APPLIED PHYSICS LETTERS 91, 202509 共2007兲
Effect of oxygen concentration on the magnetic properties of La2CoMnO6 thin films H. Z. Guo Center for Materials for Information Technology and Department of Chemistry, University of Alabama, Tuscaloosa, Alabama 35487, USA and Department of Physics, Florida International University, Miami, Florida 33199, USA
A. Guptaa兲 Center for Materials for Information Technology and Department of Chemistry, University of Alabama, Tuscaloosa, Alabama 35487, USA
Jiandi Zhang Department of Physics, Florida International University, Miami, Florida 33199, USA
M. Varela and S. J. Pennycook Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA
共Received 10 October 2007; accepted 29 October 2007; published online 16 November 2007兲 The dependence of the magnetic properties on oxygen concentration in epitaxial La2CoMnO6 thin films deposited on 共100兲-oriented SrTiO3 substrates has been investigated by varying the oxygen background pressure during growth using pulsed laser deposition. Two distinct ferromagnetic 共FM兲 phases are revealed, and the relative fraction varies with the oxygen concentration. The existence of oxygen vacancies induces the local vibronic Mn3+ – O – Co3+ superexchange interactions in direct competition with the static FM Mn4+ – O – Co2+ interactions. This results in the appearance of a new low temperature FM phase and suppression of the high-temperature FM phase, creating two distinct magnetic phase transitions. © 2007 American Institute of Physics. 关DOI: 10.1063/1.2814919兴 As a class of ferromagnetic 共FM兲 insulators, multifunctional double perovskite oxides of composition La2B⬘B⬙O6 共B⬘ = Ni and Co; B⬙ = Mn兲 have recently gained much interest, both because of their rich physics and prospects for technological applications.1 Neutron and x-ray diffraction studies of bulk stoichiometric La2CoMnO6 共LCMO兲 show that longrange ordering of Co and Mn cations leads to a transition from the orthorhombic 共Pbnm, c is the long axis兲 to the monoclinic 共P21 / n with  ⬇ 90°兲 structure.1–4 On the other hand, any disorder in the Mn/ Co atomic configuration promotes alternate magnetic interactions resulting in a reduction of the magnetic transition temperature and a decrease in the value of the saturation magnetization.2,3,5 Point disorder can create antiferromagentic 共AF兲 Mn4+ – O – Mn4+ or Co2+ – O – Co2+ interactions in LCMO with the antisite ions.3 Even in an ordered double perovskite, nucleation of atomic order at different positions results in antiphase boundaries which can introduce similar AF interations.3 Disorder at B sites also leads to the possible Co3+ and Mn3+ states in LCMO promoting AF interactions.2 In particular, the local oxygen vacancies can even introduce a new FM phase at lower temperature3 by creating a vibronic superexchange interaction.6 Therefore, controlling the B-site disorder and oxygen vacancy is the central issue in exploiting the magnetic functionalities in this class of compounds. Despite significant work in the bulk, there have only been few reports on the fabrication and characterization of LCMO thin films.7–10 A Curie temperature 共hereafter denoted as TC1兲 in the range of 225– 235 K with a saturation magnetization moment as high as 5.8 B / f.u. has recently been reported in LCMO thin films prepared under optimal condia兲
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tions using the pulsed laser deposition technique.7,8 These values are consistent with those observed in the stoichiometric bulk.3 Structural studies of LCMO thin films indicate almost complete B-site ordering of Co2+ and Mn4+ ions with nearly stoichiometric oxygen concentration.8,9 Since the oxygen content can, in principle, be more uniformly controlled in a thin film by carefully varying the growth conditions, it offers an opportunity to better understand the effect of oxygen concentration on the material properties. In this letter, we report on the structural and magnetic properties of LCMO films grown under different oxygen pressures. We embarked on this study in an attempt to reveal the effect of oxygen vacancies on the structural and magnetic properties of LCMO. In addition to the previously reported high TC1 phase 共TC1 = 226 K兲, we have identified a second ferromagnetic phase of lower Curie temperature 共TC2 = 80 K兲 which is associated with the existence of oxygen vacancies. It should be noted that we have no direct evidence for the presence of oxygen vacancies in our films and is inferred based on the oxygen stoichiometry results of bulk samples.3 LCMO films were grown epitaxially on 共001兲-oriented SrTiO3 共STO兲 substrates at different background oxygen pressures 共from 1 to 600 mTorr兲 by the pulsed laser deposition 共PLD兲 technique.7 Electron microscopy observations were carried out with an aberration corrected scanning transmission electron microscope 共STEM兲 共VG Microscopes HB501UX兲 operated at 100 keV, which is equipped with a Nion aberration corrector and a Gatan Enfina electron energy loss spectrometer 共EELS兲. Magnetic properties were measured using a Quantum Design superconducting quantum interference device magnetometer 共SQUID兲. Figure 1共a兲 presents the low-magnification crosssectional STEM Z-contrast image of a LCMO thin film
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FIG. 1. 共a兲 Low-magnification cross-section STEM Z-contrast image of LCMO film grown on STO substrate with 100 mTorr background oxygen; 共b兲 high resolution STEM Z-contrast image near the LCMO-STO interface region; and 共c兲 and 共d兲 show low and high resolution STEM Z-contrast images, respectively, indicating the formation of twin boundaries indicated by arrows.
grown on 共001兲 STO under 100 mTorr O2. The film is flat and homogeneous over relatively large lateral distances. The epitaxial nature of film growth is confirmed from the highresolution Z-contrast image shown in Fig. 1共b兲, which shows an excellent unit-cell to unit-cell epitaxial relationship between the film and the substrate. The substrate-film interface is sharp and coherent with no evidence of formation of secondary phases or any significant interdiffusion over large distances. The only observed defects are the areas of brighter contrast with halos, highlighted with arrows in Figs. 1共c兲 and 1共d兲, which we attribute to twin boundaries. They originate from the interface region and run parallel to the growth direction all the way up to the film surface. The average spacing of the twin boundaries varies but is typically of the order of 15 nm. Similar twinning domain growth has been observed in other manganese oxide-based thin films grown on cubic substrates, such as in the case of La0.73Ca0.27MnO3 films on SrTiO3.11 The EELS spectra 共not shown兲 of the sample exhibit well defined signals for the O K, Mn L, Co L, and La M edges. Preliminary analysis of the Mn oxidation state from measurement of the L23 ratio 共⬃2.11兲 for wide illumination conditions suggest the oxidation state of Mn to be +4. A more careful analysis including Co reference standards for determining the Co valence is underway, especially for the films grown at low oxygen pressure conditions. The temperature dependences of the zero-field cooled 共ZFC兲 and field cooled 共FC兲 关Fig. 2共a兲兴 response have been measured under a low applied field of 0.1 T and the temperature dependences of the FC response at higher applied fields 共1 and 5 T兲 关Fig. 2共b兲兴. We have estimated the ferromagnetic Curie temperature 共TC1 and TC2兲 of the films from the peak positions of the temperature derivative of the magnetization 共dM / dT兲 as measured in a magnetic field of 0.1 T. In the samples grown at low oxygen pressures 共1 and 10 mTorr兲, a single magnetic transition at low temperature 共TC2 ⬃ 80 K兲 is observed. On the other hand, two ferromagnetic phases exist for the samples grown at relatively high oxygen pressures 共200– 600 mTorr兲—a high TC majority ferromagnetic phase 共TC1 ⬃ 230 K兲 coexisting with a small minority component with low TC 共TC2 ⬃ 80 K兲. In the films
FIG. 2. 共a兲 Temperature dependence of the ZFC 共open symbol兲 and FC 共closed symbol兲 responses at a low applied magnetic field 共H = 0.1 T兲 for LCMO films grown in different oxygen background pressures 共1, 100, 200, and 600 mTorr兲. 共b兲 Temperature dependence of the FC 共closed symbol兲 responses at high applied magnetic fields 共H = 1 and 5 T兲 for LCMO films grown in different oxygen background pressures 共1, 100, 200, and 600 mTorr兲. The arrows mark the FM transition temperature 共either TC1 or TC2兲.
grown at intermediate oxygen pressures 共e.g., at 100 mTorr兲 two ferromagnetic phases 共TC1 ⬃ 230 K and TC2 ⬃ 80 K兲 of comparable magnitudes are observed. For all samples, the magnetization in the FC curve rises continually with decreasing temperature, whereas the ZFC curve exhibits a cusplike shape below the FM transition temperatures. With increasing applied field, the cusp in the ZFC curve tends to shift to lower temperatures, becomes broad, and eventually disappears under a sufficiently high magnetic field. This is a feature characteristic of spin glass behavior, as discussed previously.3,7 Figure 3共a兲 presents the magnetic hysteresis loops measured at 10 K for films grown at different oxygen pressures. All the measured loops exhibit characteristics of two switching fields, i.e., biloop characteristic. Figure 3共b兲 shows that the saturation magnetization 共M s兲 at first increases with increasing oxygen pressure during growth, with the highest saturation magnetization obtained in the film grown at 200 mTorr and then it decreases slightly with further increasing oxygen pressure. Meanwhile, the coercivity 共Hc兲 shows a similar dependence on oxygen pressure 关see Fig. 3共b兲兴, with a saturated value for the films grown at 200 mTorr and above. The maximum value of the saturation magnetization measured at 10 K from the sample grown under 200 mTorr O2 is ⬃5.7 B / f.u., which is close to the theoretical spinonly value of 6 B / f.u. for ferromagnetic ordering of Co2+ and Mn4+ ions. The temperature dependence of the magnetic hysteresis loops of the 200 mTorr sample is shown in Fig. 3共c兲. Both the saturation magnetization and coercivity decrease with increasing temperature. Yet, the double switching 共biloop兲 characteristic persists all the way up to the Curie temperature TC1, which is much higher than the lower magnetic transition
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FIG. 3. 共a兲 The field dependence of the magnetization measured at 10 K for LCMO thin films grown under different oxygen pressures 共1, 100, 200, and 600 mTorr兲; 共b兲 the plot of the saturation magnetization M s and coercivity Hc of LCMO thin films vs the oxygen pressure during growth; the dashed and doted lines are guides to the eye; and 共c兲 the field dependence of the magnetization recorded at different temperatures for the LCMO film grown under 200 mTorr oxygen.
temperature TC2. Singh et al.8 have also observed the biloop feature in their LCMO films and found that it exists over the whole ferromagnetic regime. They have, however, ruled out the possibility of one of the loops resulting from B-site cationic Co3+ / Mn3+ disorder structure or antisite defects, based on the fact that no clear second FM phase transition at low temperature has been observed in their samples. Instead, they suggested that the biloop hysteresis arises from the presence of two magnetic domains possessing different coercivities. This scenario may be indeed consistent with the observations in our samples. The twinning structure revealed by STEM 共see Fig. 1兲 provides the structural evidence for the magnetic domains in LCMO films. There are three issues regarding the magnetic properties observed from our films: 共1兲 a spin-glass behavior existing in both FM phases 共below TC1 and TC2兲, 共2兲 a biloop hysteresis existing in all observed samples grown at different oxygen pressures, and 共3兲 existence of two FM phase transitions with the low-temperature one 共at TC2兲 dominating in the films grown at low oxygen pressure. As we mentioned above, the existence of the biloop feature in hysteresis is likely linked to the bidomain structure in the films, independent of oxygen stoichiometry.8 The spin-glass behavior can been explained as the result of competitions between FM Mn4+ – O – Co2+ interaction and AF Mn4+ – O – Mn4+, or Co2+ – O – Co2+ interactions with antisite ions or at antiphase boundaries.3 Such behavior should occur in both oxygen-stoichiometric and nonstoichiometric case, as long as a certain amount of local B-site disorder exists in the system. Meanwhile, the coexistence of two FM phase transitions can be directly associated with the existence of oxygen vacancies. As is to be expected, a larger concentration of oxygen vacancies is created in the
films grown at low oxygen partial pressures 共1 and 10 mTorr兲. Oxygen vacancies induce transfer of an electron from the e band of Co2+ ions to the neighboring Mn4+ ions, creating intermediate-spin Co3+ 共t5e1兲 and Jahn-Teller Mn3+ ions.3 Local Jahn-Teller deformations inhibit long-range ordering of Co and Mn on different lattice sites, stabilizing vibronic, ferromagnetic superexchange interactions in an atomically disordered volume. As pointed by Dass and Goodenough,3 such a vibronic superexchange gives a small stabilization as compared with the static FM Mn4+ – O – Co2+ superexchange which provides the high-temperature FM phase 共TC1兲, thus stabilizing a FM phase with a lower Curie temperature 共TC2兲. On the other hand, the existence of oxygen vacancies, similar to the effect of B-site disordering, will diminish the long-range FM ordering which originated from the static Mn4+ – O – Co2+ superexchange interactions. This provides an explanation why the high-temperature FM phase disappears in films grown at low oxygen pressures 共such as 1 mTorr兲 and the low-temperature FM phase is dominant. In summary, LCMO thin films have been grown on STO substrates over a wide range of oxygen concentration using the PLD technique. STEM images provide direct evidence of epitaxial growth, with the film-substrate interface being sharp and coherent. The oxygen concentration plays a crucial role in determining the magnetic properties. The existence of a second FM phase with lower TC is directly associated with the oxygen vacancies which introduces new vibronic FM superexchange interaction. The findings of this study clearly indicate the importance of oxygen concentration in determining the properties of ferromagnetic double perovskite thin films and the opportunity to optimize the growth conditions in order to realize their intrinsic properties. This work was supported by ONR under Grant No. N000140610226 共C. E. Wood兲 and NSF NIRT Grant No. CMS-0609377. H.G. and J.Z. were partially supported by NSF under Grant No. DMR-0346826. The research at ORNL was sponsored by the by the Office of Basic Energy Sciences, Division of Materials Sciences and Engineering. The authors wish to thank T. Calvarese and M. A. Subramanian for providing the ceramic LCMO target used for the growth of the thin films and J. T. Luck for specimen preparation for the STEM measurements. 1
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