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Journal of Nanoscience and Nanotechnology Vol. 8, 4146–4151, 2008

Grain Size Dependent Transport and Magnetoresistance Behavior of Chemical Solution Deposition Grown Nanostructured La07Sr03MnO3 Manganite Films RESEARCH ARTICLE

R. N. Parmar∗ , J. H. Markna, P. S. Solanki, R. R. Doshi, P. S. Vachhani, and D. G. Kuberkar Department of Physics, Saurashtra University, Rajkot 360005, India Grain-size dependence of electronic transport and magnetoresistance (MR) properties of nanoby Ingenta to: 3 (100) single crystal substrates manganite thin ﬁlms on LaAlO structured La07 Sr03 MnO3 (LSMO) Delivered Jawaharlal NehruDeposition Centre for Advanced Scientific prepared using Chemical Solution (CSD) technique have beenResearch studied. The LSMO thin ﬁlms were annealed at temperatures IP in the range of 700–1000 C for different time intervals [6 h : 203.200.55.101 and 12 h] and crystallized as singlephase LSMO. Microstructural Mon, 28 Jul 2008 03:48:07 studies carried out using AFM show a marginal increase in the grain-size from 50 to 90 nm as the temperature was varied from 700 C to 1000 C respectively. It has been observed that the insulator-metal transition (TP ) and MR depend on the grain size. In zero applied ﬁeld, resistivity reduction is ∼103 at 5 K for the ﬁlms annealed at 700 C [TP ∼ 341 K] and 1000 C [TP ∼ 373 K]. MR versus H isotherms reveal that MR enhances in the vicinity of TP but decreases at low temperatures. The results obtained from the electronic and magnetotransport studies are in good agreement with the change in surface morphology of the ﬁlms studied, which shows that the randomly distributed domains are composed of faceted grains. Synthesizing conditions, annealing temperature and time control the growth and alignment of grains into the domains, which cause better conduction at grain interface.

Keywords: Nanocrystalline, Chemical Solution Deposition, Magnetoresistance.

1. INTRODUCTION Hole doped manganese oxides of the type La07 R03 MnO3 (LRMO) (R = Ca2+ , Sr2+ , Ba2+ , etc.) exhibit remarkable variation in resistivity, the phenomenon known as colossal magnetoresistance (CMR) ∼99%, when subjected to applied magnetic ﬁeld, which makes these compounds the strong candidates for application in bolometers, magnetic ﬁeld sensors, magnetoresistive read heads, magnetic memory devices etc.1–2 The nonvolatile magnetic memory heads require spin polarized read heads which can exhibit MR of about 20–30% in few hundreds of Oe magnetic ﬁeld near room temperature (RT). Thin ﬁlms of La07 Sr03 MnO3 (LSMO) and La07 Ba03 MnO3 (LBMO) manganite compounds exhibit electronic transition (TP ) and magnetic transition (TC ) above RT and hence optimum CMR effect but only under high magnetic ﬁelds (∼8–9 T) which are far higher than those required for low ﬁeld applications.3–4 The importance of CMR effect ∗

Author to whom correspondence should be addressed.

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in applications can be realized only when this effect dominates at low applied magnetic ﬁelds and near RT. In the manganites, the origin of huge magnetoresistance (MR) has been ascribed to (i) the intrinsic factor of suppression of magnetic disorder at Mn-O-Mn bonds and (ii) the extrinsic factor of magnetic ﬁeld induced reduction in scattering of carriers at the grain boundaries.5 The intrinsic factor has been explored fully and it is unanimously concluded that the MR decreases with increase in the insulator-metal transition (TP ) temperature.6 However, the effect of extrinsic factor has not been widely explored and there is a tremendous scope to enhance the low ﬁeld MR by exploiting grain boundary region either by decreasing the grain size by improved synthesis techniques or by artiﬁcially engineering the grain interfaces on epitaxial and polycrystalline manganite thin ﬁlms.7 8 Most commonly, the high quality epitaxial manganite thin ﬁlms are grown by deposition techniques like Pulsed Laser Deposition (PLD),3 9 RF sputtering,10 Metallorganic Chemical Vapour Deposition (MOCVD)11 or Molecular Beam Epitaxy (MBE).12 Physical and microstructural 1533-4880/2008/8/4146/006

doi:10.1166/jnn.2008.AN51

Parmar et al.

Chemical Solution Deposition Grown Nanostructured La07 Sr03 MnO3 Manganite Films

Calcination Annealing

350 ºC, 30 min

O2 environment 700 ºC–1000 ºC for 6 h & 12 h

Fig. 1. Flow diagram of the CSD method used for the synthesis of La07 Sr03 MnO3 (LSMO) thin ﬁlms.

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are −265% and −241% for the ﬁlms annealed at 700 C and 800 C for 12 h, while the values of mismatch for the LSMO ﬁlms annealed at 1000 C for 6 h and 12 h are −190% and −189% respectively. This suggests a considerable decrease in compressive strain in the ﬁlms with 4147

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properties of nanocrystalline manganite thin ﬁlms may difdissolved stochiometrically (0.7:0.3:1) in acetic acid and fer from those of the reported bulk and are extremely distilled water (1:1 by volume) followed by the stirring at sensitive to the thin ﬁlm processing methods, growth elevated temperature (∼80 C, 10 min) to obtain a clear conditions, post-annealing, substrate used and the ﬁlm precursor solution of 0.2–0.6 M which was used for ﬁlm thickness, etc.13 deposition. Deposition of LSMO on highly polished LAO Chemical Solution Deposition (CSD), consisting of (h00) single crystal substrate was done using automated novel sol–gel route and coprecipitation method using spin coating process at 6000 rpm for 20 s followed by metal organic decomposition (MOD) for preparation of repeated drying and calcination at 120 C and 350 C for homogeneous bulk ceramics and their thin ﬁlms, has a 30 min and ﬁnal annealing in oxygen in the temperature number of advantages over conventional vacuum deporange of 700 C–1000 C for 12 h to promote grain growth, sition techniques. CSD method is cost-effective, comwith subsequent slow cooling to RT. The annealing time positional compatible, yields stochiometrically predeﬁned was kept as 6 h and 12 h for the LSMO ﬁlms annealed compounds and provides the varieties of alternative preat 1000 C. Thickness per coating was ∼800 Å estimated cursors to choose, contains wide range of concentration, using DESJECT thickness proﬁlometer. The spin coating viscosity and pH, easier to set up and handle. The adheprocess was then repeated twice to obtain the ﬁlms with sion of the wet ﬁlms to the substrate optimizes the propdesirable ﬁlm thickness ∼1500 Å. All the LSMO ﬁlms erties of the ﬁlms grown using CSD method. However, were characterized by XRD and AFM measurements for the method does not offer particle size control but up to their structural and microstructural properties. From AFM some extent, results in the monodispersed particles. Hence, studies, the Delivered by Ingenta to: values of average grain size determined are, CSD method becomes an alternative for depositing magne50 nm, 70 nm, 80 nm and 90 nm corresponding to the Jawaharlal Nehru Centre for Advanced Scientific Research toresistive manganite thin ﬁlms. Several reports are availﬁlms annealed at 700 C, 800 C, 1000 C [6 h] and IP : 203.200.55.101 able on processing, structural, morphological, electrical C [12 h], respectively. Using d.c. four probe resistiv1000 Mon, 28 Jul 2008 03:48:07 resistivity and MR studies on LRMO (R = Ca2+ , Sr2+ ) ity technique, the electrical resistivity of all the ﬁlms was ﬁlms grown using chemical route.14 15 However, very few measured (i) as a function of temperature in the range of reports are available on the effect of varying synthesiz5–425 K in zero-ﬁeld and in applied ﬁelds of 5 T and 8 T ing conditions on the microstructure and MR properties of (5–300 K) respectively and (ii) as a function of magnetic CSD grown LRMO ﬁlms.16 17 In this context, LSMO ﬁlms ﬁeld (up to 9 Tesla) at various temperatures (5–300 K) on LaAlO3 [LAO] (h00) single crystal substrates were preusing resistance versus temperature measurements. pared by CSD method and the effect of processing parameters, annealing temperature and time on the microstructure, electronic and magnetotransport properties of these ﬁlms 3. RESULTS AND DISCUSSION were studied during this work. The typical indexed XRD patterns of LSMO thin ﬁlms on single crystalline LAO (h00) substrate annealed at 700 C 2. EXPERIMENTAL DETAILS [for 12 h], 800 C [for 12 h] and 1000 C [for 6 h and 12 h] are shown in Figure 2. All the LSMO ﬁlms are epitaxially Figure 1 shows the ﬂow diagram of CSD method used for c-axis (00l) oriented and were indexed using bulk LSMO the synthesis of LSMO thin ﬁlms. The high purity Lan´ b = 77712 Å ´ and c = cell parameters [a = 54711 Å, thanum acetate hydrate [La(CH3 COO)3 · xH2 O], Strontium ´ It can be seen from the Figure 2 that, intenacetate hydrate [Sr(CH3 COO)2 · xH2 O] and Manganese 555031 Å]. acetate hydrate [Mn(CH3 COO)2 · xH2 O] [all 99.9%] were sity of the XRD peaks increases and the values of FWHM decreases with annealing temperature from 700 C to 1000 C respectively. For instance, the FWHM of (002) peak La(CH3COO)3 · xH2O Sr(CH3COO)2 · xH2O Mn(CH3COO)2 · xH2O decreases from 0.256 for the ﬁlm annealed at 700 C to 0.121 for the ﬁlm annealed at 1000 C for 12 h. In (0.7:0.3:1) (~80 ºC,10) Mixing & stirring addition to this, the calculated values of residual microstrain present in the ﬁlm due to the mismatch between 6000 rpm, 20 sec Spin-coating single crystal substrate and material determined using the formula, Drying 120 ºC, 30 min − dthin ﬁlm d × 100 % = substrate dsubstrate

1000 ºC (12 h)

LAO (200)

La0.7 Sr0.3 MnO3 (002)

(001)

Intensity (arb. unit)

LAO (100)

Chemical Solution Deposition Grown Nanostructured La07 Sr03 MnO3 Manganite Films

LSMO 103

700 ºC (12 h) 50 nm

1000 ºC (6 h)

800 ºC (12 h) 70 nm

800 ºC (12 h)

700 ºC (12 h)

102

ρ (mΩ-cm)

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Delivered by Ingenta to: 40 50 Jawaharlal Nehru Centre for Advanced Scientific Research 2θ (degree) 102 IP : 203.200.55.101 XRD patterns of La07 Sr03 MnO3 (LSMO) thin Mon, ﬁlms annealed at 2008 03:48:07 1000 ºC (6 h) 28 Jul 20

30

Fig. 2. 700 C [12 h], 800 C [12 h] and 1000 C [6 h and 12 h].

increase in the annealing temperature and time. The substantial decrease in compressive strain along with increase in peak intensity and decrease of FWHM, clearly suggest an improvement in the crystalline structure of the ﬁlm with progressive annealing temperature and intervals. All the LSMO thin ﬁlms were characterized for electrical and magnetotransport behavior in the temperature range 5–425 K. Figure 3 shows resistivity ( ) versus temperature (T ) plots for LSMO ﬁlms in zero-ﬁeld and in a ﬁeld of 5 T and 8 T respectively. All the ﬁlms possess TP above 340 K. The values of processing parameters, resistivity at 5 K [ 5K ], peak resistivity [ p ] and TP in zero applied ﬁeld, for the ﬁlms annealed at different temperatures and time intervals are given in Table I. Thin ﬁlms annealed at lower temperatures possess broad insulator (I)-metal (M) transition which sharpens with increasing annealing temperature. The electrical resistivity decreases while TP increases with annealing temperature and time, which is in good agreement with the other reports available in literature.18 In zero applied magnetic ﬁeld, the peak resistivity suppresses by the factor of ∼102 , whereas at 5 K resistivity reduction is ∼103 as annealing temperature increases from 700 C to 1000 C respectively. For 1000 C annealed ﬁlm, in zero applied ﬁeld, the peak resistivity suppresses by the factor of ∼10, where at 5 K resistivity reduction is ∼102 as annealing time increases from 6 h to 12 h respectively. Also, it can be seen from the –T curves that, in the temperature range 5–300 K, applied ﬁelds of 5 T and 8 T do not modify resistivity behavior from that of the zero ﬁeld resistivity for 1000 C (12 h) annealed ﬁlm. Further, the LSMO thin 4148

80 nm

101

101

1000 ºC (12 h) 90 nm

100

H=0T H=5T H=8T 0

100

200

300

400

(T) Fig. 3. Resistivity ( ) versus Temperature (T ) plots for La07 Sr03 MnO3 (LSMO) thin ﬁlms annealed at 700 C [12 h], 800 C [12 h] and 1000 C [6 h and 12 h] in H = 0, 5 and 8 T ﬁelds.

ﬁlm, annealed at 700 C, exhibits low temperature resistivity minimum below 50 K which disappears in the ﬁlms annealed at 800 C and 1000 C. To obtain an insight on the origin of these effects in CSD grown ﬁlms, we have plotted magnetoresistance (MR%) versus magnetic ﬁeld (H) isotherms at various temperatures for all the LSMO ﬁlms (Fig. 4). It can be seen that, all the ﬁlms exhibit maximum MR in the vicinity of TP which decreases with decreasing temperature. J. Nanosci. Nanotechnol. 8, 4146–4151, 2008

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Chemical Solution Deposition Grown Nanostructured La07 Sr03 MnO3 Manganite Films

Table I. Values of processing parameters, resistivity at 5 K ( 5K ), peak resistivity ( p ) and TP for La07 Sr03 MnO3 thin ﬁlms. Sample LSMO LSMO LSMO LSMO

MR %

{

10

0 20

100 K

10

0 15

50 nm

5K

70 nm

10

80 nm

5

90 nm 0

0

3

6

9

H (T) Fig. 4. MR versus H isotherms at various temperatures (5–300 K) for La07 Sr03 MnO3 (LSMO) thin ﬁlms annealed at 700 C [12 h], 800 C [12 h] and 1000 C [6 h and 12 h].

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The above-mentioned results on MR behavior illustrate the suppression in MR at low temperatures and its enhancement at high temperatures around RT, underlying the importance of grain boundary contribution to MR. Moreover, suppression of LF-MR indicates further the improvement in crystallographic orientation and parallel alignment of magnetic domains. This inference is supported by increase in line intensity of XRD patterns (Fig. 2) which point towards the better orientation and crystalline order of the ﬁlms annealed at higher temperatures for longer intervals. At low temperatures, HFMR is ascribed to the reorientation of disordered spins at grain interface which in turn depends on the connectivity between the grains and on the stiffness of the aligned blocked spins at the grain surface.22 The suppression of HFMR at 5 K results into the enhancement in grain connectivity, better alignment and orientation of the grains and reduction in pinning of Mn spin at grain boundaries due to different annealing temperatures and time intervals used. Such granular contribution is also evident from the MR behavior at TP (>RT). The enhancement in MR at RT with annealing temperature and duration implies the improved magnetic order at the grain boundaries thereby allowing better electronic transport. 4149

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We discuss these MR results as a function of temperature as follows: (i) At 5 K, the ﬁlms annealed at 700 C and 800 C Annealing Annealing temperature time

5K (mcm) p (mcm) Tp (K) exhibit low ﬁeld MR (LF-MR) ∼7% (in a ﬁeld of <1 T). This uncharacteristic MR behavior of an oriented ﬁlm par700 C 12 h 294 1100 341 tially resembles the LF-MR behavior of polycrystalline 12 h 67 300 353 800 C samples, the origin of which is attributed to the inter1000 C 12 h 0.3 9 377 6h 7.1 111 355 1000 C grain spin polarized tunneling (SPT) of charge carriers that depends upon the alignment of the magnetic moment of the ferromagnetic domains.19 20 It is interesting to note that, at 5 K, LF-MR of the ﬁlm annealed at 1000 C is ∼4% for the ﬁlm annealed for 6 h while, it is negligible for the ﬁlm annealed for 12 h. Also, the high ﬁeld MR [HF-MR] LSMO (in a ﬁeld >1 T) at 5 K decreases from ∼15% for ﬁlm 20 300 K annealed at 700 C to nearly 5% for the ﬁlm annealed at 1000 C. These points toward the enhancement in connectivity between the grains, reduction in pinning of Mn-ion 10 700 ºC 12 h spins at grain boundaries, etc.21 22 800 ºC 1000 ºC (ii) At intermediate temperatures in the range of by Ingenta to:the ﬁlms annealed at 700 C and 800 C show 6h 1000Delivered ºC 100–240 K, 0 Jawaharlal Nehru Centre for Advanced Scientific Research LF-MR <5% while ﬁlms annealed at 1000 C for 6 h 240 K IP : 203.200.55.101 20 and 12 h show ∼2–3% and nearly zero MR respectively. Mon, 28 Jul 2008 The03:48:07 HF-MR is nearly ∼20–22% for ﬁlm annealed at 700 C which reduces to ∼16% for 800 C ﬁlm, and ∼10% and ∼5–7% for the ﬁlm annealed at 1000 C for 6 h and 10 12 h respectively. (iii) At RT, the LF-MR <3% is observed for all the LSMO ﬁlms annealed at different temperatures and time inter0 vals. The HF-MR observed for the 700 C annealed ﬁlm is 20 200 K ∼23%, which is slightly lower in 800 C ﬁlm. For 1000 C annealed ﬁlms, the observed HFMR is ∼15% for 6 h and 12 h annealed ﬁlm.

Chemical Solution Deposition Grown Nanostructured La07 Sr03 MnO3 Manganite Films

RESEARCH ARTICLE

To account the effect of modiﬁcations in surface morphology of the ﬁlms, AFM micrographs were recorded for all the LSMO ﬁlms annealed at different temperatures and durations (Fig. 5). The distinct grain structure is observed for the ﬁlms annealed at different temperatures and intervals which shows domain like growth composed of faceted grains. Also, it is evident from the Figure 5 that, the value of surface roughness and grain size linearly scales with the processing conditions, annealing

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temperature and annealing time. In the ﬁlm annealed at 700 C (Fig. 5(a)), grains are in preliminary stage and are partially grown. The growth, alignment and size of the grains and domains become more signiﬁcant in 800 C and 1000 C annealed ﬁlms. The observed surface roughness for 700 C ﬁlm is ∼31 nm which reduces to 13 nm and 16 nm for the LSMO ﬁlms annealed at 800 C and 1000 C respectively. The moderate reduction in the values of surface roughness along with the signiﬁcant enhancement in grain size, apparent improvement of orientation, alignment and ordering of grains and domains, points towards the improved connectivity between the grains with annealing temperature and interval. This would result into decreased grain boundary region and consequently, an improved electronic transport at grain interfaces which accounts for the enhancement in grain size vis-`avis reduction of resistivity in CSD grown nanocrystalline LSMO thin ﬁlms, disappearance of low temperature resistivity minima Ingenta to: [in 700 C] and enhancement of MR at RT.

Delivered by Jawaharlal Nehru Centre for Advanced Scientific Research IP : 203.200.55.101 4. 03:48:07 CONCLUSION Mon, 28 Jul 2008

To summarize, we have synthesized nanocrystalline La07 Sr03 MnO3 manganite thin ﬁlms on single crystalline LAO substrates using chemical solution deposition method. Increase in annealing temperatures and time periods resulted into an increase in average grain size from 50 nm to 90 nm. The reduction in ﬁlm resistivity and enhancement of TP can be correlated to the increased grain size and hence better electronic transport at grain boundaries. It is evident from the MR isotherms of all the LSMO ﬁlms that, at RT, the LFMR and HFMR increases progressively with reduction in grain size. Acknowledgment: R. N. Parmar and D. G. Kuberkar are thankful to IUAC, New Delhi, for ﬁnancial assistance in the form of UFUP project. The experimental help by Dr. V. Ganesan and Dr. R. Rawat from UGC-DAE CSR, Indore is thankfully acknowledged.

References and Notes

Fig. 5. 2 m × 2 m AFM image of the La07 Sr03 MnO3 (LSMO) thin ﬁlms annealed at (a) 700 C (b) 800 C (c) 1000 C [6 h] and (d) 1000 C [12 h] along with their respective 3D view and surface roughness histogram. The two arrowheads deﬁne an atomic step.

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Received: 7 January 2007. Accepted: 7 January 2007.