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7THIN-FILM MULTIFERROIC NANOCOMPOSITES IN THE SYSTEM LuMn03 - Pr07Sr03MnO3 OBTAINED BY MOCVD
A. R. Akbashev, O. Yu. Gorbenko, A. R. Kaul
Chemistry Departement, Moscow State University Moscow, 119991, Russia Tel.: +007-(495)-939-14-92; e-mail: kaul@inorg.chem.msu.ru
The composite multiferroic Pr^Sr^MnOj-LuMnOj thin films were grown by metallorganic chemical vapor deposition (MOCVD) on (111) Zr02(Y203) substrate. Pr(thd)3, Lu(thd)3, Mn(thd)3 and Sr(thd)2 were used as volatile precursors. A combination of the ferromagnetic Pr^Sr^MnOj with high magnetostriction and the ferroelectric LuMn03 in thin-film composite can lead to rather high magnetoelectric effect. Radiographic and electron microscopic investigations showed the presence of epitaxial hexagonal matrix LuMn03 and nanosize perovskite inclusions. The values of the lattice parameters suggest that no noticeable solubility between two components of the thin-film nanocomposite is present.
Introduction
Nowadays multiferroic materials gain increasingly more popularity due to their ability to change magnetization in applied electric field or to change electrical polarization by the applied magnetic field (so-called magnetoelectric effect) [1-3]. Multifer-roics are crystalline solids where some ordering parameters, magnetic, electric and mechanical, can co-exist [4]. Such materials can find wide applications not only in microelectronics and various sensors, but also in the new field — spin electronics (spintronics). The main problem of spintronics is an effective conversion of magnetization into electric voltage. Magnetoelectrics, the subclass of multiferroics, are predicted to become alternative giant magnetoresistance devices in the near future [5-6]. The demands for inventing and using such materials in nanotechnology are determined by towering tendency to miniaturization of electronic devices. Composite multiferroics made of the ferroelectric and ferromagnetic materials coupled by magneto or electrostriction propose an essential material flexibility and high magnetoelectric effect [7-8].
Experiment
Thin film samples in the system Pr^Sr^MnOj-LuMn03 were prepared using metalloorganic chemical vapor deposition (MOCVD) in the original experimental setup (fig. 1). A pellet made of powder mixture of precursors was fed with the constant rate into an evaporator, where the temperature was chosen as high as 250 °C to guarantee the flash evaporation of each precursor microportion [9]. Argon and oxygen were used as a carrier gas
and oxidizing agent, respectively. Gas flows were selected equal to 17 1/h of Ar and 3.5 1/h of 0, based on the result of the previous experience; therefore, the partial pressure of oxygen 2 mbar and the total pressure 8 mbar. The temperature of deposition was 700-800 °C while typical deposition rate was 0.5 mm/h. Pr(thd)3, Lu(thd)3, Mn(thd)3. Sr(thd)2 used as volatile precursors (thd = 2,2,6,6-tetramethylheptane-3,5-dionate) were sublimed in vacuum before using them for MOCVD. The surface morphology and the ratio in the films were determined using scanning electron microscope Jeol JSM-840A. The phase structure was characterized by X-ray diffraction (XRD) using a Siemens D5000 four-circle diffractometer. The cation composition was controlled by energy dispersion X-ray (EDX) analysis.
Results and discussion
According to literature data, Pr^Sr^MnOg is orthorhombically distorted perovskite with ferromagnetic ordering near the room temperature for x = 0.3 [10-14]. LuMn03 is a layered hexagonal phase which possesses ferroelectric ordering with Curie temperature Tc - 900 K owing to non-cen-trosymmetrical space group (P63cm) [15]. As it was shown in [16], there is a miscibility gap between perovskite and layered hexagonal phases of RMn03 (where R is a rare earth cation), when compounds of rare earth ions with the large difference in the ionic radia are alloyed together (like that of La and Lu, for instance). Doping cations with larger ionic radius than that of rare earth cations, like in the case of Sr2+, in such a system are concentrated completely in the perovskite phase.
Статья поступила в редакцию 11.01.2008 г. Ред. per- № 217. The article has entered in publishing office 11.01.2008. Ed. reg. No. 217.
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powder feeder
\
£
evaporator
pressure controller
substrate holder
vacuum pump
transfer line substrate
resistive oven
Ar 02
Fig. 1. Scheme of experimental MOCVD setup
Perovskite structure:
The model of the composite microstructure:
Rima
Hexagonal structure:
The epitaxial growth scheme of the liexagonal phase od
(iii)Zro3(Y3cg
¿ггавижбэк
^MeÉr
P6,mc
Fig. 2. The model of the two-phase PrlxSrtMn03-LuMnOs composite microstructure formation
Due to strong differences in ionic radia of lute-tium and praseodymium (strontium), no visible doping LuMn03 by praseodymium and strontium can be expected.
The separation of the Pr-Sr-Lu-Mn-0 films into hexagonal and perovskite phase was observed by
XRD. But we have found for the first time that films in the system Pr, JSr^MnOj-LuMnOg adopt an unusual composite microstructure on (111) Zr02(Y,03) single-crystalline substrate what is rather different compared to bulk ceramic composites described in the literature [17]. The model structure of such unusual composite is shown in fig. 2. The hexagons correspond to the epitaxial matrix of LuMn03, the inclusions contain the perovskite phase. Noteworthy, a combination of ferromagnetic nanodomains with high magnetostriction and ferroelectric matrix in composite thin films can lead to rather high value of magnetoelectric effect. The (111) Zr02(Y203) substrate plays an important role in the formation of such microstructure due to the preference for epitaxial growth of layered hexagonal structure [18]. Perovskite manganite phases do not possess so strong preference and reveals a mixture of orientations on (111) Zr02(Y203) substrate.
After few preliminary experiments the composition of the precursor mixture was adjusted to produce films corresponding to films with the component ratio corresponding to Pr1_;rSrxMn03+ +LuMn03 according to EDX, and containing no secondary phases (Mn304
and Lu203 were observed in the earlier off-stoichiometric samples) according to XRD.
Also, the XRD shows (fig. 3) that the hexagonal phase is easily identified by (004) and (008) reflections which mean pure c-orientation of the film. The calculated value of c-parameter coincided well with the c-pa-rameter of hexagonal phase reported in literature [19], what is the evidence that no significant doping of this phase with praseodymium or strontium
60 _J
(111) Zr02<YA) (222) Zr02(YA)
(004) LuMnO i I c = 1,130nrA I
(008) LuMnOj
(004) LUjOJ
(002)l*rovskite
N 25 30 36 M « »
00 45 70 75 SO S6 ЭЭ B5 Dû
28 (degree)
- » • • - Ж
. > .-.-■- •
-j Л.» • - - *
3 pm
■• v.*..-
Fig. 3. X-ray diffraction pattern of Pr, SrMnO^LuMnO, Fig. 4. SEM image of the Pr, SrxMn03-LuMn0, composite
composite
surface morphology
56
International Scientific Journal for Alternative Energy and Ecology №1
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A. R. Akbashev, 0. Yu. Gorbenko, A. R. Kaul
Thin-film multiferroic nanocomposites in the system LuMn03 - Pr0 ,Sr0 3Mn03 obtained by MOCVD
occurs in the films similar to the bulk composites [17]. The perovskite peaks were strongly broaden indicating a small size of the perovskite phase inclusions. Calculation using Debye-Sherrer formula produced size value of 5 nm, and maximum of the peak position corresponding to pseudocubic (110) reflection of Pr0 67Sr0 33Mn03. The calculated pseudocubic parameter of perovskite — a = 3.86 A (mean value from the literature: a = 3.86 A [20]). The SEM image of the surface morphology of the film (fig. 4) reveals a completely even surface. No secondary phase inclusions were sticking out of the smooth surface of the composite films.
Conclusions
Thus, we have shown for the first time the epitaxial growth of nanocomposite multiferroic Pr,_;rSr;rMn03-LuMn03 on (111) Zr02(Y203) substrate, successfully done by MOCVD. The perovskite phase grows as the small non-oriented nanoparticles inside epitaxial hexagonal matrix. The values of the lattice parameters suggest that no noticeable solubility between LuMn03 and Pr0 67Sr0 33Mn03 is present.
Acknowledgments
This work was supported by RFBR project (No. 06-03-33070).
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