Interfacial induction sustained ferromagnetism from solution chemistry to ceramics 2

By referring to the XRD patterns of the three pristine oxides shown previously in Figure 3.4, the composite could basically retain the properties of the individual oxides labelled by l –

Chapter 4

The Interface-Sustained Magnetic Properties Displayed by

the La2O3-SrO-Co3O4 Nanocomposite

4.1 Introduction

As mentioned in the earlier chapter, partial substitution of La3+ by Sr2+ in LaCoO3 leads to remarkable changes in the properties of the material With the increase in dopant concentration, the rhombohedral distortion in the perovskite structure is reduced, oxygen vacancies are generated and a small fraction of Co3+ is converted to

Co4+ The work of Raccah et al and Bhide et al showed that the rhombohedral distortion decreases with the introduction of Sr2+ until x = 0.5, after which the structure

remains cubic (Raccah and Goodenough, 1968; Bhide, et al., 1975) This is because in

the range of 0 < x < 0.5, the structure responds to strontium substitution by steadily

increasing Co4+ rather than losing lattice oxygen (Yakel, 1955) Thus, the resultant perovskite oxides, La1-xSrxCoO3-δ (x < 0.5), possess remarkably different electric and magnetic properties from their parental form

This work is a continuation of the work presented in the previous chapter As

described in the previous chapter, heterogeneous ½(1-x)La2O3-xSrO/⅓Co3O4 complex

oxide system is studied These oxide mixtures exhibit much higher coercivity than the perovskite oxide with the same composition at low temperatures Petrov reported a coercivity of 0.03T at a measured temperature of 4.2K for perovskite La0.6Sr0.4CoO3-y and the coercivity of this material is found to decrease with the increase in temperature

(Petrov, et al., 1995) On the contrary, the complex oxide with the same composition (x

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= 0.4) exhibits a coercivity of 0.165T at 74 K, which is about 5.5 times that of the perovskite La0.6Sr0.4CoO3 measured at a much lower temperature

It was reported earlier that ½(1-x)La2O3-xSrO/⅓Co3O4 complex oxide manifested

room temperature ferromagnetism and attributed it to a special interfacial phenomenon Both La2O3 and SrO phases are considered to cause, via interfacial induction, distortion of the octahedral coordination sphere in the spinel Co3O4 phase This phenomenon is known as the “Jahn-Teller” effect and becomes notable only in a highly dispersed system where a large extent of interfacial contact exists In this chapter, the influence of SrO content and chelating ligands on both the coercivity and remanence of the complex oxide at low temperature are explored

4.2.1 Chemicals

Lanthanum nitrate hydrate (La(NO3)3.yH2O, 99.99%, 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), poly(vinylbutyral) resin (Butiva-79, Monsanto), toluene (>99.5%, Merck), 2-butanone (>99.8%, Fisher Scientific), citric acid (>99.5%, Sigma), DL - malic acid (99%, Acros), lactic acid (about 90%, Merck) and Ethylene diamine tetraacetic acid (EDTA, Fluka, ≥ 98%) were used as received

4.2.2 Preparation of the hydrogel using citric acid-ethylene glycol ligands

Similar to the preparation method depicted in Section 3.2.2, a series of hydrogels containing various molar ratios of metal ions (La3+: Sr2+: Co2+ = 1-x : x : 1 and 0 < x ≤

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0.95) were prepared by the wet chemistry approach A typical procedure includes preparation of an aqueous solution of La(NO3)3.yH2O, Sr(NO3)2, Co(NO3)2.6H2O, glycine and citric acid, in which the molar ratio of total metal cations to the total functional groups (i.e amino group and carboxyl group) of citric acid and glycine was maintained at 0.154 (mass ratio of citric acid to glycine is 0.129) Ethylene glycol (77% by volume) was then added to this solution The solution was allowed to concentrate on a hot plate at 200°C to form a gel

4.2.3 Preparation of the hydrogel by using other types of ligands

Three other types of chelating reagent systems, namely the malic acid/glycine/ethylene glycol, lactic acid/glycine/ethylene glycol and ethylene diamine tetraacetic acid (EDTA) were employed respectively to synthesize the hydrogels by using the same procedure as described in the above section But only one composition of the

composite, namely x = 0.95, was used in these three types of gels for investigating the

ligand effect

4.2.4 Pyrolysis and calcination

The preparation of the testing samples is similar to that reported in Section 3.2.3 The gel, as obtained from Section 4.2.2 was heated to 400°C to execute pyrolysis to yield a black powder This black powder was calcined at 600°C for 2h under air purge to ensure complete removal of carbon residues and growth of the crystal phases in the three oxides

The resulting ½(1-x)La2O3-xSrO/⅓Co3O4 powder was then ground and added to a

polymer solution comprising of poly(vinylbutyral) dissolved in the mixture of toluene

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and 2-butanone (v/v=1) After evaporation of the solvent, the lumps were ground into fine powder (<40μ) and pressed into pellets (d ≅1cm) The pellet was then combusted

at 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

4.2.5 Characterisations

The degradation profile of the dehydrated precursor gel is obtained using the thermogravimetric analyser (TGA, TA instrument) A heating rate of 10°C/min in 100ml/min of Nitrogen gas was used and the weight against temperature chart obtained enables the study of segmental degradation of the precursor with temperature The hysteresis loops of the samples were obtained from the Vibrating Sample Magnetometer (Oxford Magnetometer VSM) with an applied field of 10kOe measured

at various temperatures X-ray Diffractometer with Cu Kα radiation (Philips X’Pert) was employed to determine the crystalline phases of the samples The electrical properties of the complex oxides and their homogeneous counterparts were investigated using an impedance analyser (Solartron 1226) The morphology of nano-composite was observed with a Field Emission Scanning Electron Microscope (FE-SEM, JEOL, JSM-6700F) and the nano-scaled phase domain morphology of the complex oxides was examined using High-Resolution Transmission Electron Microscopy (HR-TEM, Philips CM300) The X-ray photoelectron spectrometer (XPS, Kratos Axis HSi System) is employed to investigate the chemical composition and environment of the heterogeneous oxides using C 1s peak (284.6 eV) as the internal reference The electron paramagnetic resonance spectroscopy (EPR) was obtained using X-band Elexsys E500 CW-EPR Spectrometer (Bruker BioSpin GMBH)

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The first four stages (including peaks a-d), which occurred at temperatures below

550°C, are pyrolysis processes which remove the organic components and nitroxides

The last peak (labelled e) is the solid-phase reaction among the three different oxide

domains with the removal of oxygen and formation of a solid solution at about 610°C

X-ray diffraction also verifies that the heterogeneous oxide 0.1La2O3-0.8SrO/⅓Co3O4

is converted to the hexagonal type structure provided that the calcination is carried out

at temperatures above the threshold of 610°C (Figure 4.2)

The precursor of the complex metal oxides ½(1-x)La2O3-xSrO/⅓Co3O4 is a

metallo-organic hydrogel formed via chelating bonding between the three metal ions (La3+,

Sr2+ and Co2+) and the hydrophilic organic ligands (citric acid and glycine) The conversion process from precursor to ceramic is analysed using the TGA From the TGA profile shown in Figure 4.1 for precursor gel with x = 0.8, it can be observed that the conversion process involves four weight loss stages

4.3.1 Nano-scale grain-boundary structure cast by metallo-organic gel

½(1-x)La2O3-xSrO/⅓Co3O4 La1-xSrxCoO3-δ

020406080100

Figure 4.2 X-ray chart of 0.1La2O3-0.8SrO/⅓Co3O4 calcined at 600°C and 800°C respectively where H, P, s, l and c represent

hexagonal, perovskite, SrO, La2O3 and Co3O4 phase respectively

As mentioned earlier, if the pyrolytic product (x= 0.8) is calcined at a temperature

below the threshold of solid reaction, e.g at 600°C, the heterogeneous tri-oxide composite is resulted (Figure 4.2) By referring to the XRD patterns of the three pristine oxides (shown previously in Figure 3.4), the composite could basically retain

the properties of the individual oxides (labelled by l – La2O3, s – SrO, and c – Co3O4)

despite having a low content of perovskite phase (labelled by P) The formation of perovskite phase is considered to occur at the location where both La2O3 and Co3O4 oxides are in proximate contact with each other According to their XRD patterns, this

perovskite phase fades away quickly with the increase in x as shown in Figure 3.5 The

FE-SEM image of the composite reveals a grain-boundary structure with grain sizes in the range of 20-30 nm (Figure 4.3a) A closer observation of the individual grain by

HR-TEM (Figure 4.3b) shows two different major types of domains in the nanometer

scale, which cannot be specified by energy dispersive X-ray spectroscopy (EDS) due

to their minute sizes However, they could be assigned approximately using the respective radii of metal ions (Sr2+-1.18 Å; La3+-1.36 Å; Co2+-0.65 Å) and the lattice packing density (Co3O4 > SrO) as the basis We could observe that the assigned Co3O4 domain comprises of more closely assembled unit cells Since these domains mutually interpenetrate in the scale of a few nano-meters, the solid reaction among them can thus take place at the temperature (e.g 610oC) that is substantially lower than that needed for the powder blend of the three oxides

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a

2

Co3O4

1 SrO - La2O3

b

Figure 4.3 (a) FESEM and (b) HR-TEM images of 0.1La2O3-0.8SrO/⅓Co3O4

calcined at 600°C

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The maximum weight-loss rate of peak e on the TGA curve is a measure of the activity

of the solid-phase reaction Table 4.1 lists the reaction rates caused by the variation of

organic chelating ligand (hydroxycarboxylic acid) in the hydrogels with x = 0.95 It is

found that the reactivity correlates with the functionality (f = number of CO2H and

-OH groups per molecule) of the hydroxycarboxylic acid The details about the role of

organic ligands will be elaborated in Section 4.3.3

Table 4.1 Solid reaction rates on different chelating systems

Chelating ligand system Maximum weight-loss rate of peak e

(% / °C) Citric acid (f=4)/glycine/ethylene glycol 0.05580

Malic acid (f = 3)/glycine /ethylene glycol 0.04540

Lactic acid (f = 2)/glycine/ethylene glycol 0.03403

EDTA 0.04638

4.3.2 The origins of ferromagnetic properties of the heterogeneous tri-oxide

composites

As mentioned in Section 3.3.1, both the calcination temperature and the SrO content

affect the phase structure of the materials formed Perovskite solid solution is readily

generated when x ≤ 0.5 at 600°C (Figure 4.4) But at this temperature, as elucidated

above, the tri-oxide composites (x ≥ 0.8) are generated This can be further validated

through the X-ray photoelectron spectra (Figure 4.5) The Sr 3d XPS reveals rather

complicated multiple peaks The two distinct doublets, 3d 5/2 and 3d 3/2 that are observed

here were also reported by several previous studies (van der Heide, 2002; Vovk, et al.,

2005) As can be seen from the figure, the doublets appear in the XPS spectra of the

three samples (x = 0.5, 0.8 and 0.95) despite the different crystal structures between

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agreement with the fact that this heterogeneous oxide sample contains minor perovskite component (Figure 4.4) Hence, it is clear that only one set of the doublet

peaks is present in Sr 3d spectrum of the heterogeneous trioxide composite with x =

0.95 and this is because of the existence of Sr-O phase and negligible perovskite

phase On the contrary, the corresponding Sr 3d spectrum of the binary SrO/Co3O4 composite oxide displays a severely overlapped Sr 3d doublet (Figure 4.5d) This

indicates that a very low content of La2O3 phase in the former composite made Sr 3d spectrum different The electric conductivity measurement (-lg σ) also shows a leap from the perovskite solid solution to the heterogeneous tri-oxide composite (Figure 4.6) because the former has mixed-conductive structure that contains electronic conductivity (Petrov et al., 1995)

128 130

132 134

136 138

132 134

136 138

132 134

136 138

11.5

22.5

33.5

44.5

The two perovskite solid solutions La1-xSrxCoO3-y (x = 0.4 and 0.5) exhibit ferromagnetism at low temperatures (Figure 4.7) This is consistent with the earlier work done by Petrov et al (Petrov et al., 1995) In contrast to the homogeneous

perovskite samples, the heterogeneous trioxide composites (x = 0.8 and 0.9) reveals a

larger coercivity (Figure 4.8) Both figures show strong temperature dependence of magnetic properties, which is consistent with the characteristic magnetic behaviour of the perovskite oxide despite the low contents of perovskite phase in the two composite samples

In the tri-oxide composites ½(1-x)La2O3-xSrO/⅓Co3O4 with x = 0.8 and 0.9, there are

two phases that possess magnetic response They are the spinel Co3O4 and the perovskite La1-αSrαCoO3-β phase The content of perovskite phase in the composite

with x = 0.9 is lower than that in the composite with x = 0.8 (Figure 4.4), and the composite with x = 0.95 contains no perovskite phase according to the intensity of the

peak at 2θ = 33°, which is the characteristic perovskite peak Besides temperature

sensitivity, the coercivity of these three composites is also dependent on the x value Looking at Figure 4.8, it can be noted that coercivity for composite with x = 0.9 is higher than that of the composite with x = 0.8 when the measured temperature is lower than 214K For the composite with x = 0.95, its coercivity exhibits an apparent less

declination with increasing temperature than the other two composites and as it does not possessed perovskite phase, its coercivity is lower than that of x = 0.8 and 0.9 Since spinel Co3O4 phase is the only magnetic phase in this composite, the faster

decreasing trend of coercivity exhibited by the two lower-x composites should

therefore be accounted to the effect of perovskite component

Figure 4.8 Coercivity vs measurement temperature of ½(1-x)La2O3-xSrO/⅓Co3O4 with x = 0.8, 0.9 and 0.95

cry e

g g

It is known that pristine spinel Co3O4 is an antiferromagnet with a Néel temperature of 40K However, it was reported by Makhlouf that the material exhibits weak ferromagnetism at around 25K Similar effect was observed from Wang and co-workers who reported the temperature to be around 40K (Makhlouf, 2002; Wang, et al., 2005b) Since the measured temperature of 80K is much higher than the Néel temperature or any transition temperature as mentioned by Makhlouf or Wang, spinel

The three composites presented in Figure 4.8 all possess certain coercivities at 250K even though the values are rather small Since the perovskite phase loses ferromagnetism at 250K, the coercivity above this temperature is due to the coexistence of spinel Co3O4 and SrO nano-phases, which are considered to possess ferromagnetic property The EPR analysis also supports the different structure backgrounds responsible for the ferromagnetism (Figure 4.9) On the X-band EPR

spectra of composite x = 0.5 (perovskite oxide) at 200K, a multiple-splitting spectrum

is present, which is an indication of the existence of multiple-crystal field that have different crystal field splitting energies (Δcry) It is known that the Δcry value affects g factor through spin-orbital coupling ( = −αλ/Δ , where α is a parameter related

to the orientation of crystal field and type of transition metal) This EPR spectrum reflects rather intricate chemical environments (due to the participation of other metal ions) surrounding cobalt ions (Co3+ and Co4+) in the perovskite phase However, the

EPR spectrum of composite with x = 0.95 exhibits a much simpler X-band with the

magnetic cobalt ions (Co3+) in spinel phase surrounded by oxygen ions