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  • 1. Akerman, J. J.
    et al.
    Venturini, E. L.
    Siegal, M. P.
    Yun, S. H.
    Karlsson, U. O.
    Rao, K. V.
    KTH, Tidigare Institutioner                               , Materialvetenskap.
    Two-stage crossover from thermal to quantum flux creep of dilute vortex ensembles in various high-T-c superconducting thin films2001Ingår i: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 64, nr 9Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The thermal-to-quantum flux creep crossover at low vortex densities has been studied in YBa2Cu3O7, TlBa2CaCu2O7-delta, and HgBa2CaCu2O6+delta thin films using ac susceptibility. The crossover temperatures T-cr are 10-11, 17, and 30 K, respectively. Both thermal and quantum flux creep is suppressed as the vortex density is decreased. We observe a two-stage nature in the crossover behavior which appears to be a general property of all the three materials studied.

  • 2. Akerman, J. J.
    et al.
    Yun, S. H.
    Karlsson, U. O.
    Rao, K. V.
    KTH, Tidigare Institutioner                               , Materialvetenskap.
    Dislocation-mediated creep of highly separated vortices in a-axis-oriented HgBa2CaCu2O6+delta thin films2001Ingår i: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 64, nr 2Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Using ac susceptibility, we determine the critical current density J(c) and the flux creep activation energy U of an a-axis-oriented HgBa2CaCu2O6+delta thin film. The critical current density at helium temperatures is found to be 4.6 x 10(4) A/cm(2), i.e., about two orders of magnitude smaller than for corresponding films with c-axis orientation. The temperature and ac field dependent activation energy is consistent with dislocation-mediated flux creep and well described by U(T,H-ac)=U-o(1-t(4))H-ac(-1/2) with t=T/T-c, T-c=120K, and U-o = 0.77 eV Oe(1/2) for temperatures T>45 K and in the field range studied. The activation energy is of the same order as that found in c-axis-oriented films. Below T = 45 K the activation energy is observed to decrease as thermally assisted quantum creep becomes increasingly important.

  • 3. Araujo, C. Moyses
    et al.
    Nagar, Sandeep
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Ramzan, Muhammad
    Shukla, R.
    Jayakumar, O. D.
    Tyagi, A. K.
    Liu, Yi-Sheng
    Chen, Jeng-Lung
    Glans, Per-Anders
    Chang, Chinglin
    Blomqvist, Andreas
    Lizarraga, Raquel
    Holmström, Erik
    Belova, Lyubov
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Guo, Jinghua
    Ahuja, Rajeev
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik.
    Rao, K. Venkat
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Disorder-induced Room Temperature Ferromagnetism in Glassy Chromites2014Ingår i: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 4, s. 4686-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We report an unusual robust ferromagnetic order above room temperature upon amorphization of perovskite [YCrO3] in pulsed laser deposited thin films. This is contrary to the usual expected formation of a spin glass magnetic state in the resulting disordered structure. To understand the underlying physics of this phenomenon, we combine advanced spectroscopic techniques and first-principles calculations. We find that the observed order-disorder transformation is accompanied by an insulator-metal transition arising from a wide distribution of Cr-O-Cr bond angles and the consequent metallization through free carriers. Similar results also found in YbCrO3-films suggest that the observed phenomenon is more general and should, in principle, apply to a wider range of oxide systems. The ability to tailor ferromagnetic order above room temperature in oxide materials opens up many possibilities for novel technological applications of this counter intuitive effect.

  • 4.
    Araujo, C. Moysés
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Kapilashrami, Mukes
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Jun, Xu
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Jayakumar, Onattu D.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Nagar, Sandeep
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Wu, Yan
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Århammar, Cecilia
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik.
    Johansson, Börje
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik.
    Belova, Lyubov
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Ahuja, Rajeev
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik.
    Gehring, Gillian A.
    Rao, K. Venkat
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Room temperature ferromagnetism in pristine MgO thin films2010Ingår i: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 96, nr 23Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Robust ferromagnetic ordering at, and well above room temperature is observed in pure transparent MgO thin films (<170 nm thick) deposited by three different techniques. Careful study of the wide scan x-ray photoelectron spectroscopy rule out the possible presence of any magnetic contaminants. In the magnetron sputtered films, we observe magnetic phase transitions as a function of film thickness. The maximum saturation magnetization of 5.7 emu/cm(3) is measured on a 170 nm thick film. The films above 500 nm are found to be diamagnetic. Ab initio calculations suggest that the ferromagnetism is mediated by cation vacancies.

  • 5. Bhatt, P.
    et al.
    Kanciurzewska, A.
    Carlegrim, E.
    Kapilashrami, Mukes
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Belova, Lyubov M.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Rao, K. V.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Fahlman, M.
    Ferromagnetism above room temperature in nickel-tetracyanoethylene thin films2009Ingår i: Journal of Materials Chemistry, ISSN 0959-9428, E-ISSN 1364-5501, Vol. 19, nr 36, s. 6610-6615Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Room temperature ferromagnetic ordering is reported in Ni-tetracyanoethylene (TCNE) thin films fabricated on Au substrates using physical vapor deposition (PVD) under ultra high vacuum conditions. This technique enables the preparation of very clean films without having any kind of contamination from oxygen-containing species, solvents or precursor molecules. Film stoichiometry was obtained from X-ray photoelectron spectroscopy (XPS) measurements. XPS derived stoichiometry points to a similar to 1 : 2 ratio between Ni and TCNE resulting in Ni(TCNE)(x), x approximate to 2. No evidence of pure Ni metal in the in situ grown films was present in the XPS or the ultraviolet photoelectron spectroscopy (UPS) measurements within the detection limits of the techniques.

  • 6.
    Biswas, Anis
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Shirong, Wang
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Nagar, Sandeep
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Belova, Liubov
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Rao, K. Venkat
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    The effect of oxygen partial pressure during deposition in the magnetic properties of ZnO thin film2011Ingår i: Mater Res Soc Symp Proc, 2011, s. 117-122Konferensbidrag (Refereegranskat)
    Abstract [en]

    We have studied the magnetic properties of 100 nm thick ZnO thin films prepared by magnetron sputtering in different oxygen partial pressures (ratio of oxygen pressure to total pressure in deposition chamber, P Oxy). Only the films fabricated at P Oxy below ∼ 0.5 show room temperature ferromagnetism. The saturation magnetization at room temperature is initially found to increase as P Oxy increases and reaches maximum value of ∼ 5 emu/gm at P Oxy ∼ 0.3 and then starts to decrease and becomes diamagnetic for P Oxy &gt; 0.5. From small angle XRD study of structural properties of the films, we find that the lattice stress developed in the film along c-axis also exhibits a similar behavior with the variation of P Oxy. Thus, both the room temperature ferromagnetism and lattice stress appear to originate from the intrinsic defects present in the sample.

  • 7. Chastellain, M.
    et al.
    Petri, A.
    Gupta, Amita
    KTH, Tidigare Institutioner, Materialvetenskap.
    Rao, K. Venkat
    KTH, Tidigare Institutioner, Materialvetenskap.
    Hofmann, H.
    Superparamagnetic silica-iron oxide nanocomposites for application in hyperthermia2004Ingår i: Advanced Engineering Materials, ISSN 1438-1656, E-ISSN 1527-2648, Vol. 6, nr 4, s. 235-241Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Superparamagnetic iron oxide particles embedded in silica are studied for application in hyperthermia. The temperature increase is studied when submitting the samples to a weak alternating magnetic field. The influence of the iron oxide size distribution, saturation magnetization, out of phase susceptibility and anisotropy constant is discussed. A theoretical calculation of power loss is carried out and agrees with experimental data.

  • 8. Chey, Chan Oeurn
    et al.
    Masood, Ansar
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Riazanova, Anastasiia
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Liu, Xianjie
    Rao, K. Venkat
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Nur, Omer
    Willander, Magnus
    Synthesis of Fe-Doped ZnO Nanorods by Rapid Mixing Hydrothermal Method and Its Application for High Performance UV Photodetector2014Ingår i: Journal of Nanomaterials, ISSN 1687-4110, E-ISSN 1687-4129, s. 524530-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We have successfully synthesized Fe-doped ZnO nanorods by a new and simple method in which the adopted approach is by using ammonia as a continuous source of OH- for hydrolysis instead of hexamethylenetetramine (HMT). The energy dispersive X-ray (EDX) spectra revealed that the Fe peaks were presented in the grown Fe-doped ZnO nanorods samples and the X-ray photoelectron spectroscopy (XPS) results suggested that Fe3+ is incorporated into the ZnO lattice. Structural characterization indicated that the Fe-doped ZnO nanorods grow along the c-axis with a hexagonal wurtzite structure and have single crystalline nature without any secondary phases or clusters of FeO or Fe3O4 observed in the samples. The Fe-doped ZnO nanorods showed room temperature (300 K) ferromagnetic magnetization versus field (M-H) hysteresis and the magnetization increases from 2.5 mu emu to 9.1 mu emu for Zn0.99Fe0.01O and Zn0.95Fe0.05O, respectively. Moreover, the fabricated Au/Fe-doped ZnO Schottky diode based UV photodetector achieved 2.33 A/W of responsivity and 5 s of time response. Compared to other Au/ZnO nanorods Schottky devices, the presented responsivity is an improvement by a factor of 3.9.

  • 9. Dahlberg, D
    et al.
    Miller, B
    Hill, B
    Jonsson, J
    Ström, Valter
    KTH, Tidigare Institutioner                               , Fysik.
    Rao, K Venkat
    KTH, Tidigare Institutioner                               , Fysik.
    Nogués, Josep
    Schuller, K
    Measurements of the ferromagnetic/antiferromagnetic interfacial exchange energy in CO/CoO and Fe/FeF2 layers1998Ingår i: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 83, nr 11, s. 6893-6895Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Two measurement techniques, both relying on reversible rotations of the magnetization, have been used to determine the magnitude of the interfacial exchange energy (IEE) between ferromagnetic and antiferromagnetic (F/AF) layers. One technique is to use the anisotropic magnetoresistance to determine rotations of the magnetization away from the unidirectional easy axis, where the rotation is accomplished by applying external magnetic fields less than the effective F/AF exchange field. The second technique uses measurements of the ac susceptibility as a function of the angle between the ac field and the unidirectional exchange field. Both of the reversible process techniques result in values of the IEE larger (by as much as a factor of 10 in Co/CoO bilayers) than the traditional irreversible technique of measuring a shift in the hysteresis loop. The ac susceptibility technique was also used to measure one Fe/FeF2 bilayer. For this sample, the IEE values obtained by reversible and irreversible methods are equivalent.

  • 10.
    Davydova, Alexandra
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Tselikov, G.
    Dilone, D.
    Rao, K. Venkat
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Kabashin, A. V.
    Belova, Lyubov
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Fabrication of thin ZnO films with wide-range tuned optical properties by reactive magnetron sputtering2018Ingår i: Semiconductor Science and Technology, ISSN 0268-1242, E-ISSN 1361-6641, Vol. 33, nr 2, artikel-id 025004Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We report the manufacturing of thin zinc oxide films by reactive magnetron sputtering at room temperature, and examine their structural and optical properties. We show that the partial oxygen pressure in DC mode can have dramatic effect on absorption and refractive index (RI) of the films in a broad spectral range. In particular, the change of the oxygen pressure from 7% to 5% can lead to either conventional crystalline ZnO films having low absorption and characteristic descending dependence of RI from 2.4-2.7 RIU in the visible to 1.8-2 RIU in the near-infrared (1600 nm) range, or to untypical films, composed of ZnO nano-crystals embedded into amorphous matrix, exhibiting unexpectedly high absorption in the visible-infrared region and ascending dependence of RI with values varying from 1.5 RIU in the visible to 4 RIU in the IR (1600 nm), respectively. Untypical optical characteristics in the second case are explained by defects in ZnO structure arising due to under-oxidation of ZnO crystals. We also show that the observed defect-related film structure remains stable even after annealing of films under relatively high temperatures (30 min under 450 degrees C). We assume that both types of films can be of importance for photovoltaic (as contact or active layers, respectively), as well as for chemical or biological sensing, optoelectronics etc.

  • 11. Desai, R.
    et al.
    Mehta, R. V.
    Upadhyay, R. V.
    Gupta, A.
    Praneet, A.
    Rao, K. V.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Bulk magnetic properties of CdFe(2)O(4)in nano-regime2007Ingår i: Bulletin of Materials Science, ISSN 0250-4707, E-ISSN 0973-7669, Vol. 30, nr 3, s. 197-203Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Cadmium ferrite particles have been synthesized using co-precipitation technique followed by a low temperature (600 degrees C) annealing in a time scale much shorter than reported in literature. Incorporation of sodium chloride during annealing helps to form a single phase spinel structure with a final particle size of around 50 nm. Even at such a short length scale we observe the overall magnetic properties to be similar to those of the bulk. The observed magnetic properties can be explained on the basis of an anti-ferromagnetic core with a shell containing 'ferromagnetic-like', but canted spin structure.

  • 12. Emam, A. N.
    et al.
    Mohamed, M. B.
    Girgis, E.
    Rao, K. Venkat
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Hybrid magnetic-plasmonic nanocomposite: Embedding cobalt clusters in gold nanorods2015Ingår i: RSC Advances, ISSN 2046-2069, E-ISSN 2046-2069, Vol. 5, nr 44, s. 34696-34703Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We developed a method to fabricate hybrid magnetic-plasmonic nanorods (Au-Co NRs) via a modified seed mediated method. The only modification is to use cobalt ions instead of Au3+ in the preparation of the seed solution to obtain gold nanorods doped with Co clusters. By adjusting the amount of cobalt seed solution, Au-Co NRs of controlled aspect ratio can be obtained. The optical properties of the obtained Au-Co NRs were investigated and compared to those of the pure Au NRs. A slight shift and broadening were observed in the alloys compared to the pure ones, which was attributed to the presence of Co clusters leading to suppression of the dielectric properties. High resolution transmission electron microscopy (HRTEM) images indicate the existence of Co clusters in situ in the Au NR host and clearly show the metal-metal interface. The magnetic properties of the obtained Au-Co NRs increase as the concentration of dopant Co cluster seeds increases, as investigated by vibrating sample magnetometry (VSM). Our approach allows us to design nanomaterials of controlled shape, optical and magnetic properties which have many promising applications in tharanostics and photoelectronics.

  • 13.
    Fan, Jun C.
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Sreekanth, Mahadeva
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Xie, Z.
    Chang, S.L.
    Rao, K. Venkat
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    P-Type ZnO materials: Theory, growth, properties and devices2013Ingår i: Progress in Materials Science, ISSN 0079-6425, E-ISSN 1873-2208, Vol. 58, nr 6, s. 874-985Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    In the past 10 years, ZnO as a semiconductor has attracted considerable attention due to its unique properties, such as high electron mobility, wide and direct band gap and large exciton binding energy. ZnO has been considered a promising material for optoelectronic device applications, and the fabrications of high quality p-type ZnO and p-n junction are the key steps to realize these applications. However, the reliable p-type doping of the material remains a major challenge because of the self-compensation from native donor defects (VO and Zni) and/or hydrogen incorporation. Considerable efforts have been made to obtain p-type ZnO by doping different elements with various techniques. Remarkable progresses have been achieved, both theoretically and experimentally. In this paper, we discuss p-type ZnO materials: theory, growth, properties and devices, comprehensively. We first discuss the native defects in ZnO. Among the native defects in ZnO, VZn and O i act as acceptors. We then present the theory of p-type doping in ZnO, and summarize the growth techniques for p-type ZnO and the properties of p-type ZnO materials. Theoretically, the principles of selection of p-type dopant, codoping method and XZn-2VZn acceptor model are introduced. Experimentally, besides the intrinsic p-type ZnO grown at O-rich ambient, p-type ZnO (MgZnO) materials have been prepared by various techniques using Group-I, IV and V elements. We pay a special attention to the band gap of p-type ZnO by band-gap engineering and room temperature ferromagnetism observed in p-type ZnO. Finally, we summarize the devices based on p-type ZnO materials.

  • 14.
    Fang, Mei
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Aristov, Andrey
    Rao, K. Venkat
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Kabashin, Andrei V.
    Belova, Lyubov
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Particle-free inkjet printing of nanostructured porous indium tin oxide thin films2013Ingår i: RSC Advances, ISSN 2046-2069, E-ISSN 2046-2069, Vol. 3, nr 42, s. 19501-19507Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We report a simple, low-cost, single-step inkjet printing method for the fabrication of nanostructured, highly transparent and conductive ITO films, which completely avoids the use of ITO particles in the fabrication process. In our method, the inks are formed from a liquid solution presenting a properly selected mixture of indium and tin acetates. After jet printing, the ink is decomposed during a subsequent annealing step, in which the released CO2 gas bubbles control the ITO nucleation process to provide a porous film texture. We show that the fabricated ITO films are highly crystalline, stoichiometric, and nanoporous with controlled porosity. Electrical measurements show relatively low resistivity values for the films (down to 0.029 Omega cm) comparable to those of the best ITO thin films fabricated by other methods. Optical ellipsometry tests demonstrate a relatively high refractive index (1.5-1.7) and high transparency of the films over a wide region of the spectrum ranging from 500 to 1700 nm. Since the method does not require any pre-fabricated ITO particles, masks or templates, and enables the deposition of films on substrates of various materials and shapes, it can be employed for fabrication of nanoporous ITO films for a diversity of applications, including solar cell, bio- and chemical sensing, etc.

  • 15.
    Fang, Mei
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Belova, Lyuba
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Rao, K. Venkat
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    The art of tailoring inks for inkjet printing metal oxides2012Manuskript (preprint) (Övrigt vetenskapligt)
    Abstract [en]

    Inkjet printing has become a promising, efficient, inexpensive, scalable technique for materials deposition and mask-less patterning in many device applications. This article provides an introduction of the essentials of inkjet printing technology and ink preparation which remains a challenge especially for printing oxide transparent materials. After introducing the essentials of an inkjet printer and the process of the conversion of liquid ink into solid thin films of oxide materials, we present two approaches to the tailoring of inks, especially relevant for piezoelectric drop-on-demand ink jet printer: (1) the inks prepared from oxide particle suspensions (e.g., SiO2, TiO2, Fe3O4), and (2) metal-acetates precursor solutions for direct printing of thin films subsequently processed by calcination into the respective oxides like undoped and doped ZnO, MgO, ITO among others. The oxide films prepared this way using high purity precursors are free from undesirable contaminations, stoichiometric and when annealed appropriately produce smooth printed thin films. We place special emphasis upon preparation of inks that are stable without sediments over time so that the printing process is reliable and repeatable, and the obtained oxide films are dense and uniform. Also, for some of the inks containing multi-type acetates with possible phase separation even before calcinations we have developed a chelating procedure in order to tailor the films into single phase homogeneity. The films are characterized by optical microscope for micro features, high resolution SEM in a Nova600-Nanolab SEM/FIB system, and JEOL atomic force microscope for their morphology.

  • 16.
    Fang, Mei
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Belova, Lyuba
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Rao, K. Venkat
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Thermal anealing effects on Ag/TiO2 thin films prepared by ink-jet printingManuskript (preprint) (Övrigt vetenskapligt)
    Abstract [en]

    The effects of heat treatment on both the phase structure and the electronic band structure were studied for Ag/TiO2 composite films prepared by inkjet printing. Ag nanoparticles can form ‘metal-bridge’ to link TiO2 particles in the mesoporous structured films and improve the transport properties of the films. The distribution of Ag in the composite films shows dependence on the annealing conditions: Ag clusters were observed at high annealing temperature (>600 °C), and they can be annihilated by a longer time annealing. Comparing with pure TiO2 films, the decreased intensity of the photoluminescence (PL) emission spectra of Ag/TiO2 composite films indicates that the doped Ag atoms could act as traps to capture electron and inhabit the recombination of electron-hole pairs. From the identifiable PL emission peaks, the band structure of the films is deduced.

  • 17.
    Fang, Mei
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Li, Hao
    Cent S Univ, Sch Phys & Elect, Hunan Key Lab Super Microstruct & Ultrafast Proc, Changsha 410083, Hunan, Peoples R China..
    Riazanova, Anastasiia
    KTH, Skolan för kemi, bioteknologi och hälsa (CBH), Centra, Wallenberg Wood Science Center.
    Rao, K Venkat
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Belova, Lyubov
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Tuning room temperature ferromagnetism of 'in-situ' inkjet printed Fe-doped ZnO films2019Ingår i: Semiconductor Science and Technology, ISSN 0268-1242, E-ISSN 1361-6641, Vol. 34, nr 5, artikel-id 055006Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    ZnO is a wide-band gap semiconductor widely used in optical and electric devices, associating with ferromagnetism at low dimension endowing its possibility for functional applications with magneto-optical and magneto-electric properties. We prepared ZnO and Fe-doped ZnO thin films 'in-situ' on substrate by inkjet printing, and tuned the room temperature ferromagnetism (RTFM) of the film by Fe-doping concentration, film thickness and post annealing temperature. It was found that by Fe doping the saturation magnetization (M-s) of the film can be enhanced by more than 4 folds comparing with the un-doped film, i.e. from 0.9 emu g(-1) for the ZnO film to 3.8 emu g(-1) for the Fe-doped ZnO film with comparable thickness. The enhancement was attributed to the introduction of un-paired 3d electrons which formed long range ferromagnetic ordering, as well as the consequent structure changes with smaller grains which increased the interface induced magnetism. By changing the annealing temperature and the film thickness, the defect-induced ferromagnetism was investigated. The RTFM shows thickness dependence with peak saturation magnetization value of 4.44 emu g(-1) for the 45 nm thick film. The work provides an effective way of tuning magnetism in ZnO based films for functional device applications.

  • 18.
    Fang, Mei
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Riazanova, Anastasia
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Belova, Lyubov
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Rao, K. Venkat
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Magnetic properties of inkjet printed Fe-doped ZnO thin filmsManuskript (preprint) (Övrigt vetenskapligt)
    Abstract [en]

    Magnetic properties in semiconductors show dependences on the substance itself (the doped element and the matrix), the states (e.g., bulk, nanoparticles, or film) and the preparation methods, which attract huge interest for both functional applications and fundamental science. As a widespread used semiconductor, ZnO and Fe-doped ZnO thin films were prepared via calcination of the as-prepared acetates precursor films printed by inkjet technique. Their room temperature (RT) magnetic properties were investigated to obtain the insight into the origin of RT ferromagnetism (FM). It was found that the grain size of the films was reduced by Fe-doping. For ~30 nm thick films, the saturation magnetization (MS) of 10 at.% Fe-doped ZnO (3.8 emu/g) is 4 times higher than that of pure ZnO thin film (0.9 emu/g) prepared with the same route. We attribute the enhancement to: (i) the introduction of Fe atoms with unpaired 3d electrons which contribute to magnetism; and (ii) the Fe-doping increase the defect in the lattice structure of the ZnO matrix. The effects of calcination temperature on RTFM of 10 at.% Fe-doped thin films were studied, and the temperature dependent MS was observed. The RTFM depended on film thickness as well, which shows an initial increase and then decrease with the maximum MS of 4.44 emu/g obtained from the ~45 nm 10 at.% Fe-doped ZnO film. Possible reasons for the observed phenomena were discussed.

  • 19.
    Fang, Mei
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Riazanova, Anastasia
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Belova, Lyubov
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Rao, K. Venkat
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Magnetism of Fe-doped MgO thin films prepared by inkjet printingManuskript (preprint) (Övrigt vetenskapligt)
    Abstract [en]

    We prepared MgO and Fe-doped MgO thin films by inkjet printing and investigated the room temperature ferromagnetism (RTFM) of the films. Films prepared from the same route show amorphous for pure MgO films while crystals for Fe-doped MgO thin films, indicating that the doped Fe atoms can improve the crystallinity of the films. The saturation magnetization of 10 at.% Fe-doped MgO film is ~5 times as much as that of pure MgO film with same thickness (~90 nm), implying the great enhancement of magnetism introduced by Fe-doping. The RTFM of 10 at.% Fe-doped MgO films shows dependence on calcination temperature and the film thickness, where the effects of defect and crystal structure on magnetism of films were discussed. From the L2,3-edge features, the coexistence of Fe2+ and Fe3+ cations in octahedral and tetrahedral sites of the crystals was deduced, which was consistent with the two lattice structures determined from X-ray diffraction. The unpaired 3d electrons in the lattices could interact with each other directly or mediated by anions/carriers, which contribute to the enhancement of RTFM in the Fe-doped films. The saturation magnetization of ~30 nm 10at.% Fe-doped MgO film was detected to be ~6.3 emu•cm-3 and the coercively was ~50 Oe.

  • 20.
    Fang, Mei
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Ström, Valter
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Olsson, Richard T.
    KTH, Skolan för kemivetenskap (CHE), Fiber- och polymerteknik.
    Belova, Lyubov
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Rao, K. Venkat
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Particle size and magnetic properties dependence on growth temperature for rapid mixed co-precipitated magnetite nanoparticles2012Ingår i: Nanotechnology, ISSN 0957-4484, E-ISSN 1361-6528, Vol. 23, nr 14, s. 145601-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Magnetite nanoparticles have been prepared by co-precipitation using a custom-designed jet mixer to achieve rapid mixing (RM) of reactants in a timescale of milliseconds. The quick and stable nucleation obtained allows control of the particle size and size distribution via a more defined growth process. Nanoparticles of different sizes were prepared by controlling the processing temperature in the first few seconds post-mixing. The average size of the nanoparticles investigated using a Tecnai transmission electron microscope is found to increase with the temperature from 3.8 nm at 1 +/- 1 degrees C to 10.9 nm for particles grown at 95 +/- 1 degrees C. The temperature dependence of the size distribution follows the same trend and is explained in terms of Ostwald ripening of the magnetite nanoparticles during the co-precipitation of Fe2+ and Fe3+. The magnetic properties were studied by monitoring the blocking temperature via both DC and AC techniques. Strikingly, the obtained RM particles maintain the high magnetization (as high as similar to 88 A m(2) kg(-1) at 500 kA m(-1)) while the coercivity is as low as similar to 12 A m(-1) with the expected temperature dependence. Besides, by adding a drop of tetramethylammonium hydroxide, aqueous ferrofluids with long term stability are obtained, suggesting their suitability for applications in ferrofluid technology and biomedicine.

  • 21.
    Fang, Mei
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Ström, Valter
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Olsson, Richard T.
    KTH, Skolan för kemivetenskap (CHE), Fiber- och polymerteknik, Polymera material.
    Belova, Lyubov
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Rao, K. Venkat
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Rapid mixing: A route to synthesize magnetite nanoparticles with high moment2011Ingår i: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 99, nr 22, s. 222501-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We demonstrate the impact of rapid mixing of the precursors in a time scale of milliseconds on the reaction rate and magnetic properties of co-precipitated magnetite with a custom-made mixer. The mixed volume is directed into a desk-top AC susceptometer to monitor the magnetic response from the growing particles in real-time. These measurements indicate that the reaction is mostly completed within a minute. The obtained superparamagnetic nanoparticles exhibit a narrow size distribution and large magnetization (87 Am(2) kg(-1)). Transmission electron micrographs suggest that rapid mixing is the key for better crystallinity and a more uniform morphology leading to the observed magnetization values.

  • 22.
    Fang, Mei
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Ström, Valter
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Takahiko, Tamaki
    Belova, Lyubov
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Rao, K. V.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Co-precipitation of iron oxide nanoparticles by rapid mixingArtikel i tidskrift (Övrigt vetenskapligt)
    Abstract [en]

    Synthesis of Magnetite appears to be a topic of continued interest because of its versatility and the variety applications. Among the chemical techniques to synthesize Fe3O4, co-precipitation approach although very common, seems to be extremely sensitive to the consequences of nucleation, growth and most of all the rate of the reaction involved. This work is an attempt to demonstrate the complexities of obtaining monodispersed nanosized Fe3O4 particles. We consider the role of rapid mixing and its consequences on co-precipitation at ice-point, room temperature and boiling water temperatures on the magnetic properties of Fe3O4. We obtained crystallites varying in the range from 6.6 nm (grown in ice-water) to 7.9 nm (grown in boiling water) as determined from the broadening of XRD diffraction peaks using the Scherrer approach. With the increase of the particle size, the saturate magnetization of iron oxides increases from 52 emu/g to 63 emu/g, and the coercivity increases from 0.5 Oe to 7.9 Oe. Layers of nanosized magnetic particles on glass substrates show unusual wavelength dependence of Faraday rotation loops which show a reversal phenomenon in the sign of the magnetization around 550.

  • 23.
    Fang, Mei
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Voit, Wolfgang
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Kyndiah, Adrica
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Wu, Yan
    Faculty of Materials Science and Chemical Engineering, China University of Geosciences.
    Belova, Lyubov
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Rao, K. Venkat
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Room temperature ferromagnetism of Fe-doped ZnO and MgO thin films prepared by ink-jet printing2012Ingår i: MRS Proceedings, 2012Konferensbidrag (Refereegranskat)
    Abstract [en]

    Room temperature magnetic properties of un-doped, as well as 10 at.% Fe-doped ZnOand MgO single-pass layer of ink-jet printed thin films have been investigated to obtain insightinto the role of the band gaps and mechanisms for the origin of ferromagnetic order in thesematerials. It is found that on doping with Fe, the saturation magnetization is enhanced by severalfoldin both systems when compared with the respective un-doped thin films. For a ~28 nm thickfilm of Fe-doped ZnO (Diluted Magnetic Semiconductor, DMS) we observe an enhancedmoment of 0.465 μB/Fe atom while it is around 0.111μB/Fe atom for the doped MgO (DilutedMagnetic Insulator, DMI) film of comparable thickness. Also, the pure ZnO is far moreferromagnetic than pure MgO at comparable low film thicknesses which can be attributed todefect induced magnetism originating from cat-ion vacancies. However, the film thicknessdependence of the magnetization and the defect concentrations are found to be significantlydifferent in the two systems so that a comparison of the magnetism becomes more complex forthicker films.

  • 24.
    Fang, Mei
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Volotinen, Tarja
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Kulkarni, S. K.
    Indian Institute of Science Education and Research, India.
    Belova, Lyubov
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Rao, K. V.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Effect of Fe3O4 nanoparticles on the optical transmission properties of 3D magnetic photonic crystals2010Ingår i: Physics ReviewArtikel i tidskrift (Övrigt vetenskapligt)
    Abstract [en]

    Magnetic and optical properties of three-dimensional magnetic photonic crystals (MPCs), consisting of silica spheres in the size range 190-680nm embedded with 8, 9 and 13 nm Fe3O4 nanoparticles, have been investigated. All the PC-films, with and without embedded magnetic nanoparticles, show five band gaps at well defined wavelengths in their optical transmission spectrum. The band gaps are found to be a linear function of the constituent sphere size in the MPC films. From the slope of this function, the deduced refractive index for the constituents in the films is found to increase with the concentration of the embedded magnetite nanoparticles. The observed shifts in the photonic band gaps PBGs in the films is qualitatively explained in terms of the variations of refractive index and the contrast index difference arising from the concentration of the embedded nanoparticles. We also find that the angular dependence of PBG positions for MPCs at small incidence angles is strongly dependent on the p- and s- polarization states of the incident light. The polarization sensitivity of PBGs to the Fe3O4 concentration is also discussed.  

  • 25.
    Fang, Mei
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Volotinen, Tarja
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Kulkarni, S. K.
    Indian Institute of Science Education and Research, India.
    Belova, Lyubov
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Rao, K. V.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Enhanced linear sphere size dependence of photonic band gaps for 3D nanocomposite magnetic photonic crystals2010Ingår i: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550Artikel i tidskrift (Övrigt vetenskapligt)
    Abstract [en]

    Silica spheres in the size range (70-650) nm, containing embedded nano-sized magnetic iron oxide particles have been synthesized and arranged into 3D-face-centered-cubic (fcc) structured magnetic photonic crystals (MPCs) with the (111) crystal plane parallel to the glass substrate surface. Five photonic band gaps (PBGs) are observed in the optical transmission spectra measured over UV-Vis-near IR range for MPCs. The peak wavelengths of the PBGs (λC) are found to increase linearly with the sphere size (Φ). Furthermore, on embedding magnetic nanoparticles the position of PBGs is shifted to higher wavelengths. In addition, the average refractive index, 1.5 ± 0.1, obtained for the MPCs from the slopes of λC(Φ) is found to be larger than the reported value of 1.349 for pure silica PCs.

  • 26.
    Fang, Mei
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Volotinen, Tarja T.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Kulkarni, S. K.
    Belova, Lyubov
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Rao, K. Venkat
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Effect of embedding Fe3O4 nanoparticles in silica spheres on the optical transmission properties of three-dimensional magnetic photonic crystals2010Ingår i: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 108, nr 10, s. 103501-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Magnetic and optical properties of three-dimensional fcc-structured magnetic photonic crystals (MPCs), consisting of SiO2 spheres, in the size range 260-680 nm, embedded with 0-6.4 wt % Fe3O4 nanoparticles have been investigated. In the wide spatial angle transmission spectra for these crystals at normal incidence of light in the UV-visible range, five photonic band gaps (PBGs) due to Bragg diffraction from different crystal planes have been observed. The Bragg wavelengths (lambda(B)) of PBGs in both the nonmagnetic and MPCs of the same structure are found to depend linearly on the sphere size. From the slope of this linear function the calculated effective refractive index is found to increase with the concentration of the magnetite nanoparticles in the MPCs, and is consistent with the result calculated from the average dielectric constant. We also find lambda(B) of PBGs are dependent on the angle of the incidence of the light. Furthermore, for small angles this angular dependency is more strongly dependent on the polarization of incident light for MPCs than for the non-MPCs. Thus, magnetic nanocomposite PCs can be designed to incorporate additional functionality in the development of potential magneto-optical devices.

  • 27.
    Fang, Mei
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Volotinen, Tarja T.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Kulkarni, Sulabha K.
    Belova, Lyubov
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Rao, K. Venkat
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Designing photonic band gaps in SiO(2)-based face-centered cubic-structured crystals2011Ingår i: Journal of Nanophotonics, ISSN 1934-2608, Vol. 5, s. 053514-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We designed face-centered cubic-structured (fcc) photonic crystals whose lattice parameters were tuned by varying the size of the constituent spherical silica particles in the range 100 to 520 nm. From wide-angle optical transmission investigations and Gaussian fitting of the absorbance spectra over UV-Vis-Near IR range, we found that in these crystals the Bragg wavelengths of the photonic band gaps (PBGs) corresponding to the reflected crystal planes linearly increase with the size of the spheres as expected. From this data, the average refractive index along the different crystal planes of the fcc structure was found to be in the 1.24 to 1.32 range. The Bragg wavelengths were tuned between 400 and 1100 nm. Thus, photonic crystals of the same structure can be designed to tune the Bragg wavelengths of PBGs by selecting the sphere size. These studies open up possibilities to design a new class of "smart" photonic crystals consisting of dielectric entities of sub-micron silica spheres with added functionality from magnetic or piezoelectric nanoparticles embedded in them.

  • 28.
    Fang, Mej
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Voit, Wolfgang
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Wu, Y.
    Lyubov, Belova
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Rao, K. Venkat
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    'In-situ' preparation of metal oxide thin films by inkjet printing acetates solutions2013Ingår i: Mater Res Soc Symp Proc, 2013, s. 13-20Konferensbidrag (Refereegranskat)
    Abstract [en]

    Direct printing of functional oxide thin films could provide a new route to low-cost, efficient and scalable fabrications of electronic devices. One challenge that remains open is to design the inks with long term stability for effective deposition of specific oxide materials of industrial importance. In this paper, we introduce a reliable method of producing stable inks for 'in-situ' deposition of oxide thin films by inkjet printing. The inks were prepared from metal-acetates solutions and printed on a variety of substrates. The acetate precursors were decomposed into oxide films during the subsequent calcination process to achieve the 'in-situ' deposition of the desired oxide films directly on the substrate. By this procedure we have obtained room temperature contamination free ferromagnetic spintronic materials like Fe doped MgO and ZnO films from their acetate(s) solutions. We find that the origin of magnetism in ZnO, MgO and their Fe-doped films to be intrinsic. For a 28 nm thick film of Fe-doped ZnO we observe an enhanced magnetic moment of 16.0 emu/cm3 while it is 5.5 emu/cm3 for the doped MgO film of single pass printed. The origin of magnetism is attributed to cat-ion vacancies. We have also fabricated highly transparent indium tin oxide films with a transparency &gt;95% both in the visible and IR range which is rather unique compared to films grown by any other technique. The films have a nano-porous structure, an added bonus from inkjetting that makes such films advantageous for a broad range of applications.

  • 29. Fassbender, J.
    et al.
    Liedke, M. O.
    Strache, T.
    Moller, W.
    Menendez, E.
    Sort, J.
    Rao, K. V.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Deevi, S. C.
    Nogues, J.
    Ion mass dependence of irradiation-induced local creation of ferromagnetism in Fe60Al40 alloys2008Ingår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 77, nr 17Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Ion irradiation of Fe60Al40 alloys results in the phase transformation from the paramagnetic, chemically ordered B2 phase to the ferromagnetic, chemically disordered A2 phase. The magnetic phase transformation is related to the number of displacements per atom (dpa) during the irradiation. For heavy ions (Ar+, Kr+, and Xe+), a universal curve is observed with a steep increase in the fraction of the ferromagnetic phase that reaches saturation, i.e., a complete phase transformation, at about 0.5 dpa. This proves the purely ballistic nature of the disordering process. If light ions are used (He+ and Ne+), a pronounced deviation from the universal curve is observed. This is attributed to bulk vacancy diffusion from the dilute collision cascades, which leads to a partial recovery of the thermodynamically favored B2 phase. Comparing different noble gas ion irradiation experiments allows us to assess the corresponding counteracting contributions. In addition, the potential to create local ferromagnetic areas embedded in a paramagnetic matrix is demonstrated.

  • 30. Fraerman, A. A.
    et al.
    Belova, Lyubov M.
    KTH, Tidigare Institutioner, Materialvetenskap.
    Gribkov, B. A.
    Gusev, S. A.
    Klimov, A. Y.
    Mironov, V. L.
    Nikitushkin, D. S.
    Pakhomov, G. L.
    Rao, K. Venkat
    KTH, Tidigare Institutioner, Materialvetenskap.
    Shevtsov, V. B.
    Silaev, M. A.
    Vdovichev, S. N.
    Magnetic force microscopy to determine vorticity direction in elliptical Co nanoparticles2004Ingår i: Physics of low-Dimensional structures, ISSN 0204-3467, Vol. 1-2, s. 35-40Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We present a magnetic force microscopy investigation into the magnetic properties of arrays of Co nanoparticles fabricated by electron be am lithography. Vorticity directions are determined in zero applied magnetic fields. Experimental dependence of height on stable magnetic states of the particles is investigated. The statistics of the vorticity direction distribution is discussed.

  • 31. Girgis, E.
    et al.
    Adel, D.
    Tharwat, C.
    Attallah, O.
    Rao, K. Venkat
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Cobalt ferrite nanotubes and porous nanorods for dye removal2015Ingår i: ADVANCES IN NANO RESEARCH, ISSN 2287-237X, Vol. 3, nr 2, s. 111-121Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    CoFe2O4 nanotubes and porous nanorods were prepared via a simple one-pot template-free hydrothermal method and were used as an adsorbent for the removal of dye contaminants from water. The properties of the synthesized nanotubes and porous nanorods were characterized by electron diffraction, transmission electron microscopy and x-ray powder diffraction. The Adsorption characteristics of the CoFe2O4 were examined using polar red dye and the factors affecting adsorption, such as, initial dye concentration, pH and contact time were evaluated. The overall trend followed an increase of the sorption capacity reaching a maximum of 95% dye removal at low pHs of 2-4. An enhancement in the removal efficiency was also noticed upon increasing the contact time between dye molecules and CoFe2O4 nanoparticles. The final results indicated that the CoFe2O4 nanotubes and porous nanorods can be considered as an efficient low cost and recyclable adsorbent for dye removal with efficiency 94% for Cobalt ferrite nanotubes and for Cobalt ferrite porous nanorods equals 95%

  • 32. Girgis, E.
    et al.
    Fang, Mei
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Hassan, E.
    Kathab, N.
    Rao, K. Venkat
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Ink-jet-printed (ZnO)(1-x)( TiO2)(x) composite films for solar cell applications2013Ingår i: Journal of Materials Research, ISSN 0884-2914, E-ISSN 2044-5326, Vol. 28, nr 3, s. 502-506Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Ink-jet printing technique is used to prepare porous (ZnO)(1-x)(TiO2)(x) composite films on indium tin oxide-coated glass substrates. Dye-sensitized solar cells were fabricated using well-characterized printed films of thickness similar to 20 and 30 mu m, respectively. It is found that the photovoltaic performance of the cells is dependent on the film thickness and the concentrations of ZnO. The obtained results are compared with those of pure ZnO- and TiO2-based cells prepared by the same route to optimize the device efficiency. This study suggests that ink-jet printers promise an inexpensive and simple technology for manufacturing solar cell composite films.

  • 33.
    Girgis, E.
    et al.
    Natl Res Ctr, Giza, Egypt .
    Khalil, W. K. B.
    Natl Res Ctr, Giza, Egypt .
    Emam, A. N.
    Natl Res Ctr, Giza, Egypt .
    Mohamed, M. B.
    Cairo Univ, Cairo, Egypt .
    Rao, K. Venkat
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Nanotoxicity of Gold and Gold-Cobalt Nanoalloy2012Ingår i: Chemical Research in Toxicology, ISSN 0893-228X, E-ISSN 1520-5010, Vol. 25, nr 5, s. 1086-1098Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Nanotoxicology test of gold nanoparticles (Au NPs) and gold-cobalt (Au-Co) nanoalloy is an important step in their safety evaluation for biomedical applications. The Au and Au-Co NPs were prepared by reducing the metal ions using sodium borohydride (NaBH4) in the presence of polyvinyl pyrrolidone (PVP) as a capping material. The average size and shape of the nanoparticles (NPs) were characterized using high resolution transmission electron microscopy (HRTEM). Cobalt presence in the nanoalloy was confirmed by energy dispersive X-ray spectroscopy (EDX) analysis, and the magnetic properties of these particles were determined using a vibrating sample magnetometer (VSM). The Gold and gold-cobalt NPs of average size 15 +/- 1.5 nm were administered orally to mice with a dose of 80, 160, and 320 mg/kg per body weight (bw) using gavages. Samples were collected after 7 and 14 days of the treatment. The results indicated that the Au-Co NPs were able to induce significant alteration in the tumor-initiating genes associated with an increase of micronuclei (MNs) formation and generation of DNA adduct (8-hydroxy-2-deoxyguanosine, 8-OHdG) as well as a reduction in the glutathione peroxidase activity. This action of Au-Co NPs was observed using 160 and 320 mg/kg bw at both time intervals. However, Au NPs had much lower effects than Au-Co NPs on alteration in the tumor-initiating genes, frequency of MNs, and generation of 8-0HdG as well as glutathione peroxidase activity except with the highest dose of Au NPs. This study suggests that the potential to cause in vivo genetic and antioxidant enzyme alterations due to the treatment by Au-Co nanoalloy may be attributed to the increase in oxidative stress in mice.

  • 34.
    Girgis, Emad
    et al.
    Department of Solid State Physics, National Research Center, Egypt.
    Fang, Mei
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Hassan, E.
    National Research Center, Egypt.
    Kathab, N.
    Department of Solid State Physics, National Research Center, Egypt.
    Rao, K. Venkat
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Inkjet-printed (ZnO)1-x(TiO2)x composite films for solar cell applicationsIngår i: Journal of Materials Research, ISSN 0884-2914, E-ISSN 2044-5326Artikel i tidskrift (Refereegranskat)
  • 35. Girgis, Emad
    et al.
    Wahsh, Mohamed M. S.
    Othman, Atef G. M.
    Bhandu, Lokeshwar
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Rao, K. Venkat
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Synthesis, magnetic and optical properties of core/shell Co(1-x)Zn(x)Fe(2)O(4)/SiO(2) nanoparticles2011Ingår i: Nanoscale Research Letters, ISSN 1931-7573, E-ISSN 1556-276X, Vol. 6, s. 460-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The optical properties of multi-functionalized cobalt ferrite (CoFe(2)O(4)), cobalt zinc ferrite (Co(0.5)Zn(0.5)Fe(2)O(4)), and zinc ferrite (ZnFe(2)O(4)) nanoparticles have been enhanced by coating them with silica shell using a modified Stober method. The ferrites nanoparticles were prepared by a modified citrate gel technique. These core/shell ferrites nanoparticles have been fired at temperatures: 400 degrees C, 600 degrees C and 800 degrees C, respectively, for 2 h. The composition, phase, and morphology of the prepared core/shell ferrites nanoparticles were determined by X-ray diffraction and transmission electron microscopy, respectively. The diffuse reflectance and magnetic properties of the core/shell ferrites nanoparticles at room temperature were investigated using UV/VIS double-beam spectrophotometer and vibrating sample magnetometer, respectively. It was found that, by increasing the firing temperature from 400 degrees C to 800 degrees C, the average crystallite size of the core/shell ferrites nanoparticles increases. The cobalt ferrite nanoparticles fired at temperature 800 degrees C; show the highest saturation magnetization while the zinc ferrite nanoparticles coated with silica shell shows the highest diffuse reflectance. On the other hand, core/shell zinc ferrite/silica nanoparticles fired at 400 degrees C show a ferromagnetic behavior and high diffuse reflectance when compared with all the uncoated or coated ferrites nanoparticles. These characteristics of core/shell zinc ferrite/silica nanostructures make them promising candidates for magneto-optical nanodevice applications.

  • 36.
    Gu, Ziyan
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Belova, Lyubov M.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Rao, K. V.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Co-existence of Superconductivity and Ferromagnetism in melt-spun Ni-Bi Compounds2005Ingår i: Journal of Materials Research, ISSN 0884-2914, E-ISSN 2044-5326Artikel i tidskrift (Övrigt vetenskapligt)
  • 37.
    Gu, Ziyan
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Gupta, Amita
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Lopez Ponce, Enrique
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Xu, Zhihui
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Rao, K. V.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Transport and Soft Magnetic Properties of Ultra-high Strength Co43Fe20Ta5.5B31.5 Bulk Glassy alloy2005Ingår i: Journal of Materials Research, ISSN 0884-2914, E-ISSN 2044-5326Artikel i tidskrift (Övrigt vetenskapligt)
  • 38.
    Gu, Ziyan
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Owens, Frank J.
    Belova, Lyubov M.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Rao, K. V.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Magnetic Patterning of Kapton-H Polyimide films via UV laser, and Hot stamping2005Artikel i tidskrift (Övrigt vetenskapligt)
  • 39.
    Gu, Ziyan
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Ström, Valter
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Rao, K. V.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Deevi, S. C.
    Macroscopic and Local Strain-induced Ferromagnetism at a Micron-scale in FeAl Cold Rolled sheets2005Ingår i: Intermetallics (Barking), ISSN 0966-9795, E-ISSN 1879-0216Artikel i tidskrift (Övrigt vetenskapligt)
  • 40. Guo, J. H.
    et al.
    Gupta, A.
    Sharma, P.
    Rao, K. V.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Marcus, M. A.
    Dong, C. L.
    Guillen, J. M. O.
    Butorin, S. M.
    Mattesini, M.
    Glans, P. A.
    Smith, K. E.
    Chang, C. L.
    Ahuja, Rajeev
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    X-ray spectroscopic study of the charge state and local ordering of room-temperature ferromagnetic Mn-doped ZnO2007Ingår i: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 19, nr 17Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The charge state and local ordering of Mn doped into a pulsed laser deposited single-phase thin film of ZnO are investigated by using x-ray absorption spectroscopy at the O K-edge, Mn K-edge and L-edge, and x-ray emission spectroscopy at the O K-edge and Mn L-edge. This film is ferromagnetic at room temperature. EXAFS measurement shows that Mn2+ replaces the Zn site in tetrahedral symmetry, and there is no evidence for either metallic Mn or MnO in the film. Upon Mn doping, the top of O 2p valence band extends into the bandgap, indicating additional charge carriers being created.

  • 41. Gupta, A.
    et al.
    Owens, F. J.
    Rao, K. V.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Iqbal, Z.
    Guille, J. M. O.
    Ahuja, Rajeev
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    High-temperature ferromagnetism in Cu-doped GaP by SQUID magnetometry and ferromagnetic resonance measurements2006Ingår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 74, nr 22Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The search for ferromagnetism above room temperature in semiconductors doped with paramagnetic ions has intensified in recent years because of the potential of combining magnetic information storage and electronic switching in one spintronic device. Here we report an observation of ferromagnetism well above room temperature in gallium phosphide doped with Cu2+ detected by ferromagnetic resonance and SQUID magnetometry. Other important features of the results in this p-type Cu-doped GaP are the high Curie temperature above 700 K significantly higher than previous observations, the relatively simple low-temperature bulk sintering process used to synthesize the material, which will significantly reduce the cost of large-scale production, and the use of copper as the dopant rather than manganese, which precludes ferromagnetic clusters or magnetic alloy impurities as the origin of the ferromagnetism. Ab initio calculations also show the existence of ferromagnetism in Cu-doped GaP. When the spin-orbit coupling is included, the total moment is enhanced and we get a total magnetic moment of 0.31 mu(B) with a local moment on Cu 0.082 and on P 0.204 mu(B).

  • 42.
    Gupta, Amita
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Cao, Hongtao
    Parekh, Kinnari
    Rao, K. Venkat
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Raju, A. R.
    Waghmare, Umesh V.
    Room temperature ferromagnetism in transition metal (V, Cr, Ti) doped In(2)O(3)2007Ingår i: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 101, nr 9, s. 09N513-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Indium oxide is chosen as the host material for doping Ti, V, and Cr transition metal ions. Theoretical calculations based on density functional theory within a local spin density approximation show that V-V separation of 5.6 A is more stable with a strong ferromagnetic coupling. Our calculations clearly predict that substitution of vanadium for indium should yield ferromagnetism in In(2)O(3). Experimentally, (In(0.95)TM(0.05))O(3) (TM=Ti,V,Cr) were prepared using sol-gel as well as solid state reaction methods. Superconducting quantum interference device magnetization measurements as a function of field and temperature clearly showed that the V and Cr doped samples are ferromagnetic with Curie temperature well above room temperature. Thin films deposited by pulsed laser ablation using these materials on sapphire substrates exhibit a preferred 222 orientation normal to the plane of the film. The magnetic moment for (In(0.95)V(0.05))O(3) film deposited in 0.1 mbar oxygen pressure was estimated to be 1.7 mu(B)/V and is comparable to the theoretical value of 2 mu(B)/V.

  • 43. Iqbal, Z.
    et al.
    Zhang, Y.
    Grebel, H.
    Vijayalakshmi, S.
    Lahamer, A.
    Benedek, G.
    Bernasconi, M.
    Cariboni, J.
    Spagnolatti, I.
    Sharma, R.
    Owens, F. J.
    Kozlov, M. E.
    Rao, K. V.
    KTH, Tidigare Institutioner                               , Materialvetenskap.
    Muhammed, Mamoun
    KTH, Tidigare Institutioner                               , Materialvetenskap.
    Evidence for a solid phase of dodecahedral C-202003Ingår i: European Physical Journal B: Condensed Matter Physics, ISSN 1434-6028, E-ISSN 1434-6036, Vol. 31, nr 4, s. 509-515Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Evidence is presented for the formation of a solid phase based on the smallest fullerene, C-20, in thin diamond-like carbon films deposited by ultraviolet laser ablation from diamond onto nickel substrates at room temperature in the presence of 10(-4) torr of cyclohexane or benzene. Laser desorption mass spectrometry from the films shows the presence of C-20, C-21 and C-22 species, while micro-Raman spectroscopy and electron diffraction from selected particles together with first principle density-functional calculations, indicate a cubic solid with dodecahedral C-20 Cages as building blocks. Unlike solid C-60 and fully protonated C-20, which are bound by van der Waals forces, the proposed structure is stabilized by linking of the C-20 dodecahedra with bridging carbon atoms at interstitial tetrahedral sites to form a face-centered-cubic lattice with 22 carbon atoms per unit cell.

  • 44. Jayakumar, O. D.
    et al.
    Gopalakrishnan, I. K.
    Kadam, R. M.
    Vinu, A.
    Asthana, A.
    Rao, K. V.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Tyagi, A. K.
    Surfactant-induced enhanced room temperature ferromagnetism in Zn0.96Mn0.03Li0.01O nanoparticles: Prepared by solid-state pyrolitic reaction2007Ingår i: Journal of Crystal Growth, ISSN 0022-0248, E-ISSN 1873-5002, Vol. 307, nr 2, s. 315-320Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We report the synthesis of nanoparticles of Zn0.96Mn0.03Li0.01O by a low-temperature solid-state pyrolitic reaction, followed by a surfactant-assisted calcination at 400 degrees C. The X-ray diffraction and transmission electron microscopy analyses showed the formation of impurity free nanocrystals of Mn doped Li co-cloped ZnO with wurtzite structure. XPS data revealed that Mn exists in + 2 oxidation state. DC magnetization measurements as a function of field and temperature showed enhanced room temperature ferromagnetism for the surfactant-treated Zn0:96Mn0.03Li0.01O. FMR signal observed in the EPR spectrum further confirmed its ferromagnetic nature.

  • 45. Jayakumar, O. D.
    et al.
    Gopalakrishnan, I. K.
    Kulshreshtha, S. K.
    Gupta, A.
    Rao, K. V.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Louzguine-Luzgin, D. V.
    Inoue, A.
    Glans, P. A.
    Guo, J. H.
    Samanta, K.
    Singh, M. K.
    Katiyar, R. S.
    Structural and magnetic properties of (In1-xFex)(2)O-3 (0.0 <= x <= 0.25) system: Prepared by gel combustion method2007Ingår i: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 91, nr 5Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    (In1-xFex)(2)O-3 polycrystalline samples with x=(0.0,0.05,0.10,0.15,0.20, and 0.25) have been synthesized by a gel combustion method. Reitveld refinement analysis of x-ray diffraction data indicated the formation of single phase cubic bixbyite structure without any parasitic phases. This observation is further confirmed by high resolution transmission electron microscopy imaging, indexing of the selected-area electron diffraction patterns, x-ray absorption spectroscopy, and Raman Spectroscopy. dc magnetization studies as a function of temperature and field indicate that they are ferromagnetic with Curie temperature (T-C) well above room temperature.

  • 46. Jönsson, B J
    et al.
    Ström, Valter
    KTH, Tidigare Institutioner, Fysik.
    Rao, K Venkat
    KTH, Tidigare Institutioner, Fysik.
    Dahlberg, D
    Competing anistropies in exchange biased Co/CoO bilayersArtikel i tidskrift (Övrigt vetenskapligt)
  • 47.
    Kapilashrami, Mukes
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Rao, K Venkat
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Belova, Lyuba
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Effects of hole doping in the ferromagnetic semiconductor Mn-doped ZnO thin film studied by x-ray magnetic circular dichoism2009Ingår i: "Solid state devices and materials(SSDM2009)" held in Japan(7-9 october 2009), 2009Konferensbidrag (Övrigt vetenskapligt)
  • 48.
    Kapilashrami, Mukes
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Rao, K Venkat
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Belova, Lyuba
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Teknisk materialfysik.
    Kloo, Lars
    KTH, Skolan för kemivetenskap (CHE), Kemi, Oorganisk kemi.
    Åkerstedt, Josefin
    KTH, Skolan för kemivetenskap (CHE), Kemi, Oorganisk kemi.
    Defect induced magnetic anisotropy in undopted ZnO thin filmsArtikel i tidskrift (Övrigt vetenskapligt)
  • 49.
    Kapilashrami, Mukes
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Upadhyay, Ramesh Venkataramaia
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Ström, Valter
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Belova, Liubov
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Rao, K. Venkat
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Effect of synthesis techniques on the magnetic properties of Mn-doped ZnO2008Ingår i: Magnetic Materials / [ed] Ghoshray, A; Bandyopadhyay, B, 2008, Vol. 1003, s. 255-257Konferensbidrag (Refereegranskat)
    Abstract [en]

    Mn-doped ZnO thin films have been prepared using three different routes, namely, magnetron sputtering technique, Pulsed Laser Deposition technique using targets from powders synthesized by solid state, and chemical method. Films deposited using sputtering technique in absence of nitrogen pressure gives higher magnetic moment per Mn atom, than is the case with films prepared by PLD indicating that ferromagnetism in the DMS systems is highly sensitive to processing conditions.

  • 50. Kapilashrami, Mukes
    et al.
    Wang, Yung Jui
    Li, Xin
    KTH, Skolan för bioteknologi (BIO), Teoretisk kemi och biologi.
    Glans, Per-Anders
    Fang, Mei
    Riazanova, Anastasia V.
    Belova, Lyubov M.
    Rao, K. Venkat
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Luo, Yi
    Barbiellini, Bernardo
    Lin, Hsin
    Markiewicz, Robert
    Bansil, Arun
    Hussain, Zahid
    Guo, Jinghua
    Understanding the magnetic interaction between intrinsic defects and impurity ions in room-temperature ferromagnetic Mg1-xFexO thin films2016Ingår i: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 28, nr 15, artikel-id 156002Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Understanding the nature and characteristics of the intrinsic defects and impurities in the dielectric barrier separating the ferromagnetic electrodes in a magnetic tunneling junction is of great importance for understanding the often observed 'barrier-breakdown' therein. In this connection, we present herein systematic experimental (SQUID and synchrotron-radiation-based x-ray absorption spectroscopy) and computational studies on the electronic and magnetic properties of Mg1-xFexO thin films. Our studies reveal: (i) defect aggregates comprised of basic and trimer units (Fe impurity coupled to 1 or 2 Mg vacancies) and (ii) existence of two competing magnetic orders, defect- and dopant-induced, with spin densities aligning anti-parallel if the trimer is present in the oxide matrix. These findings open up new avenues for designing tunneling barriers with high endurance and tunneling effect upon tuning the concentration/distribution of the two magnetic orders.

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