Interfacial induction sustained ferromagnetism from solution chemistry to ceramics 4

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 spi

Chapter 6

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 0 < 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

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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+ ion has a larger ionic radius and d-type valence shell, {Xe}5d0, while Nd3+ has a

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 (>

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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 0 < 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 2-butanone (v/v=1) and after evaporating the solvent and drying, the lumps were ground into a fine powder (<40μ) and pressed into pellets (d ≅1cm) as mentioned in earlier chapters The pellet was then combusted to 400°C to burn away the polymer binder and calcined at 600°C or higher (e.g 750°C, 800°C and 875°C) for 1 h to conduct solid phase reaction of the three oxides

6.2.4 Instrumental Analysis

The Vibrating Sample Magnetometer (Oxford Magnetometer VSM), with an applied field of 10kOe at 298K, was employed to obtain the hysteresis loops of the oxide samples The crystalline phases were determined by X-ray diffraction with Cu Kα radiation (Philips X’Pert) The degradation profile of the dehydrated precursor gel is obtained using the thermogravimetric analyser (TA instrument, TA2050) The

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nanoscale phase domain morphology of the complex oxides was examined on a high resolution transmission electron microscope (HR-TEM, Philips CM300)

6.3 Results and Discussion

6.3.1 Interfacial Reactivity of heterogeneous ½(1-x)Nd2O3-xSrO/⅓Co3O4 Nanocomposite

When the tri-oxide composite ½(1-x)Nd2O3-xSrO/⅓Co3O4 with different

stoichiometric ratios (x = 0.4, 0.6, 0.8, 0.95 and 0.99) were calcined at different

temperatures, we observed similar phenomenon as that of the corresponding

½(1-x)La2O3-xSrO/⅓Co3O4 composite as mentioned in Chapter 3 Perovskite Nd

1-xSrxCoO3-δ single phase structure is formed at 600°C for compositions with x ≤ 0.4, whereas for x > 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

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 loss plateau until 758°C This implies that the reaction of metallic oxides sets out

-0.100.10.20.30.40.50.60.70.80.91

Figure 6.2 Thermal degradation chart of metallo-organic precursor gel (a) 0.025La2O3-0.95SrO/⅓Co3O4 and

(b) 0.025Nd2O3-0.95SrO/⅓Co3O4

020406080100120

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 Formation constants of Citrate complexes (Martell and Smith, 1977; Dean, 1992)

Metal cation Citrate (HL2-) Glycine

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

(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 1 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, 1 and 2 could be

Figure 6.3 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

Figure 6.4 XRD chart of 0.05Nd2O3-0.90SrO/⅓Co3O4 calcined at different temperatures, where H and P representing the hexagonal and perovskite phase respectively

2 theta

600°C 700°C 800°C 900°C

P

attributed to the two major phases (SrO and Co3O4) of the composite The representative dotted lines indicate the interfacial boundaries between phase 1 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

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

b

valley hill

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126128

130132

134136

138140

Figure 6.6 XPS chart of Sr 3d of 0.025Nd2O3-0.95SrO/⅓Co3O4 calcined at (a) 600°C and (b) 800°C

02004006008001000120014001600

126128

130132

134136

138140

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

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(1-

x)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

Figure 6.7 Coercivity 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

0 50 100 150 200 250 300 350 400

(1-x)La2O3/xSrO/Co3O4calcined at 600oC

(1-x)Nd2O3/xSrO/Co3O4calcined at 700oC

½ (1-x)Nd2 O 3-xSrO/⅓Co3 O 4

calcined at 600°C

½ (1-x)La2O3-xSrO/⅓Co3O4calcined at 600°C

½ (1-x)Nd2 O 3-xSrO/⅓Co3 O 4

calcined at 700°C

Figure 6.8 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

0 0.02 0.04 0.06 0.08 0.1

1/2(1-x)La2O3-xSrO/Co3O4calcined at 600oC

1/2(1-x)Nd2O3-xSrO/Co3O4calcined at 700oC

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 e g 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 ≤

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