IOP PUBLISHING NANOTECHNOLOGY

Nanotechnology 21 (2010) 075705 (11pp) doi:10.1088/0957-4484/21/7/075705

Structure and defect characterization ofmultiferroic ReMnO3 films andmultilayers by TEMNeerushana Jehanathan1, Oleg Lebedev1, Isabelle Gelard2,Catherine Dubourdieu2 and Gustaaf Van Tendeloo1

1 Electron Microscopy for Materials Research (EMAT), University of Antwerp,Groenenborgerlaan 171, B2020 Antwerpen, Belgium2 Laboratoire des Materiaux et du Genie Physique (LMGP), CNRS, INP Grenoble-INP,3 parvis Louis Neel, BP 257, 38016 Grenoble Cedex 1, France

E-mail: neerushana.jehanathan@ua.ac.be

Received 19 October 2009, in final form 10 December 2009Published 18 January 2010Online at stacks.iop.org/Nano/21/075705

AbstractEpitaxial rare earth manganite thin films (ReMnO3; Re = Tb, Ho, Er, and Y) and multilayerswere grown by liquid injection metal organic chemical vapor deposition (MOCVD) onYSZ(111) and the same systems were grown c-oriented on Pt(111) buffered Si substrates. Theyhave been structurally investigated by electron diffraction (ED) and high resolutiontransmission electron microscopy (HRTEM). Nanodomains of secondary orientation areobserved in the hexagonal YMnO3 films. They are related to a YSZ(111) and Pt(111)misorientation. The epitaxial film thickness has an influence on the defect formation. TbO2 andEr2O3 inclusions are observed in the TbMnO3 and ErMnO3 films respectively. The structureand orientation of these inclusions are correlated to the resembling symmetry and structure offilm and substrate. The type of defect formed in the YMnO3/HoMnO3 and YMnO3/ErMnO3

multilayers is also influenced by the type of substrate they are grown on. In our work, atomicgrowth models for the interface between the film/substrate are proposed and verified bycomparison with observed and computer simulated images.

1. Introduction

There is revived interest in multiferroic materials sincethe observations of the coexistence of ferromagnetic andferroelectric polarizations and of the coupling betweenthem [1, 2]. In the past few years they have attracted muchattention because of their fascinating properties and theirtechnological potential. Multiferroic materials have been usedin a vast number of devices ranging from large electricaltransformers to miniature devices used in integrated circuits.Besides, these materials are more likely to offer a range ofnovel devices and functionality because of their size-dependentphysical and chemical properties [3, 4].

Among the multiferroic materials, great interest andprogress has been made in synthesizing and characterizinghexagonal rare earth (Re) manganites ReMnO3 (withRe = Ho–Lu, Y) in bulk as well as in thin film andmultilayer form. Molecular beam epitaxy (MBE), pulsed

laser deposition (PLD), sputtering, chemical solution, andmetal organic chemical vapor deposition (MOCVD) areamong the techniques used for the synthesis of hexagonalmanganite heterostructures [5–16]. Recently, an approachcalled ‘epitaxial stabilization’ (ES) [5, 8, 10, 11] has beenused as a tool for the synthesis—in the form of epitaxialfilms—of otherwise unstable phases (as bulk compounds)such as hexagonal DyMnO3, TbMnO3, or GdMnO3 films.Epitaxial thin films often grow with structural characteristicsdifferent from their bulk counterparts because of the epitaxialrequirement.

Multiferroic multilayers open avenues to create materialsthat display both the properties of the parent compoundsand their coupling. Very often the phase composition andphysical properties of thin films differ from those of theircorresponding bulk material. Because of the interface betweendifferent materials (substrate–film and/or within multilayers),interfacial effects will ultimately influence the properties of

0957-4484/10/075705+11$30.00 © 2010 IOP Publishing Ltd Printed in the UK1

Nanotechnology 21 (2010) 075705 N Jehanathan et al

Table 1. Crystallographic information for ReMnO3 (Re = Y, Ho, Tb, and Er).

Lattice parameters (nm)

Compound Structure Space group a b c References

YMnO3 Orthorhombic (highpressure or ESform)

Pnma (62) 0.584 44 0.735 79 0.526 16 [30]

Hexagonal (lowtemp. form)

P63cm (185) 0.614 83 0.614 83 1.144 3 [31]

HoMnO3 Orthorhombic (highpressure or ES form)

Pbam (55) 0.726 43 0.847 68 0.567 00 [32]

Hexagonal (high andlow temp. form)

P63cm (185) 0.614 13 0.614 13 1.141 2 [33]

TbMnO3 Orthorhombic (highand low temp. form)

Pbnm (62) 0.530 90 0.581 18 0.738 60 [34]

Hexagonal (ES form) P63cm (185) 0.617 50 0.617 50 0.112 79 [10]ErMnO3 Hexagonal (bulk) P63cm (185) 0.611 21 0.611 21 0.114 02 [35]

the film. This is, for instance, the case for the strainmediated indirect magnetoelectric coupling [17, 18] in thinfilm heterostructures. The combination of magnetic andelectric dipolar orders in multiferroic materials, intimatelydepends on the coupling between the building blocks of thethree-dimensional crystal structure. In the case of hexagonalReMnO3 thin films, the crystal orientation is also importantbecause the magnetic ordering takes place in the ab planeand the electrical polarization is oriented along the c-axis,〈0001〉 [19–21].

The chemistry and structure of these thin film systemstherefore have direct impact on their electrical and magneticproperties. Also, growth defects in the films may either have afavorable or an unfavorable effect for the intended applications.Hence, understanding the structure and defects of these thinfilms and of their interfaces becomes of critical importance forthe subsequent performance and reliability of devices based onthese materials.

In this study, transmission electron microscopy (TEM),HRTEM, and electron diffraction (ED) are used to characterizethe structure and microstructure of hexagonal YMnO3,HoMnO3, ErMnO3, and TbMnO3 films and interfaces. Detailson the physical properties of these films have been describedelsewhere [6, 22].

2. Structural considerations

ReMnO3 are known to exist in two kinds of structuralmodifications. Depending on the size of the Re ion, ReMnO3

generally forms either in an orthorhombic (Re = La–Dy) ora hexagonal structure (Re = Ho–Lu and Y). However, it hasalso been shown in the literature that the synthesis method andpost-processing techniques may induce a phase transition inthe ReMnO3 materials. Bosak et al [7, 8], Dubourdieu et al[21], Choi et al [10], and Lee et al [11] were able to growReMnO3 such as TbMnO3, DyMnO3, GdMnO3, EuMnO3, andSmMnO3, in the hexagonal form, which normally crystallizesin a stable perovskite phase. Perovskite phase epitaxiallystabilized films of YMnO3 and HoMnO3, known in bulk formonly as high pressure phases, could be synthesized by MOCVDat much lower pressures [5].

Epitaxial hexagonal TbMnO3 and HoMnO3 thin filmshave been synthesized on Pt(111)/Al2O3(0001) and YSZ(111)by PLD and MOCVD methods [11, 22–24]. Epitaxialorthorhombic TbMnO3 and YMnO3 films have been grown onSrTiO3 and LaAlO3 substrates by PLD [25, 26]. OrthorhombicHoMnO3 crystals prepared by mild hydrothermal treatmenthave also been reported [14]. Ye et al reported thetransformation of hexagonal ErMnO3 powders, synthesized bysolid-state reaction methods, into the orthorhombic phase byhigh pressure sintering [27]. Others [16, 20, 21, 28] havereported growing hexagonal YMnO3 films on YSZ, Pt/SrTiO3,and Pt/Ti/SiO2/Si substrates by PLD or MOCVD respectively.Suzuki et al reported that the hexagonal phase they obtainedfor YMnO3 thin films on Pt(111)/TiOx /SiO2/Si was due tothe heat treatment in vacuum [13]. Choi et al on the otherhand claimed observing a single hexagonal phase for YMnO3

films when annealing in vacuum, but they obtained secondaryorthorhombic phases along with a primary hexagonal phasewhen annealing in ambient oxygen [29]. Their films weredeposited on Si(100) substrates by MOCVD. Dho et al alsoobserved a similar competition between orthorhombic andhexagonal phases for films grown on SrTiO3(111) substratesby PLD [20].

We present here the results on the structure and defectcharacterization of hexagonal YMnO3, TbMnO3, ErMnO3,and HoMnO3 films and multilayers on YSZ(111) as well as onPt(111). A literature review shows very little detailed structuraland microstructural characterization on these films. Moreover,most TEM investigations were carried out on YMnO3 epitaxialfilms.

The crystallographic information for YMnO3, TbMnO3,ErMnO3, and HoMnO3 bulk and/or thin film compounds issummarized in table 1.

3. Experimental details

YMnO3, HoMnO3, TbMnO3, and ErMnO3 films as wellas (YMnO3/HoMnO3)n and (YMnO3/ErMnO3)n multilayerswere grown on two different substrates: (111) oriented ZrO2

(Y2O3) or (111) Pt (150 nm)/TiO2(20 nm)/SiO2 (300 nm)/p-type Si(001). They will be denoted as YSZ(111) and Pt(111)

2

Nanotechnology 21 (2010) 075705 N Jehanathan et al

Figure 1. TEM images of (a) HoMnO3, (b) YMnO3, (c) TbMnO3, and (d) ErMnO3 films grown on YSZ(111). The corresponding EDpatterns show a superposition of the film and YSZ substrate.

respectively. Liquid injection MOCVD was used to synthesizethese films. The technique and experimental set-up have beendescribed elsewhere [21, 36, 37]. Mn(tmhd)3 and Re(tmhd)3

precursors were mixed in an organic solvent (monoglyme)and injected as a single mixture with a micro-valve into anevaporator heated at 250 ◦C. The vapors of the reactivespecies were transported with Ar towards the heated surfaceof the substrate, at a total pressure of 0.66 kPa and anoxygen partial pressure of 0.33 kPa. The multilayers wereprepared by using sequentially two injectors, each fed with asingle cocktail of the Re and Mn precursors. The thicknessof the films ranges between ∼10 nm and ∼500 nm. Thetotal thickness of the YMnO3/HoMnO3 and YMnO3/ErMnO3

multilayers is ∼550 nm and ∼60 nm respectively. The growthtemperature was 800 ◦C for YMnO3 and 850 ◦C for HoMnO3,ErMnO3, and TbMnO3 films. The (YMnO3/HoMnO3)n and(YMnO3/ErMnO3)n multilayers were grown at 825 and 850 ◦Crespectively. After deposition, all heterostructures were insitu annealed at a growth temperature in 1 atmosphere of di-oxygen for 15 min. The effect of the growth temperature onthe rocking curve width and surface morphology is described

elsewhere [6]. No significant temperature dependence wasfound for the hexagonal stabilized phase of TbMnO3 in therange 825–925 ◦C. For YMnO3 and HoMnO3 films, theoptimum growth temperature was found to be around 850 ◦C.Cross section samples for TEM were prepared by mechanicalgrinding to a thickness of about 20 μm followed by ion beammilling. The ion milling was done by 4 keV Ar+ ions undergrazing incidence with respect to the surface. The cross sectionsamples were cut parallel to a cubic plane of the substrateperpendicular to the contact plane. TEM investigations werecarried out using a JEOL 4000EX microscope operated at400 kV. Image simulations were made with the MacTempassoftware.

4. Results

4.1. Thin films

Figure 1 shows the TEM images of the ReMnO3 (Re = Ho,Y, Tb, and Er) films grown on a YSZ(111) substrate. Theoriginal thickness of the HoMnO3 film was estimated to be

3

Nanotechnology 21 (2010) 075705 N Jehanathan et al

Figure 2. HRTEM images showing the interface regions of(a) HoMnO3, (b) TbMnO3, and (c) ErMnO3 films on YSZ(111); theinset in (a) shows the simulation performed along the [1100] zoneaxis with t = 5 nm and � f = −60 nm. Although (a)–(c) films havethe same hexagonal structure, due to the different thickness anddefocus values of these samples they show a different contrast.Atomic surface steps at the interface region in sample (c) are possiblythe origin of the irregular contrast of the film–substrate interface.

∼12 nm. The ‘wave-like’ surface observed in figure 1(a) isattributed to TEM sample preparation. Unlike the HoMnO3

film, the thicker YMnO3 film (∼100 nm) (figure 1(b)) appearsto have a columnar structure. The TbMnO3 film in figure 1(c)has a uniform thickness of about 135 nm and exhibits a uniformcontrast. The ErMnO3 film shows the presence of Moirefringes in certain regions of the film (figure 1(d)); the thicknessof the film is about 75 nm. A relatively flat interface is observedfor all these manganite films on YSZ(111). The correspondingED patterns all show a superposition of reflections resultingfrom the ReMnO3 films and from the YSZ. The brighterreflections correspond to YSZ (cubic, Fm3m (225); a =0.5162 nm [38]). The weaker reflections are attributed to theReMnO3 film and correspond to a hexagonal structure withP63cm space group (185).

The ED patterns evidence a heteroepitaxial growth of allfilms with the substrate. All the films have the followingepitaxial relationship: [111]YSZ ‖ [0001]ReMnO3; (011)YSZ ‖(1100)ReMnO3 .

Figure 3. HRTEM image showing the interface region between thefirst and second c-axis orientation of YMnO3 and betweenYSZ(111).

HRTEM images show the interfacial structure ofHoMnO3, TbMnO3, and ErMnO3 films on YSZ(111)(figure 2). The interface between the YSZ(111) substrate andthese films appears to be relatively flat and well defined. Thec-axis oriented ReMnO3 films are perpendicular to the in-planedirection of YSZ. Image simulations (inset in figure 2(a)) basedon the HoMnO3/YSZ(111) structure are in good agreementwith the experimental image.

The YMnO3/YSZ(111) film shows a different aspect(figure 3). Although the substrate is well defined andatomically flat, the YMnO3 film reveals that, aside from themain epitaxial orientation, a secondary orientation with arotation of the c-axis results in different domains. From theED pattern shown in figure 1(b), the epitaxial relationships ofthe secondary orientation are:

[111]YSZ ‖ [0001]YMnO3; (011)YSZ ‖ (1100)YMnO3

[111]YSZ ‖ [1121]YMnO3; (011)YSZ ‖ (1100)YMnO3.

These secondary oriented nanodomains (length scaleof ∼10–15 nm) have been shown to be at the originof a significant ferroelectric contribution when measuredby the second harmonic generation (SHG) technique [39].They behave as ferroelectric nanodomains with a preferredcrystallographic orientation of their spontaneous polarization.The surrounding epitaxial c-oriented film is in a ferroelectricsingle-domain state. Such secondary oriented nanodomainsmay have an impact on the net polarization in the films.Moreover, while the linear magnetoelectric effect is not presentin bulk hexagonal manganites, magnetoelectric coupling hasbeen shown to occur between domain walls of ferroelectric andantiferromagnetic domains in YMnO3 [40]. The presence offerroelectric domains with a polarization oriented differentlyfrom the matrix could therefore have a significant impact onthe magnetoelectric effect in the films.

Also secondary inclusions may appear in some of thefilms. Figure 4(a) shows an inclusion in a TbMnO3 film

4

Nanotechnology 21 (2010) 075705 N Jehanathan et al

Figure 4. HRTEM images of an inclusion in (a) TbMnO3 and(b) ErMnO3 films on YSZ(111). The insets in (a) and (b) show theFT of the inclusions. The white dotted lines shown in the imagesenclose the region of the inclusions.

on YSZ(111). The in-plane width of this inclusion is about15 nm; other inclusions in these films were of the same orderof magnitude. The inclusions were found exclusively insidethe film and not at the film/substrate interface. The lowerinset shows the FT (Fourier transform) of the inclusion. Itcan be matched to [011] TbO2 (a = 0.5213 nm; space groupFm3m (225) [41]). Image simulations based on the TbO2

structure show a good agreement with the experimental image(not shown here).

The FT of the inclusions in the ErMnO3 film (figure 4(b))are identified as [011] Er2O3 (a = 0.5160 nm; space groupFm3m (225) [42]).

Figure 5 shows the low magnification TEM images of(a) HoMnO3 and (c) YMnO3 films grown on Pt(111). TheHRTEM image (figure 5(b)) of HoMnO3/Pt(111) and itscorresponding ED pattern are from the region marked by awhite box in figure 5(a). The thickness of the HoMnO3 andYMnO3 films is about 20 nm and 60 nm respectively. The‘wave-like’ surface of the YMnO3 film may be attributed tothe sample preparation. Step drops along Pt(111) (markedby white arrows) are observed in both films. The brighterreflections on the ED pattern are attributed to Pt (cubic, Fm3m(225); a = 0.3970 nm [43]), the other reflections belong tothe HoMnO3 film. This film also has a hexagonal structurewith P63cm space group (185). The epitaxial relationship isas follows:

[0001]HoMnO3 ‖ [111]Pt; (1100)HoMnO3 ‖ (011)Pt.

Contrast variations are observed inside the Pt layer as wellas in the HoMnO3 film. These variations appear to occur atthe boundaries between different Pt(111) crystallites. TEM

Figure 5. Low magnification TEM images of (a) HoMnO3 and(c) YMnO3 films on Pt(111); the white arrows in (a) and (c) show the‘steps’ in the Pt(111) surface. The HRTEM image and the inset EDpattern shown in (b) are from the region marked with the white box in(a). The simulated image in (b) was obtained along the [1100] zoneaxis for a thickness t = 6 nm and a defocus value � f = −60 nm.

reveals that the darker crystals in this case (figure 5(a)) areoriented along a zone axis, while the lighter Pt(111) crystallitesare slightly misoriented. Earlier x-ray diffraction work onthese films [21] reported that these Pt crystallites have anin-plane polycrystalline distribution and thus lack in-planeorientation. In spite of this, the film appears to locally maintainits epitaxial growth throughout the sample. The HRTEM imagein figure 5(b) shows a perfect epitaxial growth of the HoMnO3

film.

The HRTEM image of YMnO3/Pt(111) (not shown here)also revealed secondary c-axis orientations, but there are asignificantly larger number of nanodomains compared to theYMnO3 film grown on YSZ(111). Figure 5(a) shows that‘steps’ are observed at the boundaries between (111) Pt grains.The HRTEM image of HoMnO3/Pt(111) (figure 6) shows thatsuch a ‘step’ can induce the formation of antiphase boundaries(APBs). The film layer on the right-hand side of the APB isshifted over half a unit cell along c. Such surface step inducedAPBs have also been reported by others [44–46].

5

Nanotechnology 21 (2010) 075705 N Jehanathan et al

Figure 6. HRTEM image showing an APB in the HoMnO3 filmgrown on Pt(111). The thick black arrow shows the surface step inPt(111).

4.2. (YMnO3/ReMnO3)n (Re = Ho and Er) multilayers

Multilayers combining ReMnO3 compounds were grown fortwo purposes. From an electrical point of view, interfacescan lead to extrinsic effects such as internal barrier layercapacitance effects, which lead to an enhanced dielectricresponse. For such purpose, the system YMnO3/HoMnO3

was chosen as it combines YMnO3 compounds that exhibitthe lowest leakage currents among the prepared hexagonalmanganites with HoMnO3 that was quite leaky in thepresent preparation conditions. From a magnetic point ofview, combining ReMnO3 compounds with different magneticsymmetries in the form of ultrathin films is of interest forthe study of possible magnetic moment reorientations at theinterfaces and possible magnetic coupling between layers. Forsuch purposes, YMnO3 and ErMnO3 with different magneticsymmetries were chosen [47]. In both multilayer cases,interfaces play a major role in the resulting properties, whichmotivates a detailed local structural characterization.

4.2.1. Structure and defect characterization. Figure 7 isa low magnification TEM image of (YMnO3/HoMnO3)15

multilayers grown simultaneously on (a) YSZ(111) and (b)Pt(111). The total thickness is 545 ± 5 nm and 525 ± 5 nmon Pt(111) and YSZ(111), respectively. In the lower partof the stack (near the interface) the YMnO3 and HoMnO3

layers appear to have a less ‘wavy’ appearance compared tothe top part of the multilayer [48]. The insets in figures 7(a)and (b) are the corresponding ED patterns. The strongerreflections correspond to YSZ(111) and Pt(111), respectively,while the weaker reflections are associated with the YMnO3

and HoMnO3 films. The epitaxial relationship between layersand substrates is as follows:

[111]YSZ ‖ [0001](Ho & Y)MnO3;

(011)YSZ ‖ (1100)(Ho & Y)MnO3;

[111]Pt ‖ [0001](Ho & Y)MnO3;

(011)Pt ‖ (1100)(Ho & Y)MnO3.

Figure 7. Low magnification TEM images for the multilayeredYMnO3/HoMnO3 on (a) YSZ(111) and (b) Pt(111). The inset ineach image shows the corresponding ED pattern.

The epitaxial growth as well as the relation between themultilayers is apparent from the HRTEM image of figure 8.The image simulations (inset in figure 8(a)) based on theYMnO3/HoMnO3 structure show a good agreement with theexperimental image. The different interfaces generally appearto be defect free and well defined. However, some inclusionsin the YMnO3/HoMnO3 multilayers (figure 9(a)) occasionallynucleate at the interface between two layers and then protrudewithin the successive YMnO3 and HoMnO3 layers. The FTof the inclusion allows us to index them as [111] Mn3O4

(a = 0.5765 nm and c = 0.9442 nm; space group I 41amd(141) [49]). Image simulations based on the Mn3O4 structure(not shown here) are in good agreement with the experimentalimage. Figure 9(b) is part of the ED pattern shown in 7(a). Theextra weak reflections are associated with the inclusion and arecompatible with the Mn3O4 structure.

Figure 10 shows images of the (YMnO3/ErMnO3)10

multilayer. The total thickness is about 60 nm (figure 10(a)).The heteroepitaxial growth of YMnO3 and ErMnO3 appears tobe well defined. Figure 10(b) shows a higher magnificationimage of the multilayer. An atomically flat interface isobserved between the multilayer and YSZ. The inset EDpattern is the superposition of the different layers and YSZ.The bright reflections can be indexed as YSZ and theother reflections can be indexed based on the YMnO3 andErMnO3 structure. Some inclusions were observed insidethe multilayer as well as near the film/substrate interface(figure 10(c)). Similar to the YMnO3/HoMnO3 multilayergrown on YSZ(111), the inclusion can be identified as Mn3O4.

6

Nanotechnology 21 (2010) 075705 N Jehanathan et al

Figure 8. HRTEM images of the multilayered YMnO3/HoMnO3 film on (a) YSZ(111) and (b) Pt(111). The inset of (a) shows the simulation,performed along the [1100] zone axis with t = 5 nm and � f = −60 nm.

5. Discussion

5.1. Thin films

Based on the crystal structure and experimental evidencepresented here, the proposed atomic growth models forReMnO3 (Re = Ho, Y, Tb, and Er) films are presented infigure 11. Figures 11(a) and (b) show the ReMnO3 film grownon YSZ(111) and Pt(111), respectively. The first layer on the(111) YSZ or Pt plane is a Re–O layer; the second layer isa Mn–O layer and so on. Models for the secondary growthorientation of YMnO3 on YSZ(111) and the primary c-axisorientation of YMnO3 are shown in figures 11(c) and (d).Based on these structure models we have calculated theHRTEM simulations; they are shown in figures 11(e) and (f)and are in good agreement with the experimental images. Theschematic diagrams shown in figures 11(g) and (h) describe theprimary and secondary c-axis growth directions with respect toYSZ(111) or Pt(111).

The preferential secondary c-axis growth directionobserved in the YMnO3 film on YSZ(111) can be correlatedto the orientation of the YSZ substrate. In the (011)zone of YSZ there is a 70.5◦ angle between [111] and[111]. The experimental ED pattern in figure 1(b)shows [0001]second YMnO3 ‖ [111]YSZ, [1121]second YMnO3 ‖[0001]first YMnO3, and [1121]second YMnO3 ‖ [111]YSZ. Hence,also with the secondary c-axis growth (rotated 70.5◦ away fromthe primary c-axis), the lattice mismatch remains the same.In addition, during the annealing process the c-axis orientedYMnO3 thin films have net competing stresses owing to a highnegative thermal expansion coefficient α (−18.5 × 10−6 ◦C−1)along the a-axis direction of YMnO3 as compared to the c-axis direction (3.5 × 10−6 ◦C−1) [50] and YSZ(111) substrate

Figure 9. (a) HRTEM image of an inclusion in the YMnO3/HoMnO3

multilayer grown on YSZ(111), the inset shows the FT of theinclusion; (b) corresponding electron diffraction. The circledreflections (white arrows) are from the inclusion in (a).

(This figure is in colour only in the electronic version)

(8.8 × 10−6 ◦C−1) [51]. Therefore secondary c-axis growthorientations could contribute towards relieving it.

These remarks may explain why such secondaryorientations leading to nanodomains were suggested to be at

7

Nanotechnology 21 (2010) 075705 N Jehanathan et al

Figure 10. (a) Low magnification and (b) HRTEM image for theYMnO3/ErMnO3 multilayers grown on YSZ(111); the inset showsthe corresponding ED pattern; (c) HRTEM image showing theMn3O4 inclusion; the inset shows the FT of the inclusion.

the origin of the six-fold symmetry signal recorded by SHGwhile only a two-fold symmetry was expected for an epitaxialc-oriented film [39]. HoMnO3 also has a similar α trend in a-axis and c-axis directions as compared to YMnO3 [52], but thethickness of the HoMnO3 film (∼12 nm) is almost ten timessmaller than that of the YMnO3 film (∼100 nm) and below thecritical thickness for strain relaxation in these films.

The larger number of nanodomains observed in theYMnO3 film grown on Pt(111) is probably linked to the spreadin orientations of the Pt crystallites. Apart from the lack of asingle in-plane orientation of the substrate, contributions fromthe thermal stresses (αPt = 8.9 × 10−6 ◦C−1) [53] will alsointerfere.

The secondary phase inclusions in the TbMnO3 andErMnO3 films grown on YSZ(111) could have been generatedto relieve the excess strain energy accumulated because ofthe thickness of the film (135 nm for TbMnO3 and 75 nmfor ErMnO3). It costs excess energy to strain additionallayers of material into coherence with the substrate, unlikefor the HoMnO3 film where the thickness of the epitaxiallayer (∼12 nm) is small enough to maintain the strain energy

Figure 11. Atomic growth models for the ReMnO3 films depositedon (a) and (c) YSZ(111), and (b) Pt(111), where Re = Ho, Y, Tb, andEr. (d) First and second c-axis orientations of YMnO3 film. (e) and(f) computer simulations based on the models shown in (a) and (c),respectively. They were performed along the [1100] zone axis witht = 5 nm and � f = −65 nm; the dotted white lines show theinterface between the YMnO3 film and the YSZ substrate. (g) and (h)Schematic diagrams showing the directions of the c-axis growth for(a)–(f).

below the energy for defect formation. In the Gibb’s energy offormation equation (1):

�Gf = �gvolV + σ S + �Gstress (1)

(where �gvol is a specific free energy, V the volume of thenucleus, σ the surface energy, S the value of the nucleussurface), only �gvol is negative while σ S and �Gstress arealways positive and produce a destabilizing effect on thenucleus formation [54, 55]. The surface term (σ S) isrelatively low in the case of these epitaxial films wherethere is coherency between the film and the substrate. Thelattice mismatch between YSZ and the considered ReMnO3

films is + ∼2–3%. The lattice mismatch being defined

8

Nanotechnology 21 (2010) 075705 N Jehanathan et al

as 1 − (√

2ahex/√

3aYSZ) [21], where aYSZ = 0.5162 nmand aReMnO3 = 0.6112–0.6175 nm. Hence the significantdestabilizing role is the elastic stress (�Gstress), whichincreases with increasing thickness. Another possible origin ofthese secondary phase inclusions could be an off-stoichiometryof the films.

The inclusions have been identified as TbO2 and Er2O3.The exact composition and orientation of inclusions in anepitaxial film depends in a complex way on thermodynamics,kinetics, structure, and lattice parameters. The formation ofthese inclusions can be inferred from the subsolidus phasediagrams of Re–Mn–O systems [56]. According to the studyof Balakirev and Golikov [56], the coexistence of the twosolid phases of Re-oxide and ReMnO3 is found to be inthe temperature (t) range of 800 > t < 1400 ◦C andchemical composition (N) range of 0 < N � 0.5, whereN = NMn/(NMn + NRe). Assuming no lattice relaxationfor calculation purposes, the lattice mismatch between theReMnO3 film and TbO2 and Er2O3 inclusions is − ∼3%,where aTbO2 = 0.5213 nm and aEr2O3 = 0.5160 nm. Theorientation relationship between either inclusions and ReMnO3

film is [111]Inclusion ‖ [0001]ReMnO3 and (011)Inclusion ‖(1100)ReMnO3 . This corresponds to the same relation as thatbetween the ReMnO3 film and YSZ(111). The structure andorientation of the substrate and film govern, to a significantextent, the appearance of particular phases. Samoylenkov et al[55] and Broussard et al [57] reported that the nature of thesecondary phases in their epitaxial films is distinctly influencedby the substrate orientation they use.

5.2. (YMnO3/ReMnO3)n (Re = Ho and Er) multilayers

The strain energies due to the relatively large thickness ofthe HoMnO3/YMnO3 multilayers may be released by thearching [58] of the individual layers without destroying theorientation relationship. So, except for a few layers closeto the substrate (∼3 YMnO3 and HoMnO3 layers each), thelayers towards the surface appear to be arching more andmore. Unlike for the relatively thick individual YMnO3 filmswhere secondary c-axis orientations were observed, the thinnerYMnO3 layers in the multilayered system appear to have agood epitaxial growth similar to the HoMnO3 layers on bothsubstrates.

A possible reason for the Mn3O4 inclusions could bea small deviation of the sample stoichiometry from theideal ratios leading to the appearance of secondary phases.Too high deposition temperatures and/or too low depositionpartial pressures of oxygen relative to the ideal may alsobe a reason [55, 59, 60] for the cation Re/Mn disorder aswell as crystallographic vacancies. It has been reportedthat minor fluctuations of the composition of the depositedlayers are inherent in film growth by techniques such asMOCVD, magnetron sputtering, and laser ablation amongothers [55, 61, 62]. If, during the film growth, themanganese content in these films exceeds solubility limits,then during annealing the oxidation may stabilize the Mn-rich areas. Tetragonal Mn3O4 inclusions were also reported inepitaxial Nd1−x MnO3+δ and La1−xMnO3+δ films synthesizedby MOCVD [59, 60].

Figure 12. Twinning observed in the YMnO3/HoMnO3 multilayersgrown on Pt(111).

In contrast to the Mn3O4 inclusion in the YMnO3/ErMnO3

layers, the ‘wavy’ shaped inclusion in YMnO3/HoMnO3 re-sembles the arching of the layers. The inclusion is nucleatingat the Mn-rich interface between the YMnO3/HoMnO3 multi-layers and then grows within the individual layers. AlthoughMn3O4 inclusions were found in both multilayers grown onYSZ(111), they were not observed in the YMnO3/HoMnO3

multilayers grown simultaneously on Pt(111). Instead,twinning was present (figure 12). This twinning in theYMnO3/HoMnO3 multilayers is initiated at the boundaries ofthe Pt crystallites due to the rotation of these crystallites along〈111〉.

These observations illustrate that different types of defectsmay form within the same film, depending on the substrateand the deposition details. Also other reports [55, 63] confirmthat different inclusions can form within the same film, whenthey are grown on different substrates or on substrates with adifferent orientation.

6. Conclusions

Hexagonal YMnO3, HoMnO3, ErMnO3, and TbMnO3 filmsgrown on YSZ(111) have a good epitaxial growth. TbMnO3,which is normally stable in an orthorhombic structure, isepitaxially stabilized in its hexagonal form. A preferentialsecond orientation of the c-axis is observed in YMnO3 films(∼100 nm) and is attributed to the conforming hexagonalsymmetry in the zone axis of [1100] film and [011] YSZ.In the case of Pt(111), the secondary c-axis orientation ofYMnO3 is linked to the lack of a single in-plane orientationof the Pt crystallites. Despite the absence of an in-planeorientation of the Pt crystallites, the HoMnO3 film has a localepitaxial growth similar to that on YSZ(111). The appearanceof antiphase boundaries is attributed to the presence of surfacesteps along the Pt(111).

9

Nanotechnology 21 (2010) 075705 N Jehanathan et al

YMnO3/HoMnO3 multilayers grown on Pt(111) andYSZ(111) have a good epitaxial growth. However, due to therelatively large thickness of the films and subsequent strainrelaxation, the individual layers near the top of the film arearched. YMnO3/ErMnO3 with ultrathin interlayers grows witha high epitaxial quality on YSZ(111).

Secondary phase inclusions are found in the TbMnO3,ErMnO3 films and YMnO3/HoMnO3 and YMnO3/ErMnO3

multilayers grown on YSZ(111). They are linked tostrain relaxation and/or a possible deviation of the samplestoichiometry from its ideal ratios. The structure andorientation of the [011] TbO2 and [011] Er2O3 (Fm3m)inclusions are linked to the symmetry and structure of thefilm and the substrate and also to the thermodynamics of thesystem. [111] Mn3O4 (I 41amd) inclusions are present inthe YMnO3/HoMnO3 and YMnO3/ErMnO3 multilayers grownon YSZ(111). Twinning is observed in the YMnO3/HoMnO3

films grown on Pt(111). The kind of defect formed in thesemultilayers is associated with the type of substrate they aregrown on.

Acknowledgments

The authors acknowledge support from the EU under the FP-6 program under a contract for an Integrated InfrastructureInitiative. Reference 026019 ESTEEM. Special thanks toLudo Rossou for the TEM sample preparation. The workperformed at LMGP is within the STREP MaCoMuFi (NMP3-CT-2006-033221). Crystec (Germany) and SAFC Hitech (UK)are acknowledged for providing the YSZ substrate and CVDprecursors, respectively.

References

[1] Kimura T, Goto T, Shintani H, Ishizaka K, Arima T andTokura Y 2003 Nature 426 55–8

[2] Hur N, Park S, Sharma P A, Ahn J S, Guha S and Cheong S W2004 Nature 429 392–5

[3] Garcia N, Munoz M and Zhao Y W 1999 Phys. Rev. Lett.82 2923–6

[4] Gabureac M, Viret M, Ott F and Fermon C 2004 Phys. Rev. B69 100401

[5] Bossak A A et al 2001 Thin Solid Films 400 149–53[6] Gelard I, Huot G, Jehanathan N, Lebedev O I, Van Tendeloo G,

Ducroquet F and Dubourdieu C 2009 Structural and physicalcharacterization of hexagonal rare-earth manganitesReMnO3 films (Re = Tb, Dy, Ho, Er, Y) grown by MOCVDJ. Appl. Phys. submitted

[7] Bossak A, Dubourdieu C, Senateur J P, Gorbenko O Y andKaul A R 2002 J. Mater. Chem. 12 800–1

[8] Bosak A A, Dubourdieu C, Senateur J P, Gorbenko O andKaul A 2002 Cryst. Eng. 5 355–64

[9] Kaul A, Gorbenko O, Novojilov M, Kamenev A, Bosak A,Mikhaylov A, Boytsova O and Kartavtseva M 2005 J. Cryst.Growth 275 e2445–51

[10] Choi W S et al 2008 Phys. Rev. B 77 045137[11] Lee J H et al 2006 Adv. Mater. 18 3125–9[12] Yoo D C, Lee J Y, Kim I S and Kim Y T 2002 J. Cryst. Growth

234 454–8[13] Suzuki K, Nishizawa K, Miki T and Kato K 2002 J. Cryst.

Growth 237–239 482–6[14] Chen Y, Yuan H, Li G, Tian G and Feng S 2007 J. Cryst.

Growth 305 242–8

[15] Shigemitsu N, Sakata H, Ito D, Yoshimura T, Ashida A andFujimura N 2004 Japan. J. Appl. Phys. 43 6613–6

[16] Marti X et al 2007 J. Cryst. Growth 299 288–94[17] Rao C N R and Serrao C R 2007 J. Mater. Chem. 17 4931–8[18] Eerenstein W, Mathur N D and Scott J F 2006 Nature

442 759–65[19] Sugie H, Iwata N and Kohn K 2002 J. Phys. Soc. Japan

71 1558–64[20] Dho J, Leung C W, MacManus Driscoll J L and Blamire M G

2004 J. Cryst. Growth 267 548–53[21] Dubourdieu C, Huot G, Gelard I, Roussel H, Lebedev O I and

Van Tendeloo G 2007 Phil. Mag. Lett. 87 203–10[22] Gelard I, Dubourdieu C, Pailhes S, Petit S and Simon C 2008

Appl. Phys. Lett. 92 232506[23] Murugavel P, Lee J H, Lee D, Noh T W, Jo Y, Jung M H,

Oh Y S and Kim K H 2007 Appl. Phys. Lett. 90 142902[24] Kim J W, Schultz L, Dorr K, Aken B B V and Fiebig M 2007

Appl. Phys. Lett. 90 012502[25] Marti X, Sanchez F, Skumryev V, Laukhin V, Ferrater C,

Garcıa Cuenca M V, Varela M and Fontcuberta J 2008 ThinSolid Films 516 4899–907

[26] Cui Y, Wang C and Cao B 2005 Solid State Commun.133 641–5

[27] Ye F, Lorenz B, Huang Q, Wang Y Q, Sun Y Y, Chu C W,Fernandez Baca J A, Dai P and Mook H A 2007 Phys. Rev.B 76 060402

[28] Kim D, Killingensmith D, Dalton D, Olariu V, Gnadinger F,Rahman M, Mahmud A and Kalkur T S 2006 Mater. Lett.60 295–7

[29] Choi K J, Shin W C and Yoon S G 2001 Thin Solid Films384 146–50

[30] Iliev M N, Abrashev M V, Lee H-G, Popov V N, Sun Y Y,Thomson C, Meng R L and Chu C W 1998 Phys. Rev. B57 2872–7

[31] Katsufuji T et al 2002 Phys. Rev. B 66 134434[32] Alonso J A, Casais M T, Martinez Lope M J, Martinez J L and

Fernandez Diaz M T 1997 J. Phys.: Condens. Matter9 8515–26

[33] Munoz A, Alonso J A, Martinez Lope M J, Casais M T,Martinez J L and Fernandez Diaz M T 2001 Chem. Mater.13 1497–505

[34] Blasko J, Ritter C, Garcia J, Teresa J M d, Perez Cacho J andIbarra M R 2000 Phys. Rev. B 62 5609–18

[35] Park J, Kong U, Choi S I, Park J G, Lee C and Jo W 2002 Appl.Phys. A 74 S802–4

[36] Dubourdieu C, Rosina M, Roussel H, Weiss F, Senateur J P andHodeau J L 2001 Appl. Phys. Lett. 79 1246–8

[37] Senateur J P, Dubourdieu C, Weiss F, Rosina M andAbrutis A 2000 Adv. Mater. Opt. Electron. 10 155–61

[38] Horiuchi H, Schultz A J, Leung P C and Williams J M 1984Acta Crystallogr. B 40 367–72

[39] Kordel T, Wehrenfennig C, Meier D, Lottermoser T, Fiebig M,Gelard I, Dubourdieu C, Kim J W, Schultz L andDorr K 2009 Phys. Rev. B 80 045409

[40] Fiebig M, Lottermoser T, Frohlich D, Goltsev A V andPisarev R V 2002 Nature 419 818–20

[41] Gruen D M, Koehler W C and Katz J J 1951 J. Am. Chem. Soc.73 1475–9

[42] Leban I, Kolar D and Golic L 1972 Monatsh. Chem.103 1044–7

[43] Haglund J, Guillermet F F, Grimvall G and Korling M 1993Phys. Rev. B 48 11685–91

[44] Jiang J C, Lin Y, Chen C L, Chu C W and Meletis E I 2002J. Appl. Phys. 91 3188–92

[45] Sab T, Pietzonka I, Gottschalch V and Wagner G 1999 ThinSolid Films 348 196–201

[46] Bosak A A, Grayboy I E, Gorbenko O Y, Kaul A R andKartavtseva M S 2004 Chem. Mater. 16 1751–5

10

Nanotechnology 21 (2010) 075705 N Jehanathan et al

[47] Lottermoser T, Fiebig M, Frohlich D, Leute S andKohn K 2001 J. Magn. Magn. Mater. 226–230 1131–3

[48] Lebedev O I, Van Tendeloo G, Amelinckx S, Leibold B andHabermeier H U 1998 Phys. Rev. B 58 8065–74

[49] Jarosch D 1987 Mineral. Petrol. 37 15–23[50] Yoo D C, Lee J Y, Kim I S and Kim Y T 2001 J. Cryst. Growth

233 243–7[51] Hayashi H, Saitou T, Maruyama N, Inaba H, Kawamura K and

Mori M 2005 Solid State Ion. 176 613–9[52] Zhou H D, Denyszyn J C and Goodenough J B 2005 Phys. Rev.

B 72 224401[53] Boyer H E and Gall T L (ed) 1985 Metals Handbook-Desk

Edition (Metals Park, OH: American Society for Metals)[54] Kaul A, Gorbenko O, Graboy I, Novojilov M, Bosak A,

Kamenev A, Antonov S, Nikulin I, Mikhaylov A andKartavtzeva M 2003 Mater. Res. Soc. Symp. Proc. 755291–302

[55] Samoylenkov S V, Gorbenko O Yu, Graboy I E, Kaul A R,Zandbergen H W and Connolly E 1999 Chem. Mater.11 2417–28

[56] Balakirev V F and Golikov Y V 2003 Inorg. Mater. 39 S1–10[57] Broussard P R, Wall M A and Talvacchio J 1998 J. Mater. Res.

13 954–9[58] Holden T et al 2004 Phys. Rev. B 69 064505[59] Bosak A A, Gorbenko O Yu, Kaul A R, Graboy I E,

Dubourdieu C, Senateur J P and Zandbergen H W 2000J. Magn. Magn. Mater. 211 61–6

[60] Bosak A, Dubourdieu C, Audier M, Senateur J P andPierre J 2004 Appl. Phys. A 79 1979–84

[61] Gong J P, Kawasaki M, Fujito K, Tsuchiya R,Yoshimoto M and Koinuma H 1994 Phys. Rev. B 50 3280

[62] Samoylenkov S V, Gorbenko O Y, Kaul A R, Rebane A Y andZandbergen V L 1997 J. Alloys Compounds 251 342–6

[63] Verbist K, Vasiliev A L and Van Tendeloo G 1995 Appl. Phys.Lett. 66 1424–6

11