Session S23: Focus Session: Nanostructured Oxides and Thin Films

Synthesis, structure and properties of nanostructured manganites

Manganites form a fascinating class of multifunctional perovskite oxides with competing interactions that couple the charge, spin and the lattice. The presence of competing interactions (that often have a comparable energy scale) leads to presence of variety of phases in the manganites that can be tuned by substitution (hole concentration), pressure and magnetic field. Interesting classes of phenomena arise when the size is taken to nanoscales where the ground state can be tuned by the size. The size also tunes the nature of the phase transition and the nature of the electronic transport. Interestingly, the primary change occurs in the lattice structures on size reduction that weakens the orthorhombic distortion. The physical properties of the nanostructured manganites are very distinct and different from that seen in related nanostructured transition metal ferromagnetic oxides like the cobaltates. In this talk we will first describe the methods of synthesis of nanostructured manganites in the form of nanocrystals, nanowires and nanostructured films using soft chemical routes. Arrays of the nanostructured manganites can also be made using such techniques as dip-pen lithography using an Atomic Force Microscope. This will be followed by presentation of results on structures using synchrotron X-rays and neutrons that establish the nature of structural changes on size reduction. The structural changes (as established through the above studies) on size reduction tend to enhance the ferromagnetic interaction in the system. Next we will report a number of physical phenomena that arise as a consequence of the size reduction. This will include destabilization of the charge ordered state, change in the nature of the ferromagnetic transition and non-linear conduction that arises in the nanostructured films due to presence of a large number of grain boundaries. At very low temperatures the transport in the manganites become dominated by such effects as intergrain tunneling and Coulomb blockade.   

Epitaxial growth of complex oxide films by a chemical solution method

In the last several years, complex oxides have become the basis for many revolutionary electronic devices because they exhibit a wide range of electronic properties that conventional metallic elements and covalent semiconductors do not possess. Complex oxide films can be grown by physical vapor deposition, chemical vapor deposition, and chemical solution deposition techniques. One of the challenges in solution-based processes of oxide films has been to produce high quality films and at the same time to control the stoichiometry. Here we describe a new chemical solution method called polymer-assisted deposition (PAD) to grow epitaxial oxide films (such as Ba$_{1-x}$Sr$_{x}$TiO$_{3}$ and La$_{0.7}$Sr$_{0.3}$MnO$_{3})$. We use a new strategy to control the distribution of metals in solution at a molecular level and a mixture of metal precursor and soluble polymer to form a solution with desired viscosity. By actively binding the metal, the polymer serves to encapsulate the metal to prevent chemical reaction while maintaining a uniform distribution of the metal in solution. This ensures a homogeneous metal distribution and prevents unwanted reactivity that can lead to the formation of undesired phases. The successful growth of epitaxial complex oxide films by PAD suggests that PAD is a feasible alternative approach to the growth of high quality films with desired properties.   

Tailoring The Magnetic Properties of TiO$_{2}$ Nanobelts

Over the past decades, titanium dioxide has been discovered to exist in various novel nano-structural forms with high aspect-ratio and good crystallinity. Moreover, the addition of dopants and the self-assembling of foreign molecules on the surface can enrich the physical and chemical properties of this semiconductor, enhancing its versatility and further promoting this metal oxide to be an important nano-based functional material. Herein we report on the doping of TiO$_{2}$ nanobelts with small amount of cobalt, producing a diluted magnetic semiconductor which display a Curie temperature well above room temperature. Co-doped TiO$_{2}$ nanobelts were prepared hydrothermally by powdering a piece of Co$_{x}$Ti$_{1-x}$O$_{2}$ single crystal. The magnetic properties of these nanobelts could be tailored via different heat treatment procedure. Annealing the as-synthesized cobalt titanate in air at 1000 K produced nanobelts which are paramagnetic, while evidence of room temperature ferromagnetism could be observed after the same sample was annealed under vacuum at length. An even more pronounce ferromagnetic behavior was observed when the nanobelts were vacuum annealed direct from the titanate phase. These results again show the importance of oxygen vacancies in unison with the presence of cobalt in inducing room temperature ferromagnetism in this semiconductor.   

Surface anisotropy and orbital moment in Fe$_{3}$O$_{4}$ nanoparticles

Fe$_{3}$O$_{4}$ nanoparticles (NP) in the 5-20 nm range were synthesised in the presence of a variety of surfactants (oleic acid, PVA, {\ldots}). Saturation magnetization M$_{s}$ almost reaches the expected bulk value for those surfactants covalently bonded to the NP, being much larger than in those NP with just a protective coating. Thermo-remanence and ac susceptibility demonstrate that the surface anisotropy constant in covalently bonded NP is similar to the bulk value. XMCD confirms the dependence of the magnetic moment on the surface bond and suggests that the orbital contribution is smaller in covalently bonded NP. The occurrence of bulk M$_{s}$ in Fe$_{3}$O$_{4}$ NP may thus be related to the crystal and magnetic state at the surface. This is of relevance in biomedical applications to reduce the strength of the magnetic field required to obtain a high magnetic response, while the issue of the orbital contribution in Fe$_{3}$O$_{4}$ is under hot debate. Work funded by Spanish NAN2004-08805-CO4-02, NAN2004-08805-CO4-01, MAT2006-03999, MAT2005-02454 and CONSOLIDER CSD2006-12, and Catalan 2005SGR0969.   

Novel Oxide Glass-Based Nanocomposites: Development and Structural Characterization

Glasses can gain new functionalities when specific crystalline phases of nanometer dimensions are nucleated in them. We have developed a series of optically transparent glass-based composites, containing nanometer-sized crystals dispersed within the isotropic host matrix. The composites were characterized using conventional and synchrotron x-ray diffraction as well as synchrotron x-ray absorption spectroscopy. EXAFS studies of Er-doped, gallium-silica glasses and composites will be presented. As the glasses are treated at elevated temperatures, long-range beta-Ga$_{2}$O$_{3}$ is observed to evolve from the initially amorphous local structure. However, the same samples show no significant change in the Er local structure, possibly indicating clustering or preferential of Er in proximity to Ga atoms.   

A Memristive Switching Model of Two-Terminal Programmable Nanodevices.

The existence of a fourth passive circuit element was proposed by Chua in 1971 from fundamental symmetry arguments. Although he showed that such a device, which he called a `memristor' (acronym for memory and resistor), had many interesting and useful circuit properties, until now no one has presented a physical model or example of such an element. We show here using a simple analytical example that memristance arises naturally in systems for which electronic and atomic transport are coupled under an external bias, and we explore the range of validity of the basic nonlinear circuit model of a more general class of dynamical devices called memristive systems. These results serve as the theoretical foundation for understanding a wide range of hysteretic current-voltage behavior observed in next-generation non-volatile resistive RAM devices, including nanoscale titanium oxide crosspoint switches built in our laboratory.   

Nano-crystalline structure and catalytic activity of TiO$_{2}$ supported manganese oxide catalysts

A series of TiO$_{2}$ supported manganese oxide catalysts were prepared by wet-impregnation method. Raman spectroscopy was used to characterize the structure and chemical environment of these catalysts as well as manganese oxides. Spectra of different Mn-oxides are presented. It was found that there is strong correlation of the position and the width of E$_{2g}$ mode of anatase at 146 cm$^{-1}$ and Mn-oxide(s) concentration. Evolution of Raman spectra described in the frame of the optical confinement model. In this model, the position and width of the Raman band depend on structural disorder and size of nano-crystals. Size of nano-crystal was estimated.   

Very Low Frequency ($<$1 mHz) Magnetic Noise in La$_{0.67}$ Ca $_{0.33}$ MnO$_{3}$ Films

We report an occurrence of very low-frequency ($<$1 mHz) resistance fluctuations (noise) in a rare-earth perovskite manganite film. This fluctuation is distinct from 1/f noise and is larger than that. The fluctuation arises due to coupling to magnetization fluctuation. It reaches a peak close to the ferromagnetic Curie temperature T$_{C}$. The magnetic nature of the transition has been established by sensitivity of the noise to a very low applied magnetic field $<$0.1 Tesla. The magnetization fluctuation has been calculated from the resistance fluctuation using the directly measured magnetoresistance. The magnetization fluctuations show peak at a temperature close to but lower than T$_{C}$ and shows a nontrivial dependence on the applied magnetic field.   

Film Growth and Surface Energy of (100) CrO$_{2}$

Rutile structure CrO$_{2}$ can be grown epitaxially on a rutile TiO$_{2}$ substrate. Surprisingly, growth in the (100) direction is layer by layer even though surface energies evaluated using the VASP code [1] indicate that the surface energy of TiO$_{2}$ is less than the sum of the surface energy of CrO$_{2}$ and the CrO$_{2}$-TiO$_{2}$ interface energy[2]. It is known however, that CrO$_{2}$ (100) surfaces spontaneously decompose to form an epitaxial Cr$_{2}$O$_{3}$ phase. We have investigated the conjecture that the layer by layer growth of CrO$_{2}$ results from the formation of a surface layer substoichiometric in oxygen. If the conjectured substoichiometric layer forms, it must be converted to CrO$_{2}$ as it is covered. We calculate the energy of a (100) fully stoichiometric surface, a (100) surface with oxygen removed, and a (100) surface with an interior oxygen deficiency using the standard relaxation methods in the VASP code as well as its simulated annealing capabilities. By comparing the energies (taking into account the removed oxygen in the latter two cases), we can predict the behavior of the CrO$_{2}$ films during deposition. \newline [1] G. Kresse and J. Hafner, Phys. Rev. B {bf 47}, 558 (1993); G. Kresse and J. Hafner, Phys. Rev. B {bf 49}, 14 251 (1994). \newline [2] K. Chetry and A. Gupta, unpublished   

Study of Magneto-Transmission Spectra of La$_{0.7}$Pb$_{0.3}$MnO$_{3-\delta }$ Epitaxial Thin Film

Magneto-transmission (MT) of epitaxially grown La$_{0.7}$Pb$_{0.3}$MnO$_{3}{\rm g}$ was measured. ThermoElectron Nexus 670 FT-IR spectrometer equipped with an electromagnet was used to obtain IR spectra in the range of 350 to 15000 cm$^{-1}$ in various applied magnetic fields up to 1.0 Tesla. For optimal magneto-spectroscopic measurements in transmission, the studied film had a thickness of 190 nm, with a maximum value up to 80{\%} at 320 K in the colossal magnetoresistance (CMR) effect in 5.5 Tesla. It was observed that the MT scale proportionally with the applied magnetic field and was largest at longer wavelengths below 4000 cm$^{-1}$. In this far infrared range, the maximum observed MT value was 4.0 {\%} at 1.0 Tesla. Beyond FIR range the MT curves monotonically decrease with frequency, until the effect vanishes at 12,000 cm$^{-1}$. Such crossover of magneto-spectroscopic responses from IR to optical frequencies is the first evidence of gradual disappearance of the magento-dynamics at higher frequencies. Compared with the CMR effect, the measured MT property resembled that of the CMR closely in the field range and frequency ranges studied.   

Ferromagnetism in Co doped anatase TiO$_{2}$ thin films mediated by Co-Ti$^{+3}$-V$_{O}$ complexes

The correction of spherical aberration in the STEM has enabled sub-Angstrom imaging and spectroscopy, and, in favorable cases, direct imaging of light atoms and interstitials. We identify the origin of ferromagnetism in Co$_{0,03}$:TiO$_{2}$ anatase thin films by combining STEM, EELS and DFT calculations. The films are insulating and ferromagnetic at room temperature. Ferromagnetism is enhanced by a post growth vacuum annealing suggesting a defect-mediated mechanism in these films. DFT finds interstitial Co to be energetically preferred over substitutional Co. STEM imaging reveals the interstitials in the predicted sites, and EELS finds reduced Ti in adjacent columns, also predicted by DFT. The combination of STEM-EELS-DFT therefore identifies the defect responsible for the magnetism: an O vacancy binds to the interstitial Co to form a Co-Ti$^{+ 3}$-V$_{O}$ complex, with a magnetic moment in good agreement with the observed value. Research sponsored by Div. of Materials Sciences and Engineering US DOE, and NSF/ECS 0224138.   

Nonmetal-metal transition in anatase Nb-doped TiO$_{2}$

Anatase TiO$_{2}$ show nonmetal-metal transition on Nb doping. Epitaxial Ti$_{0.94}$Nb$_{0.06}$O$_{2}$ (TNO) thin film exhibits low electrical resistivity, \textit{$\rho $} $\sim $ 1.7 x 10$^{4} \quad \Omega $cm at 300 K, comparable to highly-conducting transition metal oxide, ReO$_{3}$ and Na$_{x}$WO$_{3}$. This TNO is an n-type degenerate semiconductor with carrier density exceeding 10$^{21}$ cm$^{-3}$. We have studied the electronic structure of this TNO system using resonant photoemission spectroscopy and compared with first-principles calculations. The first-principles calculations reveal that there is no impurity state arising from Nb doping, and partial density of states of Nb contribute to both valence band and conduction band. These results imply that Nb is highly-hybridized with Ti and O orbitals, resulting in high activation efficiency of Nb which leads to high carrier density in the TNO system. Resonant photoemission spectra clearly show wide band gap without impurity state with Fermi edge located in the conduction band.   

Carrier mediated Ferromagnetism in Cr: In$_{2}$O$_{3}$

While a number of Dilute Magnetic Semiconducting Oxides (DMSO), when doped with transition metals, exhibit ferromagnetism well above room temperature, most of them are insulating and hence the relationship between the charge carriers and ferromagnetism remains unclear. Here, we investigate a high Curie temperature DMSO, Cr:In$_{2}$O$_{3}$, which is made conductive down to low temperatures. Films of various thicknesses ranging from 200 nm to 1100 nm were grown on sapphire substrate by rf sputtering. Upon vacuum annealing, the samples became both magnetic (with the saturation magnetization $\sim $0.07$\mu $B/Cr) and highly conductive, in contrast to as-prepared samples which were both non-magnetic and insulating. Point Contact Andreev Reflection (PCAR) spectroscopy with the superconducting Sn tip demonstrated significant (50 {\%} $\pm $ 5{\%}) transport spin polarization, strongly suggesting that ferromagnetism in Cr:In$_{2}$O$_{3}$ is carrier-mediated. We will discuss the implications of these results on our understanding of the nature of ferromagnetic interactions in DMSOs, as well Cr: In$_{2}$O$_{3}$ potential applications as a spin injector.