Surface & Coatings Technology 226 (2013) 108–112

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Surface & Coatings Technology

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Simple polymer assisted deposition and strain-induced ferromagnetism of LaCoO3

epitaxial thin films

Haifeng Liu a,b, Lei Shi a,⁎, Shiming Zhou a, Jiyin Zhao a, Yuqiao Guo a, Cailin Wang a, Laifa He a

a Hefei National Laboratory for Physical Sciences at Microscale, University of Science and Technology of China, Hefei 230026, PR Chinab Analytical and Testing Center, Southwest University of Science and Technology, Mianyang 621010, PR China

⁎ Corresponding author. Tel.: +86 551 63607924; faxE-mail address: shil@ustc.edu.cn (L. Shi).

0257-8972/$ – see front matter © 2013 Elsevier B.V. Allhttp://dx.doi.org/10.1016/j.surfcoat.2013.03.042

a b s t r a c t

a r t i c l e i n f o

Article history:Received 19 December 2012Accepted in revised form 26 March 2013Available online 3 April 2013

Keywords:Polymer assisted depositionLaCoO3

epitaxial filmferromagnetism

LaCoO3 epitaxial filmswith different thicknesses (~20, 50 and 80 nm)were grown on (001) SrTiO3 substrates bya simple polymer assisted deposition method. X-ray diffraction analyses including θ/2θ symmetric scan, ω-scanand in-plane φ-scan indicate that single-phase (001) oriented LaCoO3 films with a pseudotetragonal structurewere grown on (001) SrTiO3 substrates successfully, with a biaxial tensile strain from + 2.42% to + 2.60% andtetragonal distortion from1.47% to 1.63%. Due to the lattice relaxation effect in epitaxial thinfilm, the biaxial ten-sile strain is slightly relaxedwhen the thickness of the LaCoO3 film increases, resulting in an increase of the c-axisconstant in contrast to a decrease of the in-plane constants of the film. It is different from LaCoO3 bulk with anonmagnetic ground state that all the epitaxial films exhibit a ferromagnetic transition at TC ~ 85 K. Combiningwith the structural andmagnetic analyses, it is shown that the strain-induced ferromagnetism in LaCoO3 epitax-ial films, corresponding to the higher spin states, origins from the decrease of the energy difference between egand t2g levels, which is caused by an increase of the unit-cell volume and suppression of the CoO6 octahedral ro-tations. In addition, the change of FC curve with the thickness of the film reveals that the ferromagnetism is en-hanced by the thickness decrease of LaCoO3 film due to the increase of the biaxial tensile strain.

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

Transitionmetal oxides with perovskite or perovskite-derived struc-tures exhibit a variety of unusual properties including ferromagnetism,ferroelectricity, giant/colossal magnetoresistance effects (GMR/CMR),and multiferroics (simultaneous ferroelectricity and ferromagnetism),making them excellent candidates for various devices applied in elec-tronics and sensors [1–4]. In particular, as an important member ofperovskite-type cobalt oxides, LaCoO3 (LCO) has attracted much atten-tion since being discovered several decades ago, due to its interestingspin-state transition in the temperature range 35 K b T b 100 K andsemiconductor-metal transition in the interval 300 K b T b 600 K[5–10]. Recent studies have shown that LCO epitaxial thin films grownon (LaAlO3)0.3(Sr2AlTaO6)0.7 (LSAT) [11], PMN-PT [12], and SrTiO3

(STO) [12,13] substrates exhibit ferromagnetic ordering below a criticaltemperature of TC ≈ 85 K, which is unobserved in the bulk. However,the origin and exact nature of the spin states presented in LCO epitaxialfilms are still not fully understood and need further study.

To obtain high-quality LCO epitaxial thin film for the experimentalstudy and application, on the other hand, various techniques have beenapplied, such as pulsed laser deposition (PLD) [11–14], molecular beamepitaxy (MBE) [15], magnetron sputtering deposition [16], and so on.Most of all these vacuum techniques require high-cost equipment and

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strict deposition conditions. Recently, to overcome the shortcomings, apolymer assisted deposition (PAD) method based on a surfactant-assisted sol–gel process [17–21] has been developed to grow oxide thinfilm. During the PAD process, polyethyleneimine (PEI), one of the mostcommon soluble polymers as a cationic polymer surfactant, is appliedand mixed well with the aqueous solution of metal-precursors [17–21].Due to the strong complexation reaction in the solution, metal ions orsmall metal chelates can be electrostatically bound to the dendritic mol-ecules of PEI, resulting in a homogeneous distribution ofmetal precursorsand hence the formation of uniformmetal-organic films. Compared withthe conventional sol–gel process, themain advantage of the PADmethodlies in the functions of PEI. Besides as a binding agent to themetal precur-sor, control of the solution concentration and themolecularweight of PEIcan also be helpful in achieving the desired viscosity and the thickness ofthe film. Therefore, it can provide a new way to grow crack-free and rel-atively thicker films, which are difficult or nearly impossible to prepareby conventional sol–gel process [18]. In addition, the problem that thealkoxides used in conventional sol–gel process are very water sensitiveand may be precipitated out from the solution during the preparationcan be avoided in the PAD process [22]. However, although the PADmethodhas been successfully applied in the preparation of the perovskitemanganese oxide thin films (e.g. La0.67Sr0.33MnO3/La0.67Sr0.33MnO3 filmson (001) LaAlO3) [19], to the best of our knowledge, there is no relevantreport on preparing LCO epitaxial films on STO by the simple PADmethod. Study on the preparation of LCO films on STO by the PADmethod may extend the application of the PAD method.

109H. Liu et al. / Surface & Coatings Technology 226 (2013) 108–112

In this paper, LCO epitaxial thin films with different thicknessesgrown on single-crystal (001) STO substrates were prepared by a facilePADmethod. Based on the characteristics for the thickness, morphology,component and crystal structure, the epitaxial nature and strain-inducedstructural distortion in the LCO films were investigated. Furthermore,themagneticmeasurementswere carried out, and the origin and charac-teristics of strain-induced ferromagnetism in LCO epitaxial films withdifferent thicknesses were discussed.

2. Experiment

The precursor solution was prepared as follows. First 0.25 g PEI withmolecular weight of 70,000 was dissolved in 10 mL H2O. High purity(>99.99%) metal salts La(NO3)3 nH2O (0.5 mmol) and Co(NO3)2 6H2O(0.5 mmol) were mixed and dissolved in 5 mL H2O, and then citric acidmonohydrate (1.5 mmol) was added. After 1 h chelating-reaction,10 mLPEI aqueous solutionwas slowly added into themixture by a drop-per. The mixed solution was stirred by a magnetic stirring apparatus for10 h at room temperature, and then kept being stirred in a 60 °C oilbath until the solution remaining ~5 mL.

To obtain LCO epitaxial thinfilmswith different thicknesses, the pre-cursor solution was deposited on treated single-crystal (001) STO sub-strates (10 mm × 5 mm × 0.5 mm) at 3000, 5000 and 7000 rpmover 30 s by a spin coating technique, respectively. Then the coated sub-strates were placed in a muffle furnace. According to the thermal de-composition analysis of PEI (not shown here), a low ramp rate of 1 °Cmin−1 was used from room temperature to 700 °C to make sure thewater evaporated and polymers burned up to avoid the formation ofvoids in the bulk of the films. The samples were then rapidly heated(10 °C min−1) to a temperature of 900 °C. After 2 h heat treatment,the films were cooled down to room temperature at 1 °C min−1.

The thickness and surface morphology of the film were obtainedby an FEI Sirion 200 field emission scanning electron microscope(SEM). The formation of the film sample was confirmed by anotherfield emission SEM (Ultra 55, Carl Zeiss) equipped with energy dis-persive X-ray (EDX) spectroscopy and elemental mapping facilities.The epitaxial structural measurements of the films including θ/2θsymmetric scan, ω-scan (rocking curve) as well as in-plane φ-scanwere performed by a Rigaku TTRШ X-ray diffractometer (XRD) withCu Kα radiation (λ = 1.54187 Å). In addition, the magnetic proper-ties of the films were investigated via a Quantum Design MPMS-7superconducting quantum interference device (SQUID) magnetome-ter. The field-cooled (FC) magnetization was measured in the temper-ature range 10 K ≤ T ≤ 150 K with an external magnetic field ofH = 100 Oe applied parallel to the film surface.

3. Results and discussion

3.1. Thickness, morphological and componential characteristics of theLCO film

Fig. 1 shows the SEMmicrographs of the LCO epitaxial thin films, inwhich part (a), (b) and (c) show the cross-sectional areas of LCO filmscoated at 3000, 5000 and 7000 rpm, respectively, while part(d) shows the surface morphology of LCO film coated at 5000 rpm. Asseen in Fig. 1(a–c), the thicknesses of the LCO films, obtained at spin-ning speed of 3000, 5000 and 7000 rpm, are about 80 nm, 50 nm and20 nm, respectively. Obviously, the film becomes thinner and smootherwith the spinning speed increasing. As known, the thickness of film canbe controlled by varying the spinning speed in the spin coating tech-niquewhen the other conditions are kept unchanged. Besides, it is wor-thy to note that the viscosity of the precursor solution, dependingon theconcentration and molecular weight of PEI, is also an important factor[18,20]. From Fig. 1(d), it can be found that the 50-nm-thick LCO filmshows a dense, homogeneous and crack-free surface morphology.Moreover, the EDX analysis corresponding to the 50-nm-thick LCO

film, as shown in Fig. 2, reveals that besides various intense peaks asso-ciated with Sr, Ti and O elements of STO substrate, peaks associatedwith La and Co elements can be also observed, respectively. The compo-sition of elements is confirmed to be coincidedwith the expected value.From the elementalmapping results, it can be seen that the compositionof the prepared film constitutes La and Co are distributed regularly overthe film surface.

3.2. Structural characteristics of the LCO films

In the XRD θ/2θ symmetric scans, as shown in Fig. 3, only (00l) peaksof the LCO films are observed alongwith the (00 l) peaks of the STO sub-strate, suggesting that all the LCO films have a preferential c-axis orienta-tion. Compared with the 2θ angles of STO (002) peak (~46.49°) and(001) peak (~22.78°), the (002) and (001) peaks of the LCO films allappear at higher angles, i.e., ~48.12° and ~23.52°, respectively. Accordingto the Bragg equation, it can be obtained that the out-of-plane c-axis lat-tice parameters of 80-, 50- and 20-nm-thick LCO films are 3.784 Å,3.782 Å and 3.779 Å (listed in Table 1), respectively, which are compara-ble with the reports [11–15]. It is known that the bulk LCO has a rhom-bohedral structure (R-3c) with crystal lattice parameters of a = b =5.440 Å and c = 13.100 Å, which can be considered as a pseudocubicperovskite structure corresponding to a parameter of abulk ≈ 3.805 Åwith a rhombohedral distortion along the (111) direction [11,13].Obviously, all the c-axis lattice parameters of the LCO films decrease,comparing with that of the bulk LCO, and show a continuously decreas-ing trend when the thickness of the film decreases, which can be attrib-uted to the lattice mismatch of LCO (abulk ≈ 3.805 Å) and STO (as =3.905 Å) in the ab plane. Due to the bigger lattice constants of the sub-strate, the LCO films grown on STO are under biaxial tensile strain inthe ab plane, resulting in the compressive effect along the c-axis andtetragonal distortion of CoO6 regular octahedrons. This also means thatthe LCO films are successfully grown on the (001) STO substrates as a“cube-on-cube”mode [14]. However, since the existenceof the lattice re-laxation effect in epitaxial thinfilms [12], the biaxial tensile strainmaybeslightly relaxed and the average c-axis constant will increase in a smalldegree when the LCO film becomes thicker, as shown in this work.Moreover, as far as PEI is removed during heat treatment, more or lessnarrow porosity distribution may be formed, which may be increasedin amount with the thickness increasing. It can also contribute to the re-laxation of the biaxial tensile strain. In addition, good out-of-plane orien-tation of the LCO thin films can be further confirmed by the XRD ω-scan(rocking-curve) of the (002) peak of the LCO films. The XRD rockingcurve of the (002) peak of the thickness ~50 nm LCO film is shown inFig. 4, from which it can be seen that the full-width at half-maximum(FWHM) of the peak is ~0.33°, and no indication of misorientation canbe detected.

On the other hand, in order to provide a further insight into thein-plane symmetry and lattice parameters of the LCO films, anin-plane φ-scan (asymmetric Bragg reflection) for (111) reflection ofthe 50-nm-thick LCO thin film as well as that of the STO substrate wascarried out (see Fig. 5). The four-fold symmetric diffraction peaks withan average FWHM value of 0.78° for the 50-nm-thick LCO thin filmare observed. By comparing with the average FWHM value of 0.43° forthe STO substrate, it is revealed that the films are of good epitaxial qual-ity. The epitaxial relationship between the LCO film and the STO sub-strate is confirmed to be (001)LCO|| (001)STO and [100]LCO|| [100]STO.The in-plane lattice parameters (a) of the LCO films obtained from theasymmetric Bragg scans are listed in Table 1. It can be seen that thein-plane lattice parameter a deviates from that of STO (as = 3.905 Å)and decreases slightly with the increasing thickness of LCO film,which can also be attributed to the mentioned-above lattice relaxationeffect.

For LCO epitaxial film, the room-temperature in-plane biaxial straincan be expressed as ε = (afilm − abulk)/abulk, where afilm is the in-planelattice parameter of LCO film and abulk is the pseudocubic lattice

Fig. 1. Cross-section and top-view SEMmicrographs of the LCO epitaxial thin films: (a), (b) and (c) show the cross-sectional areas of LCO films coated at 3000, 5000 and 7000 rpm,respectively, while (d) shows the surface morphology of LCO film coated at 5000 rpm.

110 H. Liu et al. / Surface & Coatings Technology 226 (2013) 108–112

parameter of bulk LCO [23]. Meanwhile, the tetragonal distortion ofCoO6 regular octahedrons can also be expressed as ΔTD = |a − c|/(a + c) [14]. Thus, based on the out-of-plane and in-plane lattice pa-rameters, the in-plane biaxial strain, unit-cell volume (V = a2c) and te-tragonal distortion of the LCO epitaxial thin films with differentthickness are obtained, as shown in Table 1, which reveal that the biax-ial strain and tetragonal distortion caused by the biaxial strain are allslightly weakened along with the increasing thickness of the LCO film,owing to the lattice relaxation effect. Simultaneity, the unit-cell size be-comes smaller. However, it is worth noting that, compared with that of

Fig. 2. EDX spectrum and elemental mapping results of the surface for 50-nm-thickLCO epitaxial thin film. Inset shows the distributions of La and Co.

bulk LCO (~55.09 Å3), each of the LCO films grown on STO shows alarger unit-cell size (~57.5 Å3). The structural distortion of the LCOfilms induced by biaxially tensile strain is expected to have an effecton the magnetic properties.

3.3. Magnetic characterization of the LCO films

Fig. 6 presents FC magnetization of the LCO epitaxial thin films withdifferent thicknesses under an external magnetic field of H = 100 Oeapplied parallel to the film surface (10 K ≤ T ≤ 150 K). The diamagnetic

Fig. 3. The (002) peaks in the XRD θ/2θ symmetric scans of the LCO epitaxial thin filmswith different thicknesses. Inset shows the (001) peaks of the LCO films.

Table 1Out-of-plane (c) and in-plane (a) lattice parameters, biaxial strain (ε), unit-cell volume(V = a2c) and tetragonal distortion (ΔTD) of the LCO epitaxial thin films with differentthicknesses.

Thickness (nm) c (Å) a (Å) ε (%) ΔTD (%) V (Å3)

80 3.784 3.897 +2.42 1.47 57.4750 3.782 3.902 +2.55 1.56 57.5820 3.779 3.904 +2.60 1.63 57.60

The pseudocubic lattice parameters of bulk LCO are abulk ≈ 3.805 Å and V ≈ 55.09 Å3,respectively.

Fig. 5. The in-plane φ-scans at (111) reflections of (a) the 50-nm-thick LCO thin filmand (b) the (001) STO substrate.

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contribution from the STO substrate was determined separately. As canbe seen from the M (T) curves, all the LCO epitaxial films clearly exhibita ferromagnetic (FM) transition in contrast to the nonmagnetic groundstate of bulk LCO. The ferromagnetism is enhanced with the decreasingthickness of the LCO film. However, TC value, determined from the min-imum of the derivative of the FC magnetization (dM/dT) is almost a con-stant, i.e., ~85 K, which is consistent with the reports [11,14].

At the beginning, it was once suspected that the ferromagnetismarises from the hole doping as observed in bulk Sr-doped LaCoO3

samples [24] or oxygen coordination reduction of the surface Co ions[25]. By the syntheses of LCO epitaxial thin films on STO and LSATunder different oxygen pressures and evaluation of the impact of oxygenvacancies on the long range magnetic order, however, Mehta et al. [26]found that the thin films grown under the higher oxygen pressureshow stronger ferromagnetic order and the observed ferromagnetism isnot a surface or interface effect but a bulk effect. Fuchs et al. [11] also re-vealed that the ferromagnetismof tensile strain induced LCOfilm extendsover thewhole volume rather than on the surface of the film. Now, it is ingeneral agreed that the appearance of ferromagnetism in LCO epitaxialfilms such asmentioned above is associatedwith the spin-state transitionfrom the nonmagnetic low-spin (LS) (t2g6 eg0, S =0) state to an intermedi-ate spin (IS) (t2g5 eg1, S = 1) state or a high spin (HS) (t2g4 eg

2, S = 2) state,where the spin state indicates the unpaired electrons of Co 3d6 electronconfiguration which mainly contribute to the ferromagnetism. Thespin-state transition of Co3+ results from a competition between thecrystal-field splitting (ΔCF) and the Hund's exchange energy (Δex). ΔCF

is the splitting between the eg and t2g energy levels of Co 3d6 electron con-figuration. Δex is the intra-atomic exchange interaction which leads to aredistribution of electrons between eg and t2g energy levels. In a simplecrystal-field picture for regular CoO6 octahedron, the LS state is stable ifΔCF > Δex, whereas higher spin state dominates if ΔCF b Δex. An IS statecan be stabilized by the Jahn–Teller distortion, a distortion of CoO6 octa-hedron,which can lead to a further split of the double degenerate eg levels

Fig. 4. LCO (002) ω-scan (rocking-curve) of the 50-nm-thick LCO epitaxial thin film;the FWHM value is shown inside the pattern.

and thereby reduces the energy difference ΔE, i.e., ΔCF − Δex [11,14]. Onthe other hand, owing to the overlap of Co (3d) derived eg and O (2p)orbits, the eg–t2g gap will be further narrowed. Thus ΔE expressed as(ΔCF − W / 2) − Δex is more accurate, where W is the bandwidth ofthe d–p bands [14,27]. Since W ∝ rCo–O

−3.5 sin(θ/2) and ΔCF ∝ rCo–O−5 [27],

where rCo–O is the Co–O distance and θ is the Co–O–Co angle, it can beconcluded that ΔE can be reduced by an increase of rCo–O and θ, i.e., thespin-state transition of LCO from LS to higher spin states can be inducedwhen rCo–O or θ increases.

Based on the above discussion, the origination of the biaxial tensilestrain-induced ferromagnetism observed in LCO epitaxial thin films canbe understood as follows. First, the larger unit-cell volume (~57.5 Å3)in LCO epitaxial film corresponds to longer rCo–O compared with that ofbulk LCO (~55.09 Å3), stabilizing the IS or HS state by a decrease ofΔCF. The similar results, i.e., the enhancement of the unit-cell volume,have also been observed in LCO nanoparticles [6] and nanowires [7],which have been taken as a key factor for the appearance of the ferro-magnetism due to the larger radius of Co ions in excited spin states[12]. Second, the CoO6 octahedral rotations are largely suppressedwhen grown on cubic STO (001) substrate especially for the thinnerfilm. It is known that there exist octahedral rotations and the Co–O–Cobond angle is ~ 164° in pseudocubic bulk LCO [7], as shown in Fig. 7(a).But in the formation of the epitaxial film, CoO6 octahedral rotations

Fig. 6. FC magnetization of the LCO epitaxial thin films with different thicknesses underan external magnetic field of H = 100 Oe parallel to the film surface.

Fig. 7. Crystal structures of (a) pseudocubic LCO bulk and (b) ideal biaxial strain in-duced LCO thin film grown on (001) STO substrate.

Fig. 8. Schematic diagram of biaxial tensile strain induced distortion of CoO6 octahe-dron: (a) the regular CoO6 octahedron in bulk LCO and (b) the compressed CoO6 octa-hedron along the c axis due to the biaxial tensile strain in the ab plane.

112 H. Liu et al. / Surface & Coatings Technology 226 (2013) 108–112

will be suppressed and θCo–O–Co tends to the ideal 180° due to the restric-tive growth on the two-dimensional square lattice (see Fig. 7(b)),despite the restrictive effect may be more or less weakened with thethickness increasing. This effect can lead to not only a decrease of ΔCF

because of larger θCo–O–Co, but also an increase of Co–O hybridizationdue to the compression of the unit cell along the c axis. The stronger hy-bridization of the Co 3d and O 2p orbits favors the ferromagneticsuperexchange [11,14]. According to James' work [28], if the octahedralrotations are allowed, strain can be accommodated through changes inthe rotation angles; thus ΔCF is largely unchanged and the diamagneticstate remains stable. Finally, the strain-induced tetragonal distortion ofCoO6 octahedron (shown as Fig. 8) can also narrow the eg–t2g gap.Furthermore, the compression of the unit cell along the c axismay reducethe energy difference of the nondegenerate eg levels caused by the Jahn–Teller effect and the spin state favors a HS configuration of Co3+. Atpresent, however, the exact spin state (IS or HS state) in the biaxial ten-sile strain induced LCO film is still unknown, which needs furtherinvestigations.

In addition, except for the same TC, the experimental results showthat the FC curves of LCOfilmswith different thicknesses are not exactlythe same (see Fig. 5). As demonstrated previously, the strain-inducedferromagnetism is not simply a surface or interface effect and rather ex-tends over the complete film thickness [11,26]. Hence, the differenceseen from the FC curves may be due to the different biaxial tensilestrain, as analyzed previously. In other words, with the thicknessincreasing, the lattice relaxation effect in epitaxial thin film leads toreducing of ε, ΔTD, in-plane parameter a as well as V (shown inTable 1), which may increase ΔCF and the eg–t2g gap and further reducethe population of higher spin states Co3+.

4. Conclusions

In summary, 20-, 50- and 80-nm-thick LCO epitaxial thin films with apseudotetragonal structure were grown on (001) STO substrates by asimple PAD method. The morphological and structural characteristicsshow that dense and homogeneous LCO epitaxialfilmswith a biaxial ten-sile strain from+ 2.42% to+ 2.60% and tetragonal distortion from 1.47%to 1.63%were grownon (001) STO substrates successfully. It is found thatall the LCO films have a larger unit-cell size compared with that of bulkLCO. The unit-cell size shows a small decrease with the increasing thick-ness of LCO film due to theweakening of biaxial tensile strain and tetrag-onal distortion caused by the lattice relaxation. It is different from thebulk LCO with a nonmagnetic ground state in that all the LCO filmsshow a ferromagnetic transition at TC ~ 85 K. Structural and magneticanalyses of the LCO epitaxial films show that the strain-induced ferro-magnetism originates from the reduction of the energy differencebetween eg and t2g level caused by an increase of the unit-cell volumeand suppression of the CoO6 octahedral rotations. However, with theincreasing thickness of the LCO film, the lattice relaxation effect in

epitaxial thin film leads to reduction of ε, ΔTD and V, which may increaseΔCF and the eg–t2g gap and further reduce the population of higher spinstates Co3+.

Acknowledgements

This project was financially supported by the National Basic ResearchProgram of China (973 Program, grant no. 2009CB939901), and theNational Science Foundation of China, grant no. 10874161.

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