Papers by Sayani Majumdar
Interfacial Properties of Organic Semiconductor–Inorganic Magnetic Oxide Hybrid Spintronic Systems Fabricated Using Pulsed Laser Deposition
ACS Applied Materials & Interfaces, 2015
We report fabrication of a hybrid organic semiconductor-inorganic complex oxide interface of rubr... more We report fabrication of a hybrid organic semiconductor-inorganic complex oxide interface of rubrene and La0.67Sr0.33MnO3 (LSMO) for spintronic devices using pulsed laser deposition (PLD) and investigate the interface structure and chemical bonding-dependent magnetic properties. Our results demonstrate that with proper control of growth parameters, thin films of organic semiconductor rubrene can be deposited without any damage to the molecular structure. Rubrene, a widely used organic semiconductor with high charge-carrier mobility and spin diffusion length, when grown as thin films on amorphous and crystalline substrates such as SiO2-glass, indium-tin oxide (ITO), and LSMO by PLD at room temperature and a laser fluence of 0.19 J/cm(2), reveals amorphous structure. The Raman spectra verify the signatures of both Ag and Bg Raman active modes of rubrene molecules. X-ray reflectivity measurements indicate a well-defined interface formation between surface-treated LSMO and rubrene, whereas X-ray photoelectron spectra indicate the signature of hybridization of the electronic states at this interface. Magnetic measurements show that the ferromagnetic property of the rubrene-LSMO interface improves by >230% compared to the pristine LSMO surface due to this proposed hybridization. Intentional disruption of the direct contact between LSMO and rubrene by insertion of a dielectric AlOx layer results in an observably decreased ferromagnetism. These experimental results demonstrate that by controlling the interface formation between organic semiconductor and half-metallic oxide thin films, it is possible to engineer the interface spin polarization properties. Results also confirm that by using PLD for consecutive growth of different layers, contamination-free interfaces can be obtained, and this finding is significant for the well-controlled and reproducible design of spin-polarized interfaces for future hybrid spintronics devices.
Magnetoresistance in bulk heterojunction solar cells
The magnetoresistance (MR) response of the poly(3-hexyl thiophene) and poly(3-hexyl thiophene):1-... more The magnetoresistance (MR) response of the poly(3-hexyl thiophene) and poly(3-hexyl thiophene):1-(3-methoxycarbonyl) propyl-1-phenyl-[6,6]-methanofullerene (PHT:PCBM) based bulk heterojunction solar cells have been studied. Positive MR was always observed at room temperature in both the devices. In both cases the magnitude of the MR signal decreases at lower temperature and shows positive to negative sign inversion at 100K for the solar cells and
We report giant magnetoresistance up to 150 percent at low bias current and low temperature as we... more We report giant magnetoresistance up to 150 percent at low bias current and low temperature as well as room temperature magnetoresistance in polymeric spin-valves having the structure LSMO/conjugated polymer/Co. The conjugated polymers, regiorandom and regioregular P3HT were used as the spacer materials. We observed an asymmetric bias voltage dependence of different devices and additional, hitherto unseen, peaks in MR vs.
Regioregular polythiophene based spintronic devices: effect of interface
Polymeric spin valves have been fabricated using regio-regular (poly 3-hexylthiophene) (RRP3HT) a... more Polymeric spin valves have been fabricated using regio-regular (poly 3-hexylthiophene) (RRP3HT) as the spacer layer sandwiched between La0.67Sr0.33MnO3 (LSMO) and Co electrodes. The devices show high spin valve magnetoresistance (MR) at 5K (80%) which reduces at room temperature to 1.5%. The spin valve behavior is quite similar to a magnetic tunnel junction although the non-magnetic spacer layer (˜100 nm) is
Synthetic Metals, 2010
In this article we review the experimental observation of ferromagnetism in metal-oxide systems. ... more In this article we review the experimental observation of ferromagnetism in metal-oxide systems. This is relevant for the application of the most commonly used metal-oxide, indium tin-oxide (ITO), in organic electronic devices where organic magnetoresistance phenomenon has recently been observed. We provide experimental evidence of ferromagnetism in commercial ITO substrates and investigate the role of impurities in giving rise to such effects. Magnetoresistance have also been observed in organic diodes without any ITO contacts. However, it is important to take into account the intrinsic ferromagnetism in the ITO when interpreting the experimental data for devices with ITO. The possible limitations of using ITO are discussed in the article.
ChemInform, 2010
In this article we review the experimental observation of ferromagnetism in metal-oxide systems. ... more In this article we review the experimental observation of ferromagnetism in metal-oxide systems. This is relevant for the application of the most commonly used metal-oxide, indium tin-oxide (ITO), in organic electronic devices where organic magnetoresistance phenomenon has recently been observed. We provide experimental evidence of ferromagnetism in commercial ITO substrates and investigate the role of impurities in giving rise to such effects. Magnetoresistance have also been observed in organic diodes without any ITO contacts. However, it is important to take into account the intrinsic ferromagnetism in the ITO when interpreting the experimental data for devices with ITO. The possible limitations of using ITO are discussed in the article.
Synthetic Metals, 2013
This article presents a comparison of spin transport mechanism in two π-conjugated organic polyme... more This article presents a comparison of spin transport mechanism in two π-conjugated organic polymers namely, regiorandom and regioregular poly (3-hexyl thiophenes) with same elemental composition but different regioregularity of the constituent atoms leading to different crystallinity and charge carrier mobility. Spin-valve devices made with both polymers show substantial low temperature giant magnetoresistance (GMR) response. However, the GMR signal decreases drastically at higher temperatures where charge carrier mobility is higher. Our results suggest that in both the polymers spin diffusion length at low temperature is almost similar, but, temperature dependence of spin diffusion length is greater in the disordered polymer compared to the more structured one. Comprehensive analysis of our experimental data suggest that at low temperature, in the VRH hopping regime (5 -50 K), spin relaxation due to hyperfine interaction and Elliot-Yafet momentum scattering is the dominant spin relaxation mechanism while in the thermally activated regime Dyakonov-Perel mechanism contribution becomes significant. However, mobility dependence of spin scattering rate in both systems differ from traditional Dyakonov-Perel model signifying that there are coexisting contributions from several spin scattering effects present in the system. Proper understanding and careful modification of spin-orbit coupling in organic semiconductors can be very useful for organic based spin devices.
Physical Review B, 2009
Magneto-electrical measurements were performed on diodes and bulk heterojunction solar cells (BHS... more Magneto-electrical measurements were performed on diodes and bulk heterojunction solar cells (BHSCs) to clarify the role of formation of coulombically bound electron-hole (e-h) pairs on the magnetoresistance (MR) response in organic thin film devices. BHSCs are suitable model systems because they effectively quench excitons but the probability of forming e-h pairs in them can be tuned over orders of magnitude by the choice of material and solvent in the blend. We have systematically varied the e-h recombination coefficients, which are directly proportional to the probability for the charge carriers to meet in space, and found that a reduced probability of electrons and holes meeting in space lead to disappearance of the MR. Our results clearly show that MR is a direct consequence of e-h pair formation. We also found that the MR line shape follows a power law-dependence of B 0.5 at higher fields.
physica status solidi (RRL) - Rapid Research Letters, 2009
We report on hysteretic organic magnetoresistance (OMAR) in polymeric diodes. We found that magni... more We report on hysteretic organic magnetoresistance (OMAR) in polymeric diodes. We found that magnitude and lineshape of OMAR depends strongly on the scan speed of the magnetic field and on the time delay between two successive measurements. The time-dependent OMAR phenomenon is universal for diodes made with various polymers. However, the width and magnitude of OMAR varied with the polymeric material. The suggestive reason for this hysteretic behavior are trapped carriers, which in presence of a magnetic field changes the ferromagnetic ground-state of the polymer leading to long spin relaxation time. These experimental observations are significant for clarification of the OMAR phenomenon.
physica status solidi (a), 2009
We report the study of magnetotransport properties of regio-regular poly (3-hexyl thiophene) base... more We report the study of magnetotransport properties of regio-regular poly (3-hexyl thiophene) based organic diodes. The devices were fabricated using two different techniques of spin coating and inkjet printing. Positive magnetoresistance (MR) effect was observed at room temperature in all the devices. The highest MR magnitude reached up to 16% for some spin-coated devices and up to 10% in inkjet printed devices. The MR magnitude and line shapes were found to depend strongly on the measuring current. We observed deviation from the theoretically predicted Lorentzian or non-Lorentzian line shape of the MR traces, which is discussed in detail in the article. Although, the printed devices exhibit MR response as high as for the spin coated ones, they still need to be optimized in terms of performance and yield for large scale applications as magnetic sensors.
New Journal of Physics, 2009
We report the effect of magnetic impurities in the spacer layer of polymeric spin valves (PSV) wi... more We report the effect of magnetic impurities in the spacer layer of polymeric spin valves (PSV) with the sandwich configuration of La 0.67 Sr 0.33 MnO 3 (LSMO)/π-conjugated polymer regio-random poly(3-hexyl thiophene)/cobalt (Co), showing giant magnetoresistance (GMR) response. Different deposition rates of Co at the top electrode resulted in two types of devices: one with lower device resistance and linear current-voltage (I-V) characteristics and the other with very low inclusion of Co and exhibiting higher device resistance and nonlinear I-V characteristics. We observed an asymmetric dc bias dependence of magnetoresistance (MR) in devices with more Co inclusion, while for the other type of device, bias dependence was more symmetric. At higher bias, %MR of both types of device showed no significant difference (5-10%), but at low dc bias it ranged between 50 and 160% MR. This can be attributed to the higher tunneling probability of spin-polarized carriers from one ferromagnetic electrode to the other. Magnetic tunnel junction-like features are observed in the devices with greater Co inclusions. Anomalous MR peaks were also observed in these devices and their origin was explained in terms of presence of additional scattering centers around the included metal ions and increased spin relaxation due to high magnetic anisotropy in the system. Both types of PSVs showed a monotonic decrease in MR with temperature at high bias currents.
Journal of Physics: Condensed Matter, 2012
Magnetoresistive double perovskite Sr 2 FeMoO 6 thin films were grown with two different depositi... more Magnetoresistive double perovskite Sr 2 FeMoO 6 thin films were grown with two different deposition pressures on SrTiO 3 , MgO and NdGaO 3 substrates by pulsed laser deposition and thorough structural, magnetic and magneto-transport characterization was made. According to x-ray diffraction, all the films were phase pure and fully textured. Indication of substrate dependent strain and low angle grain boundaries was found, especially in films on MgO. Both the deposition pressure and the choice of the substrate have a strong influence on the saturation magnetization, M s , and Curie temperature, T C . The structural and magnetic data indicate the presence of anti-site disorder (ASD) in the films. The temperature dependence of resistivity showed semiconductive behaviour at temperatures below 100 K and metallic behaviour at higher temperatures. The semiconductive behaviour was found to increase with increasing ASD. In good quality films, up to 12% negative magnetoresistance (MR) was observed and films grown on MgO and NGO substrates also showed low field MR. However, the most significant observation of this study was that the magnetoresistivity of these Sr 2 FeMoO 6 thin films could not be explained with any traditional MR mechanism, but carried the clear signature of superposition of different mechanisms, in particular low angle grain boundary tunnelling and suppression of antiferromagnetically ordered domains under a magnetic field.
Journal of Physics: Condensed Matter, 2012
The present paper reports detailed structural and magnetic characterization of the low-bandwidth ... more The present paper reports detailed structural and magnetic characterization of the low-bandwidth manganite Pr 1−x Ca x MnO 3 (with x = 0.0-0.5) (PCMO) polycrystalline samples. With increasing Ca content, reduction of the unit cell volume and improvement in perovskite structure symmetry was observed at room temperature. Magnetic characterization shows the signature of coexisting AFM-FM ordering and spin-glass phase at the low doping range (x = 0.0-0.2) while increased hole doping (x = 0.3-0.5) leads to charge ordering, training effect and an irreversible metamagnetic phenomenon. The large irreversible metamagnetism in the CO phase of PCMO and the corresponding spin memory effect is a direct consequence of hysteretic first-order phase transition arising from the weakening of the CO state under the external magnetic field and trapping of the spins due to a strong pinning potential in the material.
The European Physical Journal Conferences
Magnetoresistive Sr 2 FeMoO 6 thin films were grown by pulsed laser deposition with three differe... more Magnetoresistive Sr 2 FeMoO 6 thin films were grown by pulsed laser deposition with three different thicknesses 150 nm, 270 nm and 500 nm. Structural, magnetic and magneto-transport properties of the films were measured. Structural properties showed that impurity phases are formed when the film thickness exceed limiting thickness over 300 nm. Otherwise no major differences were observed in structural and magnetic properties between the films. The semiconductive upturn was observed in all ρ(T ) curves, but it was notably smaller for the two thickest films. At 350 K the magnetoresistive (MR) behaviour was very similar for all the films, but at 10 K the negative MR was clearly largest for the thickest film and also the shape of the curve in low fields deviated from others.
Journal of Physics: Condensed Matter, 2011
Optimization of thin films of small bandwidth manganite, Pr 1−x Ca x MnO 3 (for x = 0.1), and the... more Optimization of thin films of small bandwidth manganite, Pr 1−x Ca x MnO 3 (for x = 0.1), and their magnetic properties are investigated. Using different pulsed laser deposition (PLD) conditions, several films were deposited from the stoichiometric target material on SrTiO 3 (001) substrate and their thorough structural and magnetic characterizations were carried out using x-ray diffraction, atomic force microscopy, x-ray photoelectron spectroscopy (XPS), SQUID magnetometry and ac susceptibility measurements. A systematic investigation shows that irrespective of the growth temperature (between 550 and 750 • C), all the as-deposited films have twin boundaries and magnetic double phases. Post-annealing in partial or full oxygen pressure removes the extra phase and the twin boundaries. Zero-field-cooled magnetization data show an antiferromagnetic to paramagnetic transition at around 100 K whereas the field-cooled magnetization data exhibit a paramagnetic to ferromagnetic transition close to 120 K. However, depending on the oxygen treatments, the saturation magnetization and Curie temperature of the films change significantly. Redistribution of oxygen vacancies due to annealing treatments leading to a change in ratio of Mn 3+ and Mn 4+ in the films is observed from XPS measurements. Low temperature (below 100 K) dc magnetization of these films shows metamagnetic transition, high coercivity and irreversibility magnetizations, indicating the presence of a spin-glass phase at low temperature. The frequency dependent shift in spin-glass freezing temperature from ac susceptibility measurement confirms the coexistence of spin-glass and ferromagnetic phases in these samples at low temperature.
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Journal of Applied Physics, 2008
We report the effect of La 0.7 Sr 0.3 MnO 3 ͑LSMO͒ electrodes on the temperature dependence of th... more We report the effect of La 0.7 Sr 0.3 MnO 3 ͑LSMO͒ electrodes on the temperature dependence of the magnetoresistance ͑MR͒ of LSMO/polymer/cobalt spin valves ͑SVs͒. LSMO films have been prepared by pulsed laser deposition on three different single crystal substrates using different deposition parameters. The films were characterized for their surface morphologies, structural, magnetic, and magnetotransport properties. Low deposition rate is found to be detrimental for growth of good quality films and polycrystalline films with grain boundary effects are observed in thicker films. The films on MGO ͑100͒ substrate show a broad paramagnetic to ferromagnetic transition, accompanied with a metal-insulator transition below room temperature. This indicates growth of some strained structures due to large lattice mismatch ͑9%͒ between the substrate and the film and presence of polycrystalline grain boundaries. The deposited films on STO ͑100͒ and NGO ͑001͒ show much sharper magnetic transition and metallic behavior indicating higher spin polarization ͑SP͒ of LSMO on these substrates at room temperature. SVs made on STO ͑100͒ show improvement in switching behavior and better MR response compared to the devices made on MGO ͑100͒ at low temperatures. No difference in MR response was found at room temperature in either case. We conclude that the bulk spin polarization of LSMO films is not very important in the SV operation. The loss of most of the SP carriers at the LSMO/organic semiconductor interface at room temperature is a more dominant effect and drastically reduces the MR signal.
Materials engineering on the nanoscale by precise control of growth parameters can lead to many u... more Materials engineering on the nanoscale by precise control of growth parameters can lead to many unusual and fascinating physical properties. The development of pulsed laser deposition (PLD) 25 years ago has enabled atomistic control of thin films and interfaces and as such it has contributed significantly to advances in fundamental material science. One application area is the research field of spintronics, which requires optimized nanomaterials for the generation and transport of spin-polarized carriers. The mixed-valence manganite La 1−x Sr x MnO 3 (LSMO) is an interesting material for spintronics due to its intrinsic magnetoresistance properties, electric-field tunable metal-insulator transitions, and half-metallic band structure. Studies on LSMO thin-film growth by PLD show that the deposition temperature, oxygen pressure, laser fluence, strain due to substrate-film lattice mismatch and post-deposition annealing conditions significantly influence the magnetic and electrical transport properties of LSMO. For spintronic structures, robust ferromagnetic exchange interactions and metallic conductivity are desirable properties. In this paper, we review the physics of LSMO thin films and the important role that PLD played in advancing the field of LSMO-based spintronics. Some specific application areas including magnetic tunnel junctions, multiferroic tunnel junctions and organic spintronic devices are highlighted, and the advantages, drawbacks and opportunities of PLD-grown LSMO for next-generation spintronic devices are discussed.
Journal of Alloys and Compounds, 2012
Colossal magnetoresistive manganite La 0.7 Sr 0.3 MnO 3 (LSMO) films were prepared by pulsed lase... more Colossal magnetoresistive manganite La 0.7 Sr 0.3 MnO 3 (LSMO) films were prepared by pulsed laser deposition on three different single crystal substrates using different deposition parameters. Characterizations of their surface morphologies, structural, magnetic and magneto-transport properties show that films on MgO single crystal substrates contain higher amount of structural defects compared to those on SrTiO 3 (STO) and NdGaO 3 (NGO) substrates. Low deposition rate and thicker films give rise to polycrystallinity and grain boundaries. The films on MgO substrate showed a broad paramagnetic (PM) to ferromagnetic (FM) transition accompanied with metal -insulator transition (MIT) much below their Curie temperature (T C ) indicating growth of strained structures due to large lattice mismatch (9%) between the substrate and the film. The deposited films on STO and NGO show least effect of substrate induced strain exhibiting sharper PM-FM transition and metallic behavior below T C . The magnetoresistance (MR) † Corresponding author e-mail: sayani.majumdar@utu.fi, Phone: +358 2 3336240, Fax: +358 2 3335070
Journal of Alloys and Compounds, 2006
We report spin polarized injection and transport in organic spin-valves made from both organic sm... more We report spin polarized injection and transport in organic spin-valves made from both organic small molecules and polymers. The devices with the structure La 0.67 Sr 0.33 MnO 3 (LSMO)/organic spacer/Co showed inverse magnetoresistance and low temperature operation with the vacuum evaporated -conjugated small molecule 8-hydroxyquinoline aluminium (Alq 3 ) while normal MR and high temperature operation was observed in devices with the -conjugated polymer poly(3-hexylthiophene). Due to the proximate values of the work functions of LSMO and Co with the highest occupied molecular orbital (HOMO) energy of the polymers, spin polarized carrier injection is much more efficient in the polymeric spin-valves compared to the organic spin-valves with Alq 3 where carrier injection is hindered due to greater barrier height. Efficient spin transport is also observed in polymeric spin-valves and can be attributed to longer conjugation in the polymeric chains compared to the small molecules.
Magnetoresistance and magnetothermoelectric power of La0.5Pb0.5Mn1-xCrxO3
Physical Review B, 2001
We report the results of thermoelectric power (TEP) of a Cr doped La 0.5 Pb 0.5 Mn 1-x Cr x O 3 (... more We report the results of thermoelectric power (TEP) of a Cr doped La 0.5 Pb 0.5 Mn 1-x Cr x O 3 (x= 00.45) system measured both in the presence and the absence of magnetic field (B= 1.5 T). The small field dependence of the Seebeck coefficient is observed around the ...
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Papers by Sayani Majumdar