Enhanced magneto-electrical properties and room temperature magnetoresistance in lightly doped manganite thin films

Solid State Communications 142 (2007) 445–448 www.elsevier.com/locate/ssc

Enhanced magneto-electrical properties and room temperature magnetoresistance in lightly doped manganite thin films Ravikant Prasad a , M.P. Singh b , P.K. Siwach c , W. Prellier b , H.K. Singh a,∗ a National Physical Laboratory, Dr K S Krishnan Road, New Delhi-110012, India b Laboratoire CRISMAT, ENSICAEN, UMR 6508, Caen, France c UGC-DAE Consortium for Scientific Research, Khandwa Road, Indore-452001, India

Received 19 January 2007; received in revised form 21 March 2007; accepted 21 March 2007 by E.V. Sampathkumaran Available online 27 March 2007

Abstract We report the effect of compressive strain on magnetic and magneto-electrical properties of lightly doped manganite La0.88 Sr0.12 MnO3 thin films. Films, having 5–60 nm thickness, were grown on (001) LaAlO3 and (001) SrTiO3 substrate by DC-magnetron sputtering. These films show a magnetoresistance as high as ∼65% at room temperature and insulator–metal transition temperature (TIM ) ∼ 320 K. Further, we demonstrate that a small variation in strain causes significant changes in their properties. We have discussed the possible origin of these features and compared with the reported literature. c 2007 Elsevier Ltd. All rights reserved.

PACS: 75.47.Gk; 75.47.Lx; 71.30+h; 61.10.Nz Keywords: A. Manganite thin film; B. DC-sputtering; D. Strain effect; D. Magnetoresistance

Thin films of manganite with colossal magnetoresistance (CMR) in the vicinity of the ferromagnetic transition have been the focus of intense research activity for more than a decade [1–5]. The occurrence of CMR effect in the vicinity of paramagnetic (PM)–ferromagnetic (FM) transition temperature (TC ) is explained by the FM double exchange (DE) and the electron–phonon (e–p) interactions [6,7]. It is believed that in manganites the PM–FM and insulator–metal (IM) transitions are controlled by the √ tolerance factor (t), which is defined by t = (rA + rO )/ 2(rMn + rO ) [5,8]. Here rA , rO , and rMn are radii of the A-site cations, oxygen and manganese ions, respectively in the perovskite structure. Zhou et al. have suggested that maximum MR should be achieved for t ∼ 0.96 [9]. But manganites with t ∼ 0.96 are strongly Jahn–Teller (JT) distorted and consequently have bulk TC /TIM much below the room temperature (∼150 K) [5,8, 10]. Recently, Chen et al. reported a large enhancement in ∗ Corresponding address: National Physical Laboratory, Low Temperature/QHRS and Superconducting Devices, Room #39, Dr K S Krishnan Road, New Delhi-110012, India. Tel.: +91 011 25742610; fax: +91 011 25726952. E-mail address: [email protected] (H.K. Singh).

c 2007 Elsevier Ltd. All rights reserved. 0038-1098/$ - see front matter doi:10.1016/j.ssc.2007.03.030

TC /TIM and significant room temperature magnetoresistance (MR ∼ 20% at 3 T) in compressively strained ultra-thin manganite (t = 0.96) film [10,11]. They have attributed this to a combined effect of the interplay between the strong e–p coupling stemming from a JT splitting of the Mn3+ ion and DE. In view of the immense technical potential of enhanced room temperature MR, it is important to tailor their magnetoelectrical properties at room temperature and investigate the underlying mechanism responsible for it. Additionally, other vital properties concerning to their potential devices applications (e.g. temperature coefficient of resistance (TCR)) also need to be investigated in this class of thin films. These factors have motivated our current studies. In this letter, we report the effect of the degree of compressive strain on TC /TIM , MR and TCR in La0.88 Sr0.12 MnO3 thin films. Two methodologies have been adopted to manipulate the strain in these films: (i) films were grown on two different substrates having identical structures but different lattice parameters, viz., ˚ and LaAlO3 (LAO, a = 3.795 A); ˚ SrTiO3 (STO, a = 3.905 A) (ii) films with varying thickness were grown on each substrate. In this communication we report, our data on these films and show that a small variation in strain causes significant changes

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in properties of thin films, and can yield a large MR around room temperature. The films, having thickness in the range ∼5–60 nm, were deposited at 800 ◦ C by on-axis DC magnetron sputtering in Ar–O2 (80:20) ambient. In order to get the appropriate oxygen stoichiometric compounds, all films were annealed at 750 ◦ C under flowing oxygen for 12 h. The required La0.88 Sr0.12 MnO3 (LS8812) powder for target was prepared by a wet chemical route [12]. We have characterized in detail, the powder employed for target formation and the results are as follows. The lattice parameters of the powder were found √ √ to ˚ (a/ 2 ∼ 3.93 A), ˚ b ∼ 5.54 A ˚ (b/ 2 ∼ be a ∼ 5.56 A ˚ and c ∼ 7.74 A ˚ (c/2 ∼ 3.87 A). ˚ This gives the 3.92 A) average in plane lattice parameter (aav ) of La0.88 Sr0.12 MnO3 = ˚ which have been utilized to calculate (ab + bb )/2 ∼ 3.925 A, the lattice mismatch between film and substrate. It shows FMTC ∼ 175 K and undergoes an IM transition at TIM ∼ 145 K. TIM lower than FM-TC is explained by the fact that the former is extrinsic property and depends the nature of grain boundaries while the latter is intrinsic property independent of the nature of grain boundaries. The average lattice mismatch between substrate and film was defined by ε = (at − as ) ∗ 100/as , where at and as are lattice parameters of bulk target and substrate. It is found to be ε ∼ 0.512 and 3.42% for STO and LAO, respectively. The magnetization measurements were performed in a superconducting quantum interference device (SQUID) magnetometer and magneto-transport measurements were performed in physical properties measurement system (Quantum Design). The epitaxial nature and crystallinity of the film, was characterized by X-ray diffractometer (XRD). XRD analysis employing θ–2θ scans and rocking curves shows that all the films are grown coherently. Using the XRD data, the out-ofplane lattice parameter of the films was extracted. For 5 nm ˚ thin film on STO (S5), it was found to be very close to 3.905 A, ˚ This reveals that the film whereas for 60 nm (S60) was 3.87 A. with smaller thickness acquire the same lattice parameter as the ˚ and consequently possess a large amount substrate, i.e. 3.905 A of strain. On contrast, for the 60 nm thick film (S60), it was very close to its bulk value suggesting the relaxation of the strain with increased film thickness. However, the asymmetrical θ–2θ profile shows that the 60 nm film is still under appreciable strain. The XRD patterns of the films on STO are shown in Fig. 1. The inset shows the rocking curve of the 60 nm thin film on STO. The out-of-plane lattice parameters of the films ˚ for 6 nm film (L6) and it on LAO are observed to be 3.895 A ˚ for 35 nm (L35). The observed decrease decreases to 3.882 A in the out-of-plane lattice parameters implies certain degree of strain relaxation with increased film thickness. Further, EDAX analysis different areas of LS8812 film deposited on ZrO2 (in case of films on STO/LAO the presence of Sr/La makes it difficult to analyse the result) showed that within experimental limitations the cation composition is in close proximity to that of the target. The PM–FM transition temperature (TC ) of the films was determined from temperature dependent zero field cooled (ZFC) and field cooled (FC) DC magnetization (M)

Fig. 1. XRD pattern of 5 nm (top) and 60 nm (bottom) thin films on STO. The inset shows the rocking curve corresponding to the (002) of the 60 nm thin film.

Fig. 2. Magnetization versus temperature (M–T ) curve (a) 5 nm film grown on STO, and (b) 6 nm film grown on LAO, measured under 500 Oe magnetic fields.

measurements, which was performed at H = 500 Oe applied magnetic field parallel to the substrate. Interestingly, in each film the ZFC and FC data are found to overlap. The TC onset was observed at ∼315 K and 310 K in 5 nm (S5) and 60 nm (S60) respectively, whereas for L6 and L35, it was found to be ∼272 K and 280 K. Thus, all the films show drastic enhancements in FM-TC as compared to the bulk sample (FMTC ∼ 175 K). The FC M–T curves of 5 and 6 nm thin films on STO and LAO are shown in Fig. 2. From Fig. 2 few important features can be noted: (i) film grown on LAO possesses high remnant magnetization than the film grown on STO, (ii) LAO

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Fig. 3. Zero field resistance versus temperature (R–T ) curves of all films. The inset shows the variation in TCR as a function of temperature for all samples.

film shows a sharper phase transition than that on STO, (iii) the film grown on STO displays a significantly higher transition temperatures than the one grown at LAO. These observed features can be understood as follows: it is well known that in low divalent-doped manganites such as La0.88 Sr0.12 MnO3 , the MnO6 octahedra are heavily distorted in the in-plane directions and consequently the JT effect dominates the DE resulting in lower TC values [13]. Under compressive strain, MnO6 octahedra are compressed in the in-plane and stretched in the out-of-plane direction. This suppresses the JT distortion and causes a decrease in the in-plane Mn–Mn bond distance (din ) and at the same time the Mn–O–Mn bond angle (θin ) approaches the ideal value ∼ 180◦ . Consequently, the in-plane 3.5 ) cos(π − θ )/2 increases transfer integral tx = t y ∼ (t0 /din in and hence the DE becomes dominant resulting in higher TC . It is interesting to note that although LAO provides much larger compressive strain (ε ∼ 3.42%) than STO (ε ∼ 0.51%), the films deposited on the later possess higher TC . This suggests that the strain exceeding a certain critical value may result in certain degree of strain relaxation. Consequently, it may cause increased density of nanometric inhomogeneities and probably defects and hence the respective variations in their properties. The resistance versus temperature (R–T ) (Fig. 3) curve was measured on all samples under different magnetic fields. Irrespective of thickness and substrates, all samples display IM transition. For film grown on STO, the TIM was ∼320 K in S5 and decreases to ∼313 K for S60. Among the films on LAO, the thicker L35 (35 nm) film shows a slightly higher TIM ∼ 282 K than the 6 nm film, TIM ∼ 275 K. Clearly all the films have TIM > TC . Usually TIM in manganite films coincides or occurs below TC i.e. paramagnetic to ferromagnetic transition temperature. But there have been some reports of higher TIM than TC [14]. In our sample we have also reported TIM 2–5 K higher than their corresponding TC . Since the transition from paramagnetic to ferromagnetic is not very sharp we suggest that in the paramagnetic region short-range FM phase can co-exist with PM phase and get connected thought out the sample providing less resistive path to charge carrier than more resistive paramagnetic phase. This percolative type transport could be the reason for higher TIM than TC in our samples.

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In case of the films on the same substrate the IM transition is observed to be sharper in the thicker film. The sharpness of the transition is well evidenced by smaller full width at half maximum (FWHM) of the temperature coefficient of resistance (TCR = 100/R ∗ (dR/dT )) curves of thicker films (inset of Fig. 3). FWHM decreases from ∼80 K in S5 to ∼30 K for thicker film, S60. A similar trend is observed in the films on LAO. This has been understood due to enhanced oxygen stability. As seen in the inset of Fig. 3, films on LAO show the largest TCR > 5.5%/K but at slightly lower temperatures ∼260 K. In contrast, significant enhancement is observed in S60 which exhibits TCR ∼ 5.5%/K at 285 K. Thus, these films can be potential candidates for bolometer. As alluded to above, the increased thickness as well as larger lattice mismatch leads to strain relaxation, which may cause the inhomogenities/defects in the films that may produce local lattice distortions (distortion of the MnO6 octahedra) in the region of their occurrence. The strain relaxation can affect the conduction mechanism in the paramagnetic regime through increased localization of the electrons in films, which can be caused either by thickness or lattice mismatch. To support this hypotheses, we analyzed the resistivity data at T > TC /TIM . We have fitted the resistivity at T > TIM of all the films to the Emin–Holstein law [15–17] of adiabatic small polaron hopping given by, ρ = AT exp(E A /k B T ). Here A = 2k B /3n 2 a 2 ν, where e, n, a and ν are the electronic charge, charge carrier density, site to site hopping distance and the longitudinal phonon frequency respectively [17]. E A is the activation energy. The activation energy (E A ), extracted from the R–T curve, was found to be 71 meV and 78 meV for S5 and S60, respectively. For films on LAO, it was found to be larger; E A ∼ 96 meV for 6 nm and 101 meV for 35 nm thickness. This shows that the polaronic potential barrier and hence the electron localization increases with both the increasing film thickness as well as strain. This is a direct consequence of the variation of the microscopic features of the films as a function of strain and thickness. This is in good agreement with our hypothesis. Further, the decrease in the resistivity coefficient A from ∼1.4 × 10−6  cm for STO films to ∼4 × 10−7  cm for those on LAO suggests an increase in the site to site hopping distance. However, further work is required to explore the microstructural properties of these films by high-resolution electron microscopy, which we are taking in the near future. The magnetoresistance (M R = (R0 − R H ) ∗ 100/R0 ) was measured at different magnetic fields up to 70 kOe in the temperature range 5–400 K. Fig. 4 presents the temperature dependence of MR measured at H = 30 kOe. The 5 nm thin film on STO shows a maximum MR∼28% at 30 kOe and T ∼ 298 K. MR nearly doubles (∼50%) at the corresponding magnetic field for the 60 nm thick film at 295 K. A similar trend has been observed for the films grown on LAO. Interestingly, MR changes sign in the lower temperature regime. Such anomalous MR is suggested to be due to quantum interference effects [18]. Further, MR was also measured near room temperature, as a function of applied magnetic field (Fig. 5). The S60 sample shows MR ∼ 15%, 43% and 65% at 10, 30 and 70 kOe magnetic field at 300 K. As depicted in Fig. 5, the ultra

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stoichiometric films. Thus, our study demonstrates a way to tailor the magneto-electrical properties of these films at room temperature, which may open a route to utilizing them for potential devices. In conclusion, we have shown that compressive strain leads to large enhancement in the TC /TIM in a low divalent doped manganite having t ∼ 0.96, viz., La0.88 Sr0.12 MnO3 and enhanced magneto-electrical properties at room temperature. A small strain variation in these films leads to larger activation energy suggesting the possibility of enhanced electron localization. Further, the partially strain-relaxed films exhibit large enhancement in MR. Acknowledgements Fig. 4. Temperature dependent MR measured at H = 30 kOe.

Authors are grateful to Prof. Vikram Kumar, director NPL for his support and encouragement. Fruitful discussions with Prof. O.N. Srivastava (BHU, Varanasi) and Dr G.D. Varma (IIT, Roorkee) are thankfully acknowledged. Special thanks are due to Peter Hinze, P.T.B., Braunschweig for measuring the film thickness. RP is thankful to University Grants Commission, New Delhi for junior research fellowship. References

Fig. 5. Magnetoresistance (MR) versus applied magnetic field curves measured at (a) 300 K and (b) 275 K.

thin 5 nm film on STO has relatively smaller MR and similar trends have been observed for the films grown on LAO. Thus, the temperature and field dependence of MR clearly shows that both, increasing thickness as well as increased lattice mismatch cause large enhancement in MR. We would like to mention that the MR observed here is much larger than reported earlier for compressively strained thin films of manganites having t ∼ 0.96 [10]. However, the origin of such enhancement is not well understood but may be partially attributed to the better

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