Indian Journal of Engineering & Materials Sciences Vol. 16, April 2009, pp. 123-127

Size dependent modifications in the physical properties of chemical solution deposition and pulsed laser deposition grown La0.7Ca0.3MnO3 manganite thin films: A comparative study J H Marknaa*, P S Vachhania, N A Shahb, J Johnc, D S Ranac, S K Malikc & D G Kuberkara a

Department of Physics, bDepartment of Electronics, Saurashtra University, Rajkot 360 005, India c Tata Institute of Fundamental Research, Colaba, Mumbai 400 005, India Received 14 July 2008; accepted20 March 2009

Grain-size dependence of electronic transport and magnetoresistance (MR) properties of nanostructured La0.7Ca0.3MnO3 (LCMO) manganite thin films grown using pulsed laser deposition (PLD) and chemical solution deposition (CSD) have been compared in the context of the grain size, surface and strain modification due to the different synthesis techniques used as well as due to substrate induced magnetic anisotropy. CSD grown LCMO thin films shows the grain size ~70 nm while grain size ~150 nm is exhibited by PLD grown films resulting in an appreciable change in the strain and low temperature magnetoresistance (LTMR). In CSD grown film, nanostructured grains exhibit sharp magnetic orientation in the absence of strain resulting in the observation of large maximum TCR value ~10.1% K-1 at 235 K as compared to PLD grown film (TCR value ~2.7% K-1 at 194 K). Epitaxial thin films grown by the CSD technique are strain free and possess nanostructured grains size resulting into better physical properties as compared to PLD grown LCMO films.

Interest in the studies on manganite thin films has achieved new heights after the first observation of colossal magnetoresistance in the La0.67Ba0.33MnO3 thin films1,2. The physical properties exhibited by manganites prompts the possibility of using them in the magnetic field sensor, magnetic read head, bolometric sensor and other applications3. In all the above-mentioned applications, it is necessary that, material should be in the form of thin film. There are several techniques used to grow the manganite thin films such as PLD, CSD, MBE, chemical vapor deposition (CSD) etc. Amongst the several synthesis techniques used to grow manganite thin films, PLD and CSD methods are more often used with certain limitations4-6. In the PLD technique, the deposition conditions such as oxygen pressure, deposition temperature, fluence size and substrate to target distances drastically affects the physical properties of the film. Also, for the PLD grown thin films, in situ or ex situ oxygen annealing is necessary to obtain the films with optimized TIM, TC and magnetoresistive properties7-10. It is reported that, in the PLD grown LSMO films the microstructure and morphology changes with substrate distances11. ________________ *For correspondence [email protected])

(E-mail:

[email protected],

The main advantages of CSD method for the thin film deposition are (i) it is very easy to synthesize epitaxial and polycrystalline manganite thin films in short time using CSD technique, (ii) it is easy to grow the film on large area with lowest cost and (iii) the quality of film can be improved by controlling the growth and homogeneity of the material by the temperature parameters12 In this paper, we report the grain size dependent comparison of physical properties of the 200 nm LCMO thin films synthesized using PLD and CSD routes. The results obtained on the effect of grain size and strain (lattice mismatch) on the transport and magnetotransport properties of both the films are discussed in detail. Experimental Procedure Polycrystalline bulk target sample of La0.7Ca0.3MnO3 (LCMO) was synthesized using standard ceramic method13. Thin films of the LCMO (200 nm) were deposited on a single crystal (h00) oriented LaAlO3 (LAO) substrate by pulsed laser deposition (PLD) technique using KrF excimer laser. The deposition parameters were: substrate to target distance ~4.2 cm, substrate heating temperature ~830°C and partial oxygen pressure ~400 mTorr. Another set of LCMO films were grown on similar

INDIAN J. ENG. MATER. SCI., APRIL 2009

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substrate using chemical solution deposition (CSD) technique. The mixing, stirring and heating of appropriate stoichiometric quantities of the metal acetates in a distilled water and acetic acid resulted in a clear solution of the constituents having required molarity used for deposition. Deposition of thin films using automated spin coater was then followed by heating and annealing at 1000°C in an oxygen environment. The details are reported elsewhere14. XRD and AFM studies were carried out on these films for structural and microstructural properties. Electrical resistivity and magnetoresistance measurements with and without field were performed using dc four probe technique (PPMS, Quantum Design). Results and Discussion XRD patterns of 200 nm LCMO/LAO thin films grown by PLD and CSD techniques are shown in Fig. 1. Both the films possess single phase orthorhombic structure having the XRD peaks oriented in (h00) direction confirming the epitaxial film growth. The unit cell parameters of the bulk LCMO [a = 5.570(1) nm, b =7.736(1) nm and c = 5.497(1) nm] were used to index the XRD pattern of the thin films. The values of lattice mismatch, δ, along the interface were calculated using the formula δ=

d substrate − d thin film d substrate

Fig. 1—Typical XRD patterns of LCMO/LAO thin films (200 nm) grown by PLD and CSD methods

× 100

Positive values of δ correspond to the tensile strain while negative values correspond to the compressive strain15. In the case of PLD grown 200 nm LCMO film, the mismatch was ~ -3.798% while for the CSD grown film, the mismatch was ~ -0.0179%. The surface morphology of both the sets of LCMO films was studied using the AFM measurements. Figs 2a and 2b show the topology of the LCMO/LAO films grown by the PLD and CSD techniques respectively showing the grain size ~150 nm in the PLD grown film while ~70 nm for CSD grown film. In the same figures (Figs 2a and 2b), roughness histograms have been shown, indicating RMS roughness ~9.3 nm in the CSD grown film and ~ 17 nm in PLD grown LCMO/LAO film. Figure 3 shows the resistivity plots of the PLD and CSD grown 200 nm LCMO films in the zero applied field, while inset of this figure shows the R-T behaviour of the bulk LCMO target. It can be seen

Fig. 2—2D AFM images and histograms for the roughness analysis of the 200 nm LCMO/LAO thin films grown by (a) PLD and (b) CSD technique

Fig. 3—Resistivity versus temperature plots of the 200 nm LCMO/LAO thin films grown by PLD and CSD methods in 0 T applied field (Inset shows the R-T for bulk LCMO sample)

MARKNA et al.: CSD AND PLD GROWN La0.7Ca0.3MnO3 MANGANITE THIN FILMS

that, PLD and CSD grown films exhibit the insulators to metal transition temperature (TIM) ~239 K and 249 K respectively, while bulk LCMO target exhibit the TIM ~211K. Higher TIM value in the films as compared to the bulk LCMO indicates that both the films were grown by selecting optimal deposition parameters. The resistivity of CSD grown film is lower as compared to PLD grown film indicating good quality of film, because, it has been reported earlier that, the effect of insulating phases on the electronic transport due to phase separation appears to account for both the reduced Tp and the enhanced resistivity6. In order to investigate the transport properties, we have fitted the ρ - T data in the metallic region using magnon scattering mechanism and using different VRH models in the semi-conducting region. In metallic region, both the films obey the magnon scattering law ρ(T) = ρ0 + BTn, where B is electronmagnon coefficient of their corresponding scattering mechanism, ρ0 is residual resistivity of the sample which is a contribution of various defects, domains, grain boundaries and other temperature – independent scattering processes12. From Fig. 4 and Table 1, it can be seen that CSD grown film has higher number of Table 1—Magnetic field, coefficients of the fit to Eq.(1), and the normalized χ2 for 200 nm LCMO/LAO thin films grown by PLD and CSD methods Sample LCMO

H (T)

σ0 (mΩ cm)

B ×10-9

n

χ2 × 10-9

PLD

0

0.02690(3)

93.29

2.6(1)

500.9

CSD

0

0.00045(2)

0.086

3.3(1)

0.205

Fig. 4—Low-temperature resistivity fits to magnon-scattering law with 0 T applied field for 200 nm LCMO/LAO thin films grown by PLD and CSD methods. The symbols are the experimental data and the solid curves are the theoretical fits

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magnon number (n value) and small residual resistivity as compared to PLD grown film which shows that the connectivity between grains is very good as well as the substrate strain effect is less in CSD film having negligible effect on the Mn-O-Mn bond angle as compared to the PLD film. In the semi conducting region, we tried to fit the ρT data using both VRH model [ρ = AT exp(Ea/KT)] and small polarons model [ρ = ρ0 exp(T0/T)1/4] (ref. 16-18). In the Fig. 5, plots of the ln (ρ/T) versus T-1 shows that in the semi conducting region resistivity obeys the small polarons hopping model (Emin-Holstein theory) (Figs 5a and 5c) while VRH type of model fails for the PLD and CSD made thin films (Figs 5b and 5d). The values of activation energy (Ea) and the resistivity coefficient (B) calculated using Emin-Holstein equation are given in Table 2. In PLD grown film, B value is higher as compared to CSD grown film which is due to the pronounced effect of dead layer formed in PLD Table 2—Comparison of activation energy and resistivity coefficient (B) of the 200 nm LCMO/LAO thin films grown by PLD and CSD methods Sample LCMO

TIM (K)

Ea (eV)

PLD

239

0.052

180

CSD

249

0.118

1.23

B (10-6Ω cm /K)

Fig. 5—Resistivity fits in the semi-conducting region for the plolarons hopping model and VRH model applied field for 200 nm LCMO/LAO thin films grown by PLD and CSD methods. The symbols are the experimental data and the solid curves are the theoretical fits.

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INDIAN J. ENG. MATER. SCI., APRIL 2009

Fig. 6—MR% versus H (T) isotherms for 200 nm LCMO/LAO thin films grown by PLD and CSD methods

Fig. 7—Temperature coefficient of resistance of the for 200 nm LCMO/LAO thin films grown by PLD and CSD methods

grown film while the activation energy (Ea) is lower in the PLD grown film which can be correlated to the large grain size in the film. These observations clearly show the effect of different growth techniques on the properties of LCMO films. Figure 6 shows the MR isotherms of both the type of films as a function of the external applied field (0 - 9 T). Magnetoresistance is defined as, MR (%) = (ρ0 – ρH)/ρ0 × 100. Maximum MR% ~90% is observed in PLD grown film near 200 K while CSD grown film exhibit MR% ~91% at 240 K. Low temperature magnetoresistance (LFMR) in the thin film is mainly due to two reasons, first one is grain boundary effect which is less pronounced in the epitaxial film and second is spin fluctuations at Mn-O-Mn bond angle

due to the strain and magnetic anisotropy in the grains. At low temperature (5 K), in the PLD grown LCMO film, LFMR increases with the applied field while it remains constant with field in the CSD grown film. This behaviour points towards the magnetic anisotropy generated by the substrate in the PLD grown film, the effect which is negligible in the CSD grown film. In the CSD grown film, due to nanostructured grains, strain is released as compared to the PLD grown film. This shows that the strain plays an important role in modifying the magnetotransport behaviour of films. Temperature coefficient of resistance (TCR) values for both the films were determined 1 dR × 100 K-1. There is a large using, TCR (%) = R dT disparity seen in the TCR values on both the positive and the negative sides of y-axis in 200 nm LCMO films grown using both the techniques. In the CSD thin film, maximum positive TCR value ~10.1% K-1 at 235 K is observed while in PLD grown film, maximum TCR value exhibited is ~2.7% K-1 at 194 K (Fig. 7). This feature is highly useful for the bolometric sensor applications. Sudden decrease in the resistivity at Tc results in higher TCR value and sharp insulator to metal transition .The reason for which is due to the presence of highly oriented magnetic domains and strain free structure3. Conclusions In this paper, the properties of LCMO/LAO thin films synthesized using PLD and CSD techniques have been compared. It is shown that CSD grown LCMO film has smaller nano-structured grains (70 nm) which reduces the magnetic inhomogeneity and strain in CSD grown film resulting into negligible LFMR at low temperature as compared to PLD grown film with larger grains. The high TCR value of CSD film shows good quality of film and possibility of its use in the bolometric applications. In conclusion, it can be said that the film growth technique has an appreciable effect on the physical properties of the film by way of modifications in grain morphology, strain and magnetotransport behaviour. Acknowledgement This work was carried out under DAE-BRNS project. One of the authors (JHM) is thankful to DST, New Delhi for the award of DST-Young Scientist Project.

MARKNA et al.: CSD AND PLD GROWN La0.7Ca0.3MnO3 MANGANITE THIN FILMS

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