Franklin J. Wong

Enhanced magnetism in epitaxial SrRuO3 thin films

We observed enhanced magnetization in epitaxial SrRuO3 thin films compared to previously reported bulk and thin film values. The enhancement is strongly dependent on the orientation of the lattice distortions imposed by (001), (110), and (111) oriented SrTiO3 substrates. A larger magnetization enhancement for coherently strained SrRuO3 films on (111) and (110) oriented SrTiO3 compared to films on (001) SrTiO3 confirms the importance of the strain state in determining the magnetic ground state of the Ru ion. Moreover, SrRuO3 films on (111) SrTiO3 exhibit enhanced moments as high as 3.4 μB/Ru ion, suggesting the stabilization of a high-spin Ru4+ state.

Enhanced magnetization in epitaxial SrRuO3 thin films via substrate-induced strain

Epitaxial SrRuO3 thin films were grown on SrTiO3, (LaAlO3)0.3(Sr2AlTaO6)0.7, and LaAlO3 substrates inducing different compressive strains. Coherently strained SrRuO3 films exhibit enhanced magnetization compared to previously reported bulk and thin film values of 1.1–1.6μB per formula unit. A comparison of (001) SrRuO3 films on each substrate indicates that strained films have consistently higher saturated moments than corresponding relaxed films, which exhibit bulk moments. These observations indicate the importance of lattice distortions in controlling the magnetic ground state in this transitional metal oxide.Epitaxial SrRuO3 thin films were grown on SrTiO3, (LaAlO3)0.3(Sr2AlTaO6)0.7, and LaAlO3 substrates inducing different compressive strains. Coherently strained SrRuO3 films exhibit enhanced magnetization compared to previously reported bulk and thin film values of 1.1–1.6μB per formula unit. A comparison of (001) SrRuO3 films on each substrate indicates that strained films have consistently higher saturated moments than corresponding relaxed films, which exhibit bulk moments. These observations indicate the importance of lattice distortions in controlling the magnetic ground state in this transitional metal oxide.

Ferromagnetism in tetragonally distorted LaCoO3 thin films

Thin films of epitaxial LaCoO{sub 3} were synthesized on SrTiO{sub 3} and (La, Sr)(Al, Ta)O{sub 3} substrates varying the oxygen background pressure in order to evaluate the impact of epitaxial growth as well as oxygen vacancies on the long range magnetic order. The epitaxial constraints from the substrate impose a tetragonal distortion compared to the bulk form. X-ray absorption and x-ray magnetic circular dichroism measurements confirmed that the ferromagnetism arises from the Co ions and persists through the entire thickness of the film. It was found that for the thin films to show ferromagnetic order they have to be grown under the higher oxygen pressures, since a decrease in oxygen deposition pressure alters the film structure and suppresses ferromagnetism in the LaCoO{sub 3} films. A correlation of the structure and magnetism suggests that the tetragonal distortions induce the ferromagnetism.

Gigahertz-frequency operation of a LaAlO3/SrTiO3-based nanotransistor

Nanoscale control of the metal-insulator transition of the LaAlO3/SrTiO3 interface with a conductive-atomic force microscope (c-AFM) technique has enabled a variety of electrical and photonic device concepts. While previous devices have demonstrated sub-10 nm critical features, information processing applications also require high operating speeds. Here we show that a “sketched” nanoscale transistor (“SketchFET”) can operate at frequencies in excess of 2 GHz. The combination of high speed and high conductance with a small footprint make these devices and this platform attractive for sub-10 nm computing and storage architectures.

Growth and characterization of superconducting spinel oxide LiTi2O4 thin films

LiTi2O4 is a unique material in that it is the only known oxide spinel superconductor. Although bulk studies have demonstrated that superconductivity can be generally described by the BardeenCooper-Schreiffer theory, the microscopic mechanisms of superconductivity are not yet resolved fully. The sensitivity of the superconducting properties to various defects of the spinel crystal structure provides insight into such mechanisms. Epitaxial films of LiTi2O4 on single crystalline substrates of MgAl2O4, MgO, and SrTiO3 provide model systems to systematically explore the effects of lattice strain and microstructural disorder. Lattice strain that affects bandwidth gives rise to limited variations in the superconducting and normal state properties. Microstructural disorder such as antiphase boundaries that give rise to Ti network disorder can reduce the critical temperature, but Ti network disorder combined with Mg interdiffusion can affect the superconducting state much more dramatically. Thickness dependent transport studies indicate a superconductorinsulator transition as a function of film thickness regardless of lattice strain and microstructure. In addition, surface sensitive X-ray absorption spectroscopy has identified Ti to retain site symmetry and average valence of the bulk material regardless of film thickness. PACS numbers: 74.78.-w, 61.10.Ht

Enhanced Magnetization of CuCr2O4 Thin Films by Substrate-Induced Strain

Enhanced Magnetization of CuCr 2 O 4 Thin Films by Substrate-Induced Strain Jodi M. Iwata 1 , Rajesh V. Chopdekar 2, 1 , Franklin Wong 1 , Brittany B. Nelson-Cheeseman 1 , Elke Arenholz 3 , and Yuri Suzuki 1 Dept. Materials Science and Engineering, UC Berkeley, Berkeley, CA 94720 School of Applied Physics, Cornell University, Ithaca, NY 14853 Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, CA 9472 Abstract We report the synthesis of epitaxial spinel CuCr 2 O 4 thin films that display enhanced magnetization in excess of 200% of the bulk values when grown on single-crystal (110) MgAl 2 O 4 substrates. Bulk CuCr 2 O 4 is a ferrimagnetic insulator with a net magnetic moment of 0.5μ B due to its distorted tetragonal unit cell (c/a= 1.29) and frustrated triangular moment configuration. We show that through epitaxial growth and substrate-induced strain, it is possible to tune the magnetic functionality of our films by reducing the tetragonal distortion of the unit cell which effectively decreases the frustration of the magnetic moments allowing for an overall greater net moment.

Quasi-two-dimensional electron gas behavior in doped LaAlO3 thin films on SrTiO3 substrates

We have demonstrated the growth of Tm and Lu doped LaAlO3 epitaxial thin films on single crystal (001) SrTiO3 substrates. These rare-earth dopants potentially act as sources of localized moment and spin-orbit scattering centers at the interface. Through structural and chemical characterization, we confirm the incorporation of Tm and Lu dopants into highly crystalline LaAlO3 films. The rare earth doping of the La site does not significantly modify the sheet carrier concentration or mobility compared to undoped samples despite the evolution of sheet carrier concentration, mobility, and sheet resistance with LaAlO3 thickness in undoped LaAlO3 films on SrTiO3.

Electronic tuning of La2/3Sr1/3MnO3 thin films via heteroepitaxy

Epitaxial La2/3Sr1/3MnO3 films grown on LaAlO3 substrates of various orientations exhibit a range of magnetoresistive properties, demonstrating the utility of strain as an electronic tuning parameter for manganites. Large magnetoresistance over a broad range of temperatures—highest (−64% at 50 kOe) at the lowest temperatures measured—is observed in a coherently strained La2/3Sr1/3MnO3 film on a (001) LaAlO3 substrate. In addition to higher magnetoresistance, its reduced magnetization and conductance suggest the stabilization of a more insulating ground state and the possibility of strain-induced phase coexistence. Similar field-dependent magnetotransport features at low temperatures, distinct from those exhibited by bulk manganites, are also seen in a partially strained film on a (110) LaAlO3 substrate, but bulk-like magnetoresistive behavior is observed in a relaxed La2/3Sr1/3MnO3 film on a (111) LaAlO3 substrate.

Interfacial magnetism in CaRuO3/CaMnO3 superlattices grown on (001) SrTiO3

We have studied epitaxially grown superlattices of CaRuO3/CaMnO3 as well as an alloy film of CaMn0.5Ru0.5O3 on (001) SrTiO3 substrates. In contrast to previous experiments, we have studied CRO/CMO superlattices with a constant CRO thickness and variable CMO thickness. All superlattices exhibit Curie temperatures (TC) of 110 K. The saturated magnetization per interfacial Mn cation has been found to be 1.1 μB/Mn ion. The TC’s of the superlattices are much lower than the TC of the alloy film while the saturated magnetization values are larger than that of the alloy film. These observations suggest that interdiffusion alone cannot account for ferromagnetism in the superlattices and that double exchange induced FM must play a role at the interfaces.

Structure and magnetism of nanocrystalline and epitaxial (Mn,Zn,Fe)3O4 thin films

Nanocrystalline (NC) textured Mn0.5Zn0.6Fe1.9O4 (MZFO) films, grown at room temperature on both isostructural and non-isostructural substrates, show magnetization values significantly suppressed from epitaxial MZFO films. X-ray absorption spectroscopy and x-ray magnetic circular dichroism measurements indicate larger ratios of Fe3+ to Fe2+ ions on the tetrahedral sites in the NC films compared to the epitaxial films. The magnetization loops of the NC films are shifted by 200−400 Oe at low temperatures. No such effect is observed in the epitaxial films. We hypothesize that the presence of a more structurally disordered, possibly magnetically frustrated, matrix exchange biases the crystalline regions.