Alkaline-doped manganese perovskite thin films grown by MOCVD

Journal of Magnetism and Magnetic Materials 211 (2000) 47}53

Alkaline-doped manganese perovskite thin "lms grown by MOCVD M. Bibes!, O. Gorbenko", B. MartmH nez!, A. Kaul", J. Fontcuberta!,* !Institut de Cie% ncia de Materials de Barcelona, Consejo Superior de Investigaciones Cientn& xcas, Campus Universitat Auto% noma de Barcelona, E-08193 Bellaterra, Catalunya, Spain "Department of Chemistry, Moscow State University, 119899 Moscow, Russia

Abstract We report on the preparation and characterization of La Na MnO thin "lms grown by MOCVD on various 1~x x 3 single-crystalline substrates. Under appropriate conditions epitaxial thin "lms have been obtained. The Curie temperatures of the "lms, which are very similar to those of bulk samples of similar composition, re#ect the residual strain caused by the substrate. The anisotropic magnetoresistance AMR of the "lms has been analyzed in some detail, and it has been found that it has a two-fold symmetry at any temperature. Its temperature dependence mimics that of the electrical resistivity and magnetoresistance measured at similar "elds, thus suggesting that the real structure of the material contributes to the measured AMR besides the intrinsic component. ( 2000 Elsevier Science B.V. All rights reserved. Keywords: Manganese perovskites; Alkaline doping; Magnetoresistance anisotropy

1. Introduction Alkaline doping resulting in Re A MnO 1~x x 3 (A"Li, Na, K, Rb) perovskites is an alternative route of doping the parent antiferromagnetic ReMnO structure to promote ferromagnetism 3 and metallic behavior much as the commonly used alkaline-earth doping (A"Ca, Sr, Ba). However, there have been only few reports on this subject using ceramic samples [1}4] or thin "lms [5,6]. Still, there is a lack of systematic information on composition and real-structure e!ects on mag-

* Corresponding author: Tel.: 34-93-580-18-53; fax: 34-93580-57-29. E-mail address: [email protected] (J. Fontcuberta)

netoresistive properties. The system La 1~x Na MnO is especially of interest due to the high x 3 Curie temperature ¹ and signi"cant magC netoresistance close to room temperature, which is favorable for practical applications. In this work structure and transport properties of thin "lms with nearly identical composition on various substrates were studied.

2. Experimental La Na MnO (LNMO) thin "lms (all 1~x x 3`d about 400 nm thick) were grown by powder #ash MOCVD using 2,2,6,6-tetramethylheptane-3,5dionates of corresponding metals, M(thd) , as n volatile precursors. The "lms were obtained on

0304-8853/00/$ - see front matter ( 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 9 9 ) 0 0 7 1 2 - X

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M. Bibes et al. / Journal of Magnetism and Magnetic Materials 211 (2000) 47}53

single-crystalline substrates LaAlO (LAO), 3 SrTiO (STO), NdGaO (NGO) with perovskite 3 3 structure, and ZrO (Y O ) (YSZ) of #uorite struc2 2 3 ture, all being cut along (0 0 1) plane (for LAO and NGO in pseudocubic notations). After deposition at 7503C and P 2 "5 mbar the "lms were annealed O for 0.5 h in oxygen #ow (1 atm) and then cooled down in #owing oxygen. The cationic stoichiometry was determined by energy dispersive X-ray analysis (EDAX). It is found that the sodium content corresponds to x"0.16$0.02 for all samples, and the (La#Na)/Mn ratio is about 0.51$0.02. Because the Na oxidation state is #1, we expect that the carrier concentration caused by doping should be close to that in alkaline-earth (Ca or Sr) doped compositions with x"1. 3 X-ray di!raction study was accomplished with Siemens D5000 four-circle di!ractometer using Cu K radiation and secondary graphite monoa chromator. h}2h scans were measured with the step 0.023/s, u scans with the step 0.23/s. h}2h scans for "lms grown on NGO, LAO and STO reveals perfect (0 0 1) orientation (in pseudocubic notation) and the u-scans indicate that cube-on-cube epitaxy takes place on perovskite substrates. The lattice parameters normal to the "lm surface extracted from the h}2h scans are: 0.3889, 0.3886 and 0.3881 nm for "lms on NGO, LAO and STO, respectively. The measuring error is estimated to be of about $0.0003 nm with the re#ections of the substrate used as an inner standard. The small di!erences in cell parameters is probably due to the residual lattice strain caused by the "lm}substrate lattice mismatch. Taking into account that for the thin manganite "lm the di!erence of the out-ofplane lattice parameters for the "lms on LAO and STO is about 0.005 nm, the La Na MnO 1~x x 3~d "lms are close to the complete relaxation. The conclusion about the lattice symmetry cannot be done from h}2h scans in the symmetrical geometry, as (0 0 l) pseudocubic re#ections do not split in the case of the rhombohedral distortion of the perovskite lattice. The corresponding La 1~x Na MnO ceramics are rhombohedral [4]. x 3`d The LNMO "lm on YSZ has (1 1 0) orientation similar to the other perovskite manganites [7]. The in-plane orientation variants were found by XRD scan. All the epitaxial variants have a volume

diagonal of the perovskite cube parallel to the face diagonal of the #uorite cube. There are four equivalent variants which satisfy the condition and they produce 8 re#ections (4 pair of the two-fold symmetry re#ections) of nearly the same intensity in the u scan (Fig. 1). They form a network of the large angle boundaries with the misorientation angles about 903, 713 and 193. Such boundaries are all strictly coherent along the normal to the substrate surface. There is no TEM observations of the particular LNMO samples but as the XRD results are very similar we can suppose that the in-plane domains have the same size as such domains in La Ca MnO "lms on YSZ (&30}40 nm). 0.7 0.3 3 Thus, the LNMO "lm on YSZ has a strict lattice alignment to the substrate as the LNMO "lms on the perovskite substrates have, but contains

Fig. 1. XRD di!raction study of LNMO "lm on YSZ: (A) u-scan for (2 2 0) re#ection at the tilt angle of 603 reveals inplane orientation variants in the "lm; (B) rocking curve for (0 2 8) re#ection (hexagonal setting) demonstrates perfect alignment of the domains along the normal to the "lm}substrate interface; (C) h}2h scan proves the rhombohedral distortion of the perovskite phase } pseudocubic re#ection (3 3 0) is split into (3 3 0) and (0 3 1 2) re#ections (hexagonal setting), non-symmetrical peaks shape is due to a #a doublets. 1 2

M. Bibes et al. / Journal of Magnetism and Magnetic Materials 211 (2000) 47}53

large-angle boundaries that were not detected in the "lms on the perovskite substrates. At the same time the "lm on YSZ is di!erent from the random ceramic materials as the large-angle boundaries are not random and their density is by a factor 100 higher in the "lm on YSZ as compared to the random ceramic materials. The latter results in the higher resistivity of the manganite "lms on YSZ than that of the random ceramics. The pseudocubic (h h 0) re#ections are split in well-resolved doublets, as it should be in the case of the rhombohedral distortion (Fig. 1, inset). Because the split re#ections are from (h h 0) and (0 1 4 1) planes in the hexagonal setting, the parameters of the hexagonal cell can be calculated. We have found for the LNMO "lm on YSZ: a"0.5507$ 0.0002 nm, c"1.3351$0.0007 nm. Next, we have calculated the (1 0 2) lattice spacing which corresponds to the apparent pseudocubic lattice parameters found for the "lms on the perovskite substrates. The value calculated (0.3881 nm) is the same as that found for the "lm on STO and lower than those found for the "lms on LAO and NGO. Thus, we can conclude that at the thickness 400 nm only "lms on LAO and NGO keep some lattice strain due to the "lm}substrate lattice mismatch but the "lm on STO is relaxed. The increase of the out-of-plane lattice parameters in the "lm on LAO and NGO can be understood as a compensation of

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the "lm lattice contraction in the plane of the interface with the substrate. Magnetotransport properties have been measured under magnetic "elds up to H"70 kOe. The electrical resistivity was determined using the conventional four-probe method. The magnetic "eld is applied within the "lm plane. Magnetoresistance anisotropy has been measured by applying a current J and rotating the in-plane "eld H. We de"ne U as the angle between H at a given measuring point and H at the beginning of the measurement. In order to have a well-de"ned magnetic state of the "lm, these measurements have been performed after saturating the sample at the maximum available "eld and then reducing it to the desired value H.

3. Results and discussion In Fig. 2 the temperature dependence of the resistivity o(¹) for various "lms is presented. A well-de"ned resistivity peak occurs close to the Curie temperature and a metallic behavior is observed at lower temperatures. The temperatures ¹ where the maximum of resistivity occurs are: 1 296.9 K, 291.0 K, 289.3 K for the (0 0 1) epitaxial "lms on NGO, LAO and STO, respectively, and 225 K for the (1 1 0) "lm on YSZ.

Fig. 2. (a) Zero-"eld resistivity versus temperature of the La Na MnO "lms grown on LAO, NGO, STO and YSZ substrates; 0.84 0.16 3~d (b) Dependence of ¹ on the c-axis cell parameters. P

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M. Bibes et al. / Journal of Magnetism and Magnetic Materials 211 (2000) 47}53

We "rst note that these ¹ values are close 1 to that reported for the bulk material with the controlled oxygen content (290 K for La 0.85 Na MnO with 31% Mn4` in Ref. [4], 308 K 0.15 3.02 for La Na MnO with 38% Mn4` in Ref. 0.82 0.16 3.00 [8]. The result implies that the oxygen content of our "lms is close to that of the bulk material. One should observe that these ¹ values are similar to 1 those reported for (La Nd ) Sr MnO , 1@4 3@4 2@3 1@3 3 which nominally should have nearly the same hole doping, and where ¹ is of about 294.5 K [9]. It is C well known that ¹ is closely connected to the C Mn}O}Mn bond angle, which is mainly controlled by the mean size R of the cations occupying the O larger site of the perovskite structure [10,11]. Using the standard values of ionic sizes for the distinct cations present [12] in our "lms it turns out that R "1.163 As , which is very close to the correO sponding R value for (La Nd ) Sr MnO O 1@4 3@4 2@3 1@3 3 [9]. On the other hand, when comparing the ¹ of 1 our epitaxial "lms one observes that it varies systematically with the out-of-plane lattice parameter c as determined from the h}2h scans. In Fig. 2b we show this dependence. The fact that ¹ (+¹ ) is 1 C larger when increasing c can be understood in terms of the in-plane cell parameters contraction resulting from the cell-volume conservation. It is well known that under pressure, the Curie temperature rises due to bandwidth broadening [13]. In the (1 1 0) "lm grown on YSZ, ¹ is signi"cantly 1 lower probably due to the presence of a intergranular resistance caused by the scattering at the large

angle boundaries. In fact, the mere observation of the room-temperature "lm resistivities reveals that whereas the epitaxial "lms have o+ 1.1}1.6 m) cm, the "lm grown on YSZ has o+ 37 m) cm. Similar correlation of the electrical resistances was reported for the (La,Pr) Ca MnO 0,7 0,3 3 "lms [14] and La Sr MnO "lms [15] on per1~x x 3 ovskite and YSZ substrates. We turn now to the "lm magnetoresistance MR. The isothermal MR"(o(H)!o(H"0))/o(H"0) data measured at various temperatures (10 and 300 K) for "lms grown on STO and YSZ substrates are collected in Fig. 3. The most remarkable di!erence appears in the MR measured at low temperature. Whereas for the epitaxial "lm on STO MR is almost negligible, this is not the case for the "lm on YSZ where the existence of signi"cant low "eld response (about !21%) re#ects the tunnel magnetoresistance due to the high density of the large angle grain boundaries. Finally, we will focus on the dependence of the magnetoresistance on the angle formed by the measuring current and the direction of the in-plane applied magnetic "eld. At "elds well above the inplane anisotropy "eld H , the "lm magnetization A M follows the applied "eld H, and thus any dependence of the magnetoresistance AMR will re#ect the so-called anisotropic magnetoresistance. AMR is de"ned as AMR(H, ¹,U)"(o(H, ¹,U)! o(H, ¹,U"0))/o(H, ¹,U)). Therefore, AMR(U) can be most easily determined from isothermal measurements performed at "elds H
Fig. 3. Field dependent magnetoresistance of the "lms on: (a) STO and (b) YSZ at di!erent temperatures (10 and 270 K).

M. Bibes et al. / Journal of Magnetism and Magnetic Materials 211 (2000) 47}53

the magnetocrystalline anisotropy will be absent and AMR should re#ect the two-fold symmetry on U. Indeed, AMR is predicted to have an angular dependence given by: AMR(U)"A # 0 A sin2(U#/) where A and A are in general, are 1 0 1 "eld and temperature-dependent parameters, and the / initial angle between H and J. Of course, when reducing the "eld amplitude the increasing contribution of the magnetocrystalline anisotropy will lead to a more complex angular dependence of AMR. In this paper we will restrict ourselves to the H
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"lms. In this plot, and in order to allow comparison with other measurements, A (¹) has been nor1 malized by its maximum value. Inspection of the results of Fig. 5 immediately reveals that the AMR, or more precisely its amplitude A (¹), has a well1 de"ned maximum at ¹+¹ (50 kOe) and dep creases when cooling the samples further. One should note however that the low-temperature amplitude of the anisotropic magnetoresistance is higher for the YSZ "lm than for the STO "lm. In fact the temperature dependence of the AMR amplitude mimics that of the high "eld magnetoresistance MR(¹)"(o(H, ¹)!o(H, ¹))/o(H"0, ¹) as well as that of the electrical resistivity. To emphasize this result we include in Figs. 5a and b the magnetoresistance measured for both "lms at a comparable "eld (40 kOe). These results indicate that the anisotropic magnetoresistance is intimately connected to the transport properties of carriers across the ferromagnetic transition, where the socalled colossal magnetoresistance occurs. We note that the AMR is not roughly proportional to the sample magnetization M, as observed in other ferromagnetic systems [16] where it grows monotonously when cooling the sample below ¹ . C This is not the observed behavior in Fig. 5. Recently, similar temperature dependence of A (¹) 1 has been reported by Ziese et al. [17] and J.O'Donnell et al. [18] on epitaxial "lms. However, detailed inspection of the data reveals signi"cant di!erences. For instance, the low-temperature data of J.O'Donnell et al. [18] indicated the presence of a four-fold component in AMR(U) which is clearly

Fig. 4. Anisotropic magnetoresistance AMR measured at 300 and 60 K under a 50 kOe "eld for: (a) STO and (b) YSZ "lms. The solid lines through the data are the results of the "ts using a AMR &sin2(U) dependence.

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M. Bibes et al. / Journal of Magnetism and Magnetic Materials 211 (2000) 47}53

Fig. 5. Temperature dependence of the amplitude A (¹) of the AMR(50 kOe) (solid symbols) for: (a) the STO "lm free of the large angle 1 boundaries and (b) the YSZ "lm with the high density of large angle boundaries. The magnetoresistance MR(40 kOe) measured for the same "lms are also included (open symbols).

absent in our data. The development of a four-fold component was attributed to the existence of large polarons at ¹;¹ , which wave function extends C through several lattice sites in any direction (along the Mn}O bonds) thus introducing this higherorder symmetry component in AMR. There is no obvious reason to assume that in our "lms the size of any existing polaron below ¹ may be signi"C cantly reduced in such a way that only hopping matrix elements between neighboring sites along the transport current become relevant. Therefore we should conclude that the characteristics (size) of polarons cannot be essential in order to account for the reported four-fold component to the AMR at low temperatures. On the other hand, regarding the temperature dependence of A (¹), the signi"cant observation is 1 that the YSZ "lm, has a substantially larger AMR amplitude at low temperature. In fact we have observed that A (¹) becomes larger at low temper1 ature in polycrystalline "lms in such a way that the pronounced peak occurring at ¹+¹ becomes 1 smeared out and a temperature dependence more similar to that of the magnetization M(¹) is measured [17,19]. If the magnetoresistance MR found in polycrystalline samples is related to the spin depolarization at interfaces [20], then the similar temperature dependence of the high "eld MR and AMR is suggestive of a interface contribution to the AMR.

Irrespectively on the precise mechanism leading to the observed AMR, a nonzero AMR occurs due to the presence of spin-coupling: the spin}dipole moments becomes oriented by the "eld and results via the spin}orbit coupling to a reorientation of the orbital moments. The nonspherical charge distribution of the involved orbitals (Mn e and O 2p in the ' conduction band in manganites) "nally leads to an asymmetry in the "eld-dependent resistivity. Therefore the observation of a sizable AMR indicates that the orbital momentum in manganites is not completely quenched. Distinct orbital momentum for the bulk of the "lms and from the grain interfaces may thus provide the source of two components to the measured AMR: one having a maximum at ¹ and the other growing at lower temperature. 1 In summary, we have reported on the preparation and characterization of La Na MnO 0.84 0.16 3 thin "lms grown by MOCVD on various substrates. Under appropriate conditions epitaxial thin "lms have been obtained. The anisotropic magnetoresistance of the "lms have been analyzed in some detail, and it has been found that it has a two-fold symmetry at any temperature. Its temperature dependence mimics that of the electrical resistivity and magnetoresistance measured at similar "elds, thus suggesting that two components contribute to the measured AMR, one of them being intrinsic and another one being coupled with the real structure of the material.

M. Bibes et al. / Journal of Magnetism and Magnetic Materials 211 (2000) 47}53

Acknowledgements Financial support by the INTAS (IR-97-11954), CICYT (MAT97-0699), the CEE OXSEN projects and the Generalitat de Catalunya (GRQ95-8029) are acknowledged.

References [1] (T. Shimura, T. Hayashi, Y. Inaguma, M. Itoh, J. Solid State Chem. 124 (1996) 250. [2] Z. Jirak, J. Hejtmanek, K. Knizek, J. Solid State Chem. 132 (1997) 98. [3] G.H. Rao, J.R. Sun, K. Bearner, N. Hamad, J. Phys.: Condens. Matter 11 (1999) 1523. [4] T. Boix, F. Sapin8 a, Z. El-Fadli, E. Martinez, A. BeltraH n, J. Vergara, R.J. Ortega, K.V. Rao, Chem. Mater. 10 B (1998) 1569. [5] M. Sahana, R.N. Singh, C. Shivakumara, N.Y. Vasanthacharya, M.S. Hegde, S. Subramanian, V. Prasad, S.V. Subramanyam, Appl. Phys. Lett. 70 (1997) 2909. [6] C.-C. Chen, A. de Lozanne, Appl. Phys. Lett. 71 (1997) 1424. [7] O.Yu. Gorbenko, A.R. Kaul, V.N. Fu#yigin, A.A. Molodyk, M.A. Novozhilov, A.A. Bosak, V.A. Amelichev, G. Wahl, U. Krause, J. Alloys and Compounds 251 (1997) 337.

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[8] M. Itoh, T. Shimura, J.D. Yu, T. Hayashi, Y. Inaguma, Phys. Rev. B 52 (1995) 12522. [9] J. Fontcuberta, V. Laukhin, X. Obradors, Appl. Phys. Lett 72 (1998) 2607. [10] H.Y. Hwang, S.-W. Cheong, P.G. Radaelli, M. Marezio, B. Batlogg, Phys. Rev. Lett. 75 (1995) 914. [11] J. Fontcuberta, B. MartmH nez, A. Se!ar, S. Pin8 ol, J.L. GarcmH a-Mun8 oz, X. Obradors, Phys. Rev. Lett 76 (1996) 1122. [12] R.D. Shannon, Acta Crystallogr. A 32 (1976) 751. [13] V. Laukhin, J. Fontcuberta, J.L. GarcmH a-Mun8 oz, X. Obradors, Phys. Rev. B 56 (1997) R10009. [14] E. Ganshina, O. Gorbenko, N. Babushkina, A. Kaul, A. Smechova, L. Belova, in: V. Kose, J. Sievert (Eds.), Non-linear Electromagnetic Systems, Elsevier Studies in Applied Electromagnetics in Materials, Vol. 13, 1998, p. 325. [15] O. Gorbenko, P. Demin, A. Kaul, L. Koroleva, R. Shimchak, Fiz. Tv. Tela 40 (1998) 290. [16] P.A. Stampe, H.P. Kunkel, Z. Wang, G. Williams, Phys. Rev. B 52 (1995) 335. [17] M. Ziese, S.P. Sena, J Phys.: Condens. Matter 10 (1998) 2727. [18] J. O'Donnell, M. Onellion, M.S. Rzchowski, J.N. Eckstein, I. Bozovic, unpublished. [19] M. Bibes, B. MartmH nez, J. Fontcuberta, V. Trtik, C. Ferrater, F. SaH nchez, M. Varela, R. Hiergeist, K. Steenbeck, G Presented at the EMRS-Spring meeting. Strasbourg, 1999. [20] L. Balcells, J. Fontcuberta, B. MartmH nez, X. Obradors, Phys. Rev. B 58 (1998) RC14697.