Preparation and characterization of LaMnO3 thin films grown by ...

JOURNAL OF APPLIED PHYSICS 100, 023910 共2006兲

Preparation and characterization of LaMnO3 thin films grown by pulsed laser deposition C. Aruta,a兲 M. Angeloni, G. Balestrino, N. G. Boggio, P. G. Medaglia, and A. Tebano Coherentia CNR-INFM and Dipartimento di Ingegneria Meccanica, Università di Roma “Tor Vergata,” Via del Politecnico 1, I-00133 Roma, Italy

B. Davidson TASC CNR-INFM National Laboratory, Area Science Park, Basovizza, 34012 Trieste, Italy

M. Baldini, D. Di Castro, P. Postorino, and P. Dore Coherentia CNR-INFM and Dipartimento di Fisica, Università di Roma “La Sapienza,” Piazzale Aldo Moro 5, I-00185 Roma, Italy

A. Sidorenko, G. Allodi, and R. De Renzi Dipartimento di Fisica and Unità CNISM, Universita di Parma, Parco Area delle Scienze 7A, I-43100 Parma, Italy

共Received 21 September 2006; accepted 9 May 2006; published online 28 July 2006兲 We have grown LaMnO3 thin films on 共001兲 LaAlO3 substrates by pulsed laser deposition. X-ray diffraction confirms that the films are only slightly relaxed and are oriented “square on square” relative to the substrate. The measured Raman spectra closely resemble that observed in bulk LaMnO3, which indicates no relevant distortions of the MnO6 octahedra induced by the epitaxial strain. Therefore, no detectable changes in the lattice dynamics occurred in our LaMnO3 strained films relative to the bulk case. 55Mn nuclear magnetic resonance identifies the presence of localized Mn4+ states. Superconducting quantum interference device magnetization measures TN = 131共3兲 K and a saturation moment ␮ = 1.09␮B / Mn, revealing a small concentration of Mn4+ and placing our films within the antiferromagnetic insulating phase. © 2006 American Institute of Physics. 关DOI: 10.1063/1.2217983兴 INTRODUCTION

The discovery of the colossal magnetoresistance 共CMR兲 effect in manganite compounds1 has renewed the interest in this class of materials. The chemical formula of CMR manganites can be written as RE1−xAxMnO3, where RE stands for a 共trivalent兲 rare earth, A for a 共divalent兲 alkaline earth 共Ca, Sr, or Ba兲, and x ranges from 0 to 1. For 0.2⬍ x ⬍ 0.5, doped manganites show a number of extraordinary magnetotransport properties related to the fully spin polarized character of the electrical conduction. These properties, in view of possible applications, have triggered an increasing effort toward film growth and optimization. Beside technical problems related to film deposition, lattice distortions induced by epitaxial strain have been shown to play a relevant role in connection with physical properties of thin manganite films— epitaxial strain influences the local symmetry of the octahedral Mn ions that is crucial in determining their magnetotransport properties.2 An exhaustive overview of manganite properties is given in Ref. 3. In studying manganite physics, of particular importance is the investigation of the parent compound LaMnO3 共LMO兲. Such a compound is an A-type antiferromagnetic insulator where the orbital ordering is established due to the cooperative Jahn-Teller 共JT兲 interaction. LMO was shown to have the O⬘-type orthorhombic structure4 共space group Pbnm兲 a兲

Author to whom correspondence should be addressed; electronic mail: [email protected]

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with a = 0.554 nm, b = 0.572 nm, and c = 0.770 nm, at room temperature. The MnO6 octahedron is Jahn-Teller distorted with the distortion axis lying in the a-b plane: longer and shorter Mn–O bonds in the a-b plane are 0.218 and 0.191 nm, respectively, along two orthogonal in-plane directions, while the Mn–O bond length is 0.196 nm along the c axis. Therefore, the Mn–Mn distances in plane and out of plane are 0.398 and 0.385 nm, respectively. The interest in this specific manganite compound has been renewed by a recent paper by Yamada et al.5 In this paper it was shown that a thin 共few unit cell兲 LMO buffer layer inserted at a SrTiO3 / La0.6Sr0.4MnO3 interface can improve the interfacial magnetic properties near the manganite’s Curie temperature. Such a result could open important perspectives in the field of manganite spin valves operating at room temperature. In bulk LMO samples defects associated with excess oxygen, because of the difficulty of accommodating interstitial excess oxygen ions in the close-packed perovskite structure, take the form of La- and Mn-cation vacancies.6–8 LMO can be either lanthanum or manganese deficient, or both.9 The chemical formula for samples having the correct stoichiometric ratio between Mn and La 共namely, La: Mn = 1 : 1兲, assuming that the oxygen is O2−, can be written as 3+ 3+ 4+ 共La1−x 关兴x兲共Mn1−7x Mn6x 关兴x兲O2− 3 . In these conditions, the fraction of Mn4+ over the total is 6x / 共1 − x兲, so that even such a small value of x = 0.05 corresponds to about 1 / 3 Mn4+. Samples having the correct oxygen stoichiometry can be prepared at low oxygen pressure 共艋10−3 Pa兲共Ref. 10兲 or in

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reducing atmosphere.11,12 La deficiency also converts Mn3+ 3+ into Mn4+ according to the formula 共La1−z 关兴 z兲 3+ 4+ 2− 4+ ⫻共Mn1−3zMn3z 兲O3 so that the Mn content is three times the La deficiency. Cation-deficient samples tend to be ferromagnetic and metallic.13–15 Because of such critical stoichiometry issues, the growth of stoichiometric LMO thin films is not a trivial achievement. Owing to the strong interplay between magnetic, transport, and structural properties, different experimental techniques must be employed in studying the manganite properties. In particular, improved experimental microscopic data are necessary in order to understand the role of distortions induced by the substrate in thin manganite films. In this framework micro-Raman spectroscopy can be an important investigation tool for these systems. Namely, the most intense peaks of the Raman-active phonon spectrum are ascribed to vibrational modes of the MnO6 octahedra, which are crucial in determining transport properties of manganites. A number of Raman papers dealing with bulk and thick film samples witnesse the relevance and sensibility of this technique.16 For instance, in a recent paper it was shown that a strong hardening of the phonon frequencies of the bending and stretching modes is apparent in ultra-thin films 共d ⬍ 100 Å兲. This behavior, strongly connected with the measured d dependence of the insulator-to-metal transition temperature, is ascribed to co-operative effects of MnO6 octahedra rotation and charge localization.17 From the magnetic point of view pure LMO is an A-type antiferromagnet with TNx=0 = 139.5 K 共where x is the Mn4+ concentration兲 and a small spin canting due to the antisymmetric Dzialoshinski-Moriya interaction 共less than 2° from collinearity between the two sublattices兲, giving rise to a weak ferromagnetic behavior18 with a saturation moment of ␮ = 0.16␮B / Mn. The behavior of lightly doped La1−␦Mn1−␦O3 is coarsely similar to that of other lightly doped pseudocubic manganites,19 such as La1−xSrxMnO3 共Ref. 20兲 and La1−xCaxMnO3,21 where small angle neutron scattering has demonstrated the presence of nanoscopic ferromagnetic clusters embedded in an antiferromagnetic bulk background at low temperatures.22 These clusters diffuse at higher temperatures,23 giving rise to a complex magnetic behavior characterized by a large ferromagnetic macroscopic static susceptibility. For an effective concentration of Mn4+x ⬍ 0.07 the antiferromagnetic interactions dominate and the Neel temperature slightly decreases from TNx=0 down to 115 K 共Ref. 19兲. Above x ⬎ 0.07 both the saturation moment ␮共x兲 and the ordering temperature TC共x兲 increase rapidly with x reaching, e.g., TC = 220 K and ␮ = 3.8␮B / Mn for La0.8Ca0.2MnO3, and the phase is often identified as insulating ferromagnetic 共FI兲.24 In the present work, we prepared thin films of the LMO compound by pulsed laser deposition 共PLD兲. Films were characterized through different techniques, such as Rutherford back scattering 共RBS兲, x-ray diffraction, electrical resistivity as a function of temperature, and superconducting quantum interference device 共SQUID兲 magnetometry. We also utilized Raman spectroscopy to study the LMO phonon spectrum. In principle, in this compound, all Mn ions have the same 3 + valence and no structural disorder, deriving from

random chemical substitution on La sites, is expected. Raman phonon peaks much sharper than those observed in substituted manganites can then be observed.16,25 In the case of thin films, this should allow a precise measurement of small frequency shifts of these peaks related to substrate induced crystallographic distortions which affect the MnO6 octahedral structure. 55Mn zero field nuclear magnetic resonance 共NMR兲 was also employed: nuclear spin echoes can be generated in the presence of the hyperfine field, due to the ordered electron moments. Distinct spectral contributions may be easily assigned to the three possible electronic environments: localized Mn4+, localized Mn3+, and mixed valence band MnDE, in the double exchange 共DE兲 metallic phase.26–28 Therefore NMR can directly confirm the presence of localized Mn4+ ions. EXPERIMENT

The PLD of the LMO films was carried out using an excimer laser charged with KrF 共wavelength of 248 nm, pulse width of 25 ns, and repetition rate of 3 Hz兲. The laser beam, with an energy of 150 mJ per pulse, was focused onto a target in a vacuum chamber. LaAlO3 共LAO兲 substrates 共001兲 oriented were placed at a distance of about 50 mm from the target on a heated holder. LAO was selected as substrate since its low intensity and simply structured Raman signal allows to extract the phonon spectrum of LMO even for thin films.17 La:Mn stoichiometric targets were prepared in air by solid state reaction starting from MnO and La2O3 high purity powders. The final sintering treatments were carried out in air at 1500 ° C for 24 h. LMO films were grown in oxygen atmosphere. Optimal molecular oxygen pressure for compensated LMO films resulted to be about 10−2 mbar with an initial base pressure of 10−5 mbar. Film growth temperature Tg was about 700 ° C. Film growth rate was about 0.1 nm per laser shot. After the growth, samples were cooled to room temperature, in about 10 min, in the growth atmosphere. In order to determine the cation stoichiometry, LMO films were grown on MgO substrates in the identical growth conditions and investigated by RBS. Lattice parameters were measured by x-ray diffraction. Electrical resistivity as a function of temperature was measured by the standard four-probe technique. SQUID measurements were performed on a commercial Quantum MPMS apparatus. Raman spectra were measured in backscattering geometry, using a micro-Raman spectrometer 共LabRam by Jobin-Yvon兲 equipped with a charge coupled device detector and a notch filter to reject the elastic contribution. The sample was excited by the 632.8 nm line of a He–Ne laser. The confocal microscope was equipped with a 50⫻ objective, which allows obtaining a laser spot about 2 ␮m2 wide at the sample surface. Spectra have been collected within the 200– 1100 cm−1 frequency range with a spectral resolution of about 3 cm−1. In order to reduce as much as possible the signal from the substrate, a very small confocal diaphragm 共50 ␮m兲 was used to limit the scattering volume. The 55Mn NMR spectrum was collected with the homebuilt phase coherent spectrometer HyReSpect 共Ref. 29兲 by a solid spin echo sequence on a tuned


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FIG. 1. Arrhenius plot of an optimized LMO film between 360 and 530 K, together with the best linear fit. In the inset, the fit of the resistivity data with Holstein’s model has been reported.

probe circuit. The amplitude is recorded at each frequency to obtain the zero frequency component of the fast Fourier of the echo.

RESULTS AND DISCUSSION

The crucial point in obtaining stoichiometric LMO film is to optimize the oxygen content. Therefore, several films were grown at the same temperature Tg of 700 ° C varying the pressure of the oxygen background gas between 5 ⫻ 10−3 and 0.2 mbar. The oxygen content in the film was monitored indirectly measuring the film resistivity: films having higher resistivity were assumed to have a better oxygen stoichiometry. Following this approach, a growth pressure of 10−2 mbar was chosen. In Fig. 1 we show the resistivity of a LMO film, 73 nm thick 共determined by RBS measurements兲, grown at an oxygen pressure of 10−2 mbar. Resistivity is reported in an Arrhenius plot 共ln ␳ vs 1 / T兲 in the temperature range between 360 and 530 K. A semiconducting behavior is evident with an activation energy of about 0.32 eV, in reasonable agreement with values reported in literature for bulk LMO compounds,30,31 showing that films grown at an oxygen pressure of 10−2 mbar are quite well oxygen compensated. In the inset of Fig. 1 we report the fit of the resistivity data with Holstein’s model32 共ln ␳ / T ⬀ 1 / T兲. Within such a model, the conduction takes place through thermal-activated polaron hopping. In our opinion experimental data do not allow to discriminate between the two models. However, a fit of the experimental data according to Holstein’s model resulted in a hopping energy of ⬇0.355 eV, which corresponds to about 5% of Mn4+.33 As shown below by RBS, magnetization, and NMR measurements, a small but measurable concentration of Mn4+ has been also estimated. In spite of the general trend that the presence of Mn4+ favors metallic conductivity, the resistivity of these films is notably higher 共by a factor of 5–50 at 360 K兲 than typical values for high-quality bulk samples with small Mn4+ concentration4 共less than a few percent兲 and nominally stoichiometric thin films.34,35 The Raman and NMR results discussed below demonstrate that significant

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FIG. 2. ␪-2␪ diffraction spectrum of an optimized LMO film across the 共001兲 and the 共002兲 reflections of the LMO film and the LAO substrate. In the inset, the rocking curve 共␪ scan兲 across the 共002兲 peak of the film is shown.

structural disorder, at levels high enough to cause the observed resistivities, is not present in these films. The cation stoichiometry of a film grown at such an oxygen pressure was investigated by RBS measurements, yielding a La:Mn ratio of 0.95:1, with a relative error of 3% on the estimate of both La and Mn. This result suggests, within the experimental error, a slight La deficiency with respect to the stoichiometric target, in agreement with the magnetization and NMR results discussed below. A description of the crystallographic structure of the film on the basis of the structure of orthorhombic bulk LMO is not straightforward. The pseudoperovskite lattice parameters of the orthorhombic unit cell of bulk, stoichiometric LMO are a1 = a2 = 0.398 nm and a3 = 0.385. In strained films, the in-plane lattice parameters are both compressed to match the substrate, yielding a tetragonal unit cell. The structural properties of the LMO films were investigated by x-ray diffraction at Cu K␣ wavelength with a Bragg-Brentano diffractometer. In Fig. 2 a ␪-2␪ diffraction spectrum of a film grown in the optimized conditions is shown. Peaks in the spectrum belong either to the LAO 共001兲 oriented substrates or to the LMO film. The LMO film is 共001兲 oriented. No spurious phases can be detected from x-ray diffraction. From the symmetric diffraction spectra it can be noticed that the 00l peaks are quite broad and a perpendicular lattice parameter c of 0.399± 0.001 nm can be estimated. In the inset of the figure the rocking curve, taken from the 共002兲 peak of the film, is shown. The full width at half maximum of the rocking curve is about 0.5°, showing a satisfactory structural order in the perpendicular direction. The in-plane order was investigated by asymmetric diffraction measurements. ␾ scan at 共103兲 Bragg angles reported in the inset of Fig. 3 shows that LMO films result to be oriented “square on square” relative to the LAO substrates. In Fig. 3 a reciprocal space map around the 共103兲 reflection is reported. The data are given in reciprocal lattice units 共r.l.u.兲 and normalized relative to the lattice parameters of LAO substrate 共a = b = c = 0.379 nm兲. The typical twin structure of the LAO substrate and the presence of a very broad peak from the LMO film can be observed in the map. The major diffraction contribution of the 共103兲 reflection of the film occurs at the same H = 1 value of the sub-


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FIG. 3. Isointensity contour plot on a logarithmic scale of 共103兲 reciprocal space map of a LMO film grown on LAO substrate. The range of the logarithmic scale is from 4 to 30 000. In the inset the ␾-scan measurement around the 共103兲 reflection is reported.

strate, thus indicating that the film is mostly in-plane matched with the substrate. The weak tail at lower H values is a consequence of the partial relaxation of the lattice parameter toward the bulk values, possibly in the topmost layer of the film, together with the presence of different domains mainly influenced by the twin structure of the substrate. By rotating the sample of 90°, the reciprocal space map does not significantly change, in agreement with the in-plane square symmetry of our LMO film. These results indicate that the strained unit cell has a volume that is much less 共about 5%兲 than the pseudocubic unit cell of bulk stoichiometric LMO. The typical Raman spectrum of a LMO film is shown in Fig. 4. Owing to the film thickness 共about 75 nm兲 and to the small confocal diaphragm, no Raman signal from the LAO substrate 关which, within the investigated spectral range, mainly consists of a sharp peak at 485 cm−1 共Ref. 36兲兴 is detectable. The measured spectrum closely resembles that observed in bulk LMO samples36 and mainly consists of two components around 500 and 600 cm−1, usually ascribed to bending 共B兲 and stretching 共S兲 modes of the MnO6 octahedron, respectively.36,37 In order to distinguish the different components, we have applied a fitting procedure by employ-

FIG. 4. Typical Raman spectrum of a LMO film. Best fit profile and components are shown separately. The low frequency region of the spectrum is shown in expanded scale in the inset.

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FIG. 5. Magnetization vs field at T = 2 K of a LMO film, with the external field oriented in the film plane.

ing a standard model function S共␯兲, given by the linear combination of damped harmonic oscillators.37,38 Figure 4 shows that a very good description of the measured spectrum can be obtained by including in the model S共␯兲 four components 共1–4兲 besides the B 共at 490 cm−1兲 and S 共at 614 cm−1兲 ones. The uncertainties on the peak frequency values reported in Fig. 4, as estimated on the basis of reproducibility of fit results for the different LMO films we investigated, are ±3 and ±6 cm−1 for the sharper 共1, B, and S兲 and broader 共2, 3, and 4兲 components, respectively. The peak frequency values we determine for LMO films are in very good agreement with those obtained for LMO bulk samples.36,39 Figure 5 shows the field dependence of the magnetization measured by SQUID on a 75 nm thick film with the external field oriented in the film plane. The background contribution from the substrate and the sample holder has been subtracted. The measured saturation magnetization is M s = 1.59⫻ 105 A / m, which corresponds to ␮ = 1.09␮B / Mn, to be compared with the values of ␮ = 4␮B / Mn expected for Mn3+, of ␮ = 3.8␮B / Mn in FI compositions, and finally with ␮ = 0.16␮B on pure LMO. The temperature dependence of the magnetization is shown in Fig. 6, obtained both by field cooling 共FC兲 the film in a magnetic field of H = 1 kOe and by

FIG. 6. Temperature dependence of FC and ZFC magnetizations of a LMO film with an external field of H = 1 kOe. Inset: Blowup of the ZFC data.


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FIG. 7. 55Mn NMR spectrum of a LMO film between 310 and 410 MHz at T = 1.6 K.

zero field cooling 共ZFC兲, applying the same field at T = 2 K and measuring while heating. The difference between the two curves is typical of very lightly doped manganites, due to the presence of ferromagnetic contributions from localized Mn4+ moments in a canted antiferromagnetic bulk background. The two data sets merge at TN = 131共3兲 K, where the ordered moments vanish both in the antiferromagnetic bulk and at localized Mn4+. A peak in the ZFC curve around TN is evident from the inset of Fig. 6. The ordering temperature is very close to that of the pure LMO, but the saturation value of the average ferromagnetic moment indicates that the samples must lie closer to the lower edge of the FI phase. Assuming that substrate induced strain does not alter strongly the phase diagram, the transition temperature itself, which is lower than that of pure LMO, indicates that the films are probably not yet within the FI phase. This is also consistent with the minute value of the ZFC magnetization in H = 1 kOe, which is instead a sizable fraction of the FC value for samples within the FI phase. The number of Bohr magnetons per Mn versus Mn4+ concentration x in bulk La1−␦Mn1−␦O3 共Ref. 40兲共Fig. 6 therein兲 gives a rough estimate of x = 0.05 for our sample. Such an estimate, together with the RBS results indicating a slight lanthanum deficiency, leads to the following formula 共within 0.005 atoms/ 3+ 3+ 4+ 兲共Mn0.95 Mn0.05 兲O2− cell兲 unit for our film 共La0.98 3 . The presence of a sizable amount of localized Mn4+ ions is directly witnessed by the NMR spectrum of Fig. 7 共on the same film兲. The most prominent feature is the peak around 320 MHz, corresponding to the nuclei of localized Mn4+ ions. The Mn3+ is mostly escaping observation, although it probably contributes with very poor signal/noise ratio to the higher frequency portion of the NMR spectrum. This is commonly seen41–43 in nanoscopically confined manganites and it is due to a combined effect of the faster relaxation rates, larger broadening, and lower enhancement of the nuclei of Mn3+. Relative to the pseudoperovskite bulk lattice parameters for stoichiometric, antiferromagnetic LMO, our films show significant decrease of the unit cell volume that is not readily ascribed to the slight off-stoichiometry 共Mn4+ ⬃ 5 % 兲 discussed above. However, a detailed neutron diffraction study

of the structure and magnetic ordering in LMO samples prepared under different conditions7 shows that the low concentrations of Mn4+ 共⬍2 % 兲 are typically achieved only when annealing in a nonoxidizing environment; the presence of a small amount of oxygen during preparation can produce a number of different unit cells, all associated with slight nonstoichiometries. In particular, when annealing in air a second orthorhombic 共Pnma兲 phase results that has a unit cell volume about 4% less than the stoichiometric orthorhombic phase. This second orthorhombic phase is slightly rich in Mn4+ and is ferromagnetic with TC = 131 K and about 1.54␮B / Mn. One possible explanation for the structural 共⬃5% smaller unit cell兲 and magnetic properties 共AF, TN = 131 K, 1.09␮B / Mn兲 of these films is that, in the oxygen environment during PLD, the epitaxial strain stabilizes this second orthorhombic phase with slightly less Mn4+ than reported in Ref. 7, leaving the film insulating but on the AF side of the AF/F boundary. The epitaxial stabilization of slightly different unit cell structures under different growth conditions has been seen, for example, in cuprate thin films.44 Another possible explanation can be found in the paper of Ritter et al.,4 where a decrease of the unit cell volume of LaMnO3+␦ is observed with increasing the lanthanum deficiency and the Mn4+ content. CONCLUSIONS

We have shown that LMO films grown by PLD at an oxygen pressure of 10−2 mbar are nearly stoichiometric and quite well oxygen compensated. The structural parameters have been obtained by x-ray diffraction measurements: films resulted epitaxial, with both in-plane lattice parameters equal to the in-plane lattice parameter of the LAO substrate. Only a partial relaxation in the topmost layer is suggested. Moreover, the LMO in-plane cell orientation was “square on square” relative to the substrate. The perpendicular-to-thesurface lattice parameter resulted to be 0.399 nm. Raman measurements did not show any sizable shift or broadening of the peaks relative to the typical bulk spectra. Therefore we can conclude that the epitaxial strain does not induce relevant tilting or rotation of the MnO6 octahedra enough to influence the Raman spectra. A significant effect on the Raman spectra would also be expected in the case of heavy cation or oxygen disorder that can be ruled out in our LMO films16 as evidenced by the structural, electrical transport, and magnetic measurements. ACKNOWLEDGMENT

Partial support from “THIOX” ESF network is acknowledged. 1

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