Vanadium dioxide (VO2) shows a phase transition above room temperature accompanied with a gigantic resistance jump. High temperature metallic phase is of tetragonal structure and undergoes a metal-insulator transition (MIT) into insulating monoclinic phase at lower temperature. The MIT temperature could be controlled with W doping to reveal metallic ground state at around 9% doping [1]. The crystal distortion in insulating phase is quite robust even if a few unit-cells of VO2 layers were confined with tetragonal TiO2 layers in superlattices [2]. However, W doping promotes phase separation, of which stability could be tuned by X-ray irradiation [3]. The over-doping of W promote reentrant of insulating phase, of which crystal electronic structures are distinct from the pristine phase as revealed by x-ray diffraction [4] and infra-red spectroscopy [5]. Charge accumulation in VO2 through an electric double layer gating was successfully applied to induce MIT by an electric field effect [6]. This work has been conducted in collaboration with K. Shibuya, D. Okuyama, J.-S. Lee, M. Nakano, Y. Taguchi, T. Arima, Y. Iwasa, and Y. Tokura. [1] K. Shibuya et. al, Appl. Phys. Lett. 96, 022102 (2010) [2] K. Shibuya et al., Phys. Rev. B 82, 205118 (2010) [3] K. Shibuya et. al., Phys. Rev. B 84, 165108 (2011) [4] D. Okuyama et. al., unpublished [5] J.-S Lee et. al., unpublished [6] M. Nakano et. al., unpublished

Recent advances in the growth of correlated electron oxide-based nanowire have opened up new possibility of these material systems for their use as building blocks of functional nanoscale device. Because they exhibit significantly different properties compared with their bulk counterparts due to surface effects and unique dimensionality. In case of VO2, which undergoes a first-order metal-insulator transition (MIT), a single metal-insulator domain wall (DW) driven phase transition has been observed in nanowire [1], while it is driven by nucleation and percolation of multiple domains in polycrystalline film. The interaction between conduction electron and DW could be very significant in a 1-dimensional nanowire. Particularly, the current induced motion of DW due to the local thermoelectric effect [2]. Here we studied electrical detection of a single DW dynamics in VO2 nanowire and the effect of current on the DW motion. The VO2 nanowires are grown by vapor transfer growth method [1]. Using pure Ar as a carrier gass, and VO2 flake as a source material, single crystal VO2 nanowires are grown on the 1 μm SiO2 /Si substrate. The temperature and vacuum pressure of tube furnace are controlled to 1000 C and 3 Torr, respectively under the Ar flow of 25 sccm. Electrical contact made of 200 nm Au/100 nm Cr are fabricated on the nanowires by using standard EB lithography and EB evaporation. The distance between the contacts is varied from 5 to 20 μm. After the fabrication of electrical contact, SiO2 layer is removed by a buffered HF solution to make nanowire to be suspended structures. Resistance of the nanowire is detected by applying constant voltage of 100 mV between the contacts and measuring the current through the nanowire with current amplifier. The metal-insulator transition of both as-deposited and suspended nanowire is detected with an optical microscope and a resistance measurement. We confirmed that the single phase domain is stabilized in the suspended nanowire, and therefore phase transition in the nanowire is driven by the injection and motion of a single metal-insulator DW. On the other hand, stripe domain structure is stabilized in case of as-deposited nanowire due to the strain from the substrate. These differences are more pronounced in the resistance measurement. Suspended nanowire exhibits non hysteresis region during phase transition due to the single DW motion, while very broad hysteresis loop is observed in case of as-deposited nanowire. By using suspended nanowire, we could precisely detect the motion of DW with electrical measurement during the application of an electric current. The effect of current on the DW motion is systematically studied. [1] J. Wei, Z. Wang, W. Chen, and D. H. Cobden, Nature Nanotech. 4, 420 (2009). [2] Fisher, B. J. Phys. C: Solid State Phys. 9, 1201 (1976).

Vanadium dioxide (VO₂) is a strongly correlated electrons material which shows a huge metal-insulator transition (MIT) at 340 K and it has mixed electronic phases consisting of metallic and insulating domains in the nano-spatial area near the MIT temperature. These nano-scaled domains will play an important role in their physical properties. From a view point of applications to electronic devices such as a Mott-FET, electrical control of the individual nano electronic domain is interesting to expect drastic change of physical properties by small perturbations such as electric field. In fact, each domain behavior against temperature or a magnetic field remarkably reflects to the electronic properties in nano-sized thin films which are a comparable order with domain size. In this research, we investigated the relationship between individual domain behavior and the electronic properties in high quality epitaxial VO₂ thin films through observation of micro-scaled metal/insulator domains, and fabricated epitaxial VO₂ nano-structures toward high performance nano-device application.VO₂ thin films were deposited on TiO₂(001) substrates or Al₂O₃(0001) substrates using a pulsed laser deposition technique. For the nanostructuring of the thin films, we performed nanoimprint lithography after spin-coating of organic resists on the thin films and the imprinted patterns on the resists were transferred to the underlayer VO2 film using reactive ion etching process. It was found that domain size in the VO₂ film on TiO₂(001) was intrinsically larger than that in reported VO₂ domains, which reflect strain effect from the substrate. Behavior of these micro size domains clearly affects their electronic properties as multiple resistive changes. We also perform nanoimprint lithography on VO₂ thin film with precise positioning control, and report their nanoscopic transport properties in VO₂ nanostructures.

Vanadium dioxide (VO2) has been proposed for several applications based on the nonlinearity and tunability of its electrical, dielectric and optical properties across its metal-insulator transition (MIT). We recently reported the potential of using VO2 thin-films for terahertz modulation with a transmission intensity modulation depth of 95%, which is significantly higher than the reported ~ 50% modulation depth of the-state-of-art THz modulators. To achieve electrically control of VO2 MIT for such device applications, it is critical to elucidate the evolution process of VO2 MIT under electrical control. Here we report the evolution process of VO2 thin films from the insulating phase to the metallic phase under current injection for the two-electrode based thin-film devices. Based on electrical characterization and Raman microscopic detection, it was found that there exist two critical current densities, based on which the insulator-to-metal transition process can be divided into three stages. In stage I with low current injection, VO2 film in the insulating (semiconducting) phase acts as a resistor until the first critical current density, above which the insulator-metal transition is a percolation process with metallic rutile and insulating monoclinic phases coexisting (stage II); while beyond a second critical current density, a filamentary current path with pure metallic phase is formed with the remaining part outside of the current path receding back to the pure insulating phase (stage III). We found that a critical current density is required for the onset of electrically induced insulator-to-metal transition in VO2 thin-films. Pulse measurement also confirmed that the phase transition can occur in sub-microsecond.

Amorphous oxide semiconductors has a long history comparable to crystalline semiconductors, leading to emerging research on transparent amorphous oxide semiconductors(TAOS) aiming at applying backplane TFTs for next generation FPDs utilizing large electron mobility and easy fabrication by sputtering. In 1953 a research group at Sheffield University, UK, published a paper reporting semiconducting glasses containing a large amount of V₂O5. This is a first report on semiconducting glass. Since then, many papers have been published on transition metal oxide-based glasses to date. The mechanism for semiconductivity in these glasses is due to electron hopping among the transition metal cations with different valence states such as V⁴⁺ and V⁵⁺. Here we report a novel class of glassy semiconductors, eletride glass. An electride is a crystal in which an electrons serves as an anion. First electride was synthesized in 1983 by J.Dye who successfully crystallized a crown-ether solution containing solvated electrons derived from alkali metals. Unfortunately, organic electrides synthesized were so thermally and chemical unstable that details on physical properties remained unclear. We reported thermally and chemical stable electrides in 2003 employing 12CaOâ-7Al₂O₃(C12A7) composed of 3D-connected sub-nanometer-sized crystallographic cages. The resulting C12A7 electride shows metallic conduction with (300K)=~1,500Scm^-1 and metal-superconductor transition at 0.2K. We found that a single crystalline C12A7 electride can be grown directly from the melt of polycrystalline C12A7 electride seed by floating zone technique. This result suggests that solvated electrons are persistently present even in the high temperature melt of C12A7 electride under a well controlled atmosphere. We measured electrical conduction in C12A7 melt in PO₂=10_-24 atm, fining that metallic conduction occurs in a whole temperature range 2-1600K. Subsequently, the melt was rapidly quenched by a twin-roller to obtain a glass. The resulting glass looks a block colored and exhibited a variable range hopping type conduction obeying logâ^T-1/4 at temperatures below ~700K. The concentration of electrons in C12A7 electride crystal and C12A7 electride determined by iodometry demonstrates that electrons accommodated in the cages of crystal retains even in the glass. The concentration of unpaired electrons evaluated by ESR is only 2% of the total electrons, implying the majority of electrons exists a spin paired state. It was obtained by Resonance Raman scattering that electrons are trapped in the cage like structure. The present result indicates that C12A7 electide glass is a novel class of amorphous semiconductors based on interstitial electrons in the insulating amorphous matrix.

Epitaxial perovskite-type ABO₃ thin films exhibit rich variety of electronic and magnetic phenomena that are closely coupled to strain-induced structural distortions of a perovskite unit cell. Mostly, such distortions are caused by B-O-B bond angle variations and/or changes of the B-O bond length resulting in rotations and/or deformations of BO₆ octahedra. The corner-sharing BO₆ octahedral distortions represent an effective way for substrate-induced strain accommodation and are an important tuning mechanism of the physical properties in epitaxial ABO₃ thin films [1]. Due to an inherent inter-atomic coupling across the film-substrate interface, to some degree, the coherently strained layer is expected to inherit the octahedral rotation pattern of the underlying substrate. This coupling effect is believed to be more pronounced at the vicinity of the interface, i.e. in ultrathin layers, and diminish with film thickness. As a result, functional properties of oxide thin films are expected to vary with thickness. Here we demonstrate that the octahedral distortions in ABO₃ oxide thin films where B = Mn or Ni are largely responsible for the metal to insulator transition as thin film thickness decreases. Transport measurements and x-ray diffraction data demonstrate that thicker films are metallic and possess monoclinic unit cell which is determined by rigid BO₆ octahedral rotations. As films get thinner, the unit cell symmetry increases from monoclinic to tetragonal by reducing the degree of octahedral rotations and inducing BO₆ octahedral deformations. The symmetry change caused by the proximity of the substrate alters the overall pattern of octahedral distortions and is believed to be a source of insulating behavior of ultrathin films and complex oxide heterostructures.

[1] A. Vailionis, et al., Phys. Rev. B 83, 064101 (2011).

Artificial heterostructures with functional perovskite oxides as building blocks can now be synthesized using advanced thin film deposition techniques [1]. Furthermore, strain engineering of these oxide heterostructures opens up routes for the creation of novel electronic phases that do not exist in the bulk [2,3]. To fully exploit the functionalities of the oxide, understanding its electronic and structural response to epitaxial strain is crucial. One example is the recent demonstration of biaxial tensile strain stabilizing an insulating ferromagnetic ground state in normally non-magnetic LaCoO3 [4,5]. However, theoretical understanding of the spin structure and ferromagnetic correlation in strained LaCoO3 is incomplete. In this talk, using spin density functional theory with the Hubbard U correction we discuss the origin of strain induced transition to insulating ferromagnetic ground state in LaCoO3. We show that beyond biaxial tensile strain of 2.5% local magnetic moments, originating from high spin state (t2g4eg2) of Co3+, emerge in low spin (t2g6eg0) Co3+ matrix. We further show that these local moments are ferromagnetically coupled via superexchange interaction. In contrast, we find that compressive strain by itself is not able to stabilize a magnetic state, that agrees with recent experiment [6]. Ferromagnetism found in tensile-strained LaCoO3 is tightly coupled to the materialâ?Ts orbital and structural response to applied strain. We discuss how LaCoO3 accommodates tensile strain via spin state disproportionation, resulting in an unusual sublattice structure. References [1] A. Ohtomo, D. A. Muller, J. L. Grazul, and H. Y. Hwang, â?oArtificial charge-modulation in atomic-scale perovskite titanate superlatticesâ?, Nature 419, 378-380 (2002). [2] J. H. Lee et al. â?oA strong ferroelectric ferromagnet created by means of spin-lattice couplingâ?, Nature 466, 954-958 (2010). [3] R. J. Zeches et al. â?oA strain-driven morphotropic phase boundary in BiFeO3â?, Science 326, 977 (2009). [4] D. Fuchs, C. Pinta, T. Schwarz, P. Schweiss, P. Nagel, S. Schuppler, R. Schneider, M. Merz, G. Roth, and H. v. Löhneysen, â?oFerromagnetic order in epitaxially strained LaCoO3 thin filmsâ?, Phys. Rev. B 75, 144402 (2007). [5] A. Posadas, M. Berg, H. Seo, A. de Lozanne, A.A. Demkov, D.J. Smith, A.P. Kirk, D. Zhernokletov, and R.M. Wallace,â? Strain-induced ferromagnetism in correlated oxide LaCoO3 epitaxially grown on Si (100)â?, Appl. Phys. Lett. 98, 053104 (2011). [6] V. Mehta and Y. Suzuki, â?oFerromagnetism enhanced by structural relaxation of biaxially compressed LaCoO3 filmsâ?, J. Appl. Phys. 109, 07D717 (2011).

Transition metal oxides display a wide range of physical properties arising from the complex interplay between their spin, charge, orbital and lattice degrees of freedom. The combination of these materials in artificially layered structures enables not only further tuning of their already outstanding properties, but also gives access to hidden phases and emergent physical phenomena. Here, we study LaNiO₃, the only member in the perovskite nickelates family which, in the bulk, lacks any ordering phenomena and remains a paramagnetic metal at all temperatures. We show that ultrathin films of LaNiO₃ undergo a metal-insulator transition as their thickness is reduced to just a few unit cells and that magnetotransport in the vicinity of the transition can be explained by 2D weak localization in the presence of significant spin fluctuations [1]. We also investigate high quality superlattices that combines LaNiO₃ with ferromagnetic LaMnO₃, grown along the [111] orientation . Surprisingly, exchange bias is observed in the magnetic properties of these superlattices, indicating not only that there is a strong interfacial coupling between the Mn and Ni cations, but also that magnetic order develops in the LaNiO₃ layers. The experimental results are in agreement with independently performed first-principles calculations, which point towards a spin-density wave as the most plausible type of magnetic order in LaNiO₃ [2]. Other possibles scenarios will be discussed. [1] R. Scherwitzl et al., PRL, 106, 246403 (2011) [2] M. Gibert et al., submitted

With recent advances in epitaxial thin film growth of the perovskite rare earth nickelates RNiO₃ (R = La, Pr, Nd, â?¦), experimental and theoretical efforts have explored the prospect of tuning the strongly correlated ground state through external parameters such as strain or electric field. In particular, a prototype â?oMott transistorâ? has been proposed [1] that would operate by electric field control of the RNiO₃ insulator-metal transition. This device would utilize the Schottky barrier between NdNiO₃ and SrTiO₃ [2] to accumulate charge in the NdNiO₃ [3], modulating its transition temperature. An important parameter for such a device is the Schottky barrier height, which we probe with vertical photocurrent measurements in two types of RNiO₃/SrTiO₃ heterojunctions. In the first structure, Al or Au contacts are evaporated on sputter deposited SrTiO₃/LaNiO₃ bilayers to form capacitive juntions. The zero bias photocurrent indicates the presence of a built-in electric field attributed to a larger barrier height on the nickelate side of the junction. Several energy dependent features are observed and are attributed to the band gap in SrTiO₃ and internal photoemission from the nickelate into the titanate. This data suggests an offset of 1.0 eV between the Fermi level of LaNiO₃ and the conduction band of SrTiO₃. In the second structure type, LaNiO₃ and NdNiO₃ films are sputter-deposited on MBE-grown SrTiO₃ films with controlled doping levels. The photocurrent spectra in the two structure types are compared to expose contributions from defects in the sputter deposited SrTiO₃ and from the (Au,Al)/SrTiO₃ interface. Additionally, we use very similar structures to address the underlying Mott physics of RNiO₃ through measurements of the (dark) tunnel current. A zero-bias feature is attributed to incipient localization. [1] J. Son et al. J. Appl. Phys 110, 084503 (2011). [2] Y. Kozuka, T. Susaki, and H. Y. Hwang, Appl. Phys. Lett. 88, 142111 (2006). [3] J. Son et al. arXiv:1110.4134.

Rare earth nickelates (RNiO₃) exhibit a first order metal insulator transition upon cooling. The transition temperature (T_MIT) can be strongly modified by chemical doping with heterovalent ions on the R site. Bulk studies on chemical doping indicated that both divalent ions (â?ohole dopingâ?) and quatrovalent ions (â?oelectron dopingâ?) were effective in shifting T_MIT to lower temperatures by ~ 25 to 50°K for 1 % electron and hole doping, respectively. However, chemical doping also affects the Ni-O bond angles and lengths. Separating the influence of structural distortions from band filling is particular important for the nickelates because the metal-insulator transition temperature is also a strong function of the rare earth ionic radius and of applied pressure. Here we present a new approach to control the band-filling in nanoscale NdNiO₃ thin films by modulation (or remote) doping. The Mott material NdNiO₃ is remotely doped by interfacing it with a degenerately doped conventional band insulator, La-doped SrTiO₃. We show that the remote doping approach allows for purely electronic modulation of a carrier density in the absence of other structural changes. The proposed approach is experimentally tested using ultrathin (2.5 nm) NdNiO₃ films grown on La-doped SrTiO₃ films with different carrier concentrations. We show that remote doping systematically changes the charge carrier density in the NdNiO₃ film and causes a moderate shift (20 â?" 30 K) in the metal-insulator transition temperature. These results will be discussed in the context of theoretical models of this class of materials exhibiting a metal-insulator transition. We show that an applied voltage can be used to modulate the electron transfer from the doped band insulator to the Mott material and produces depletion mode transistor action by inducing an insulator-to-metal transition. We show that the modulation-doped Mott field effect transistor (MM-FET) is characterized by unique â?ocharge gainâ? characteristics as well as competitive transconductance, small signal gain and current drive.

Multilayer systems of transition metal oxides have recently come to the fore due to their ability to form correlated electron interfaces [1, 2]. In addition to the chemical composition, their electronic and magnetic properties can be affected by the substrate material by means of an input of strain [3] or by varying the layer thicknesses [4]. Because of its strongly correlated conduction electrons, LaNiO3 is a promising candidate as one component of such superlattices. Separated by an insulating layer, as LaAlO3, superconductivity is predicted to emerge if the electronic dimensionality and the orbital polarization are correctly controlled [5]. We have characterized nickelate superlattices on an atomic scale using scanning transmission electron microscopy in combination with electron energy loss spectroscopy. The superlattices with two or four unit cell thick single layers were epitaxially grown on two substrates, SrTiO3 and LaSrAlO4, which differ in the strain they induce as well as in their polarity. In the case of the non-polar SrTiO3, a polar discontinuity develops at the interface between the substrate and the polar LaNiO3. Our results show that the atomic structure at the interface is significantly influenced by this polar discontinuity. Here, nanometer-sized nickel oxide precipitates form on non-polar substrates [6]. References: [1] A. Ohtomo, H. Y. Hwang, Nature 427 (2004), p. 423-426. [2] S. Okamoto, A. J. Millis, Nature 428 (2004), p. 630-633. [3] J. Son et. al., Appl. Phys. Lett. 96, 062114 (2010). [4] A.V. Boris et. al., Science 332, 937 (2011). [5] P. Hansmann et. al., Phys. Rev. B 82, 235123 (2010). [6] E. Detemple et. al., Appl. Phys. Lett., submitted

Oxide heterojunctions are an emerging area of interest in electronics and energy conversion. Microscopic mechanisms arising due to electron correlations versus point defect mediated phenomena require special attention. In this presentation, we consider a representative charge ordered system of (La,Sr)2NiO4 in thin film form grown on conducting oxide substrates and their semiconducting properties. We report on the synthesis of epitaxial La1.875Sr0.125NiO4 (LSNO) films on Nb-doped SrTiO3 (Nb:STO) single crystals and a detailed study of their electrical properties. The junctions measured displayed highly rectifying current-voltage characteristic from 298 to 373 K. Mechanism for rectification is likely due to the formation of a pn junction between heavily doped n-type Nb:STO and semiconducting p-type LSNO. Rectification ratio of approximately 300 was observed. Junction ideality of the interface was seen to decrease from approx. 2.8 to 2.3 as a function of temperature. Capacitance-voltage (C-V) measurements as a function of temperature and frequency were used to determine the admittance of the pn junction. Capacitance values did not scale with thickness indicating the role of the interface in dielectric response. From junction ideality values and built-in potential variation with frequency, we infer the role of defects in affecting the electronic properties of complex oxide pn junctions.

Magnetic nanostructures with graded anisotropy offer a solution to both thermal stability and writability challenges in advanced magnetic recording media. The interlayer exchange coupling lowers the overall coercivity, facilitating the writing process, while the magnetically hard layer provides pinning for the media and ensures its thermal stability. Magnetization reversal in such materials can be influenced by both the magnetic anisotropy gradient along the film depth and the lateral feature size. We have explored magnetization reversal in Co/Pd films and patterned structures. Perpendicular magnetic anisotropy is varied by changing the Co thicknesses or sputtering pressure during growth. Effects of deposition order and ion irradiation have been studied by x-ray diffraction, transmission electron microscopy, magnetometry, and first-order reversal curves. Structural integrity and amount of disorders are found to sensitively influence the magnetic properties. Reversal in highly ordered films is dominated by nucleation, propagation, and annihilation of domain walls while in disordered films magnetization reversal is largely by domain wall pinning and magnetization rotation. Depth-dependent magnetization profiles and magnetic anisotropy have been confirmed by polarized neutron reflectivity. Effects of lateral patterning have been investigated in patterned nanodots (down to 60nm diameter). An increase in coercivity and a modified switching field distribution are observed in patterned structures. This is due to the reduced lateral dimensions which limit the domain nucleation and propagation commonly found in unpatterned films. These results demonstrate attractive features of nanostructures with graded anisotropy towards future magnetic recording applications. This work has been supported by the US NSF (DMR-1008791 & ECCS-0925626).

The synthesis of silica mesoporous materials with ordered arrangements of mesopores (pore diameter ranging from 2 to 50 nm) and large surface-to-volume ratio is becoming increasingly appealing due to the potential applications in a broad range of fields. To gain in functionality, silica templates have been filled with different types of materials and transition metal oxides have been obtained as negative replicas of mesoporous silica templates. Here we present the structural and magnetic properties of two types of ordered mesoporous systems formed by two dissimilar (ferrimagnetic(FiM)-antiferromagnetic(AFM)) magnetic materials. The first approach is based on the synthesis of SBA-15 templated mesoporous NiO/NiCo₂O₄/Co₄O₂ composite replicas synthesized by filling a SBA-15 silica template with different proportions of nickel and cobalt nitrate salts followed by calcination. As a final step the SiO₂ template is etched away, leading to a mixed NiO - Co₃O₄ (AFM) and NiCo₂O₄ (FiM) replica. In a second approach a Co₃O₄ mesoporous replica was formed from a KIT-6 SiO₂ template. This replica-is subsequently filled with a Fe nitrate salt, which after calcination leads to a highly ordered AFM Co₃O₄ mesoporous host filled with FiM Fe_xCo_(3-x)O₄ nanostructures. Theses novel concepts allow increased functionality not only because of the synergetic combination of the properties of the constituents but also due to the AFM-FiM exchange interactions. Importantly, the transmission electron microscopy (TEM) and the small angle x-ray scattering (SAXS) investigations certify the high structural quality of the replicas. The magnetic properties show that the saturation magnetization, M_S and the coercivity, H_C, can be tuned by the relative amounts of the diverse phases. Moreover, when the material is cooled below the Néel temperature (T_N) of the AFM H_C can be further enhanced. Both systems also exhibit a loop shift in the field axis (i.e., exchange bias, H_E) below T_N, which is characteristic of AFM-FiM coupling. Remarkably, in the Fe_xCo_(3-x)O₄@Co₃O₄ case, H_C, H_E and the remanence (i.e., the magnetization at zero field) can be controlled, post synthesis, with the cooling field conditions.

Core|Shell nanoparticles (CS) have been recently proven to be interesting multicomponent systems that combine, in an effective and synergic manner, the distinct properties of the diverse components in a single modal structure [1-4]. However, in most cases bi-magnetic CS nanoparticles, where both the core and the shell are magnetic, are composed by the same transition metal ion [5,6]. In this work we present the study of heterostructured bi-magnetic CS systems based on two different transition metal oxides Fe₃O₄|Mn₃O₄. The nanoparticles were synthesized following a multi-step procedure, where preformed iron oxide nanoparticles were used as seeds for the subsequent growth of manganese (II) oxide. The Fe₃O₄ seeds were prepared by thermolysis of the iron (III) oleate [7] and the manganese oxide layer was laid on them modifying an earlier reported procedure used for the synthesis of MnO|Mn₃O₄ nanoparticles [5]. The system was studied by X-ray diffraction, transmission electron microscopy, electron energy loss spectroscopy and magnetic measurements. The structural characterization of the CS nanoparticles reveals a diffusion of the Mn and Fe ions in the particle during the high temperature exposure to air, leading to Mn being incorporated in the core and Fe in the shell, i.e., Mn_xFe_3-xO₄|Fe_xMn_3-xO₄. Given the soft and hard behaviour of Mn_xFe_3-xO₄ and Fe_xMn_3-xO₄, respectively, the structure can be considered an inverted soft|hard structure, in contrast to the conventional hard|soft arrangements. The magnetic measurements reveal that the CS nanoparticles exhibit an extra magnetic transition and lower values of MS compared to Fe₃O₄ seeds, consistent with the growth of a Fe_xMn_3-xO₄ shell, which has a Curie temperature of ~ 40 K. The hysteresis loops exhibit a smooth appearance implying the strong coupling between the core and the shell. Moreover, samples with different core sizes and equal shell thickness evidence that for the nanoparticles with smaller cores the coercivity increases while the overall MS decreases. This is expected from relative larger ratio of hard/soft phases in the particles with smaller cores and the hard character (higher coercivity and smaller MS) of the shell. References: [1] W. Schärtl, Nanoscale, 2010, 2, 829 [2] V. Skumryev, et al. Nature. 2003, 423, 850 [3] J. Nogués, et al. Phys. Rep. 2005, 422, 65 [4] G. Salazar-Alvarez, et al. J. Am. Chem. Soc. 2011, 133, 16738 [5] G. Salazar-Alvarez, et al. J. Am.Chem. Soc. 2007, 129, 9102 [6] A. López-Ortega, et al. J. Am. Chem. Soc. 2010, 132, 9398 [7] J. Park, et al. Nat. Mater. 2004, 3, 891

For a half century there has been significant fundamental research interest in the transition metal oxides that exhibit temperature-driven insulator-to-metal phase transition. Two-terminal devices based on such materials display current-controlled negative differential resistance (CC-NDR), or threshold switching, because Joule heating induces filamentary metallic phase formation causing a dramatic drop in resistance above a bias threshold. Although threshold switching could be valuable for applications ranging from signal processing to isolation in memristor crossbars, widespread use of phase transition oxides has been limited to date because the energy and time required to heat traditional device volumes is prohibitive. Here we experimentally demonstrate that a 30 nm effective radius NbO2 device can switch to the low resistance state within a nanosecond and with a total energy input less than 100 femtoJoules. We additionally report on a new dynamical model for Joule-heating-induced CC-NDR, calibrate it against nanoscale NbO2 devices and use this model to predict that a 10 nm radius device could switch with an enthalpy input of 10 femtoJoules within 100 picoseconds. These results demonstrate that, at the nanoscale, Joule-heating-induced CC-NDR devices are compatible to transistors in terms of switching speed and energy requirements and thus could be extremely valuable in hybrid CMOS-oxide integrated circuits.

Complex oxides have served as promising candidates for discovery of novel functional materials. One of the most successful examples is the mixed-valence lanthanum manganite in which very large magnetoresistance (MR) effects are observed. Among them, La1-xCaxMnO3, La1-xSrxMnO3 (LCMO, LSMO) are two most-studied compounds of the mixed-valence manganite family for its robust and large MR effect than other manganite members. In this work, manipulations of the both electronic and magnetic characteristics in colossal magnetoresistance (CMR) materials and corresponding MR effects through vertical heteroepitaxial nanostructures are carried out. Firstly, self-assembled La1-xCaxMnO3-NiFe2O4 and La1-xSrxMnO3- NiFe2O4 (LCMO-NFO and LSMO- NFO) nanostructures are deposited by using pulsed laser deposition. The spinel-structured ferromagnetic NFO nano-pillars are uniformly embedded in the perovskite-structured LCMO/LSMO matrix, which are distinguished by atomic force microscopy (AFM) and transmission electron microscopy (TEM). HRXRD and X-ray reciprocal space mapping have been used to confirm the phase-separation and the epitaxial relationships of the two functional materials, where a consistent four-fold symmetry epitaxial correlations for all species has been revealed. Such an elegant system enables us to gain advanced control of the CMR materials since the 3-D epitaxial strain and coupling have served as the bridges across the borders of two functional materials. Vibrating sample magnetometer and S.Q.U.I.D. are used to measure the magnetic properties and anisotropy of the nanostructures. X-ray magnetic circular dichroism (XMCD) of Mn element has further revealed the magnetism in both LCMO and LSMO at room temperature, implying a strong enhancement of the intrinsic Curie temperature via the coupling of the ferromagnetic nano-pillars is achieved. In addition, magnetotransport as functions of temperatures and magnetic fields have been probed. The spontaneous ordered NFO nanostructure form uniform-dispersed grains in LCMO/LSMO matrix; an enhanced low-field MR would be expected and explored based on spin-polarized tunneling theory. Through the coupling of the self-assembled vertical nanostructures of ferromagnetic and CMR materials, we successfully demonstrate the approach to manipulate one order parameters through the other. Such results pave a new pathway to manipulate the intriguing physical properties through the correlations between functional materials, which lead to new generation multifunctional applications and devices.

Transparent conducting oxides (TCOs) have attractive physical properties, including structural versatility, transparency and high conductivity. Imparting new functionalities into these complex materials is therefore of significant fundamental and technological importance. In this presentation I will describe the correlation between the growth, composition, metastability and defect structure of colloidal Ga2O3 and In2O3 TCO nanocrystals (NCs). The formation of native defects (i.e. oxygen vacancies) in these nanostructures was used to manipulate their optical emission and radiative decay dynamics by controlling the defect interactions. A strong blue emission (quantum yield ~ 25 %) observed in metastable γ-Ga2O3 NCs was assigned to the donor-acceptor pair (DAP) recombination mechanism. The emission energy and lifetime are generally determined by the donor and acceptor binding energies (which are dependent on the nanocrystal structure), and the attractive Coulombic interactions between charged donor and acceptor sites (which are dependent on the defect concentration). The formation and electronic structure of localized defects sites in Ga2O3 NCs can be controlled by synthesis conditions and composition manipulation (i.e. alloying with In), allowing for further manipulation of the photoluminescence properties. I will also present our recent progress in theoretical modeling of this size dependent photoluminescence decay in the framework of the DAP model. The described native defects are responsible for other functional properties of TCOs, particularly conductivity, and the ability to control their formation and interaction lends a new path for enhancement of the inherent functionalities of complex TCOs NCs.

Nonlinear optical (NLO) effects occur when strong light fields interact with matter. The applications include phenomena like parametric frequency generation, self-phase modulation and nonlinear light transmission. It has been shown that nanostructured materials are efficient media for NLO. When materials are combined to form a nanocomposite, under proper conditions the composite can be more NLO active than its constituents. Optical properties of composite materials are related to local field effects, and the third order nonlinear susceptibility of a nanocomposite is proportional to the fourth power of the local field correction factor. In the past few years there has been substantial interest in the development of nonlinear-optical media based on oxide glasses. Even though second order optical nonlinearities are absent in normal glass due to its inversion symmetry which nullifies even-order nonlinearities, this limitation can be removed by thermal treatment of glass in an electric field, oriented crystallization of non-centro-symmetric (NCS) phases on the glass surface, and nanostructuring of glass using semiconductor, metallic, or NSC oxide crystals. Rare-earth doping is another way to modify oxide glasses for nonlinear optics. A possible way for developing glass-based nonlinear optical media is the introduction of nonlinear optical crystals into a vitreous melt to produce a transparent composite. An introduction to optical nonlinearities in oxide nanocomposite materials will be given in this talk. Experimental techniques in NLO for measuring second order and third order nonlinearities will be briefly mentioned. Results obtained from our own work as well as those reported in literature will be presented, and the potential applications of oxide nanocomposites in optoelectronics and photonics will be discussed.

Phosphor screens made from powder phosphors such as Gd₂O₂S:Ce,Tb are efficient X-ray converters and widely used in X-ray imaging. However, the micron-sized phosphor particles embedded within the matrix cause strong light scattering, which limits spatial resolution, and subsequently prevents light-propagation thus reducing efficiency and resolution in thick screens. In this paper Tb doped gadolinium halide nanophosphors embedded in an aluminosilicate glass matrix will be reported for X-ray imaging applications. With negligible light scattering from nanoparticles, the transparent glass-matrix nanocomposite X-ray converter will potentially exhibit higher spatial resolution than screens made of micron-sized particles. Oxide glass-matrix scintillators are chemically and mechanically stable and can be easily shaped into large-sized plates for applications within adverse environments. The nanocomposite scintillators were prepared by a melt-quench method followed by an annealing treatment. All materials were mixed together and heated to above the melting temperature to prepare a homogenous melt, and then quenched to a low temperature to form a transparent glass sample. The halide nanophosphors were precipitated from and within the oxide glass matrix during the quench process or the following the anneal treatment. The quench and anneal treatment conditions were investigated to control the nanoparticle size between 1-20nm. Various high-Z gadolinium halide materials, including the fluorides, chlorides, and bromides were studied and Tb doping concentrations were then optimized by photoluminescence and X-ray scintillation studies. Co-doping of Ce₃⁺ and Tb₃⁺ will also be investigated to improve the luminescence efficiency.

Optical proccessing of information has many advantages, however Si, the dominating material in the modern microelectonics, is not suitable for the fabrication of optoelectronic devices because of its indirect energy band. This physical limitation can be overcome by employing the quantum-size Si nanocrystals. Light-emitting Si nanocrystals may be synthesized by the high-temperature heat treatment of silicon oxides. Swift heavy ions (SHI) have unique feature of creating in solids long narrow ion tracks with the extremely high levels of ionization and the temperatures up to 5000 K. Their cooling rates are about 10-10 K/s providing the annealing time of ~10-12 s. The SiO2 layers are widely used in microelectronics owing to their excellent dielectric properties and to the perfect Si-SiO2 interface. They are very promising for creation of optical waveguides. Here we attempted to form the light-emitting Si nanostructures site-selectively by disproportionation of SiO2 in the tracks of SHI. The 320 nm-thick SiO2 layers on Si substrates were irradiated with 167 MeV Xe ions to the fluences ranging between 1012 cm-2 and 1014 cm-2, or with 700 MeV Bi ions in the fluence range of 3x1012 â?" 1x1013 cm-2. After irradiation the yellow-orange photoluminescence (PL) band appeared. In the case of Xe bombardment its intensity grew continuously with the dose, while for Bi ions the PL intensity saturated by the dose of 5x1012cm-2. In parallel optical absorption in the region of 950-1150 cm-1, Raman scattering and X-ray photoelectron spectroscopy evidenced a decrease in the number of Si-O bonds and an increase in the number of Si-coordinated atoms. The irradiated layers were annealed in hydrogen at 500 -700 oC for defect passivation, or in nitrogen at 800 â?" 1100 oC for 30 min. It was found that the low-temperature passivation increased the fluence-dependent PL intensity. According to the high-resolution electron microscopy high temperature annealing promoted the formation of well defined nanoprecipitates with mean dimensions of 3 - 4 nm. The results obtained are interpreted as the formation of the light-emitting Si nanostructures inside the SHI tracks. High ionization losses of the ions are regarded as responsible for the effects observed. Difference between the dependences of the PL intensity on the fluences of Xe and Bi ions are ascribed to their different stopping energies, namely 14.5 keV/nm for Xe and 24 keV/nm for Bi. From the fluence dependences the track diameters for Xe and Bi ions were assessed as ~3 nm and ~10 nm, respectively. Position of the SHI induced PL band agrees with the predictions of the quantum confinement theory for Si nanoinclusions.

Domain structures of ferroelectric nanodots are still controversial discussed. Simulations showed the formation of particular domain patterns, like toroidal shapes or â?ocâ?-shapes due to the changes in surface to volume ratio and the changed boundary conditions of single nanodots. In this work we used nanosphere lithography (NSL) to pattern various samples of Pb(Zr,Ti)O₃ (PZT) thin films which were magnetron-sputtered on silicon substrates with a (111)-oriented platinum/titanium bottom electrode. XRD investigations showed that the resulting polycrystalline PZT films are also mainly (111)-oriented. Piezoresponse force microscopy (PFM) measurements on non-patterned films showed grain sizes around 50nm in random orientation and an average surface roughness of about 7.3nm. The nanosphere sizes and ion milling times were adjusted so that the final structures had diameters of down to 100nm and heights from 20nm to 100nm. PZT thin films were used for the fabrication of well-ordered nanodot arrays by means of NSL. This method is based on a two step etch process to, firstly, adjust the self-assembled polymeric nanosphere mask in diameter by reactive ion etching and, secondly, transfer the mask to the substrate by ion milling. This two step process allows the facile fabrication of nanostructures with various diameters and aspect ratios. The structures were measured by means of PFM while simultaneously the normal force and lateral force components were recorded which correspond to the out-of-plane and in-plane polarization, respectively. While nanodots with low aspect ratio showing grains and domains comparable with these one find in non-structured films, nanodots with higher aspect ratio show the formation of particular structures. The in-plane amplitude images show mostly a bisection domain assembly, while the out-of-plane amplitude images show three or more domains which form in some cases a ring of domains around a central part. The patterning of the ferroelectric material has an impact on the formation of ferroelectric domains. The initial polycrystalline, randomly-ordered films are influenced by the patterning process. It leads to a re-orientation of the single grains and shows more or less predictable formations depending on diameter and aspect ratio of the structures. This may offer the possibility to tailor ferroelectric materials in theire piezoelectric and pyroelectric properties just by patterning.

Synthesis of FeC₂O₄ 2H₂O nano particles was carried out by thermal double decomposition of solutions of oxalic acid dihydrate (C₂H₂O₄ 2H₂O) and FeSO₄ 7H₂O employing microwave reactor. Structural elucidation was carried out by employing X-ray diffraction (XRD). This results indicate that oxalate is in rhombic symmetry with cell constants a = 12.675â"«, b = 5.40â"«, c = 9.98â"«. Particle size and shape were studied by transmission electron microscopy (TEM). The particles are cylindrical in shape and have a diameter of 50 nm and length of 266 nm. Nature of bonding was investigated by electron paramagnetic resonance (EPR). EPR results indicate that the sample exhibits rhombic symmetry. Optical absorption spectrum is due to both iron +2 and +3 and are in distorted octahedral environment.NIR results are due water fundamentals which indicate that the water molecules are not free but are bounded to the metal ion as ligands. The powder resulting from this method is pure and possesses distorted rhombic octahedral structure. The particle grain size is about 50 nm.