Two-phase structure of ultra-thin La–Sr–MnO films

Thin Solid Films 515 (2006) 691 – 694 www.elsevier.com/locate/tsf

Two-phase structure of ultra-thin La–Sr–MnO films S. Balevi*ius a,*, P. Cimmperman a, V. Petrauskas a, V. Stankevi* a, E.E. Tornau a, N. Zˇurauskiene˙ a, A. Abrutis b, V. Plauxinaitiene˙ b, M. Sawicki c, T. Dietl c, M. Aleszkiewicz c a

Semiconductor Physics Institute, Vilnius, Lithuania b Vilnius University, Vilnius, Lithuania c Institute of Physics, PAS, Warsaw, Poland Available online 23 January 2006

Abstract The structural, electrical and magnetic properties of ultra-thin La0.83Sr0.17MnO3 (LSMO) films, deposited on NdGaO3 substrate by using the MOCVD technique, were studied. The film thickness d varied in the range from 4 to 140 nm. X-ray and RHEED measurements demonstrated that the films had a two-phase structure. One phase had an orthorhombic face centred structure (a = 0.406 nm and c = 0.46 nm), while the other one had a cubic perovskite-like structure with a = 0.388 nm. Low field dc resistance and magnetization vs. temperature dependences were investigated in the temperature range from 5 to 300 K using a conventional four-probe method and a SQUID magnetometer. It was found that the temperature of the resistivity maximum, T m, increases with increasing film thickness and that the value of the Curie temperature T C estimated from the temperature dependence of magnetization is very close to T m. Modelling of the remanent magnetization vs. temperature dependence based on a two-phase model was in agreement with experimental results. This model also explains the T m shift to lower temperatures with decreasing film thickness. D 2005 Elsevier B.V. All rights reserved. PACS: 75.47.Lx; 75.70.Ak; 75.60.Ej Keywords: Manganite thin films; Electrical resistivity; Magnetization

1. Introduction The properties of ultra-thin manganese oxide films with thicknesses less than 100 nm were intensively studied due to their possible application in vertical or planar tunnel junction devices [1,2]. Investigations of such films reported in [3 –8] showed that a decrease in film thickness strongly influences the electrical resistivity and its peak temperature (T m) of the film. Moreover, it was found that La 0.67 Sr 0.33 MnO 3 [3,4], La0.75Sr0.25MnO3 [5], and La0.7Sr0.3MnO3 [8] films, prepared by laser deposition, and La0.6Sr0.4MnO3 [6,7], fabricated by magnetron sputtering on LaAlO3 [3 – 5,8], NdGaO3 [4,8], SrTiO3 [5– 8] and MgO [6,7] substrates, exhibited a region at the substrate –film interface having structural distortion and compositional inhomogeneity. A similar structure was also found in La0.67Ca0.33MnO3 films [9] grown on LaAlO3 and

* Corresponding author. Goxtauto st. 11, LT-01108 Vilnius, Lithuania. Tel.: +370 5 2617546; fax: +370 5 2627123. E-mail address: [email protected] (S. Balevi*ius). 0040-6090/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2005.12.240

NdGaO3 by pulsed-laser deposition and La0.83Sr0.17MnO3 prepared by MOCVD technique on NdGaO3 (001) substrates [10]. It was found that this region consists of two phases, which have a strong impact on the properties of ultra-thin films. In this paper, we report on our investigations that demonstrate how two-phase structure influences the T m and magnetic properties of La0.83Sr0.17MnO3 films. 2. Experimental The films were deposited on NdGaO3 (NGO) (001) substrate at 825 -C using an injection MOCVD technique, from a solution with composition La0.78Sr0.22Mn0.733. Metal2,2,6,6-tetramethyl-3,5-heptandionates dissolved in monoglyme were used as precursor materials. The temperature of the sublimation was kept at 290 -C which corresponded to a film growth rate of 8.3 nm/min. The thickness of the film ranged from 4 to 140 nm. The chemical composition of the film was measured by means of EDX and was found to be La0.83Sr0.17MnO3. It should be noted that thin LSMO films grown on (001) NGO substrate are anisotropically

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strained in plane (in contrary to the films deposited on (110) NGO surface which are free from strain). These films have different lattice mismatch signs for different crystallographic directions. The samples, having a length ranging from 6 to 8 mm, were designed to have co-planar shape with a width of 2 or 5 mm. Thin Ag films as electrodes were deposited by thermal evaporation at 200 -C and then annealed for 1 h at 1 atm pressure in an argon gas atmosphere at 400 -C. The electrodes were spaced by a 50 Am gap, which was the active area of the sample. Measurements of dc resistance and magnetization vs. temperature dependence were performed at temperatures ranging from 5 to 300 K using a standard four-probe method and a SQUID magnetometer, respectively. The structure of the film, as well as the ratio between the volumes of the phases having different lattice parameters, were investigated using reflection high-energy electron diffraction (RHEED) at room temperature (290 K). 3. Results It was found that the RHEED patterns recorded for samples with 4 nm thickness corresponded to an orthorhombic face centered structure with a lattice constant of a = 0.406 nm in plane and c = 0.46 nm out of plane (phase 1). The vector [100] of the reciprocal lattice in this phase is aligned parallel to the vector [110] of orthorhombic cell of the NGO substrate. The patterns found for films with a thickness greater than 60 nm (see inset in Fig. 1 for d = 140 nm) revealed that the film consisted mainly of a phase having a cubic perovskite-like structure with a lattice constant of about a = 0.388 nm (phase 2). The vector [100]c of the reciprocal lattice of this phase is also parallel to the vector [110] of the NGO substrate. Here and below the subscript ‘‘c’’ indicates the pseudocubic

Fig. 2. Temperature dependence of the resistivity of LSMO for films of different thicknesses.

perovskite unit cell. The diffraction pattern found for [100]c zone-axis films with thicknesses ranging from 8 to 20 nm revealed a two-fold superstructure (see inset in Fig. 1 for d = 20 nm) which consists of orthorhombic face centered and perovskite-like cubic phases. Fig. 1 shows the relative volume of the phase 2 fraction c 2 in the film, obtained from RHEED data analysis, as a function of film thickness d. It was found that experimental data fit well the following formula: c 2 = 1 / {1 + exp[ k(d  d 0)]}, here k = 0.05, d 0 = 9.65. The resistivity (q) vs. temperature (T) dependences of our films are presented in Fig. 2. All curves exhibit a maximum at temperature T m, and the value of T m increases with the thickness of the film up to 20 nm until it starts to saturate (see also inset in Fig. 5). Moreover, for films of low thickness the resistivity is anisotropic in the substrate plane: it is higher in the compressed [100] and lower in the tensed [010] directions, respectively.

Fig. 1. The relative volume of the majority phase fraction c 2 for film thickness d = 8, 20, 58 and 140 nm (squares) obtained from RHEED measurements. Solid line shows that the best fit is given by the function c 2 = 1 / {1 + exp[k(d  d 0)]}, where k = 0.05 and d 0 = 9.65 nm. Inset: RHEED images for d = 20 and 140 nm samples.

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Typical results for remanent magnetization M REM of these films (field-cooled at 1000 Oe) as a function of temperature are presented in Fig. 3. As it can be seen, the M REM measured for films of 20 nm thickness along the [010] and [100] axes is different both in absolute value and behaviour. The signal of the M REM along the [010] axis is relatively high and depends non-linearly on the temperature. However, the signal of the M REM along the [100] axis is significantly lower and increases linearly as temperature decreases. The temperature at which the M REM signal becomes zero could be associated with the Curie temperature (T C). Results presented in Figs. 2 and 3 show that for 20 nm films the T C is very close to T m (about 295 K). The strong anisotropy of the magnetic properties in two perpendicular directions in-plane to the substrate was also detected from measurement of the magnetic hysteresis loop. The magnetization vs. magnetic field for 20 nm film at T = 80 K is presented in Fig. 4. The saturation of magnetization was obtained at about 400 emu/cc. Taking into consideration that 20 nm thickness film contains a 64% of phase 2 and a 36% of phase 1, we calculated the value of the saturation magnetization M s and found it to be å 2.82 l B. The expected M s for ideal structure for Sr composition of our films x = 0.17 is 3xl B + 4(1  x)l B = 3.83 l B. This result demonstrates that part of the film material is in a non-magnetized state. 4. Discussion In order to explain how this two-phase structure could influence the magnetization and T m behaviour of the film, we performed the calculation based on results presented in Figs. 2 and 3 assuming that T m å T C. The simplest model for magnetization of a two-phase system is the model for an alloy. In this model, the spin at any site in the lattice experiences the same mean-field (MF), which is proportional to the concentration of both ingredients. In contrast, thin films, most likely, have to be characterized by a more strained phase, closer to the

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Fig. 4. Magnetization hysteresis loops at 80 K for magnetic fields applied parallel to the film surface along [010] (open squares) and [100] (open triangles) axes.

substrate (phase 1) and the ‘‘strain-relaxed’’ phase 2, with magnetic properties closer to those of the bulk system, near the top of the film. In this case, the fraction of the first phase c 1 has to decrease and the fraction of the second phase c 2 = 1  c 1 has to increase with increasing film thickness d. As it was already mentioned, this dependence can be well described by the Boltzmann function c 2 = 1 / {1 + exp[ k(d  d 0)]}, where d 0 is the thickness at which the concentrations of the phases are equal (c 1 = c 2 = 0.5) and k is a parameter, in some way, characterizing the fabrication (or phase separation) conditions: high values of k correspond to abruptly separated phases, while low values indicate high degree of mixing in the system. The parameter k is normalized to lattice constant. In such a system and for simple cubic lattices, the interaction of the mean fields and the external magnetic field H with the mean magnetic moments M 1i = gl B and M 2i = gl B of both phases in the i-th plane can be described by the following equations:  < s1i > ¼ s1 B s1 ½J11 c1 ð4 < s1i > þ < s1iþ1 > þ < s1i1 > Þ þ J12 c2 ð4 < s2i > þ < s2iþ1 > þ < s2i1 > Þ þ glB H=kB T g   < s2i > ¼ s2 B s2 J21 c1 4 < s1i > þ < s1iþ1 > þ < s1i1 > Þ þ J22 c2 4 < s2i > þ < s2iþ1 > þ < s2i1 > Þ þ glB H=kB T g:

Fig. 3. Temperature dependence of remanent magnetization for 20 nm film in magnetic fields applied along [010] (open circles) and [100] (open triangles) axes.

ð1Þ

Here B{. . .} is the Brillouin function, k B and g are Boltzmann and Lande factors, respectively, l B is Bohr magnetron, and T is the temperature. The spin moments of both phases are denoted as s 1 and s 2 ( and are their thermal averages in the ith plane). The J 11, J 22 and J 12 = J 21 are the exchange constants for the interactions between spins of the first component, between spins of the second component, and between spins of the first and second components, respectively. We assume that s 1 = s 2 = s(Mn3+) = 2. The solution for magnetization in the i-th plane of components M 1i and M 2i and plane magnetization M i = M 1i + M 2i is related to the magnetization in the neighbouring planes. Thus, to obtain total magnetization of the film vs. its thickness d = Na = d 0  (1 / k)ln[(1  c 2) / c 2], we have to

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5. Conclusions

Fig. 5. Temperature dependence of normalized magnetization for d = 20 nm sample (circles) and its fit by solid line obtained in mean-field (MF) approximation by using formula (1) and J 11 / J 22 = 0.4, J 12 / J 22 = 0.8, k = 0.05 and d 0 = 9.65 nm. Inset: experimental T m vs. d dependence (squares) and its fit to the T C vs. d curve.

solve the system of Eq. (1) for N planes, where a = a i  a iT1 is the lattice parameter. In Fig. 5 experimentally obtained temperature dependence of magnetization, normalized to saturation magnetization, for d = 20 nm sample (circles) and its fit by a solid line obtained using formula (1), where J 11 / J 22 = 0.4, J 12 / J 22 = 0.8, k = 0.05 and d 0 = 9.65 nm, are presented. Thus, the interphase interactions as well as interactions of both phases are chosen to be ferromagnetic. The inset in Fig. 5 shows the experimentally measured T m vs. d dependence (squares) and its fit to the T C vs. d curve, assuming that T C å T m and that the values of interaction parameters for the d = 20 nm sample are valid for other thicknesses as well. T C is calculated from Eq. (1) for the same set of parameters. The discrepancies between experimental and MF curve in Fig. 5 might be due to the insufficient accuracy of the MF approach. However, the specific behaviour, close to linear, of the experimental curve at T < 150 K does not allow one to exclude the possibility that the minority (strained) phase 1, which for d = 20 nm occupies 1/3 of the sample, might be of paramagnetic or even antiferromagnetic origin (see e.g. [9] for two phase behaviour of LCMO). We tried to fit the experimental curve with the numerical curve for a paramagnetic set of interactions in the minority phase ( J 11 = J 12 å 0), and fairly good agreement was achieved, but only for T < 220 K. At higher temperatures, the T C of both curves are very different (T Cexp ¨300 K while T Ccal is close to 370 K).

In conclusion, the two-phase structure has strong influence on the electric and magnetic properties, peak of resistivity maximum in q(T) dependences in particular, of ultra-thin films of lanthanum manganites. This peak increases with decrease of film thickness and shifts to lower temperatures. It was demonstrated for 20 nm films that Curie temperature is very close to the resistivity peak temperature (about 295 K). Magnetic measurements revealed strong anisotropy of magnetization along two perpendicular in-plane, [010] and [100], axes. The RHEED imaging showed that the ultra-thin films are multilayered, each layer being a mixture of two phases with different structure. Our modelling demonstrated that large decrease in T m with the decrease of film thickness might be due to such a two-phase nature of the thin film. The two-phase model proposed is sufficient to qualitatively explain the main magnetization vs. temperature dependences of such films. Further investigations are needed to clear up the origin and structure of the phase 1 found at the vicinity of the substrate. Acknowledgements The authors would like to thank L.L. Altgilbers (US Army Space and Missile Defense Command, Huntsville AL, USA) for fruitful discussions. Support by the Swedish Institute and the EC project ‘‘The Centre in Processing, Research and Application of Advanced Materials (PRAMA),’’ contract Nr. G5MA-CT-2002-04014, is gratefully acknowledged. References [1] Y. Lu, X.W. Li, G.Q. Gong, G. Xiao, A. Gupta, Ph. Lecoeur, J.-Z. Sun, Y.Y. Wang, V.P. Dravid, Phys. Rev. B 54 (1996) R8357. [2] J.B. Philipp, C. Ho¨fener, S. Thiehaus, J. Klein, L. Alff, R. Gross, Phys. Rev. B 62 (14) (2000) R9248. [3] H.L. Ju, Kannan M. Krishnan, D. Lederman, J. Appl. Phys. 83 (1998) 7073. [4] J.Z. Sun, D.W. Abraham, R.A. Rao, C.B. Eom, Appl. Phys. Lett. 74 (1999) 3017. [5] S.I. Khartsev, P. Johnsson, A.M. Grishin, J. Appl. Phys. 87 (2000) 2394. [6] L.B. Steren, M. Sirena, J. Guimpel, J. Magn. Magn. Mater. 211 (2000) 28. [7] M. Sirena, L.B. Steren, J. Guimpel, Thin Solid Films 373 (2000) 102. [8] Joonghoe Dho, N.H. Hur, I.S. Kim, Y.K. Park, J. Appl. Phys. 94 (2003) 7670. [9] Amlan Biswas, M. Rajeswari, R.C. Srivastava, Y.H. Li, T. Venkatesan, R.L. Greene, A.J. Millis, Phys. Rev. B 61 (2000) 9665. [10] S. Balevi*ius, V. Stankevi*, N. Zˇurauskiene˙, Cˇ. Sˇimkevi*ius, J. Parxeliu¯nas, P. Cimmperman, A. Abrutis, V. Plauxinaitiene˙, Acta Phys. Pol. A 107 (2005) 203.