UTRIP 2014 Report: Topotactic Fluorination of Perovskite Manganese Oxide Thin Films

Transcription

1 UTRIP 2014 Report: Topotactic Fluorination of Perovskite Manganese Oxide Thin Films Alpin Novianus Tatan Supervisors: Prof. Tetsuya Hasegawa, Assistant Prof. Akira Chikamatsu Department of Chemistry, School of Science, The University of Tokyo Hongo, Bunkyo-ku, Tokyo , JAPAN Abstract Fluorination of La 0.6Sr 0.4MnO 3 (LSMO) and SrMnO 3 δ (SMO) samples with Polyvinylidene Fluoride were performed to look for changes in their physical properties. Various experiments were conducted in order to discover the best conditions for fabricating and uorinating thin lms of such perovskite manganese oxides on (001) SrT io 3 substrate. Deposition of SMO at temperature of 700 o C with 15 mj of 2 Hz, KrF laser energy for 1 hour under background pressure below Torr proved better than any other sample conditions used in this project. Oxygen vacancies were proposed to have an important role in determining success of uorination along with other possible factors such as coherent growth. Magnetic characterization of the sample suggested an antiferromagnetic-to-ferromagnetic transition with Curie temperature above 380K as the consequence of uorination. 1

2 1 Introduction Perovksite manganese oxides, La 1 x Sr x MnO 3 exhibit a wide range of interesting physical phenomena, such as colossal magnetoresistance, metal-insulator transition, and half-metallic behavior. Since these properties are closely associated with metal-oxide-interaction, they can be drastically modied by moderate replacement of O 2 by F. One of the most versatile methods to obtain oxyuorides is topotactic reaction using polyvinylidene uoride (PVDF), where guest species (uorine) can be introduced into a host crystalline structure without destroying the initial crystalline framework. Thin-lm samples are expected to show considerably higher reactivity with PVDF rather than bulk ones, because they have larger surface area/volume ratio. Here, the study of uorine doping into La 0.6 Sr 0.4 MnO 3 and SrMnO 3 thin lms through topotactic reaction is reported. 1.1 The SrT io 3 substrate SrT io 3 (STO), which has cubic unit cell with lattice parameter of 3.905Å at room temperature[1], is chosen as the substrate for this experiment. STO is well lattice-matched to a wide selection of perovskite oxides, enabling epitaxial growth of various materials systems [2].Its structure does not change between room temperature and typical deposition temperatures of C [2] and has a thermal expansion coecient of C 1 at room temperature. 1.2 La 0.6 Sr 0.4 MnO 3 (LSMO) Bulk crystal of LSMO has rhombohedral symmetry (see Figure 1.1), with a lattice parameter of 3.87Åin pseudocubic setting [3]. It can be considered as an alternative stack of La(Sr)O and MnO 2 atomic layers along [001] direction [3]. In LSMO, the A-site is populated randomly by La 3+ and Sr 2+, allowing Mn cation on the B-site to have a mixed-valence of Mn 3+ /Mn 4+ [4]. LSMO is metallic and showing ferromagnetic transition at a certain temperature between K [4]. Its thin lm was reported to to adopt tetragonal structure when grown on STO as due to strain introduced by the substrate [3]. Figure 1.1: perovskite and rhombohedral perovskite structure [5, 6] 1.3 SrMnO 3 δ (SMO) SMO is band-like insulator with high-spin Mn 4+ t 3 2ge 0 g conguration [4]. Bulk SMO can be stabilized in hexagonal, orthorhombic, or cubic phases, depending upon oxygen content and 2

3 temperature[4]. Its thin lm grown on STO has been reported by other groups to have its outof-plane lattice parameter dependent of the target's Sr/M n ratio[7], e.g. for Sr/M n = 56/44 the lattice parameter was reported to be 3.78(5)Å [8]. The Sr/Mn ratio of the lm, in turn, show a weak dependence to laser uence used and hence enable ratio tuning for Mn-excess lms [7]. In order to make stoichiometric or Sr - excess lms, a Sr-excess target has to be used [8]. The lattice parameter is also sensitive to the amount of oxygen vacancies in the lm δ [8]. Thus measurement of lattice expansion will allow one to know the amount of vacancies in the lm. 1.4 Polyvinylidene Fluoride (PVDF) Polyvinylidene uoride is used as topotactic uorinating agent as it is stable in air with melting point of 170 C [9]. It is also more preferable than other uorinating agents such as the toxic F 2 gas, NH 4 F, and MF 2, (M = Cu, Zn, Ni, Ag) as these agents normally form unwanted metal byproducts [9, 10].The use of PVDF as topotactic uorinating agent has also achieved success in both oxidative [10] and non-oxidative reactions [9]. Thus, its versatility makes it a convenient uorinating material. 1.5 Project Aim In this project, we aim to fabricate thin lms of perovskite manganese oxides on top of STO substrate and then uorinate them with PVDF. We also wish to identify the possible determining parameters for the success of these two processes. Finally, we would like to investigate the change in its magnetic properties through measurement of its magnetization over a wide range of temperature. 2 Experimental Method The thin lms were to be fabricated on top of the STO substrate using Pulsed Laser Deposition (PLD) method from their bulk targets. Firstly, the STO substrate was cut to 5mm 5mm pieces by a diamond saucer. The cut substrate was then washed successively in acetone and ethanol for 5 minutes using an ultrasonic cleaner. Atomic Force Microscopy (AFM) was performed to investigate the surface of the substrate. Annealing of the substrate at high temperatures for few hours were performed to obtain at surface, which is advantageous to control the growth of the thin lm. Step-featured STO would be obtained after this heat treatment, as conrmed via AFM observation. The LSMO sample has been deposited in a previous occasion (see Table 1) and therefore was ready for uorination. Table 1: LSMO deposition conditions in terms of background pressure (B.P), temperature (T), Nd : Y AG laser energy (L.E), frequency (f), duration (d) Sample T ( C) B.P (10 4 T orr) L.E f (Hz) d (hour) LSMO mj 1 1 The SrMnO 3 δ (SMO) samples were to be fabricated in four dierent conditions (see Table 2), as the best condition was not yet known. Before the PLD took place, the STO substrate was washed again successively in acetone, ethanol, (EL) acetone, and (EL) ethanol via ultrasonic 3

4 cleaner and dried by a jet of nitrogen gas. It is then mounted on top of graphite support using platinum or silver paste, and heated in two phases: up to 100 C for 30 minutes and up to 350 C for 1 hour. The STO-graphite unit is then placed on the sample holder and tightly secured. Table 2: SMO deposition conditions in terms of background pressure (B.P), temperature (T), KrF laser energy (L.E). all processes were performed in 2 Hz of laser frequency for 1 hour sample # B.P (bef/aft) (10 7 T orr) T ( C) L.E (bef/aft) (mj) / / / / / / / /13 In order to remove impurities from the bulk SMO target, a pre-ablation step had to be performed. This was done by shooting laser pulses for 10 minutes to vaporize dust and other impurities. During this process, the sample was masked to prevent unwanted compounds being deposited. Afterwards, the actual deposition was performed once the desired conditions have been fullled. The fabricated lms were then analysed using Out-of-plane, 1D X-Ray Diractometry (Bruker D8 Discover, Cu K α1 radiation). Having known the peak positions for STO substrate, analysis of additional peaks would inform whether the deposition was successful. Successfully deposited samples were then uorinated with Polyvinylidene uoride (PVDF). The sample was rst wrapped with aluminium foil, then was put in a ask together with pieces of PVDF tablets. The ask was further wrapped in aluminium foil and placed on an alumina boat. They were inserted into a pipe and placed into the center of the uorination furnace. Argon gas was introduced to ow in the pipe during uorination with rate of 0.1l/min in order to prevent oxygen from reacting with the samples. The uorination was performed for 12 hours at 250 C, with 1 hour each to reach to and cool down from the desired temperature. Subsequently, the uorinated samples were re-analyzed using XRD to verify their crystal structure. This can be done via comparison of out-of-plane lattice parameters before and after uorination, as the introduced uorine atoms should alter the interplanar distance of the crystal. Energy Dispersive X-Ray Spectroscopy (EDS) measurement was also performed to investigate the chemical composition of the lm under vacuum condition P a. Successfully uorinated samples were then characterized by Magnetic Properties Measurement System (MPMS). The measurement was performed longitudinally in sweep mode at 1000 Oe of magnetic eld with increments of 3.97 K/min for temperature range between 10 K K. 3 Results 3.1 AFM studies of STO substrate Atomic force microscopy images of the substrate are depicted Figure 3.1. The images signies the importance of annealing process as step-terrace pattern could be obtained. The at surface features of the annealed substrate was desirable in order to successfully fabricating thin lms. 4

5 Figure 3.1: AFM images and pro les of STO: before anneal (left/top) and after anneal (right/bottom) 3.2 XRD studies of LSMO sample Figure 3.2 shows X-ray Di raction spectra of LSMO sample before and after uorination. From the spectra, the out-of-plane lattice parameters of LSMO before and after uorination are determined to be Åand Å respectively. From this lattice value and small number of peaks generated, it can be inferred that non-layered LSMO was fabricated. The change in lattice parameter after uorination, however, was minute. Hence, it is concluded that uorination is not successful for this non-layered perovskite sample. Figure 3.2: XRD spectra of LSMO sample: before and after uorination 3.3 XRD studies of SMO Figure 3.3 shows X-Ray Di raction Spectra for SMO samples fabricated in di erent conditions. From the spectra, indications of SMO lm can be observed for samples fabricated at 600 C 5

6 (#220) and 700 C (#215). The SMO peaks observed correspond to (002) planes [8]. These peaks then lead to the calculation of out-of-plane lattice parameter, which produces value of 3.871Å and 3.839Å for #220 and #215 samples, respectively. Figure 3.3: XRD spectra of fabricated (left) and uorinated (right) SMO samples. 3.4 Fluorination of SMO Samples #220 and #215 are then uorinated with PVDF through identical procedure as LSMO. Comparison of XRD spectra before and after uorination (Figure 3.3 (right)) shows a peak shift for (002) plane for sample #215, while SMO traces have disappeared from sample #220 after uorination. The new peak correspond to out-of-plane lattice parameter of Å, which means that the lattice has expanded in c-direction as an result of uorination process of sample # EDS studies of uorinated SMO Such lattice expansion elaborated in the previous section could be caused by introduction of uorine atoms. In order to verify this hypothesis, the chemical composition of the uorinated sample is studied using EDS measurements. The EDS result is shown in Figure 3.4. From this result, it is veri ed that uorine has been successfully introduced into the lm. Figure 3.4: SEM picture and EDS of sample #215 6

7 3.6 Size of uorinated SMO lm The thickness of the lm was measured via X-Ray Reectivity (XRR) measurement and its lateral dimensions were estimated using graph paper and a digital camera (Figure 3.5). The lm was measured to be nm thick with an area of 22.0 mm 2. This corresponds to cm 3 of deposited lm. Figure 3.5: Measurement of lm size 3.7 Magnetic Properties of uorinated SMO The M T curve of the uorinated SMO sample is shown in Figure 3.6. The curve suggests ferromagnetic behavior with Curie temperature larger than 380 K. Simple conversion of the units of the vertical scale in Figure 3.6 led to calculation of magnetic moment to be 0.46µ B /Mn atom at low temperature. Figure 3.6: Temperature dependence of uorinated SMO sample's magnetization. Measurement was taken at 1000 Oe of magnetic eld. 4 Discussion In order to epitaxially grow thin lms of perovskite manganese oxides, atomically at substrates are required to maintain crystallinity of the deposited lm. Rough surfaces would result in uneven distributions of deposited materials, which results in grain boundaries and other defects 7

8 which tend to form polycrystalline or even amorphous lms. Flat surfaces, on the other hand, facilitate epitaxial growth as well as minimize the strain induced from grain boundaries. In optimum conditions, this may lead to coherence growth, for which the crystal structure of the deposited lm adopt the same size and shape of the substrate. For these reasons, the condition of the substrate has been improved by performing heat treatments to obtain surfaces as at as possible. XRD studies of LSMO sample does not suggest a successful uorination. Based on the knowledge of our sample, this result suggests the importance of oxygen vacancies in the success of a uorination process. The existence of such vacancies are expected to facilitate the inclusion of uorine into the lm's lattice structure. This can be veried from our results for SMO samples which are fabricated to have some oxygen vacancies with them. The best condition for deposition and uorination we have discovered in this project is found to belong to Sample #215. The observed lattice expansion upon uorination is most likely due to weaker electrostatic interaction attributed to uorine's lower valence compared to oxygen ions. Such expansion has been reported elsewhere [10] for uorination experiments and hence is expected. This success then again recommends oxygen vacancies as one important factor in uorination process, while the otherwise result of sample #220 implies the existence of other determining factors, for instance, the coherence growth of the lm. SMO has been reported elsewhere [11] to show antiferromagnetic behavior. Thus, our obtained M T curve suggests that an antiferromagnetic-to-ferromagnetic transition has occurred as the consequence of uorination. While the underlying mechanism of this transition is not yet understood, possible candidates include the double-exchange mechanism, as this governs similar transitions occurring in other perovsite manganese oxides, as well as concepts on critical bonding angles and distances. On the other hand, calculation of magnetic moment per M n atom suggests some room for improvement in future experiments regarding magnetic properties of the sample. One such improvement, for instance, is to increase the external applied magnetic eld. By doing so, it is expected that the gradient of magnetization curve would approach zero at low temperatures, i.e. the magnetization saturates. As such, higher value of magnetic moments per M n atom could be achieved. More rigorous theoretical calculations to reveal the actual governing mechanism behind the results we have obtained, however, are unfortunately out of the scope of this project. Nevertheless, the resulting ferromagnetic material with relatively high curie temperature does highlight the drastic alterations in physical properties due to uorination, which satises our objective. 5 Conclusion In this project, we have studied the properties of perovskite manganese oxides. We looked for changes in physical properties of these materials in their thin lm state upon uorination with PVDF. Our experiment with LSMO sample highlighted the importance of oxygen vacancies in determining the success of uorination process. In our eort to fabricate SMO samples with oxygen vacancy, deposition conditions of samples #215 and #220 were found to best allow the growth of SMO lms among others. Fluorination of these samples revealed the condition of sample #215 as the best of our samples of study in this project as conrmed via XRD and EDS measurements. Antiferromagnetic-to-ferromagnetic transition was observed in MPMS measurement as the impact of uorination to our sample. Further experiments to look for other physical property alterations and to understand the relation between deposition and uorination conditions with the resulting properties are highly encouraged to be performed as a continuation of this project. 8

9 6 Acknowledgement I would like to thank Mr. Yuji Kurauchi, Mr. Keisuke Kawahara, Mr. Tomoya Onozuka, and Ms. Thantip S.Krasienapibal for all their support during this program. I would also like to thank my supervisors: Prof. Tetsuya Hasegawa and Assistant Prof. Akira Chikamatsu for their valuable advice and discussions. References [1] F.W Lytle. (1964). J.Appl.Phys, 35 (1964), [2] T.Tybell, C.Eom, Synthesis of epitaxial and multiferroic oxide thin lms, E.Tsymbal,et.al (Ed). Multifunctional Oxide Heterostructures,(2012). Oxford: Oxford University Press. [3] M.Izumi, et.al. Appl. Phys. Lett. Vol.73. No.17. (1998), [4] A.Bhattacharya, S.Dong, R.Yu, Manganite Multilayers, E.Tsymbal,et.al (Ed). Multifunctional Oxide Heterostructures,(2012). Oxford: Oxford University Press. [5] S. Majumdar, S. van Dijken, J. Phys. D: Appl. Phys. 47 No.3 (2014) [6] H.Naganuma, Multifunctional Characteristics of B-site Substituted BiF eo 3 Films,M.Lallart (Ed), Ferroelectrics - Physical Eects, (2011), InTech, DOI: /17538 [7] S.Kobayashi, et.al. J.Mater. Sci, 46 (2011), [8] S.Kobayashi, Y.Ikuhara, T.Yamamoto. Appl.Phys.Lett,102 (2013), [9] T. Katayama, et.al. J. Mater. Chem. C, 2 (2014), 5350 [10] T. Sivakumar, J.B. Wiley, Materials Research Bulletin, 44 (2009) [11] R.Søndenå, P. Ravindran, S. Stølen. Phys.Rev.B, 74 (2006)