Emission studies of the gas-phase oxidation of Mn during pulsed laser deposition of manganates in O 2 and N 2 O atmospheres

Transcription

1 Emission studies of the gas-phase oxidation of Mn during pulsed laser deposition of manganates in O 2 and N 2 O atmospheres P. Lecoeur, A. Gupta, a) and P. R. Duncombe IBM Research Division, T. J. Watson Research Center, Yorktown Heights, New York G. Q. Gong Department of Physics, Brown University, Providence, Rhode Island Gang Xiao IBM Research Division, T. J. Watson Research Center, Yorktown Heights, New York and Department of Physics, Brown University, Providence, Rhode Island Received 22 January 1996; accepted for publication 22 March 1996 The plasma produced during pulsed laser deposition of manganate films has been probed using optical emission spectroscopy. The studies have been carried out using Mn, Mn 2 O 3, and La 0.67 Sr 0.33 MnO 3 LSMO as target materials in the presence of two different oxidizing gases: nitrous oxide N 2 O and oxygen O 2. Emission from excited MnO MnO* has been observed in all cases resulting primarily from reaction of the ablated Mn atoms with the background gas. Consistent with the oxidation reaction energetics, the emission intensity from MnO* is found to be about an order of magnitude stronger with N 2 O than with O 2. Magnetization measurements of LSMO films show improved magnetic properties of films prepared in N 2 O compared to O 2 at low pressures. The improvement in film quality can be attributed, at least in part, to the increased oxidation of Mn in the plasma plume American Institute of Physics. S I. INTRODUCTION The recent observation of colossal magnetoresistance CMR in the La 1 x D x MnO 3 D Ba, Sr, Ca, Pb, or vacancies systems has spurred renewed interest in studying these novel materials. 1 5 The pulsed laser deposition technique PLD has been the most commonly used method for growth of these multicomponent oxide films because of its apparent simplicity and versatility. An oxidizing ambient O 2 or N 2 O has been used for the in situ growth of the films at temperatures of C to aid in the formation and stabilization of the perovskite phase. Nevertheless, a postannealing step at higher temperatures is often necessary for further improving the magnetoresistance MR properties of the films. It has been suggested that the as-grown films are oxygen deficient and postannealing increases the oxygen content, thereby optimizing the mixed Mn 3 /Mn 4 ratio which is important for CMR. 2,3,5 Depending on the background gas pressure used during in situ growth of the La 1 x D x MnO 3 films, both gas-phase and surface reactions will be important for oxidation. For typical oxidant pressures mtorr used during PLD, there will be significant chemical interaction of the expanding cation species in the plume with the background gas leading to the formation of diatomic oxides. The oxidation reaction of La, being highly exothermic, occurs quite readily, with some of the oxide products being produced in electronically excited states. 6 Efficient gas-phase oxidation of the alkaline earth s Ba, Sr, Ca has also been observed during ablation of high-t c oxide materials. 7 On the other hand, because of the low bond energy of MnO, the oxidation of Mn is expected to be much less favorable. The reaction a Electronic mail: agupta@watson.ibm.com with O 2 is, in fact, endothermic ev and will not occur at normal collision energies. 8 Nonetheless, the hyperthermal Mn atoms in the plume can be effective in surmounting the substantial energy barrier and promote the formation of MnO. The situation is similar to the oxidation of copper during ablation of high-t c oxides. 9 Energetically, the reaction of Mn with N 2 O is much more favorable, being exothermic by ev. 8 It would, therefore, be interesting to compare the relative effectiveness of O 2 and N 2 O to form gas-phase MnO during ablation of the manganese oxides, and determine whether it correlates with their oxidation capability as deduced from the magnetic properties of the films prepared using these gases. In this article we report on the observation of emission from excited state MnO MnO* in the reactions of Mn with O 2 and N 2 O during laser ablation of Mn, Mn 2 O 3, and La 0.67 Sr 0.33 MnO 3 LSMO. We find that N 2 O is indeed more effective in the formation of gas-phase MnO* than O 2. This correlates well with the magnetic characteristics of the LSMO films prepared at low pressures using the two gases. II. EXPERIMENT The experimental system used for deposition of the manganate films has been described previously. 5 The emission measurements were carried out on the plasma plume produced with 355 nm laser frequency-tripled Nd:YAG laser ablation of Mn, Mn 2 O 3, and La 0.67 Sr 0.33 MnO 3 targets at a fluence of 2 J/cm 2. The ablation was done in O 2,N 2 O, and Ar atmospheres at pressures ranging from mtorr. An optical multichannel analyzer PAR Model 1420 attached to a 1/4 m Instruments SA monochromator with a 1200 grooves/mm grating was used for spectra acquisition. The optical emission from the plume, at a distance of 6.5 cm from the target, was collected by a pair of right-angle lenses J. Appl. Phys. 80 (1), 1 July /96/80(1)/513/5/$ American Institute of Physics 513

2 and imaged on the entrance slit of the spectrometer. The distance corresponds to the position of the substrate during film deposition. The multichannel analyzer was replaced by a fast photomultiplier for recording the time-of-flight TOF spectra of Mn and MnO, with the spectrometer tuned to the appropriate emission wavelength. About 30 laser pulses were averaged using a transient digitizer to obtain a single TOF spectrum. For comparison of magnetic properties, 3000-Åthick LSMO films were deposited on 100 -oriented SrTiO 3 substrates at a substrate temperature of 700 C, with different background pressures of O 2 and N 2 O. They were subsequently cooled at room temperature at 15 C/min in 700 Torr O 2. The films were characterized using x-ray and magnetization measurements. The magnetization was measured as a function of temperature using a Quantum Design superconducting quantum interference device SQUID magnetometer in a magnetic field of 5000 Oe. III. RESULTS AND DISCUSSION Our initial experiments on spectral emission characteristics were performed using a LSMO target in the presence of different background gases. Strong emission from atomic, ionic, and some molecular species was observed over the whole spectral range, with many overlapping lines and bands. In order to assign the emission spectra, and in particular to identify the Mn and MnO emission features, we subsequently made detailed measurements of the plume emission using Mn and Mn 2 O 3 targets. We found that the spectral and temporal characteristics of the emission for the manganese species were qualitatively similar for the different target materials. In the following we restrict ourselves to discussing the emission results obtained primarily using the Mn 2 O 3 target. The plume spectra of Mn and MnO from a Mn 2 O 3 target, for two ranges of wavelength, as a function of background N 2 O pressure are displayed in Fig. 1. The sharp emission lines observed in Fig. 1 at lower wavelengths can be assigned to excited Mn atoms Mn* I. 10 The spectral broadband around nm corresponds to oxide emission MnO* and can be assigned to the unresolved 0 0 band of the A 6 X 6 transition. 11 Additionally, overlapping spectra emission from atomic Mn* and MnO* are observed in the region nm. Emission lines corresponding to Mn ion Mn* II have also been detected in the UV range, but are not shown in Fig. 1. It is observed that both the metallic Mn* and the oxide MnO* peaks initially increase with pressure, with the MnO* increasing much more rapidly starting from almost zero intensity. The intensity increase of the metal lines is due to the effect of cooling by the background gas resulting in confinement of the plasma plume. At pressures above mtorr the Mn* and MnO* emissions again drop in intensity, with the former decreasing at a much faster rate. The observed decrease in intensity results from a combination of the overall reduction in the plume length at higher pressures due to hydrodynamic effects and increased conversion to the oxide from reaction. When the experiments were done in an O 2 atmosphere, the intensity and pressure dependence of the metallic lines were found to be quite similar. However, the intensity of the FIG. 1. Emission spectra of Mn and MnO from ablation of a Mn 2 O 3 target as a function of background N 2 O pressure recorded at a distance of 6.5 cm from the target. The spectral range nm has been scaled up by a factor of 3 for comparison of emission intensities. MnO* emission was about an order of magnitude lower than in N 2 O. In an Ar atmosphere or in vacuum, the oxide band emission was barely observable, suggesting that very little MnO is formed directly by ablation of the Mn 2 O 3 target. To conclusively identify the oxide emission, experiments were also done using a metallic Mn target. In this case no oxide emission was observed in vacuum or in an inert atmosphere. On the other hand, the intensity evolution of the oxide band as a function of O 2 and N 2 O pressure was similar to that observed with the Mn 2 O 3 target. It is worth noting that the observed greenish-yellow MnO* emission in an oxidizing ambient is confined primarily in the expanding front of the plasma. The strong selective enhancement of MnO emission at the edge of the plasma plume can be attributed to excitation of MnO during recombination and oxidation reactions of atomic Mn in the outer contact front of the expanding plasma with the surrounding oxidizing atmosphere. The much higher intensity of the MnO emission observed in N 2 O than in O 2 atmosphere is in agreement with the energetics of the respective oxidation reaction. Since the plume dynamics has a strong influence on the emission intensities of Mn* and MnO* as a function of background gas pressure, it is more meaningful to study the evolution of the MnO*/Mn* emission ratio instead. Figure 2 shows the normalized MnO */ Mn * intensity ratio as a function of N 2 O pressure for ablation from Mn 2 O 3 and Mn targets. It is observed in both cases that the ratio increases almost linearly with pressure before gradually leveling off at pressures higher than 300 mtorr. This is very similar to the behavior observed by Dye et al. for the oxidation of Y from ablation of Y 2 O 3 in an O 2 atmosphere. 12 In fact, the simple kinetic model developed by the authors to describe their results also appears to be relevant for the oxidation of Mn. The solid and dotted lines in Fig. 2 are fits to the two sets of data using the equation 514 J. Appl. Phys., Vol. 80, No. 1, 1 July 1996 Lecoeur et al.

3 FIG. 2. Plot of the MnO */ Mn * signal ratio as a function of N 2 O pressure for ablation from Mn 2 O 3 and Mn targets. The solid and dotted lines are model fits to the data using the equation MnO* / Mn* A N 2 O /(B C N 2 O. MnO* Mn* A N 2O B C N 2 O, where N 2 O is the concentration of the oxidizing gas and A,B, and C are constants which represent combinations of rate constants for the different elementary reaction steps as described in Ref. 12. The equation provides a very good fit to the data for both the Mn 2 O 3 and Mn targets. The slightly higher value of MnO* / Mn* for ablation from Mn 2 O 3 can be explained in terms of a larger value of A in the equation resulting from a higher rate of evaporation from this target than the Mn target due to increased absorption. Further insight into the oxidation dynamics can be gained by examining the temporal evolution of the oxide emission. Figure 3 shows the temporal evolution of the Mn* nm and MnO* nm emission from ablation of the Mn 2 O 3 target in different N 2 O pressures. An initial increase in the peak MnO* intensity is observed with pressure. At low pressures, the average velocity of the species calculated from the peak arrival time is in the range of 10 6 cm/s. This is similar to what has been observed during ablation of other oxide materials With increasing pressure, the emission intensity decreases for both MnO* and Mn* and the maxima are increasingly delayed because of the deceleration and attenuation of the ablated species. Nevertheless, the tail of the distribution remains largely unaffected with rise in pressure. It should be noted that the observed distribution function for Mn and MnO are quite similar, particularly at high pressures. This is to be expected if the latter is formed by oxidation at the contact front of the expanding plasma. Previous experiments have shown that laser ablation of a solid in a high-density background gas leads to the formation of a nonsteady shock wave called a blast wave. 15,16 The propagation velocity of the blast wave at a fixed radius, R, is FIG. 3. Temporal profiles of the Mn and MnO emission intensities as a function of N 2 O pressure. The small initial spike near time zero is from scattering of the laser light. proportional to (E/ 0 ) 0.5 R 1.5, where E is the amount of energy released during the ablation and 0 is the ambient gas density. For low background pressures the collisional mean free path of the expanding plasma is large and the kinetic energy of the fragments will be transferred over a relatively large volume. The dynamics will then more closely resemble free expansion into a vacuum with some deceleration due to viscous force. 17 The blast wave model should be applicable at higher pressures, particularly at larger distances. An approximation (1/P) 0.5 dependence of the peak velocity of the fragments, v p, with ambient pressure, P, would be expected in this case. 15 Figure 4 shows the variation in v p as a function of background oxidizing gas. The peak velocity has been calculated from the temporal spectra in Fig. 3 obtained at a distance of 6.5 cm from the target. We have included the velocity data for emission from both Mn* and MnO* using the Mn 2 O 3 target with O 2 and N 2 O as background gases. In all cases the blast wave model seems to provide a satisfactory fit for pressures above 50 mtorr. We finally discuss the magnetic properties of LSMO films prepared using the two gases. X-ray diffraction analysis of the films grown on 100 -oriented SrTiO 3 substrates show that they are epitaxial. No significant changes in the lattice parameter perpendicular to the substrate 3.85 Å has been observed as a function of background gas pressure. Figure 5 shows the temperature dependence of the magnetization M in an applied field of 5000 Oe for LSMO films deposited at different pressures of O 2 and N 2 O. The magnetization at 5 K increases with increasing pressure for both gases and saturates at a value of 630 emu/cm 3 at high pressures. In addition, the ferromagnetic transition temperature T c also in- J. Appl. Phys., Vol. 80, No. 1, 1 July 1996 Lecoeur et al. 515

4 FIG. 4. Dependence of the peak velocity, v p, determined from the temporal profiles in Fig. 3 as a function of background gas O 2 or N 2 O pressure. The solid line is a (1/P) 0.5 fit to the data. FIG. 5. The temperature dependence of the magnetization M for in situ epitaxial La 0.67 Sr 0.33 MnO 3. LSMO films deposited with different background pressures of O 2 and N 2 O. All the films were deposited at 700 C using 355 nm laser radiation. creases substantially with pressure. The saturation value of M at low temperatures is close to the theoretical value 670 emu/cc for spin only contribution from all Mn ions. It is worth noting that, at low pressures, films prepared in N 2 O exhibit significantly higher M values than those prepared in O 2. This is consistent with the higher effectiveness of N 2 Oin the production of gas-phase MnO. It is quite likely that in addition to gas-phase oxidation, surface oxidation will also play an important role in determining the oxygen content of the LSMO films. In fact, with increasing pressure, the behavior is very similar for the two gases as the magnetization approaches its theoretical limit. Films prepared at high pressures exhibit excellent ferromagnetic properties. As seen from Fig. 5, for 400 mtorr O 2, the magnetization increases very sharply below T c and quickly reaches a saturation value similar to a conventional ferromagnet. IV. SUMMARY In summary, we have monitored the gas-phase emission from Mn and MnO during ablation of manganese-containing targets in N 2 O and O 2 atmosphere. We find that N 2 O is about an order of magnitude more effective than O 2 in the production of gas-phase excited MnO resulting from oxidation of the ablated Mn atoms. The gas-phase oxidation behavior has been qualitatively correlated with the magnetic properties of LSMO films prepared using these gases at different pressures. ACKNOWLEDGMENT G. X. was supported by the National Science Foundation under the Materials Research Group Grant No. DMR and partially by Grant No. DMR R. von Helmont, J. Wecker, B. Holzapfel, L. Schultz, and K. Samwer, Phys. Rev. Lett. 71, ; K. Chahara, T. Ohno, M. Kasai, and Y. Kozono, Appl. Phys. Lett. 63, S. Jin, T. H. Tiefel, M. McCormack, R. A. Fastnacht, R. Ramesh, and L. H. Chen, Science 264, ; S. Manoharan, N. Y. Vasantacharya, M. S. Hegde, K. M. Satyalaksmi, V. Prasad, and S. V. Subramanyam, J. Appl. Phys. 76, G. C. Xiong, Q. Li, H. L. Ju, R. L. Greene, and T. Venkatesan, Appl. Phys. Lett. 66, G. Gong, C. Canedy, G. Xiao, J. Z. Sun, A. Gupta, and W. J. Gallagher, Appl. Phys. Lett. 67, A. Gupta, T. R. McGuire, P. R. Duncombe, M. Rupp, J. Z. Sun, W. J. Gallagher, and G. Xiao, Appl. Phys. Lett. 67, J. L. Gole and D. R. Preuss, J. Chem. Phys. 66, ; J. L. Gole and G. L. Chalek, ibid. 65, P. Engst, P. Kubat, P. Bohacek, and J. Wind, Appl. Phys. Lett. 64, ; D. Fried, G. P. Reck, T. Kushida, and E. W. Rothe, J. Appl. Phys. 70, Value calculated using the dissociation energy of ev for MnO: J. B. Pedley and E. M. Marshall, J. Phys. Chem. Ref. Data 12, C. E. Otis and R. W. Dreyfus, Phys. Rev. Lett. 67, ; C.E. Otis, A. Gupta, and B. Braren, Appl. Phys. Lett. 62, CRC Handbook of Chemistry and Physics, edited by D. R. Lide, 75th ed. CRC, Boca Raton, FL, B. Pinchemel and J. Schamps, Can. J. Phys. 53, ; J.M.Das Sarma, Z. Phys. 157, R. C. Dye, R. E. Muenchausen, and N. S. Nogar, Chem. Phys. Lett. 181, J. Appl. Phys., Vol. 80, No. 1, 1 July 1996 Lecoeur et al.

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