Thin films and superlattices of multiferroic hexagonal rare earth ...

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Rare earth manganites, ReMnO3 with Re ¼ La to Dy, crystallize in a ..... should be confirmed before any hypothesis can be drawn for the origin of such a.
Philosophical Magazine Letters, Vol. 87, Nos. 3–4, March–April 2007, 203–210

Thin films and superlattices of multiferroic hexagonal rare earth manganites C. DUBOURDIEU*y, G. HUOTy, I. GELARDy, H. ROUSSELy, O.I. LEBEDEVz and G. VAN TENDELOOz yLaboratoire des Mate´riaux et du Ge´nie Physique (LMGP), UMR CNRS/INPG 5628, INPG-Minatec, 3 parvis Louis Ne´el, BP 257, 38016 Grenoble cedex 1, France zElectron Microscopy for Materials Research (EMAT), University of Antwerp, Groenenborgerlaan 171, B-2020 Antwerpen, Belgium

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(Received 6 October 2006; in final form 23 November 2006) Hexagonal YMnO3 and HoMnO3 as well as (YMnO3/HoMnO3)15 superlattices were grown on (111) ZrO2 (Y2O3) and (111) Pt/TiO2/SiO2/p-type (100)Si substrates. Epitaxial stabilization was used on the same substrates to grow DyMnO3 or TbMnO3 (which normally crystallize in an orthorhombic, perovskite-type, structure). All heterostructures were obtained oriented, with the c-axis of the hexagonal cell perpendicular to the substrate plane. This orientation is desired since it is the direction of the ferroelectric polarization in hexagonal manganites. The strain state of the films grown on conductive Pt electrodes, tracked by the evolution of the c parameter as a function of film thickness, was found to be completely different than the one obtained on YSZ. Such a study is important for future strain engineering in multilayers. Highresolution transmission electron microscopy on superlattices grown on (111)Pt indicates a high crystalline quality along the c-axis and sharp interfaces.

1. Introduction Rare earth manganites, ReMnO3 with Re ¼ La to Dy, crystallize in a perovskite-type structure (orthorhombic, space group Pnma), whereas the manganites with Re3þ of smaller ionic radius (Ho, Er, Tm, Yb, Lu) as well as Y and Sc manganites crystallize in a hexagonal structure of LuMnO3-type (space group P63cm). This structure can be described as dense oxygen-ion packing (ABCACB), with Mn3þ ions having coordination number of 5 (fivefold trigonal bipyramidal coordination), and Rþ3 having a coordination number of 7 (sevenfold mono-capped octahedral coordination) [1, 2]. The hexagonal manganites are both ferroelectric with a transition temperature to the non-ferroelectric state, TFE, of the order of 800–1000 K and antiferromagnetic with a transition temperature to the paramagnetic state, TN, of the order of 70–100 K. These hexagonal manganites thus belong to the class of the few single-phase compounds, in which two ferroic orders (more precisely one ferroic and one antiferroic) coexist [3, 4 and references therein]. YMnO3 has been the most studied compound of the series and has been proposed as a suitable candidate for *Corresponding author. Email: [email protected] Philosophical Magazine Letters ISSN 0950-0839 print/ISSN 1362-3036 online ß 2007 Taylor & Francis http://www.tandf.co.uk/journals DOI: 10.1080/09500830601137173

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metal–ferroelectric–semiconductor structures for non-volatile memories [5]. The coupling between the electric and magnetic order parameter has been demonstrated [6, 7] but is expected to be quite weak in thin films. However, combining different ReMnO3 compounds in superlattices can be an alternative strategy for enhancing the magnetoelectric response. Different effects can be expected from epitaxial multilayers: strain, which may contribute to an indirect coupling between the electrical and magnetic response, coupling effects between layers, or interface effects (disorder), which may lower the ferroelectric ordering temperature or contribute to the dielectric response by introducing internal barrier layer capacitance effects. If one can grow the whole range of ReMnO3 compounds as high quality films, there are then many possibilities of playing with these different effects. Since the ferroelectric polarization develops along the h001i directions, whereas the Mn magnetic moments order in the (a,b) plane in a 120 C triangular antiferromagnetic configuration, the films should be grown with the c-axis of the hexagonal cell perpendicular to the substrate plane. In this paper, we focus on the growth and structural characterization of YMnO3 and HoMnO3 films and superlattices. Epitaxial stabilization of DyMnO3 and TbMnO3 is also presented.

2. Experimental details Thin films of YMnO3 and ReMnO3 with Re ¼ Tb, Dy and Ho were grown on ZrO2(Y2O3) (denoted YSZ) cut in the (111) plane and on (111)Pt (150 nm)/TiO2 (20 nm)/SiO2 (300 nm)/p-type (001)Si substrates (TbMnO3 was only deposited on YSZ). (HoMnO3/YMnO3)15 superlattices were also grown on the same substrates. All samples were synthesized by metal organic chemical vapour deposition using a liquid delivery scheme. The set-up has been described previously [8]. Briefly, for the synthesis of a given ReMnO3 compound, the Re(tmhd)3 and Mn(tmhd)3 precursors are mixed in a solvent (monoglyme) and injected using a microvalve into an evaporator held at 250 C. Flash evaporation occurs and the vapours of reactive species are transported to the substrate. The heterostructures were grown in the temperature range 800–900 C depending on the considered rare earth manganite, in a total pressure of 0.66 kPa and oxygen partial pressure of 0.33 kPa. After deposition, the samples were in situ annealed in 1 bar of O2. Films of thickness 2–150 nm were prepared. The crystalline structure was studied by X-ray diffraction (/2 scans, !-scans, pole figures and -scans) using CuK radiation. X-ray reflectometry was used for films thickness calibration. The films morphology was studied by atomic force microscopy. Transmission electron microscopy was used on selected heterostructures.

3. Results and discussion Figure 1 shows /2 scans of films grown on (111)YSZ and (111)Pt. On YSZ, the films are oriented with the c-axis perpendicular to the substrate, which is desired

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since it is also the direction along which the electrical polarization develops in hexagonal ferroelectric ReMnO3. Secondary orientations 110 and 112 appear for thicker films (>100 nm). Rocking curves (!-scans) were performed on the 004 peak. The full width at half maximum measured on 25 nm thick films is typically of 0.08–0.20 for all compounds. Our values for YMnO3 are slightly larger than those reported for PLD films [9]. Pole figure and -scans show that the films are textured, with the following epitaxial relationship: (001)hex.RMnO3//(111)YSZ and  hex.RMnO3//h110i  YSZ. The choice for YSZ was motivated by the fact that h110i the (111) plane presents in-plane lattice parameters reasonably close to the in-plane parameters of the hexagonal phase of LuMnO3-type and excellent coincidence of p the oxygen atoms at the interface. The lattice mismatch is calculated as 1  ( 2a hex/ p 3aYSZ), with aYSZ ¼ 5.14 A˚. The in-plane lattice parameter, ahex, for hexagonal YMnO3 and HoMnO3 is about 6.14 A˚. The lattice mismatch is in the range 4

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to þ 4% for all rare earths (Re ¼ La to Lu) [10], using extrapolated data for the set of rare earths La to Dy, for which the stable phase at room temperature is of perovskite-type. For TbMnO3 and DyMnO3, the metastable hexagonal phase is obtained via epitaxial stabilization. The formation of a coherent interface allows to decrease the surface energy term and thus the non-equilibrium state (hexagonal) can be stabilized by the free energy gained [10, 11]. (111)Pt/TiO2/SiO2/p-type (001)Si was chosen as it is a conductive substrate, which will allow electrical measurements. There are few reports on the growth of YMnO3 on such a stack (or similar stack such as (111)Pt/ZrO2/SiO2/Si) [12–14]. Other reports on (111)Pt deal with stacks starting from (0001)Al2O3 or (111)MgO [15, 16]. As shown in figure 1, films grown on (111)Pt/TiO2/SiO2/Si are also oriented, with the c-axis perpendicular to the substrate plane. However, secondary orientations appear such as 110 and 112 in a larger amount than on YSZ as the film thickness increases. In the case of YMnO3, the 111 peak of the perovskite phase was also observed. It is remarkable that despite the absence of in-plane texture of the (111)Pt surface, the stabilization of DyMnO3 is achieved. The FWHM of the !-scans performed on the 004 peak of 25 nm thick films is typically of 1.7 to 2.0 for YMnO3 or HoMnO3 and increases up to 2.6 for DyMnO3. These values, which indicate a much larger spread of the orientation of the c-axis direction as the one found for epitaxial films on (111)YSZ, directly reflect the spread also found for the 111 axis of the Pt film (the FWHM of the 111 peak is 2.3 ). The out-of-plane lattice parameter, c, was calculated from the 008 peak and the substrate was used as a reference (444 YSZ peak and 004 Si peak). The films thickness for YMnO3 and HoMnO3 was determined from a calibration set of superlattices (see below). On (111)YSZ, a similar behaviour is found for both compounds: the c parameter value increases with films thickness, as shown in figure 2. The shortened values as compared to bulk indicate an in-plane bi-axial

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Figure 2. Out-of-plane lattice parameter c value as a function of film thickness for YMnO3 and HoMnO3 deposited on YSZ(111) and (111)Pt/TiO2/SiO2/(100)Si substrates. The dotted lines correspond to the bulk values. For YMnO3, the bulk parameter was taken from Ne´nert et al. [18].

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tensile strain, which is consistent with the in-plane lattice mismatch. For YMnO3, both on YSZ and Pt, thickest films exhibit a c lattice parameter of 11.454 A˚. This value is larger than the value of 11.407 A˚ reported for YMnO3 single crystals grown in a Bi2O3 flux [17]. However, as discussed by Palstra’s group [18, 19], YMnO3 single crystals grown by the floating zone technique or YMnO3 powders (from powder synthesis) are found to have a much larger c parameter (11.468 A˚ in Ne´nert et al. [18]). The reason for such a difference is not known. In the case of YMnO3 films grown by pulsed laser deposition on (111)YSZ [9], a lattice parameter of about 11.40 A˚ was reported for thick films; however, the c value for thin films was larger than the bulk value and was decreasing with increasing film thickness, which was quite surprising according to the in-plane lattice mismatch. For HoMnO3, the bulk c parameter (for powder) is reported to be of 11.412 A˚ [20]. The c value of our films does not level off as thickness increases, indicating that 150 nm thick films are still significantly strained. On (111)Pt/TiO2/SiO2/Si (001) substrates, the behaviour is very much different. Since the Pt crystallites exhibit no in-plane orientation (polycrystalline distribution as shown by -scans), the strain in the manganite films may be governed by thermal strain caused by the difference in the thermal expansion coefficients between Pt and the manganite. The evolution of the c parameter with film thickness is, however, difficult to predict. Our results show that YMnO3 and HoMnO3 behave differently and that strain engineering can be envisaged by combining both compounds in superlattices. The surface morphology of the different films was studied by AFM. On YSZ substrates, 25 nm films have a typical rms roughness of 0.2–1.5 nm, thus exhibiting a smooth surface (see figure 3). HoMnO3 and YMnO3 appear rougher than DyMnO3

Figure 3. AFM images of 25 nm films of (a) HoMnO3 and (b) DyMnO3 both deposited on (111)YSZ (shown with the same z-scale: 15 nm). The root mean square value of the roughness is of 1.5 and 0.4 nm for HoMnO3 and DyMnO3, respectively.

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Figure 4. X-ray diffraction 2/ scans for (YMnO3/HoMO3)15 superlattices deposited on (111)Pt/TiO2/SiO2/(100)Si substrates. The thickness of HoMnO3 was held constant (5.6 nm) and the thickness of YMnO3 was varied (1.1, 9.5 and 15.4 nm). The sequence starts with HoMnO3.

and TbMnO3. The bare Pt/TiO2/SiO2/Si stack exhibits an rms roughness of 1.3 nm. Films grown on this stack have an rms roughness of 3.0–6.0 nm. Superlattices of (YMnO3/HoMO3)15 were grown on both YSZ and (111)Pt/TiO2/ SiO2/Si substrates. Superlattices peaks are observed on both substrates. On YSZ, the in-plane texture is found to be similar as the one observed for thin films. Figure 4 shows the /2 scans for three superlattices grown on (111)Pt with a constant thickness of HoMnO3 (5.6 nm) and varying thickness of YMnO3 (1.1, 9.5 and 15.4 nm). From the relationship between the period of the stacking and the number of injections, the average growth rates were calculated to be 0.050 and 0.052 nm per injection for YMnO3 and HoMnO3, respectively. They were similar to those calculated for the stacks grown on YSZ. A high-resolution transmission electron microscopy image in cross-section is shown on figure 5 for a superlattice grown on (111)Pt/TiO2/SiO2/Si. This image shows a high crystalline quality along the growing direction and sharp interfaces. Larger scale images show some waving of the layers, which may be related to strain relaxation. A more detailed study on the structure at the interface and on the defects in these materials will be reported later. Electrical I(V) measurements performed on the superlattices (square top gold electrodes of area 70  70 mm2 were evaporated through a shadow mask) show typical leakage current densities of 102 A cm2 at 1 V. Preliminary magnetocapacitance measurements show a peak in the capacitance versus temperature curve at about 260 K, the amplitude of which decreases with applied magnetic field (it completely disappears under an applied field of 10 T). These preliminary results should be confirmed before any hypothesis can be drawn for the origin of such a peak near room temperature.

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Figure 5. HRTEM image of a (YMnO3/HoMO3)15 superlattice grown on (111)Pt/TiO2/ SiO2/(100)Si substrate. The sequence starts with YMnO3.

4. Conclusion Hexagonal YMnO3 and HoMnO3 as well as (YMnO3/HoMnO3)15 superlattices were grown on (111)ZrO2(Y2O3) and (111)Pt/TiO2/SiO2/(001)Si substrates. Epitaxial stabilization was used on the same substrates to grow DyMnO3 or TbMnO3 (which normally crystallize in an orthorhombic, perovskite-type, structure). The hexagonal phase is obtained with the [001] direction perpendicular to the substrate plane.

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The ability to grow hexagonal manganite multilayers on (111)Pt/TiO2/SiO2/(001)Si opens up the route to the study of possible magnetoelectric effects in systems, in which strain can be engineered via the choice of the rare earth manganite and of the interlayer thicknesses.

Acknowledgements

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This work has been performed in the frame of the European Network of Excellence FAME ‘‘Functionalized Advanced Materials and Engineering: Hybrids and Ceramics’’ (FP6-500159-1). C.D. and G.H. thank B. Kundys and Ch. Simon for access to their PPMS equipment at CRISMAT (Caen) and for assistance in the measurements.

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