Journal of Alloys and Compounds xxx (2013) xxx–xxx

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Journal of Alloys and Compounds

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Crystal structure and surface morphology of magnetron sputteringdeposited hexagonal and perovskite-like YbMnO3 thin films

0925-8388/$ - see front matter � 2013 Published by Elsevier B.V.http://dx.doi.org/10.1016/j.jallcom.2013.03.156

⇑ Corresponding author. Tel.: +7 917 577 46 94.E-mail address: andreevn.misa@gmail.com (N.V. Andreev).

Please cite this article in press as: N.V. Andreev et al., J. Alloys Comp. (2013), http://dx.doi.org/10.1016/j.jallcom.2013.03.156

N.V. Andreev a,⇑, T.A. Sviridova a, V.I. Chichkov a, A.P. Volodin b, C. Van Haesendonck b, Ya.M. Mukovskii a

a National Science and Technology University ‘‘MISiS’’, Leninskii prosp., 4, Moscow 119049, Russiab KU Leuven, Lab. Solid State Phys & Magnetism, Celestijnenlaan 200 D, BE-3001 Leuven, Belgium

a r t i c l e i n f o

Article history:Available online xxxx

Keywords:A. ManganitesA. Thin filmsA. MultiferroicsB. Nanofabrications C. Crystal structureC. Electric polarizationD. Atomic force microscopyD. X-ray diffraction

a b s t r a c t

Thin YbMnO3 films with thickness 50–100 nm were grown by RF-magnetron sputtering on NdGaO3

(001), SrTiO3(001), SrTiO3(110), LaAlO3(001) and on buffer layered Pt(111)/SrTiO3(111) substrates.X-ray analysis reveals that the platinum buffer layer grown on a single crystalline SrTiO3(111) substrateas well as the hexagonal YbMnO3 film with c-axis normal to the substrate plane grow epitaxially. Theorthorhombic (perovskite-like) modification of YbMnO3, which otherwise forms in a bulk hexagonalstructure, has been epitaxially stabilized on NdGaO3, SrTiO3 and LaAlO3 single crystal substrates. Thesurface topography of the films was studied by atomic force microscopy. The surface of the hexagonalfilms reveals spiral-shaped growth terraces with a step height of half the c-lattice parameter. The perov-skite-like films have a smooth surface with small 50–100 nm islands. Piezoresponse force microscopywas used to demonstrate the ferroelectric behavior of the hexagonal films. Thin films of a compound witha structure different from the bulk one can also show different physical properties. In the case of multif-erroics it can lead to enhancing of magnetoelectric coupling.

� 2013 Published by Elsevier B.V.

1. Introduction

The structure of the rare-earth manganites RMnO3 depends onthe size of the rare-earth ion so they have either an orthorhombicperovskite-like structure (space group Pbnm) in the case of a largeionic radius (R = La–Dy) or a noncentrosymmetric hexagonal struc-ture (space group P63cm) in the case of a small ionic radius (R = Ho–Lu, Y, Sc) [1,2]. The difference between Gibbs free energies for theformation of orthorhombic and hexagonal phases for compoundsnear the structural phase boundary is small for the compounds withthe mentioned ions. This allows controlling the crystal structure byvariation of the external synthesis parameters. Manganites thathave hexagonal structure under normal conditions can be synthe-sized as metastable orthorhombic perovskites using either highpressure [3], low temperature soft-chemistry procedure [4] or epi-taxial thin film growth [5–7].

The YbMnO3 compound studied belongs to so named multifer-roic where electrical and magnetic subsystems are tightly linkedone with another, so applying of an external magnetic field canchange its electrical properties and vice versa. Elaborating of suchmaterial with high coupling between electrical and magnetic sub-systems can give basis for new electronic devices design.

Under normal conditions the YbMnO3 compound belongs to thehexagonal rare-earth manganite family and is multiferroic, i.e. a

material that exhibits simultaneously electric polarization andmagnetic ordering. Its ferroelectric Curie temperature is above1350 K [8]. The spontaneous electrical polarization exists belowthis temperature along the hexagonal c-axis. The temperature ofthe antiferromagnetic ordering for Mn ions in the ab-plane is81–90 K [9–11]. It was found [12] that for orthorhombic YbMnO3

bulk samples prepared under high pressure the Mn lattice has anantiferromagnetic transition. Below 43 K an incommensurate anti-ferromagnetic structure appears, followed by a lock-in transition toan E-type antiferromagnetic ordering at lower temperatures.Theoretical studies [13,14] suggest that the E-type antiferromag-netic structure induces a ferroelectric state. However, due to diffi-culties in synthesis of this metastable phase dielectric andmagnetoelectric characterization of manganites with the E-typestructure could not be performed and the mentioned hypothesiscould not be confirmed. Magnetodielectric coupling was observedat the temperature of the antiferromagnetic transition in thinorthorhombic YbMnO3 films deposited on SrTiO3 substrates [7].

Here we describe the synthesis, crystal structure and surfacemorphology characterization of epitaxial thin films of YbMnO3

prepared by magnetron sputtering. As already mentioned theproperties of YbMnO3 critically depend on the crystal structure.One also expects that similar to other orthorhombic manganitesits properties have strong anisotropy in different crystallographicdirections. Therefore we used different substrates in order toobtain films with different crystal symmetry (hexagonal andepitaxially stabilized orthorhombic) and texture of the films.

Fig. 1. (a) h–2h XRD scan of a h-YbMO(0 01) film on the Pt(111)||STO(111)substrate. XRD /-scans of the (b) Pt(111) and STO(111), (c) h-YbMO(111), (d) h-YbMO(102).

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2. Experimental

Single-phase YbMnO3 targets were prepared by solid-state synthesis using stoi-chiometric quantities of Yb2O3 and MnO2 powders as the starting materials. Magne-tron sputtering was used to deposit the YbMnO3 thin films. For the growth ofhexagonal YbMnO3 (h-YbMO) films (111) oriented SrTiO3 (STO) single-crystal sub-strates were used. A 10-nm-thick platinum functional sublayer was deposited onthe STO with cathode sputtering at an Ar pressure of 10 mTorr and 400 �C. Next,h-YbMO thin films were grown from the prepared targets by RF-magnetron sputter-ing with the ‘‘facing-target’’ scheme [15] at a temperature of 700 �C. A mixture of Arand O2 with pressure 1–2 mTorr was used as the working atmosphere. After depo-sition the films were post-annealed for 20 min at the same oxygen partial pressureand at 500 �C.

The orthorhombic (perovskite-like) modification of YbMnO3 (o-YbMO) was epi-taxially stabilized on STO, LaAlO3 (LAO) and NdGaO3 (NGO) single-crystal sub-strates. Substrate temperature and working atmosphere pressure were the sameas for the hexagonal films. Post annealing was not used. The thickness of the ob-tained YbMO films was 50–100 nm.

The structure of the grown thin films was studied by X-ray diffraction (RigakuUltima, using Cu Ka and Co Ka radiation). The surface topography of the films wasanalyzed by atomic force microscopy (AFM). Piezoresponse force microscopy (PFM,Dimension 3100 from ‘‘Bruker’’) was used for mapping the ferroelectric propertiesof the films.

3. Results and discussion

For the films of hexagonal ytterbium manganite epitaxialgrowth with the c-axis normal to the substrate planes was ob-served. Fig. 1a shows the h–2h scan of a h-YbMO(001)||Pt||STOsample. The peaks correspond to the (111) reflections of the sub-strate and the platinum sublayer, as well as to the multiple reflec-tions from the (001) plane of the hexagonal ytterbium manganiteh-YbMO. The out-of-plane d(001), i.e. the c lattice parameter ofYbMO is 11.56 Å, somewhat larger than the value for bulk material,i.e. cbulk = 11.35 Å [16]. The /-scan measurements confirm the epi-taxial growth of the films. Fig. 1b shows the /-scan of the (002)peaks for platinum. There are six platinum peaks and three of thesereflections overlap with the strong reflections from the substrate,separated from each other by 120�. This allows us to conclude thatthe platinum grows epitaxially on the STO substrate with the(111) plane parallel to the substrate plane. The epitaxial relation-ships with the substrate are [11�2]Pt(111)||[11�2]STO(111) and[�1 �12]Pt(111)||[11�2]STO(111). Thus the film of the hexagonalYbMO phase grows epitaxially on the platinum. Fig. 1c displayssix distinct (111) h-YbMO peaks with 60� separation. The samepattern, with an offset of 30�, is observed for the (104) h-YbMOscan (see Fig. 1d). Comparing this pattern with the distribution ofpeaks for platinum we find that the hexagonal YbMO phase growsepitaxially on platinum and the epitaxial relationships are [100]YbMO(001)||[11�2]Pt(111) and [100]YbMO(001)||[�1 �12]Pt(111).

Fig. 2a and b presents the AFM images of the sputtered hexag-onal YbMO film on STO(111) substrate with a platinum sublayer.The surface of the film, which has a root-mean-square (rms) rough-ness of 0.5–2 nm, reveals spiral-shaped growth terraces. Thegrowth spirals originate from screw dislocations (left-handed andright-handed), which are randomly distributed over the surfaceof the sample. The value of the steps between terraces approxi-mately corresponds to half the c-lattice parameter, allowing us toconclude that the screw dislocation Burgers vector component per-pendicular to the surface is equal to c/2. The mechanism of filmgrowth involves addition of adatoms to the growth steps that al-ways exist along the edge of a spiral-shaped growth terrace. Thisis consistent with a classical spiral growth mechanism for crystalsgrown under conditions of low supersaturation [17,18].

Piezoresponse force microscopy (PFM) [19] was used to demon-strate the ferroelectric behavior of the hexagonal films. The exper-iment was performed as follows. First a small 1 lm � 1 lm squarewas polarized during a scan with a �10 V DC voltage applied to thePFM tip. Next, an AC-modulated voltage was applied to the PFM tipand the PFM response was harmonically detected during scanning

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over a larger area. The PFM phase image of a h-YbMO film (Fig. 2c)reveals clear contrast over the polarized square. This results fromthe different amplitudes of the PFM response for the electricallypolarized and unpolarized domains and confirms the piezoelectricbehavior of the film.

The bulk orthorhombic perovskite-like structure of YbMnO3

(space group Pbnm) has lattice parameters a = 5.2208 Å,b = 5.8033 Å, and c = 7.3053 Å [20]. The microstructure of thin filmsis sensitive to lattice mismatch between film and substrate. There-fore we used a set of lattice matched crystals such as cubic STO(a = 3.90 Å), cubic LAO (a = 3.79 Å) and orthorhombic NGO (Pbnm;a = 5.431 Å, b = 5.499 Å and c = 7.710 Å [21]) as substrates for theo-YbMO thin film growth.

Fig. 3 shows h–2h scans for YbMO films deposited on NGO(001),STO(001), STO(110) and LAO(001) substrates. In all cases, inaddition to the reflections from the substrates, we observe thereflections associated with the orthorhombic ytterbium manganite

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Fig. 2. AFM images of the sputtered h-YbMO(001) film grown on Pt(111)||STO(111) substrate: (a) 1 lm2 area. (b) 0.25 lm2 area. (c) PFM image of h-YbMO(001)||Pt||STOafter poling an area of 1 lm � 1 lm by scanning with a PFM tip voltage of �10 V.

Fig. 3. h–2h XRD scans of o-YbMO films on NGO(001), STO(001), STO(1 10) andLAO(001) substrates.

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phase. The films are textured, with the out-of-plane orientationdifferent for each substrate. Comparison of the h–2h scans andthe XRD data for the o-YbMO phase suggests that the films are tex-tured in the following ways: o-YbMO(001)||NGO(001); oYbMO(001)||STO(001) and o-YbMO(110)||STO(001); o-YbMO(112)||S-TO(110) and o-YbMO(010)||STO(110); o-YbMO(110)||LAO(001).

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To determine the exact orientation of the in-plane lattice vec-tors of film and substrate, pole figures were constructed. Fig. 4ashows the pole figure corresponding to the (111) reflections ofthe o-YbMO film on the NGO(001) substrate. Four o-YbMO(111)peaks are observed at /-positions 48�/136�/229�/315� andw = 62�. Comparison with the NGO substrate (111) reflections at/-positions 46.75�/136.75�/227.5�/316.0� (not shown) indicatesthat the o-YbMO film, which was grown on the rectangular NGOlattice (aNGO = 5.431 Å � bNGO = 5.499 Å), accommodates to the filmin-plane parameters a and b. The corresponding epitaxial relation-ship [100] o-YbMO(001)||[100]NGO(001) (Fig. 5a). The value ofthe mismatch between the film and the substrate has the oppositesign in perpendicular directions, so along the [100] o-YbMO it isequal f[100] = 100%�(d[100]substrate–d[100]film)/d[100]film) = 3.96%, andthere is the tension force that acts to the film, and correspondinglyalong the [010] o-YbMO direction f[010] = �5.21% – compressionforce acting to the film.

The pole figure of the (111) o-YbMO reflections of the o-Yb-MO||STO(001) is shown in Fig. 4b. The presence of two groups ofpeaks at w = 28� and w = 62� confirms our hypothesis that the filmhas two different out-of-plane orientations: o-YbMO(001)||S-TO(001) (peaks at w = 62�) and o-YbMO(110)||STO(001) (peaksat w = 28�). This hypothesis is not obvious due to the coincidenceof the (110)YbMO and (001)STO reflections on the h–2h scan.The first group of peaks at w = 62� can be considered as four 90�shifted sets of pair of peaks separated by 4.6�. A similar situationwas observed for the case of orthorhombic YMnO3(001)||STO(001)films [22]. This is consistent with the coexistence of two types ofdomains with the same out-of-plane orientation and rotated inplane by 90�. The smaller mismatch between the film and the sub-strate lattices is due to fact that the lattice of the film is rotated by45� with respect to the substrate (001) plane, i.e. YbMO grows onthe aSTO

ffiffiffi2p

= 3.9ffiffiffi2p

Å � aSTO

ffiffiffi2p

= 3.9ffiffiffi2p

Å square STO lattice. Theformation of the two in-plane 90� rotated domains is expecteddue to the fourfold symmetry in the plane of the substrate. The cor-responding epitaxial relationships are [100]o-YbMO(001)||[110]STO(001) and [010] o-YbMO(001)||[110]STO(001) (Fig. 5b).

Another possibility of surface energy minimization is providedby film growth where the (110) film plane is parallel to the(001) STO plane. In this plane the film has a rectangular lattice

with sides cYbMO = 7.305 Å �ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffia2

YbMO þ b2YbMO

q= 7.806 Å, which is

placed within the spacings 2a and 2b of STO(001). This orientationcorresponds to the peaks at w = 28�. The epitaxial relationships are[001]YbMO(110)||[100]STO(001) and [1�10]YbMO(110)||[100]S-TO(001) (Fig. 5b).

In the case of the first out-of-plane orientation lattice mismatchalong [100] o-YbMO is tensile (f[100] = 5.64%), and compressive(f[010] = �4.91%) along [010] o-YbMO. In the case of the secondout-of-plane orientation. Lattice mismatch also has a different signfor perpendicular directions: f[001] = 6.78% and f[110] = �0.05%.

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Fig. 4. XRD pole figures of the (111) o-YbMO reflections of the (a) o-YbMO||NGO(001), (b) o-YbMO||STO(001), (c) o-YbMO||LAO(001) and (d) o-YbMO||STO(110) films.

Fig. 6. AFM images and topography line profiles of o-YbMO(001) films grown on (aand b) NGO(001), (c) STO(001) and (d) STO(110) substrates.

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The pole figure of the (111) o-YbMO reflections of the o-Yb-MO||LAO (001) film is shown in Fig. 4c. LAO also has a cubicstructure with a square lattice in the (001) plane. The group ofpeaks at w = 28� corresponds to a case which is similar to the caseof the o-YbMO(110)||STO(001) orientation for the STO substrate.

The rectangular film lattice cYbMO �ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffia2

YbMO þ b2YbMO

qlies on the

square substrate lattice 2aLAO = 2 � 3.79 Å � 2aLAO = 2 � 3.79 Å,and the corresponding epitaxial relationships are [001]o-Yb-MO(110)||[100]LAO(001) and [1�10]o-YbMO(110)||[100]LAO(001) (Fig. 5c). Similar to the previous cases, the lattice mismatchhas opposite sign for perpendicular directions: f[001] = 3.76% andf[110] = �2.87%. In the case of films on the LAO substrate the secondout-of-plane orientation o-YbMO(001)||LAO(001) is absent. Inter-atomic distance along the [110] LAO is more than a and b latticeparameters of o-YbMO, so if the film was grown in the orientation[100]o-YbMO(001)||[110]LAO(001) (as in the case of the (001)STO substrate), then along [100]o-YbMO and [010]o-YbMO direc-tions the film would be stretched, that is less convenient, than anopposite sign of the lattice mismatch for perpendicular directions.

A more complex situation occurs in the case of the STO(110)substrate. Fig. 4d shows the pole figure corresponding to the(111) o-YbMO reflections of the o-YbMO||STO(110) film. Weobserve three types of peaks marked as A, B and C. The A peaksindicate out-of-plane orientation o-YbMO(010)||STO(110), and

Fig. 5. Schematic illustrations of the epitaxial relationships for (a) o-YbMO||NGO(001)

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the presence of four peaks is the result of the orthorhombic sym-metry of YbMO. The absence of additional in-plane domains with(100) out-of-plane texture is confirmed by the absence ofadditional peaks in the pole figure. The corresponding epitaxialrelationship is [001]YbMO(010)||[001]STO(110). In this case thelattice mismatch is tensile along [001]o-YbMO (f[001] = 6.78%)and compressive along [010]o-YbMO (f[010] = �4.91%). The groupsof peaks B and C indicate out-of-plane orientation o-Yb-MO(112)||STO(110). For peaks B the epitaxial relationship is[1�10]YbMO(112)||[001]STO(110), and for peaks C this is [1�10]Yb-MO(112)||[001]STO(110) (Fig. 5d).

, (b) o-YbMO||STO(001), (c) o-YbMO||LAO(001) and (d) o-YbMO||STO(110) films.

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The AFM images and section analysis of a sputtered o-YbMOfilms grown on NGO (001) (a and b), STO(001) (c) and STO(110)(d) substrates are shown in Fig. 6. The orthorhombic films onSTO and LAO substrates display smooth surfaces with rms rough-ness varying from 0.5–1 nm (for STO(001) and LAO(001)) to1–2 nm (for STO(110)) with small 50–100 nm island-like grains.Films on NGO substrates have less homogeneous surfaces withperiodic lens-like inclusions (3000 nm � 300 nm and heightaround 40 nm). The inclusions are situated perpendicularly oneto another along the [100] and [010] substrate directions. Theseinclusions probably have out-of-plane YbMO(110)||NGO(001)orientation that is not revealed by X-ray analysis, but is allowedin the considered system.

4. Conclusion

We have grown by magnetron sputtering films of hexagonaland epitaxially stabilized orthorhombic perovskite-like YbMnO3.The surface of the hexagonal YbMO films reveals spiral-shapedgrowth terraces and the local PFM response confirms the ferroelec-tric behavior of the films. Epitaxial orthorhombic YbMO films havedifferent textures depending on the type and orientation of thesubstrates and have a smooth surface. The surface morphology isdetermined by the film texture. Detailed study of physical proper-ties of the samples obtained is in progress.

Acknowledgments

This work has been supported by the Flemish Concerted Action(GOA) research program and the RFBR Grant No. 12-02-00717_a.We thank A.E. Pestun for preparing the targets for magnetronspattering.

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