Interfacial induction sustained ferromagnetism from solution chemistry to ceramics 5

Chapter A Study on Doped-TiO2 Magnetic Thin Films Fabricated by the Sol-Gel Technique 7.1 Introduction As indicated in Section 2.4, the prospect of integrating information processing and storage functionalities in one material triggers the development of spintronics (Ohno, 1998; Prinz, 1998; Twardowski, 2000; Fukumura, et al., 2005). Recently, diluted magnetic semiconductor (DMS) has received intense attention due to their potential applications in this rapidly developing area. Co-doped titanium dioxide, since its discovery in 2001, has been widely researched upon due to its room temperature ferromagnetism that manifested tremendous potential as DMS. Titanium oxide exists in three crystalline structures, namely anatase, rutile and brookite as shown in Figure 7.1. Earlier work by Matsumoto et al. reported room temperature ferromagnetism of anatase Co-TiO2 film deposited by laser molecular beam epitaxy technique and the same property was revealed in the thermodynamically more stable rutile state using the same fabrication technique (Matsumoto, et al., 2001a; Matsumoto, et al., 2001b). 168 Figure 7.1 Crystal structure of TiO2 (a) rutile, (b) anatase and (c) brookite (Mo and Ching, 1995) As mentioned earlier in Section 2.4.3, Co-doped TiO2 thin films can be fabricated using either physical methods (like pulsed laser deposition and sputtering) or chemical methods (like chemical vapour deposition). As compared with these vacuum-relied techniques, the solution (wet) chemistry approach has its own superiority in growing functional oxide films because it is a straight-forward fabrication technique which enables us to achieve the desired composition and crystallinity of the films via maneuvering the stoichiometry of feedstock. Since this method is not a line-of-sight deposition process, irregular or porous substrates can also be coated evenly. However, the major drawback is the film cracks caused by thermal stress emanated from the drying and curing process (Prasad, et al., 2003). In this work, through appropriate usage of an organic additive in the sol-gel process, continuous dense films of Codoped TiO2 and SrO-Co-doped TiO2 have been obtained from the conventional threestep procedure: coating, drying and calcination. From there, we are able to evaluate the 169 room temperature ferromagnetism of the thin films fabricated by the solution chemistry approach. Many studies have been carried out to determine the origin of ferromagnetism in Codoped TiO2 films. Our study in this work shows unanimously no metallic cobalt clusters in the films, and therefore the ferromagnetism should not be due to the zero valence cobalt. Researchers like S.A. Chambers explained the origin in terms of ferromagnetic exchange interaction, attributing this room temperature ferromagnetism to intrinsic stimulation (Chambers, 2002; Chambers, et al., 2003; Toyosaki, et al., 2004). Cui et el. also reported that the ferromagnetism comes from the exchange mechanism between Co2+-O2--Co3+ (Cui, et al., 2004; Shutthanandan, et al., 2004). Coey and coworkers proposed that the ferromagnetic exchange here (in dilute ferromagnetic oxides and nitrides) is different from the conventional super-exchange or double-exchange interaction as such interactions cannot produce long-range magnetic order at such low magnetic cationic concentrations. They thus proposed that the ferromagnetic exchange is due to the indirect exchange via shallow donors (associated with oxygen vacancies) (Coey, et al., 2005). From the earlier chapters on heterogeneous metallic oxide composite ½(1-x)La2O3xSrO/⅓Co3O4, it was found that such oxide composite manifested ferromagnetism at room temperature with the stipulation that the oxides must be mixed in nano-domains. On the contrary, Sr-doped lanthanum cobalt oxide (namely perovskite LSCO structure), studied by many materials scientists, has a Curie temperature much lower than room temperature (Raccah and Goodenough, 1968; Rao, et al., 1977; Petrov, et al., 1995). The uniqueness of the heterogeneous composite system lies in its vast interfacial phase 170 between SrO and spinel Co3O4 nano-domains. In order to maintain the oxide phases in a few nanometers range during the thermal curing process required for cultivating crystal structures in the respective oxide phases, applying a low calcination temperature is imperative. The generation of this ferromagnetism is attributed to interfacial stimulation happening at the boundary between SrO and Co3O4 domains and presumed via the interfacial “Jahn-Teller” effect. An attempt to implement this interfacial stimulation mechanism into the Co-TiO2 matrix for enhancing its room temperature ferromagnetism has been found viable. Through our research on this tricomponent (SrO-Co-TiO2) composite, it has been found that SrO exhibits a surfactantlike effect on maintaining an extremely high dispersion of the three oxide phases in the matrix of thin film in addition to the induction effect. 7.2 Experimental 7.2.1 Chemicals Titanium (IV) isopropoxide (C12H28O4Ti, 97%, Aldrich), cobalt (II) 2- methoxyethoxide (Co(OCH2CH2OCH3)2, 99%, Alfa Aesar), strontium nitrate (Sr(NO3)2, >99%, Acros Organic), hydrochloric acid (37%, Fisher Chemicals), triethanolamine (98%, Aldrich), ethanol (≥ 99.9%, Merck), isopropanol (≥ 99.5%, Tedia), Brij 30 (Aldrich) and polyethylene glycol 400 (Merck) were used as received. 7.2.2 Preparation of precursors for thin film fabrication Two different sol-gel processes, which employ the acidic and the basic catalysts respectively, were designed to fabricate the dense TiO2-based films. 171 7.2.2.1 The acidic method For formulating the sol-solution to synthesize Co-TiO2 film (CTO), the sol solution was first prepared by adding titanium tetra-isopropoxide (TTIP, 6.384-7.728 mmol) and cobalt methoxyethoxide (CMEO, 0.672-2.016 mmol) into an acidified ethanolic solution, which was made up of 0.3ml concentrated hydrochloric acid (37%) in 9.8ml ethanol. Three different compositions were attempted, namely CoxTi1-xO2-y with x = 0.08, 0.16 and 0.24. A given amount (5.4 % by volume) of polyethylene glycol (PEG 400) was then added into it, and the resulting mixture was stirred for 1hr at ambient temperature (~25°C). The sol solution generated in this controlled hydrolysis step contains nano CTO particles (< 5nm) as shown in Figure 7.2. Figure 7.2 HR-TEM image of nano CTO particles in the sol solution 172 7.2.2.2 The alkaline method For formulating the sol-solution for fabrication of SrO-Co-TiO2 film (SCTO), an aqueous solution containing the desired amount of strontium nitrate was added dropwise to an isopropanol solution containing titanium tetraisopropoxide, cobalt methoxyethoxide (with the stoichiometry Ti/Co = 1.0 / 0.16) and an organic base. Triethanolamine (TEA, 9.8% by vol), which acts as the coordination ligand to stabilize the metal ions (in particular Sr2+ and Ti4+ ions), were used as the organic base here. Two drops of the non-ionic surfactant, Brij-30, was then added into the solution. Similar to the acidic based method, the same amount of PEG 400 was introduced to the solution, and the resulting solution was stirred for 1h at ambient temperature (~25°C) to obtain a transparent sol solution. For comparison purposes, it was found that the acidic sol-gel approach is inappropriate in formulating the sol solution. This is because Sr2+ and Cl- are likely to form an electron donor-acceptor bridged complex and such bridged complex can be extended in a dimensional network, leading to the heavy gelation in the acidic medium. 7.2.3 Development of film The substrate used here is silicon wafer. The silicon substrate was cleaned and degreased in acetone in an ultrasonic cleaner. Spin coating was performed with a spin rate of 2000 rpm for to develop a liquid film of the sol solution. The resultant liquid film was then dried for 30 at 80°C before heated up to 500oC using a heating rate of 2°C/min, where the curing sample was dwelled for h before allowed to cool at 2°C/min. The coating and curing procedure was repeated several times to get a thicker film. 173 7.2.4 Characterizations The degradation profile of the precursor gel was obtained on a thermogravimetric analyser (TA Instrument, TGA 2050). The samples for TGA were prepared by drying a small volume of the sol-solution in a dehumidifier chamber at 60°C for days to obtain a gel like form. These samples were also used to record their IR spectra on a FTIR spectrometer (Bio-Rad FTS-3500 ARX). Crystal structure of the film was determined by the X-ray diffractometer (XRD, Bruker AXS) using Cu Kα radiation. Field emission scanning electron microscope (FESEM, JEOL, JSM-6700F) and high resolution Transmission electron microscope (HR-TEM, Philips CM300) were used to obtain the morphology of the samples. Valence states of Co 2p, Ti 2p and Sr 3d were analysed with the X-ray photoelectron spectroscopy using C 1s (284.6eV) as reference (XPS, Kratos Axis HSi System) and Vibrating Sample Magnetometer (VSM, Lake Shore 735, noise floor 10-7 emu for 10 sec/pt) was used to measure the magnetic properties of the films at room temperature. 7.3 Results and Discussion 7.3.1 Fabrication of continuous dense Co-TiO2 films From FESEM, it can be observed that both the CTO films (x = 0.08 and 0.16) have a uniform membrane morphology and good adhesion to the substrate shown by the intimate interface (Figure 7.3 a and b). The PEG present in the sol solution yields a lubricating effect on sol particles such that they are able to maneuver among themselves when the solvent is being removed. By adding small amount of PEG, conducting a gradual combustion of the organic entity and setting a slow calcination rate to consolidate the amorphous oxide composite generated from the combustion, a crack-free film could be ultimately fabricated. 174 a b 175 c Figure 7.3 FESEM images of CTO films with (a) x = 0.08, (b) x = 0.16 and its cross sectional view (c) Besides the control of processing conditions, the stoichiometry of cobalt is also crucial to the quality of film. Severe cracks, as can be observed from Figure 7.4, were resulted with the usage of high dopant content (x = 0.24). When the Co content is too high, crystallites (CoTiO3) with high cobalt oxide content will be formed. As these high Cocontent crystallites have different coefficients of thermal expansion from the TiO2 phase, the material stress that was generated during the cooling course (after calcination at 500°C) led to the cracking of film. This conclusion can be supported from the IR spectra of the dried precursor gels (Figure 7.5). From the IR spectrum b, which still contains the characteristic vibration absorption peaks of organic moiety that could be assigned to methoxyethoxide ligand, it can be inferred that the sol-gel reaction (hydrolysis and condensation) rate of the cobalt-containing precursor (CMEO) is slower than that of the titanium precursor (TTIP). Therefore, with increasing CMEO content, the CoO-enriched oxide phase will be easily generated. 176 Figure 7.4 SEM image of CTO film of x = 0.24 The thermal degradation graphs of the dried precursor gel (Figure 7.6) were collected to analyze the dehydration and condensation course of how the dried gel was converted to the crystalline CTO. From the thermal degradation graph for CTO with x = 0.08, it was observed that the first major weight loss took place in the range from 200°C to 325°C which was followed by a small weight loss from 325°C to 450°C. 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Magnetic and Transport Properties of the System La1-xSrxCoO3-y (0[...]... c b a 15 20 25 30 35 40 45 50 55 60 65 70 75 2 theta Figure 7.7 0.24 XRD chart of TiO2 and CTO films after calcination at 50 0°C for 1 hr, (a) TiO2, (b) x = 0.08, (c) x = 0.16 and (d) x = 180 With a curing (calcination) temperature of 50 0°C, it can be observed from Figure 7.7 that the major phase of the three films is anatase Though TGA analysis of the dried gel showed that a temperature above 50 0°C... when x = 0.16, where two factors, which are the content of CoTiO3 phase that weakens ferromagnetism and the extent of Co-doping anatase TiO2 that stimulates ferromagnetism, seem to be balanced Table 7.1 Magnetic properties of the CTO and SCTO films Remanence (% wrt x Coercivity (Oe) saturation magnetization) 0.08 43 .5 8.6 0.16 68.0 13.9 0.24 49.0 5. 7 SCTO 155 .6 18 .5 In addition to XRD characterization,... 36% 40 70 Deriv Weight 0.6 80 0.3 0.2 20 50 0oC 0.1 0 0 Figure 7.10 200 400 600 o Temperature ( C) 800 0 1000 Thermal degradation chart of SCTO precursor with x = 0.16 (inset: XRD chart of SCTO calcined at 50 0°C) 187 Intensity (Arbitrary units) a Rutile TiO2 a CoTiO3 a a x = 0.08 r x = 0.16 15 r r s-c r s-c 25 s-c 35 45 55 65 75 2 theta Figure 7.11 XRD of SCTO film calcined at 800°C with r, a and s-c... not originate from the presence of metallic Co This magnetic response is deemed to originate from the occupation of high spin Co3+ ions in the cationic sites of anatase TiO2 Alternatively, 189 Intensity (Arbitrary units) 457 .9eV 463.7eV c b a 464.1eV 458 .3eV 4 75 470 4 65 460 455 450 Binding energy (eV) Figure 7.12 X-ray photoelectron spectroscopy of Ti 2p of (a) CTO film with x = 0.08; (b) CTO film with... Co 2p of CTO films 770 183 7.3.2 Implanting the SrO nano domains into the matrix of CTO film Our previous finding of the interfacial induction, which takes place between SrO and Co3O4 nano-domains and triggers the unique room temperature ferromagnetism, has led us pursue the same type of induction mechanism in the CTO film via implanting of nano-size SrO domains into its matrix The above CTO film (x... Capacitors on Silicon with Conducting Barrier Layers, Appl Phys Lett., 68, pp.1 350 -1 352 1996 Feynman, R.P There's Plenty of Room at the Bottom, J MEMS, 1, pp.60-66 1992 Flin, R.A and P.K Trojan (19 95) Engineering Materials and Their Applications Boston, Houghton Miffin Company Fluitman, J.H and H Guckel (1996) Micro-Actuator Principles Proceedings Actuator '96, Bremen, Germany Fukumura, T., H Toyosaki... extent of TiO2 as well as the formation of unwanted CoTiO3 phase The calcination of SCTO at 50 0°C leads to an interfacial (or tiny individual oxide domains) predominant system because of the dispersing effect of SrO SCTO manifests greater magnetic properties than CTO This enhancement is originated from the interfacial induction between SrO and Co3O4 oxide domains 192 Chapter 8 Conclusions and Recommendations... to 200K, the interfacial phase still retains weak coercivity and remanence while the perovskite phase becomes paramagnetic 3 Since the room temperature ferromagnetism originates from the interfacial induction between SrO-Co3O4, further work is done on the effect of chelating agents on the 0.025La2O3-0.95SrO/⅓Co3O4 The organic ligands used in the formation of the precursor for the oxide affect the ferromagnetism. .. CTO film with x = 0.08; (b) CTO film with x = 0.24 and (c) SCTO film with x = 0.16 190 3000 3d5/2 133.02 eV 250 0 3d3/2 134.72 eV Intensity 2000 150 0 1000 50 0 0 140 138 136 134 132 130 128 Binding energy (eV) Figure 7.13 X-ray photoelectron spectroscopy of Sr 3d of SCTO film (x = 0.16) calcined at 50 0°C 126 191 the introduction of Sr2+ into the Co-TiO2 system significantly deterred the crystallization... could be fitted satisfactorily by adding a shoulder peak at 781.5eV, which is indicative of the presence of Co3+ ion According to the areas of the two fitting peaks, the ratio of Co2+ to Co3+ is about 0.8 Since anatase TiO2 has the tetragonal bipyramids structure with an elongated distortion, when Ti4+ (r = 0. 75 ) is substituted by both Co2+ and Co3+, to minimize structural distortion, Co2+ ion would . TiO 2 and CTO films after calcination at 50 0°C for 1 hr, (a) TiO 2 , (b) x = 0.08, (c) x = 0.16 and (d) x = 0.24 15 20 25 30 35 40 45 50 55 60 65 70 75 2 theta Intensity A CoTiO 3 A A A A CoTiO 3 a d c b 180 . properties of the CTO and SCTO films x Coercivity (Oe) Remanence (% wrt saturation magnetization) 0.08 43 .5 8.6 0.16 68.0 13.9 0.24 49.0 5. 7 SCTO 155 .6 18 .5 In addition to XRD characterization,. converted to the crystalline CTO. From the thermal degradation graph for CTO with x = 0.08, it was observed that the first major weight loss took place in the range from 200°C to 3 25 C which