Interfacial induction sustained ferromagnetism from solution chemistry to ceramics 4

Chapter A Study of Interface-Sustained Ferromagnetism in ½(1-x)Ln2O3-xSrO/⅓Co3O4 Nano Composite 6.1 Introduction From the earlier chapters, it was found that room temperature ferromagnetism was observed in heterogeneous ½(1-x)La2O3-xSrO/⅓Co3O4. This unique ferromagnetic response is interpreted as the result of interfacial induction, presumably through the Jahn-Teller distortion that happened at the octahedral interstices of spinel Co3O4 adjacent to the SrO phase. It was also found that this ferromagnetism can be enhanced when the spinel phase of the composite is doped by a small amount of La2O3. In this work, tri-oxide composites, ½(1-x)Ln2O3-xSrO/⅓Co3O4 where < 1-x < 0.2 and Ln = La and Nd, were studied by focusing on three areas: (i) Generation of nano-composite dominant by interfacial phase via the pyrolysis of preceramic metallo-organic gel (ii) Influence of post-pyrolysis calcination and Ln2O3 content on the phase composition of the composite (iii) Elucidation of different magnetic responses caused by the nature of Ln2O3 dissolved in the Co3O4 phase. The Ln3+-doped Co3O4 oxide displays only paramagnetic behavior at room temperature, but the ferromagnetic response is attained upon its mixing with SrO in 143 nano-scale. The SrO phase plays the role in assisting Co3O4 phase by aligning unpaired electrons through interfacial induction. This work is thus focused on the different doping effects between La2O3 and Nd2O3. Neodymium, similar to lanthanum, belongs to the lanthanide series. However, the primary differences between them are their ionic sizes and valence shell configurations as depicted in Table 6.1 (Shannon, 1976). Table 6.1 Ionic radii and electronic configurations of Ln3+ used Ln Ionic radius (Å) Electronic Configuration La3+ 1.36 {Xe}5d0 Nd3+ 1.27 {Xe}4f3 La3+ ion has a larger ionic radius and d-type valence shell, {Xe}5d0, while Nd3+ has a f-type valence shell, {Xe}4f3. In principle, f orbital has a weaker capability than d orbital in shielding of nuclear charges. Therefore, Nd3+ ion has a stronger effective nuclear charge than La3+. These basic structural differences indeed make the two lanthanide ions reveal different results according to the study of the variation of room temperature ferromagnetism with dopant content. 6.2 Experimental 6.2.1 Chemicals Lanthanum nitrate hydrate (La(NO3)3.yH2O, 99.99%, Aldrich), neodymium nitrate hexahydrate (Nd(NO3)3.6H2O, > 99.9%, Aldrich), strontium nitrate (Sr(NO3)2, >99%, Acros Organic), cobalt (II) nitrate hexahydrate (Co(NO3)2.6H2O, 99%, Acros Organic), Glycine (≥98.5%, Fluka), ethylene glycol (Mallinckrodt, AR), citric acid (> 144 99.5%, Sigma), poly(vinylbutyral) resin (Butiva-79, Monsanto), toluene (> 99.5%, Merck) and 2-butanone (>99.8%, Fisher Scientific), were used as received. 6.2.2 Preparation of ½(1-x)Ln2O3-xSrO/⅓Co3O4 Complex Oxide Powders The tri-oxide mixtures with < x ≤ 0.99 were prepared using the Pechini method (Pechini, 1967). A similar procedure as described in Section 3.2.2 is employed though Nd(NO3)3.6H2O is used instead of La(NO3)3.yH2O for the preparation of Nd3+-oxide composite. 6.2.3 Preparation of Testing Samples The ½(1-x)Ln2O3-xSrO/⅓Co3O4 powder was then ground and added to a polymer solution comprising of poly(vinylbutyral) dissolved in the mixture of toluene and 2butanone (v/v=1) and after evaporating the solvent and drying, the lumps were ground into a fine powder ( 0.4, the heterogeneous composite still remains (Figure 6.1). As concluded in our previous work, SrO phase is the least reactive component of the three in the solid phase reaction that yields perovskite or hexagonal structure (depending on the x value). In addition, the identity of these two trivalent lanthanide ions also affects the solid phase reaction, which has been investigated by TGA analysis (Figure 6.2). The two metallo-organic (MO) gels, preceramics for 0.025Ln2O3-0.95SrO/⅓Co3O4, are selected to study their pyrolytic process. The TGA profile of the preceramic gel of 0.025La2O3-0.95SrO/⅓Co3O4 (Figure 6.2a) shows a plateau corresponding to around 22% of the mass retained, which accounts for the metallic oxides after complete combustion of the organic carbon residues in the temperature range of 530–600°C. For this composite, the solid phase reaction takes place in the range of 580–680°C. The TGA profile of 0.025Nd2O3-0.95SrO/⅓Co3O4 (Figure 6.2b) does not display a massloss plateau until 758°C. This implies that the reaction of metallic oxides sets out 146 P P Intensity (Arbitrary units) P x = 0.6 x = 0.4 20 30 40 50 theta 60 70 80 147 Intensity (Arbitrary units) x = 0.90 x = 0.85 x = 0.80 20 30 40 50 60 70 theta Figure 6.1 XRD chart of ½(1-x)Nd2O3-xSrO/⅓Co3O4 calcined at 600°C using different dopant content 80 148 120 a 0.9 100 o 0.7 Derivative weight (%/ C) 0.8 Weight (%) 80 0.6 0.5 60 0.4 0.3 40 0.2 0.1 20 0 100 200 300 400 500 o Temperature ( C) 600 700 -0.1 800 149 b 120 0.9 100 o 0.7 Derivative weight (%/ C) 0.8 Weight (%) 80 0.6 0.5 60 0.4 0.3 40 0.2 0.1 20 0 200 400 600 800 -0.1 1000 o Temperature ( C) Figure 6.2 Thermal degradation (b) 0.025Nd2O3-0.95SrO/⅓Co3O4 chart of metallo-organic precursor gel (a) 0.025La2O3-0.95SrO/⅓Co3O4 and 150 before complete combustion of the carbon residues. To understand the above phenomenon, we scrutinized the coordination chemistry of La3+ and Nd3+ in the preceramic MO gels. Both cations possess very similar thermodynamic affinities for formation of chelating complexes with the two chelating ligands, i.e. citric acid and glycine (Table 6.2), due to their close proximity to each other in the periodic table and having the same charge state. This indicates that both metal cations could achieve similar distribution uniformity in the respective preceramic MO gel. However, these two MO gels exhibited different thermal degradation rates. For instance, La3+containing MO retains about 22% mass after calcining at 600°C while Nd3+-containing MO retains around 29% of its original mass. This difference is due to the amount of carbon residue rather than the difference in the atomic mass of the two cations since these two cations have close atomic numbers. It is clear that Nd3+-containing MO gel undergoes a sluggish pyrolytic rate relative to the La3+-containing counterpart. As a result, the growth of SrO and Co3O4 into bigger domains were impeded by the partitioning action of the carbon residue left behind, or in other words, the mixing extent of these two major oxides is promoted. Table 6.2 1992) Formation constants of Citrate complexes (Martell and Smith, 1977; Dean, Metal cation Citrate (HL2-) Glycine La3+ log K = 6.65 log K = 11.2 Nd3+ log K = 6.32 log K = 11.6 Sr2+ log K = 2.80 log K1 = 0.91 Co2+ log K1 = 4.83 log K1K K = 25.24 151 The analysis of these two TGA diagrams could also find support from the XRD patterns of the calcined oxide composite with the composition 0.025Nd2O3-0.95SrO/⅓Co3O4. From the XRD shown in Figure 6.3a, it can be concluded that a hexagonal Nd0.05Sr0.95CoO3 crystalline structure is obtained only when the temperature is 800°C or higher. In contrast, the hexagonal structure can be realized at a lower temperature (700°C from Figure 6.3b) if La3+ is used instead of Nd3+. It is interesting to note that for the composites with relatively higher Nd-contents, the hexagonal structure appears as an intermediate between the trioxide composite and the perovskite structure with increasing calcination temperature. For instance, composite 0.05Nd2O3-0.90SrO/⅓Co3O4 (1-x = 0.10) has a hexagonal structure after calcination at 700°C. Upon calcination at 900°C, the hexagonal phase is converted further to the perovskite structure (Figure 6.4). However, this is not the case for the composition with 1-x = 0.05 (Figure 6.3a), in which the hexagonal phase is the final destination. 6.3.2 Interfacial Characteristics of Nano-domains Following the above elucidation, we moved forward to examine the lattice patterns of the 0.025Nd2O3-0.95SrO/⅓Co3O4 composite (a) and the hexagonal Nd0.05Sr0.95CoO2 solution (b) by means of the HR-TEM (Figure 6.5). After careful inspection of Figure 6.5a, it is found to contain three types of nano-domains with different texture-like patterns labelled by 1, 2, and 1-2; the last one reveals a texture resembling a mixture of and 2. As regard to this, the preceding TGA study has pointed out that the slow burning rate of Nd3+containing MO gel favours mixing of SrO and Co3O4. In this context, and could be 152 a Intensity (Arbitrary units) 600°C 700°C H H H 800°C 900°C 20 30 40 theta 50 60 70 80 153 b Intensity (arbitrary units) H H H 800oC 700oC 600oC 20 Figure 6.3 30 40 theta 50 60 70 80 XRD chart of (a)0.025Nd2O3-0.95SrO/⅓Co3O4 and (b) 0.025La2O3-0.95SrO/⅓Co3O4 calcined at different temperatures, where H representing the hexagonal phase 154 Intensity (Arbitrary units) P P H P 900°C H H 800°C 700°C 600°C 20 Figure 6.4 30 40 50 theta 60 70 80 XRD chart of 0.05Nd2O3-0.90SrO/⅓Co3O4 calcined at different temperatures, where H and P representing the hexagonal and perovskite phase respectively 155 attributed to the two major phases (SrO and Co3O4) of the composite. The representative dotted lines indicate the interfacial boundaries between phase and 2. In comparison, the matrix after calcination at 800°C (Figure 6.5b) displays a single texture pattern, and this is consistent with the XRD result (Figure 6.3a) that confirms the occurrence of single phase structure. Another point to note is the presence of “valleys” and “hills” like morphologies (Figure 6.5b) that represent the surface roughness occurring at nano-scale. In short, the presence of 1-2 nano-domains in the 0.025Nd2O3-0.95SrO/⅓Co3O4 composite suggested that it is a meta-stable solid solution and phase-separation takes place only in the scale of several elementary cells. a 2 Mixing 1-2 156 b hill valley Figure 6.5 HR-TEM images of surface morphology of 0.025Nd2O3-0.95SrO/⅓Co3O4 calcined at (a) 600°C and (b) 800°C To confirm the above assignment, the XPS identifications of Sr(II) in the two oxide systems as inspected in Figure 6.5 were collated. The two distinct doublets (3d5/2 and 3d3/2) of Sr2+ (van der Heide, 2002; Vovk, et al., 2005) were observed on the spectra shown in Figure 6.6. Since it has been verified by TGA that Nd3+-containing oxide composite (Figure 6.6a) has a greater extent of mixing between SrO and Co3O4 than that of La3+- containing one, it possessed a higher interfacial area between SrO and Co3O4 nano-domains. As Sr2+ ions located at the interfacial region contribute to the higher binding energy peak due to their cross coordination environments (i.e. sharing of oxygen 157 a 1400 3d5/2 1200 Intensity 1000 3d3/2 800 600 400 200 142 140 138 136 134 132 Binding energy (eV) 130 128 126 158 b 1600 3d5/2 1400 Intensity 1200 3d3/2 1000 800 600 131.91 eV 400 200 142 Figure 6.6 140 138 136 134 132 Binding energy (eV) 130 XPS chart of Sr 3d of 0.025Nd2O3-0.95SrO/⅓Co3O4 calcined at (a) 600°C and (b) 800°C 128 126 159 ligand with Co ion in the Co3O4 nano-domain), it is expected that the binding energy of Sr 3d5/2 and 3d3/2 doublet in 0.025Nd2O3-0.95SrO/⅓Co3O4 will be higher than that in 0.025La2O3-0.95SrO/⅓Co3O4 (Figure 4.5c). The Sr 3d5/2 and 3d3/2 doublet of the hexagonal lattice (Figure 6.6b) displays a slightly lower binding energy than that of the SrO constituting the composite (Figure 6.6a). For the Nd3+-containing hexagonal structure (Figure 6.6b), Sr2+ located in the octahedral coordination environment of O2- shares the O2- ligand with Co2+ in the hexagonal lattice. Thus, it manifested a lower binding energy than its heterogeneous counterparts. The difference in the chemical environment between the hexagonal and heterogeneous 0.025Nd2O3-0.95SrO/⅓Co3O4 system is also reflected by the presence of shoulder peak of 3d5/2 at 131.91 eV. This shoulder peak, which is only observed in the Nd3+-containing hexagonal system, is deemed to be associated with the presence of Nd3+ in the hexagonal cell since it does not appear in the XPS spectra of the un-doped hexagonal lattice of SrCoO2. 6.3.3 Interfacial induction phenomenon in ½(1-x)Nd2O3-xSrO/⅓Co3O4 composite The magnetic hysteresis loops of structurally similar heterogeneous trioxide composites, ½(1-x)Nd2O3-xSrO/⅓Co3O4 and ½(1-x)La2O3-xSrO/⅓Co3O4 (x > 0.8), were obtained using VSM (with an applied field of 10kOe) at room temperature and the coercivity and remanence were collated and depicted in Figures 6.7 and 6.8. Similar to the latter, ½(1x)Nd2O3-xSrO/⅓Co3O4 exhibits room temperature ferromagnetism due to the interfacial induction through Jahn-Teller distortion that occurred at the octahedral interstices of spinel Co3O4 adjacent to the SrO phase. At the interfacial region, the octahedral cells of Sr2+ and Co3+ are linked to each other through oxygen bridge of Sr2+-O2--Co3+. Due to the 160 400 ½ (1-x)Nd2O3-xSrO/⅓Co3O4 (1-x)Nd2O3/xSrO/Co3O4 calcined 600°C calcinedatat 600oC ½ (1-x)La2O3-xSrO/⅓Co3O4 (1-x)La2O3/xSrO/Co3O4 calcined atat 600°C calcined 600oC ½ (1-x)Nd2O3-xSrO/⅓Co3O4 (1-x)Nd2O3/xSrO/Co3O4 calcined 700°C calcinedatat 700oC 350 Coercivity (Oe) 300 250 200 150 100 50 0 Figure 6.7 at 600°C 0.05 0.1 1-x 0.15 0.2 0.25 0.3 Coercivity chart of ½(1-x)Nd2O3-xSrO/⅓Co3O4 calcined at 600°C and 700°C and ½(1-x)La2O3-xSrO/⅓Co3O4 calcined 161 0.1 ½ (1-x)Nd2O3-xSrO/⅓Co3O4 1/2(1-x)Nd2O3-xSrO/Co3O4 calcined atat600°C calcined 600oC ½ (1-x)La2O3-xSrO/⅓Co3O4 1/2(1-x)La2O3-xSrO/Co3O4 calcined atat600°C calcined 600oC Remanence (emu/g) 0.08 ½ (1-x)Nd2O3-xSrO/⅓Co3O4 1/2(1-x)Nd2O3-xSrO/Co3O4 calcined atat700°C calcined 700oC 0.06 0.04 0.02 0 Figure 6.8 0.05 0.1 1-x 0.15 0.2 0.25 Magnetic remanence chart of ½(1-x)Nd2O3-xSrO/⅓Co3O4 calcined at 600°C and 700°C and ½(1-x)La2O3- xSrO/⅓Co3O4 calcined at 600°C 162 size difference between the two cells, in order to fulfil the different size requirement, the Co3+-O2- bond is elongated and this elongation results in a further splitting of either eg or t2g orbitals of Co3+ and the effect is the lowering of energy gap between the frontier orbitals. Thus, the paired electrons could migrate to the nearby empty orbital(s) to become spin-unpaired electrons. As mentioned in Chapter 3.3.2, the presence of a high content of La2O3 is undesirable since it would form perovskite phase at the interfacial region with the two major oxides and impede such interfacial induction mechanism. Nevertheless, reducing the content of La2O3 to near 1-x = 0.15 brings about a rapid increase in coercivity, and correspondingly, a special phase is observed on the XRD chart (2θ = 35.3° and 35.7° of Figure 3.6). This special phase is ascribed to be the doping of La3+ in the spinel Co3O4 phase and the content of La3+ in the ½(1-x)La2O3-xSrO/⅓Co3O4 appeared to be crucial. The La3+ could be brought into Co3O4 phase during pyrolytic process only when its concentration is below a certain level, and above this level, individual La2O3 domains would be formed. It is logical that only small amount of La3+ (i.e. 1-x < 0.15) can be dissolved in Co3O4 phase since this dissolution (doping) causes slender distortion of Co3O4 phase, which has been observed by HR-TEM in our previous work (Figure 3.12), and was suggested to augment the interfacial induction effect. However, too low a doping concentration (1-x < 0.05) would dilute such impact, leading to a weaker ferromagnetism. As far as the variation of coercivity with the increase in Nd2O3 constituent in the composite is concerned, a general decreasing trend has been observed in the two sets of samples, which were prepared by calcination at 600°C and 700°C, respectively (Figure 6.7). The profile of 700°C entails samples with the hexagonal structure Nd1-xSrxCoO3 (x ≤ 163 0.9, Figure 6.4a) and the tri-oxide composite (x = 0.95, Figure 6.3), and consequently an inflexion point is seen at about 1-x = 0.1. Hexagonal Nd1-xSrxCoO3 manifests room temperature ferromagnetism, while the hexagonal La1-xSrxCoO3 is paramagnetic (refer to Section 3.3.3). Hence, Nd3+-containing samples reveal stronger coercivity than the La3+containing counterparts in the range of 1-x > 0.15. In light of the coercivity~x profiles displayed by oxides made by calcination at 600°C, there are two pensive points. Firstly, in the range 0.15 [...]... spinel Co3O4 adjacent to the SrO phase At the interfacial region, the octahedral cells of Sr2+ and Co3+ are linked to each other through oxygen bridge of Sr2+-O2 Co3+ Due to the 160 40 0 ½ (1-x)Nd2O3-xSrO/⅓Co3O4 (1-x)Nd2O3/xSrO/Co3O4 calcined atat 600oC calcined 600°C ½ (1-x)La2O3-xSrO/⅓Co3O4 (1-x)La2O3/xSrO/Co3O4 calcined at 600°C calcined at 600oC ½ (1-x)Nd2O3-xSrO/⅓Co3O4 (1-x)Nd2O3/xSrO/Co3O4 calcined... mixing between SrO and Co3O4 than that of La3+- containing one, it possessed a higher interfacial area between SrO and Co3O4 nano-domains As Sr2+ ions located at the interfacial region contribute to the higher binding energy peak due to their cross coordination environments (i.e sharing of oxygen 157 a 140 0 3d5/2 1200 Intensity 1000 3d3/2 800 600 40 0 200 0 142 140 138 136 1 34 132 Binding energy (eV)... composites, ½(1-x)Nd2O3-xSrO/⅓Co3O4 and ½(1-x)La2O3-xSrO/⅓Co3O4 (x > 0.8), were obtained using VSM (with an applied field of 10kOe) at room temperature and the coercivity and remanence were collated and depicted in Figures 6.7 and 6.8 Similar to the latter, ½(1x)Nd2O3-xSrO/⅓Co3O4 exhibits room temperature ferromagnetism due to the interfacial induction through Jahn-Teller distortion that occurred at the... ½(1-x)Nd2O3-xSrO/⅓Co3O4 calcined at 600°C and 700°C and ½(1-x)La2O3-xSrO/⅓Co3O4 calcined 161 0.1 ½ (1-x)Nd2O3-xSrO/⅓Co3O4 1/2(1-x)Nd2O3-xSrO/Co3O4 calcined at 600oC calcined at 600°C ½ (1-x)La2O3-xSrO/⅓Co3O4 1/2(1-x)La2O3-xSrO/Co3O4 calcined at 600oC calcined at 600°C Remanence (emu/g) 0.08 ½ (1-x)Nd2O3-xSrO/⅓Co3O4 1/2(1-x)Nd2O3-xSrO/Co3O4 calcined at 700oC calcined at 700°C 0.06 0. 04 0.02 0 0 Figure 6.8... 1-x < 0.15) can be dissolved in Co3O4 phase since this dissolution (doping) causes slender distortion of Co3O4 phase, which has been observed by HR-TEM in our previous work (Figure 3.12), and was suggested to augment the interfacial induction effect However, too low a doping concentration (1-x < 0.05) would dilute such impact, leading to a weaker ferromagnetism As far as the variation of coercivity with... 128 126 158 b 1600 3d5/2 140 0 Intensity 1200 3d3/2 1000 800 600 131.91 eV 40 0 200 0 142 Figure 6.6 140 138 136 1 34 132 Binding energy (eV) 130 XPS chart of Sr 3d of 0.025Nd2O3-0.95SrO/⅓Co3O4 calcined at (a) 600°C and (b) 800°C 128 126 159 ligand with Co ion in the Co3O4 nano-domain), it is expected that the binding energy of Sr 3d5/2 and 3d3/2 doublet in 0.025Nd2O3-0.95SrO/⅓Co3O4 will be higher than that... attributed to its smaller ionic radius (refer to Table 6.1) and its f valence shell However, when 0.01 < 1-x < 0.05, the substantially low doping concentrations of Nd3+ (in Co3O4), could still hold a positive impact on interfacial induction Such impact is supposed to be related to the high effective nuclear charge Z eff of Nd3+ ion (i.e the lanthanide contraction), which might be the key factor in tuning... key factor in tuning the magnetic susceptibility of Co3O4 to the interfacial induction and requires the 1 64 P P P Intensity 900oC 800oC H H H 700oC 600oC 25 Figure 6.9 35 45 2 theta 55 65 75 XRD of 0.1Nd2O3-0.8SrO/⅓Co3O4 calcined at different temperatures where H and P represent the hexagonal and perovskite phase respectively 165 maximum dose close to 1-x = 0.05 The Z eff affects the lowest unoccupied... to be the doping of La3+ in the spinel Co3O4 phase and the content of La3+ in the ½(1-x)La2O3-xSrO/⅓Co3O4 appeared to be crucial The La3+ could be brought into Co3O4 phase during pyrolytic process only when its concentration is below a certain level, and above this level, individual La2O3 domains would be formed It is logical that only small amount of La3+ (i.e 1-x < 0.15) can be dissolved in Co3O4... H H 800°C 900°C 20 30 40 2 theta 50 60 70 80 153 b Intensity (arbitrary units) H H H 800oC 700oC 600oC 20 Figure 6.3 30 40 2 theta 50 60 70 80 XRD chart of (a)0.025Nd2O3-0.95SrO/⅓Co3O4 and (b) 0.025La2O3-0.95SrO/⅓Co3O4 calcined at different temperatures, where H representing the hexagonal phase 1 54 Intensity (Arbitrary units) P P H P 900°C H H 800°C 700°C 600°C 20 Figure 6 .4 30 40 50 2 theta 60 70 80 . 600oC 1/2(1-x)Nd2O3-xSrO/Co3O4 calcined at 700oC ½ (1-x)Nd 2 O 3 -xSrO/⅓Co 3 O 4 calcined at 600°C ½ (1-x)La 2 O 3 -xSrO/⅓Co 3 O 4 calcined at 600°C ½ (1-x)Nd 2 O 3 -xSrO/⅓Co 3 O 4 calcined. ½(1-x)La 2 O 3 - xSrO/⅓Co 3 O 4 calcined at 600°C 0 0.02 0. 04 0.06 0.08 0.1 0 0.05 0.1 0.15 0.2 0.25 1-x Remanence (emu/g) 1/2(1-x)Nd2O3-xSrO/Co3O4 calcined at 600oC 1/2(1-x)La2O3-xSrO/Co3O4 calcined. (Oe) (1-x)Nd2O3/xSrO/Co3O4 calcined at 600oC (1-x)La2O3/xSrO/Co3O4 calcined at 600oC (1-x)Nd2O3/xSrO/Co3O4 calcined at 700oC ½ (1-x)Nd 2 O 3 -xSrO/⅓Co 3 O 4 calcined at 600°C ½ (1-x)La 2 O 3 -xSrO/⅓Co 3 O 4