INDO SWISS JOINT RESEARCH PROGRAMME (ISJRP) RESEARCH FELLOWSHIPS EXCHANGE GRANT REPORT. Grant No.: RF30

Transcription

1 INDO SWISS JOINT RESEARCH PROGRAMME (ISJRP) RESEARCH FELLOWSHIPS EXCHANGE GRANT REPORT Grant No.: RF30 Part 1 - General Information Project Title: Phenomenological studies of some strongly correlated system Keywords: Correlated system, Thin films Start date: 01 st February, 2012 Duration: 11 months Part 2 - Exchange Participant(s) Details VISITING SCIENTIST [Name] Proloy Taran Das [Institute], University] Indian Institute of Technology Kharagpur [Address] Department of Physics & Meteorology, IIT Kharagpur [postal code, city, COUNTRY]721302,Kharagpur, India [ address] dasproloy@phy.iitkgp.ernet.in, dasproloy2007@gmail.com HOSTING SCIENTIST [Name] Prof. Christian Bernhard [Institute, University] University Of Fribourg [Address] Chemin du Museé 3 [postal code, city, COUNTRY]CH-1700, Fribourg, Switzerland [ address] Christian.bernhard@unifr.ch

2 Part 3 - Scientific & Technical Information 3.1 Purpose of visit The experimental investigation of cooperative phenomena in strongly correlated electron systems with reduced dimensionality is a hot topic in modern solid state physics. Prominent examples are the perovskite-type manganites which exhibit a colossal magneto-resistance (CMR) effect where the resistance changes by orders of magnitude at a transition from a paramagnetic and insulating or badly conducting high temperature state to a ferromagnetic and metallic low temperature state. This CMR effect holds great promises for applications in sensors and electronic devices. The goal of my research was the growth and the investigation of thin films of LaMnO 3 which is the parent compound of the CMR manganites. In the bulk form LaMnO 3 is an antiferromagnetic (A-type) insulator. The CMR effect appears upon hole doping which is achieved by substituting divalent alkaline cations, like Ca 2+, Ba 2+, or Sr 2+, on the La-site. Alternatively, this hole doping can be obtained in La- or Mn-deficient La 1-δ Mn 1-δ O 3 which is often written as LaMnO 3+δ. As a function of increasing δ, the material evolves through a spinglass regime at 0.0 δ 0.1, to a ferromagnetic insulating state at 0.11 δ 0.14 and, at even larger δ values, to metamagnetic and eventually ferromagnetic metallic state. This behavior is well-established for LaMnO 3+δ bulk ceramics [1, 2]; however it is not systematically explored and thus poorly understood in thin films for which the electronic and magnetic properties are found to vary strongly depending on the details of the growth parameters. The goal of my research was to investigate these trends systematically. The purpose of the visit can be summarized in the following: 1. To learn new experimental techniques like pulsed laser deposition and spectroscopic ellipsometry. 2. Growth of epitaxial LaMnO 3+δ (LMO) thin films by pulsed laser deposition using different growth parameters. 3. Study the structural, electrical, magnetic and optical properties of these LMO thin films and, in particular, the influence of deposition conditions on these properties. 4. Investigate the influence of post-deposition oxygen annealing treatments. 5. Study the optical properties of hole doped manganite nano particles. 6. To interpret and analyze the obtained experimental data. 3.2 Short description of the work carried out during the visit Growth and experimental of LaMnO 3+d, LaMnO3+d/La 2/3 Ca 1/3 MnO 3 : In this work, we present a systematic study of LaMnO 3+δ thin films that have been grown by pulsed laser deposition with a KrF 248 nm excimer laser at a fluence of 2 Jcm -2 and pulse frequency of 2 Hz. The substrate heated to C and we used different background gases (i.e. O 2 and N 2 O) and gas pressures ( mbar).

3 Fig. 1 Schematic diagram of a Pulsed Laser Deposition (PLD) system. Some of these as grown films were post annealed at C for 15 hours in oxygen atmosphere to increase the oxygen content in the films. The film thickness of 100 nm was controlled with Reflection high-energy electron diffraction analysis during the layer by layer growth. The microstructural, magnetic, and electromagnetic properties have been studied with X ray diffraction and reflectometry, atomic force microscopy and with electric conductivity and magnetization measurements. In addition, the optical spectra have been measured with broad-band (far-infrared to ultraviolet range) ellipsometry. During my stay, I developed technical skills in Pulsed laser deposition growth, Physical property measurement system, Atomic force microscopy, and the Ellipsometry technique. Besides, I gained some knowledge on defect chemistry, physics of the proximity effect, and the role of a 2D electron gas at interface in manganite systems. 3.3 Outcomes The oxygen content as well as the magnetic, optical, and electronic properties of the films can be controlled in-situ by selecting the background gas for deposition (O 2, and N 2 O) as well as by varying the depositing parameters: background gas pressure, substrate temperature, laser fluence, and film thickness. High quality thin films with different oxygen contents were obtained covering the range from the antiferromagnetic-insulating state to the ferromagneticmetallic state on the phase diagram (i.e. with δ from 0 to 0.14). The films were shown to have saturation magnetic moments at 10 K in the range from 0 to ~ 4 μ B (per Mn ion). The observed changes suggest that the films tend to form cationic vacancies whose type and concentration systematically depend on the PLD growth parameters [2, 3]. An important parameter is the type and the pressure of the background gas in which the deposition is performed. A Rutherford back scattering (RBS) analysis reveals a different La/Mn ratio in the samples deposited in different oxygen background pressure: the films grown in lower P(O 2 )

4 are Mn deficient (relative to the La content), whereas the films grown in higher P(O 2 ) tend to be La deficient. This difference has a distinct influence on the electrical and magnetic properties of the films. Figure 2 (a), shows the magnetization behavior of samples grown in 0.3 mbar background pressure of oxygen. The figure depicts that all samples show a ferromagnetic behavior at low temperatures. The paramagnetic to ferromagnetic transition temperature and magnetic moment increase upon post-annealing in oxygen. However, the samples which are grown in lower oxygen background pressure of 0.1 mbar (both annealed and not annealed) exhibit a weak ferromagnetic insulating or canted antiferromagnetic insulating state in the temperature range from 10 K to ~150 K. The frequency dependent inplane optical conductivity σ 1 (ω) was measured with ellipsometry for a set of four representative LMO films. Figure 2 (b) shows the temperature-dependent spectral weight redistribution of an annealed sample that was grown at 0.3 mbar oxygen background pressure. M (μ Β / Mn atom) P O2 = 0.30 mbar Not Annealed C C (a) LMO-1047 P (O 2 )= 0.30 mbar 300 K 15 K 100 K 150 K 200 K 250 K (b) T (K) Photon energy (ev) Fig. 2. (a) Magnetization as a function of temperature for 100 nm thick LaMnO 3+d films grown in oxygen at a pressure of 0.3 mbar and annealed at different temperatures as indicated in the graph; (b) Spectra of the in-plane optical conductivity, σ 1 (ω), of the film annealed at C for 15 hr σ 1 (Ω 1 cm -1 ) We have also measured the in plane optical conductivity, σ 1 (ω) of manganite Pr 0.8 Sr 0.2 MnO 3, and Nd 0.8 Sr 0.2 MnO 3 nano particles in the far infrared and mid infrared range by a home-made ellipsometer with rotating analyzer type. We have observed polaronic feature in the mid infrared range. The observed data may explain the small polaronic hopping model of this kind of strongly correlated system. References: 1. J. Töpfer and J. B. Goodenough, J. Solid State Chem. 130, 117(1997). 2. J. Töpfer and J. B. Goodenough, Chem. Mater. 9, 1467(1997). 3. J. A. M. Van Rossmalen, E. H. P. Cordfunke, R. B. Helmholdt, and H. W. Zandbergen, J. Soild State Chem. 110, 100(1994).

5 3.4 Future collaboration with host institution In the near future, we plan to envisage innovative scientific research on perovskite thin films, multilayers, and superlattices of manganite and high T C superconductor to explore the underlying physics of different novel quantum states at the interface. Study of extremely fascinating phenomena such as multiferroicity, magnetism and superconductivity properties on perovskite thin films and oxide heterostructures may have a strong potential in applied research field. This visit will also help us to open a gateway between these two institutions for long term collaboration. 3.5 Various comments The visit for 11 months through the Indo-Swiss Joint Research Fellowship to the Solid State Physics Lab at the University of Fribourg, Switzerland was very successful. I was able to perform some very interesting research work. We have also completed this project within due time, a publication is in progress. Besides that, I have seen the majestic beauty of Switzerland and enjoyed a lot the hospitality, and cultural activities. Acknowledgement: I would like to acknowledge earnestly the support of Prof. C. Bernhard and of the Indo-Swiss Joint research program (ISJRP) without whom I would miss this excellent research platform. Christian s way of thinking, encouragement, and personal guidance has provided an asset for my personality and knowledge. I would also like to give a special thanks to the group members Dr. I. Marozau and Dr. M. Rössle who supported me in my work. Also I wish to express my sincere gratitude to Dr. Laura Nuccio, Dr. Premysl Marsik, Miguel, Saikat, Chennan, Kaushik, Meghdad, and Thomas. I am very much grateful to Director, and Dean (PGS &R) of IIT Kharagpur, Head of the Department of Physics and Meteorology, IIT Kharagpur and obviously to my thesis supervisors Prof. A. Taraphder and Prof. T. K. Nath to avail this opportunity. In a word, it was an amazing experience to my life. 3.6 Projected publications/articles resulting or to result from the exchange 1. An abstract of the work was presented in the JEMS-2012, held in Parma, Italy during September 9-14, An abstract of the work is accepted for poster presentation in the ISJPS-2013, will be held in IIT Kharagpur, India during February 25-27, Full page manuscripts are in process.