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This article was published in an Elsevier journal. The attached copyis furnished to the author for non-commercial research and
education use, including for instruction at the author’s institution,sharing with colleagues and providing to institution administration.
Other uses, including reproduction and distribution, or selling orlicensing copies, or posting to personal, institutional or third party
websites are prohibited.
In most cases authors are permitted to post their version of thearticle (e.g. in Word or Tex form) to their personal website orinstitutional repository. Authors requiring further information
regarding Elsevier’s archiving and manuscript policies areencouraged to visit:
Keywords: Thin films; Nanostructured bulk materials; Magnetic properties; Transport properties
1. Introduction
The large interest arouses by the discovery of a HTc supercon-ductivity (1986) and a colossal magnetoresistance (CMR) effect(1993) in oxides derived from the perovskite structure has ope-ned the route to the synthesis of new functional materials both inthe form of bulk and thin films. In the last 10 years, the colossalmagnetoresistive effect observed in manganese oxides has cau-sed a renewed interest concerning the study of these compoundsthat often undergo several phase transition in relation with theexistence of strongly correlated magnetic and electric properties.Interestingly these properties can be tuned by cationic substitu-tion onto the A and/or B-site of the perovskite but also by theintroduction of vacancies on both cationic and anionic sublat-tices. The physics behind these materials is related to their elec-tronic configuration, crystal structure and eventually microstruc-ture where many competing parameters complicate any interpre-tation of the observed behaviour leading, often, to controversy.
In this respect, the ordered double perovskite La2MnB’O6(B’ = Ni, Co) is interesting [1]. La2NiMnO6 is a ferromagne-tic semiconductor with a Curie temperature of 280 K. In thebulk form, La2NiMnO6 has been studied extensively both forits structure and physical properties. However, the actual nuclearstructure and oxidation states of both transition metals are stillcontroversial [2,3]. Several papers have been focussed on this
issue, leading to two main questions: first, the Ni/Mn ordering atthe B-site of the perovskite lattice (i.e. the existence of a so-called“double perovskite”) and second, the oxidation state of each B-site cations (i.e. couple Ni2+/Mn4+ or Ni3+/Mn3+). Recently,La2NiMnO6 films have been grown on various substrates by thepulsed laser deposition (PLD) [4,5]. Both authors found that thefilms display a ferromagnetic insulating behaviour “compatiblewith the Ni/Mn ordering” considering the similarity with theproperties observed in La2NiMnO6 bulk samples [1]. This hasmotivated us to investigate the structural and physical propertiesof La–Ni–Mn–O film that will be presented here in a first part.
Without having the remarkable properties of the manga-nese perovskite, the oxygen deficient double cobalt perovskiteREBaCo2O5+x (so-called 112, RE = rare earth) have a broadmagnetoresistive effect with a similar behaviour [6] and, in parti-cular, a transition of the metal-insulator type (MI) with a chargeordering (CO) and spin ordering. Recently the phase diagramof the system GdBaCo2O5+x was established [7] showing thatmagnetic and transport properties seem to be dominated by ananoscopic phase separation. In the second part of this paper,some recent results on the 112 bulk cobalt oxides LaBaCo2O6will illustrate how structural and microstructural parameters mayaffect notably the properties in such compounds.
2. Experimental
La–Ni–Mn–O 70 nm thin films were grown on (0 0 1)-oriented SrTiO3 (STO) at 650–750 ◦C by the PLD technique
50 Ph. Boullay et al. / Materials Science and Engineering B 144 (2007) 49–53
Fig. 1. (a) θ–2θ XRD scan of a typical film. S indicated the reflections of the SrTiO3 (STO) substrate. Insert shows a rocking-curve recorded around the (0 2 0)reflection, (b) (M–T) curve recorded with 500 Oe. Insert displays the (M–H) loop recorded at 10 K. Magnetic field is applied parallel to the [1 0 0] direction of thesubstrate.
in a flowing 100 mTorr O2 ambient. Stoichiometric La2NiMnO6was employed as a target, which was synthesized by the standardsolid-state chemistry routes [8].
Ordered and nanostructured ordered LaBaCo2O6 compoundswere prepared by sol–gel method from nitrates. Precursor cal-cinated at 900 ◦C for 20 h, sintered at 1150 ◦C for 48 h in Aratmosphere. By controlling the partial O2 pressure during thesynthesis, it is possible to obtain both materials. DisorderedLaBaCo2O6 compounds were prepared by solid-state methodfrom oxides and carbonates [9]. The as-synthesized samples arethen oxygenated at elevated O2 pressure (100 bar) at 350 ◦Cto obtain fully oxidised samples as confirmed by iodometrictitration.
X-ray diffraction (XRD) patterns were collected using aSeifert 3000P diffractometer (Cu K�1, λ = 0.15406 nm) and aPhilips X’pert Pro diffractometer (Cu K�). The magnetizationmeasurements were measured from 5 K to 400 K using a SQUIDmagnetometer (MPMS, Quantum Design).
The transmission electron microscopy (TEM) study was car-ried out using JEOL 200CX, JEOL 2010, JEOL 2010F and FEITecnai F30 electron microscopes. For the sample preparation,small fragments of the bulk or thin film were crush in an agatemortar and put in a suspension in alcohol. A drop of the suspen-sion was then deposited and dried on a copper grid previouslycoated with a thin film of amorphous carbon.
3. LaNi0.5Mn0.5O3 thin films
Fig. 1a shows a typical XRD pattern of a film grown at720 ◦C. It has been indexed based on the pseudo-cubic sym-metry. Film displays only the peaks corresponding to (0 k 0)reflections (where k = 1, 2, 3), which indicates that the out-of-plane lattice parameters is a multiple of the perovskite subcellparameter (asub ∼ 0.39 nm), which is confirmed by the elec-tron microscopy study. Quality of the films is confirmed bythe rocking-curve (see insert of Fig. 1a) measured around the(0 2 0) reflection. The full-width-at-half-maxima (FWHM) ofthe rocking-curve is found to be around 0.19◦. �-Scan, recor-ded around the (1 0 3)sub reflection of the cubic subcell, showsfour peaks separated by 90◦ from each other, confirming thatthe film has grown epitaxialy with respect to the substrate andthat the film possesses a pseudo-cubic symmetry. In the insert ofFig. 1b, the in-plane loop recorded at 10 K shows a well-defined
hysteresis with a saturation magnetization close to 5 �B/f.u. Fur-thermore, the M(T) (Fig. 1b) recorded under 500 Oe appliedmagnetic field, shows a clear magnetic transition at 250 K anda minor one, close to 150 K. Both transitions have already beenreported in LaNi0.5Mn0.5O3 bulk compound and associated tothe existence of different spin states of Ni and Mn [2]. In the lightof recent literature, a high temperature magnetic transition hasbeen associated with a B-site ordering in bulk materials [1] andalso assumed in the case of thin films [4,5]. The DC-electricalproperties of the films, measured in four probe configurationusing a PPMS system, revealed a semiconducting behaviour.These results indicate that the properties of the present films aresimilar to those reported previously and one can now addressthe following questions: what structure these films do possessand further, what is the role of Ni/Mn ordering in the magneticand transport properties?
In order to address them, we have studied at room temperaturethe microstructural state by transmission electron microscopy(TEM). The EDS analyses confirm the compositional homoge-neity, with a cationic ratio La/Mn/Ni close to 2/1/1, in the limitof the accuracy of the technique (∼5%). The electron diffraction(ED) patterns exhibit a set of intense reflections correspondingto the perovskite subcell observed by XRD. The perfect super-position of the film and substrate patterns implies that the subcellparameter asub is very close to 0.389 nm. The reconstruction ofthe reciprocal space evidenced a supercell with a ∼ c ∼ asub*
√2
and b ∼ 2asub. The conditions limiting the reflection show a Icentred cell with the supplementary conditions (0 k l): (k + l = 2n)and, at 90◦ tilting around [0 1 0]* (h k 0): h = 2n (Fig. 2). Suchconditions imply the absence of fourfold axis and, are consistentwith the space groups Imma, Im2a or I2/a (c unique axis withγ ∼ 90◦), commonly observed in the distorted perovskites. In
Fig. 2. ED patterns obtained along: (a) [1 0 0] and (b) [0 0 1] zone axes, respec-tively.
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Fig. 3. HREM images observed along one of the 〈1 0 0〉sub direction: (a) atroom temperature most of the film exhibits a “I-type” matrix where, for such adirection, only the perovskite subcell is evidenced (see Fourier transform werepre-transitional effects are also observed). In this image, a nano-sized “P-type”domain exhibits a 0.79 nm periodicity as attested by the Fourier transform ofthe area. The darker blotches are point defects and (b) at 95 K, the HT phase hasdisappeared and only orientation variants (denoted Pi) of the LT “P-type” phaseare observed.
addition, at room temperature, SAED patterns exhibit faintedextra reflections, keeping the cell parameters unchanged butwithout centering of the cell. HREM images show that this“P-type” phase is stabilized in the form of nano-sized domains(Fig. 3a), spread out in the “I-type” matrix. A temperature inves-tigation (JEOL2010/2010F and GATAN cryo-holder) evidencesthat the “P-type” domains are absent at 360 K and, cooling down,grow to the detriment of the “I-type” high temperature phasewith a “burst” in good correlation with the magnetic transition
observed at 250 K. At low temperature (95 K Fig. 3b) the “P-type” phase is found in the form of large domains related toorientation variants derived from the 〈1 0 0〉 directions of the per-ovskite subcell. On the symmetry of the low temperature phase,an ED study performed at 95 K suggests that the conditions limi-ting the reflections are compatible with a Pnma space group.Nonetheless, the 0.788 nm fringes (Fig. 3) clearly evidences forthe P-type phase on the HREM images is not compatible witha Pnma space group. Already observed in the CMR manganeseoxides, such an image contrast is compatible with a monoclinicsymmetry reduction [10].
4. LaBaCo2O6 bulk compounds
In the CMR manganites, the modifications of the A-site cationwas found to play an important role on the control of theirphysical properties as illustrate by the complexity of the phasediagram of several RE1−xAxMnO3 systems [11]. Not as wellknown, is the role of quenched disorder in the apparition of theCMR effect as illustrated in the manganites RE0.5Ba0.5MnO3[12] where two possible structures exist for one compositiondepending on the synthesis condition [13]. One case correspondsto a disordered A-site perovskite structure while the second oneis A-site ordered 112-type structure with alternating layer ofREO and BaO sheets. Notably, only the disordered structureshows the CMR effect with a drastic reduction of the Curie tem-perature as compared to the ordered structure. This possibilityto obtain two structures (A-site ordered and A-site disorde-red) is also observed for the cobaltites RE0.5Ba0.5CoO3 andrecently reported for RE = La [14]. In [14], the A-site orde-red LaBaCo2O6 and disordered La0.5Ba0.5CoO3 are reportedto have a ferromagnetic transition at, respectively Tc = 175 and190 K, i.e. the disordered structure display an increase of theCurie temperature and controversial results are given regardingthe existence of a MR effect for both ordered and disorderedcompound [9,14]. This behaviour has attracted our interest andagain address the question about the relation between cationicorder or disorder and the magnetic and transport properties?We have thus decided to investigate these materials but usinga different synthesis condition using a sol–gel method anddiscover that actually a third structure somewhat intermediatebetween the ordered and the disordered ones can be obtainedfor La0.5Ba0.5CoO3.
In Fig. 4a, a part of the XRPD patterns typically observed forthe cubic ap disordered structure is shown. Under appropriatesynthesis condition, some La0.5Ba0.5CoO3 powders preparedby sol–gel exhibit a clear hkl-dependent peaks broadening indi-cative of the existence of some disorder at a microstructurallevel. In Fig. 4b, the temperature dependence of the magneti-zation, M(T), for this sample shows a ferromagnetic behaviourwith a Tc close to 190 K very similar to the one measured fordisordered La0.5Ba0.5CoO3. Nonetheless the sample exhibits astrong magnetic anisotropy, much stronger than the one obser-ved in both disordered La0.5Ba0.5CoO3 and ordered LaBaCo2O6[14]. In order to investigate in more details, the actual struc-ture and microstructure of this sample, TEM observations wereperformed.
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Fig. 4. (a) part of the XRD powder patterns recorded for a cubic disordered La0.5Ba0.5CoO3 perovskite structure prepared by solid state reaction and oneLa0.5Ba0.5CoO3 sample prepared by sol–gel. This last compound exhibits clearly a hkl-dependent peak broadening and (b) temperature dependence of themagnetization M(T) recorded with 100 Oe for one La0.5Ba0.5CoO3 sample having XRD peak broadenings. Inset figure shows M–H behaviour at three differenttemperatures.
Fig. 5. (a) Typical HREM image corresponding to one of the perovskite {1 0 0} planes showing that the La0.5Ba0.5CoO3 sample having XRD peak broadening isactually a nanostructured bulk material where the 112-type ordered LaBaCo2O6 structure is in the form of 90◦ oriented domains fitted into each other; (b) Fouriertransform of the (a). Besides the strong reflections related to the perovskite subcell, three supplementary sets of reflections are observed. Two sets are related to theexistence of 90◦ oriented domains having the apx2ap 112-type structure. The last one is related to faint reflections compatible with a “2apx2ap surstructure”; (c)Fourier filtered image of (a) obtained taking all the reflections noted in (b) except the ones from the perovskite subcell. The 112-type structure is present all over thearea in the form of 90◦ oriented domains. The “2apx2ap surstructure” is found mainly at the 90◦ domain boundaries but can be extended (see for instance the twozones delimited in white).
Selected area ED patterns exhibit strong reflections corres-ponding to the cubic perovskite subcell but associated with theexistence of extra spots related to a quadratic apx apx2ap super-cell related to the 112-type ordered LaBaCo2O6. Systematicallytwo sets of supercell reflections 90◦ oriented are observed inthe SAED patterns. HREM observations at room temperaturerevealed that this sample is actually a nanostructured bulk mate-rial made of 90◦ oriented domains of ordered LaBaCo2O6 asshown in the representative area Fig. 5a. The Fourier trans-form of the area (Fig. 5b) indicates, in addition to the apx2ap90◦ oriented supercell reflection already observed by SAED,the existence of faint reflections compatible with a “2apx2apsupercell”. By selecting some reflections related to both typeof supercell (as indicated in Fig. 5b) and performing an inverseFourier transform (Fig. 5c), we obtain a Fourier filtered imagewhere zones having a perovskite superstructure are evidenced.First, the 2ap 90◦ oriented domains extend all over the viewing
area with a variation in size (5 nm for the smallest) and shape.Most likely, the magnetic anisotropy is strongly increased bythe nanostructure presented by this material and the existence of90◦ oriented domains fitted into each other at a very fine scale asrevealed by TEM observations. Second, the “2apx2ap supercell”appears to be found mostly at the boundary between intersec-ting apx2ap 90◦ oriented domains with some local extension tofew nanometer-sized zones. The possibility that this “2apx2apsupercell” corresponds actually to an artefact due to the superim-position of two apx2ap 90◦ oriented domains can not be excludedand need further investigations.
5. Conclusion
Regarding the LaNi0.5Mn0.5O3 thin films, the TEM observa-tions reveal several points. First, the substrate-induced strainsthat stabilize an “I-type” phase at room temperature. Second,
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the presence of “P-type” nano-sized domains is associated tothe seeding of the low temperature phase. Third, the magnetictransition at 250 K is associated to a structural phase transitioncompatible with a distortion of the perovskite from a 2-tilt sys-tem (a−b0a−) to a 3-tilt one (a−a−c+). Regarding the Ni/Mnordering, at room temperature, the observation of a a mirrorplane (Imma, Im2a or I2/a) does not support the long-range B-site ordering. Consequently, this study leads us, to conclude thata 1:1 long-range ordering between Ni and Mn is not a neces-sary condition for the observation of high Curie temperatureat 250 K [4,5] that may rather be dependent of modification ofcharge distribution onto the B-site irrespective of the nature ofthe cation.
Regarding the La0.5Ba0.5CoO3 bulk compounds, in additionto the known ordered 112 and disordered perovskite structures,the TEM observations reveal the possibility to synthesize anovel and somewhat intermediate structure. This nanostructu-red LaBaCo2O6 compound is similar in some aspects to thedisordered La0.5Ba0.5CoO3 (XRD patterns and Tc) but possessthe 112 ordered LaBaCo2O6 structure in the form of 90◦ orien-ted domains fitted into each other at a nanometer scale leadingto the existence of a strong magnetic anisotropy characteristicfor this compound.
As illustrated by these two examples, TEM is a valuabletool to characterize the structure and microstructure for samplesuch as thin films or nanostructured bulk materials. It providescomplementary diffraction, imaging and even spectroscopictechniques that would allow to reveal structural inhomogeneityover range from micrometer to atomic resolution. Nonetheless,finding correlations between the observed structural features andthe physical properties in strongly correlated electronic systemsappear as a challenging task.
Acknowledgements
W. Prellier, B. Mercey, M. Hervieu, V. Pralong, V. Cai-gnaert and B. Raveau for their contributions to this work.
The French Ministry of Education and the Finnish MagnusEhrnrooth Foundation for supporting some of the authors. Thework on the LaNi0.5Mn0.5O3 thin films is carried out in theframe of the NoE FAME (FP6-500159-1), the STREP MaCo-MuFi (NMP3-CT-2006-033221) and the STREP CoMePhS(NMP4-CT-2005-517039) supported by the European commu-nity.
References
[1] N.S. Rogado, J. Li, A.W. Sleight, M.A. Subramanian, Adv. Mater. 17 (2005)2225.
[2] V.L. Joseph Joly, P.A. Joy, S.K. Date, C.S. Gopinath, Phys. Rev. B 65 (2002)184416.
[3] J. Blasco, J. Garcia, M.C. Sanchez, J. Campo, G. Subias, J. Perez-Cacho,Eur. Phys. J. B 30 (2002) 469;J. Blasco, M.C. Sanchez, J. Perez-Cacho, J. Garcia, G. Subias, J. Campo,J. Phys. Chem. Solids 63 (2002) 781.
[4] H. Guo, J. Burgess, S. Street, A. Gupta, T.G. Calvarese, M.A. Subramanian,Appl. Phys. Lett. 89 (2006) 022509;H.Z. Guo, A. Gupta, T.G. Calvarese, M.A. Subramanian, Idib 89 (2006)262503.
[5] M. Hashisaka, D. Kan, A. Masuno, M. Takano, Y. Shimakawa, T. Tera-shima, K. Mibu, Appl. Phys. Lett. 89 (2006) 032504.
[6] C. Martin, A. Maignan, D. Pelloquin, N. Nguyen, B. Raveau, Appl. Phys.Lett. 71 (1997) 1421.
[7] A.A. Taskin, A.N. Lavrov, Y. Ando, Phys. Rev. B 71 (2005)134414.
[8] M.P. Singh, C. Grygiel, W.C. Sheets, Ph. Boullay, M. Hervieu, W. Prel-lier, B. Mercey, Ch. Simon, B. Raveau, Appl. Phys. Lett. 91 (2007)012503.
[9] F. Fauth, E. Suard, V. Caignaert, Phys. Rev. B 65 (2001) 60401.[10] M. Hervieu, G. Van Tendeloo, V. Caignaert, A. Maignan, B. Raveau, Phys.
Rev. B 53 (1996) 14274.[11] G. Van Tendeloo, O.I. Lebedev, M. Hervieu, B. Raveau, Rep. Prog. Phys.
67 (2004) 1315, and references therein.[12] D. Akahoshi, M. Uchida, Y. Tomioka, Y. Matsui, Y. Tokura, Phys. Rev.
Lett. 90 (2003) 177203.[13] F. Millange, V. Caignaert, B. Domenges, B. Raveau, E. Suard, Chem. Mater.
10 (1998) 1974.[14] T. Nakajima, M. Ichihara, Y. Ueda, J. Phys. Soc. Jpn. 74 (2005)
