction patterns, which might belong to TiO 2 with oxygen defects (i.e., Magneli, Ti n O 2n 1 ), were observed for the films prepared under 10 8 torr. Inaba et al. [7] confirmed that the full width at half maximum (FWHM) of low angle peak is , , and for the films deposited for 10 4,10 6, and 10 8 torr, respectively,

4 1068 K. Nomura, K. Inaba, et al. Figure 4 CEMS spectra of transparent titanium oxides deposited on sapphire heated at 650 C in a 10 1 torr, b 10 6 torr, and c 10 8 torr by pulsed laser ablation of 6% 57 Fe 2 O 3 +TiO 2 target. The substrate temperature: 650 C. indicating that the lower crystallinity is obtained at the lower oxygen pressure. The distance between (101) faces increased with the decrease of the oxygen pressure (d = , and nm) for 10 1, and 10 6 torr, respectively, showing either higher oxygen defect concentration or incorporation of Fe species into the lattice (or both). These results lead to a conclusion that the crystallinity is not directly related to the ferromagnetism. Hysteresis of the Kerr rotation angle on magnetic field and light wavelengths are shown in Figure 3, before and after post-annealing at 300 C for 20 h in air and for 6 h in oxygen atmosphere. The hysteresis curve became narrow by post-annealing, suggesting that the weaker magnetic fields can be applied. As shown by the CEMS spectra in Figure 4, a doublet of paramagnetic Fe 3+ was observed for transparent TiO 2 films prepared under the low vacuum condition of 10 1 torr. Two magnetic sextets and also a small amount of paramagnetic peaks were observed for the films prepared under 10 6 and 10 8 torr. The Mössbauer parameters are shown in Tables I and II. The parameters of paramagnetic peaks showing up together with the sextets were around isomer shift, δ = 0.37(2) mm/s and quadrupole splitting, Δ = 0.9(2) mm/s. It is assumed that the sextet with the larger magnetic field (B hf ) is due to metallic iron with δ = 0.0 mm/s and B hf = 33.0 T (α-fe). The other sextet with smaller B hf = 29.5 T is ambiguous because it may be due to either finely dispersed metallic iron, Fe(Ti) alloy or high spin Fe 4+ species incorporated in TiO 2 (the δ value of the latter is very close to 0). The assignment of this sextet will be further discussed later. The peak intensities of the sextet with the smaller B hf as well as those of the paramagnetic components were low for the TiO 2 films prepared under 10 8 torr. The question arises why the metallic iron detected in the samples prepared under 10 8 torr was not sensitive to scanning SQUID. It is assumed that, if the magnetic domains are much smaller than the probing scale of 5 μm in diameter, the vertical up and down magnetic moments due to spontaneous magnetization cancel each other and cannot be detected. The internal relative peak intensities of the sextet with the smaller B hf were different from those of the sextet with 33 T. The former indicates that the magnetic moments tend to align parallel to the surface by post annealing, whereas those with the large B hf are rather random. If the sextet with a small B hf is due to fine particles of metallic iron, post-annealing

5 CEMS study on diluted magneto titanium oxide films prepared by pulsed laser deposition 1069 Table I Mössbauer parameters of 6%Fe doped TiO 2 films prepared at 10 1 and 10 8 torr Sample; 6% Fe + TiO 2 films Partial δ (mm/s) Δ (mm/s) B hf (T) Γ (mm/s) Area (%) 650 C torr C torr Table II CEMS parameters of 6%Fe doped TiO 2 films prepared at 650 C in 10 6 torr, and post-annealed at 300 C for various hours Sample; 6% Fe + TiO 2 films Partial δ (mm/s) Δ (mm/s) B hf (T) Γ (mm/s) Area (%) As prepared Post-annealed 300 C 20 h in air (b) C 3 h in O (b) C 6 h in O C 16 h in O of the films may induce agglomeration of small particles to form large ones, or the easy oxidation of small particles. In the Mössbauer spectra, the subspectra with the smaller B hf would disappear, and only the other sextet would be observed. Therefore we applied thermal treatment of this sample in air and also in oxygen for several hours at 300 C. As shown in Figure 5, the intensity of the sextet of smaller B hf did not change so much although the sextet with 33 T decreased a little and the doublet of Fe 3+ increased. This suggests that the sextet with broad peaks and B hf = 29.3 T may not be due to finely dispersed iron species. The sextets with 33 T and 29 T disappeared after heating on oxygen atmosphere at 400 C for 16 h, and a broad sextet with δ = 0.41 mm/s, Δ = 0.14 mm/s and B hf = 48.3 was observed together with a doublet of Fe 3+. This broad sextet is considered to be due to hematite containing Ti atoms because the B hf of the broad sextet was a little smaller than that of pure hematite. The area intensity of 31% was almost the same as that of the sextet with 29 T. It may suggest that the sextet with 29 T contained Ti atoms. This point is of great importance considering that the magnetic sextet components are still present in the films annealed for long time at 300 C in air and oxygen i.e. in strongly oxidizing conditions. It is known that laser ablated species such as atoms, ions and clusters have high kinetic energy (<100 ev) as compared with that used in physical films deposited by a normal resistance heater ( 0.1 ev). On the process of the film formation, the kinetic

6 1070 K. Nomura, K. Inaba, et al. Figure 5 CEMS spectra of TiO 2 films after post-annealing at 300 o C for various durations; a as prepared on substrate at 650 C, b annealing for 20 h in air, c treatment b followed by annealing for 3 h in O 2 atmosphere, and d treatment b followed by annealing for 6 h in O 2 atmosphere. energy may affect the diffusion of element species. We have to mention here that the applied vacuum conditions are significantly different in the sense that at 10 1 torr, the mean-free path length of the evaporated atoms is much shorter than the target-sample distance, while at 10 6 and 10 8 torr, it is much longer. Thus at the lower vacuum not only the availability of oxygen is larger at the deposition, but also the deposited atoms or clusters of atoms have much lower kinetic energy when hitting the sample surface. In order to check if the Mössbauer sextet with B hf = 33 T can really be assigned to metallic iron, we performed one more experiment heating the sample in reductive ambient. CEMS spectra of the (previously already oxidized) samples did not show substantial change in the intensity of this sextet upon heat treatment at 400 C for 2 h in 5%H 2 +Ar atmosphere. This confirmed that, really, metallic Fe was incorporated into structure of the TiO 2 film. A large part of the metallic iron must be due to oxygen loss during the high vacuum laser ablation process. Then it is logical to assume that TiO 2 might also have suffered some reduction and a small amount of metallic Ti formed. The sextet with 29 T may then be Fe alloy doped with Ti at the interface of TiO 2, which covers the metallic iron particles and may explain the strong corrosion resistance. Inaba et al. [7] reported that low spin Fe 3+ species were observed by XPS spectra and magneto-optical measurements. XPS observed only top surface of the films, which might correspond to the paramagnetic Fe 3+ peaks observed in CEMS. We need to reconsider these

7 CEMS study on diluted magneto titanium oxide films prepared by pulsed laser deposition 1071 assignments further, taking into account of other mechanism such as those involving the role of delocalized electrons. 4 Summary Fe doped TiO 2 epitaxial films prepared by PLD at 10 1, 10 6 and 10 8 torr were characterized by CEMS. It was confirmed by Kerr magneto-optical measurement and scanning SQUID microscope that the films prepared under 10 6 torr show strong ferromagnetic behavior. Two magnetic components and one doublet were observed in CEMS spectra of the films prepared under the high vacuums, whereas only one doublet of Fe 3+ was observed for the film prepared under 10 1 torr. Two sextet components of B hf = 33 T and 29 T were stable against the post-annealing for long-term oxidation at 300 C. Tentatively, these subspectra may be largely due to metallic Fe, and Fe (Ti) alloy, produced in the transparent semiconductor of TiO 2. Further studies are needed to confirm these assignments. References 1. Ohno, H.: Science 281, 955 (1998) 2. Nazmul, A.M., et al.: Phys. Rev. Lett. 95, (2005) 3. Matsumoto, Y., et al.: Science 291, 854 (2001) 4. Wang, Z., et al.: J. Appl. Phys. 93, 7870 (2003) 5. Lee, H.M., et al.: IEEE Trans. Magn. 39, 2788 (2003) 6. Kim, Y.J., et al.: Appl. Phys. Lett. 84, 3531 (2004) 7. Inaba, K., et al.: Jpn. J. Appl. Phys. 45, L114 (2006) 8. Nomura, K., et al.: Spectrochim. Acta, Part B: Atom. Spectrosc. 59, 1259 (2004)