Crystallographic defects introduced in the YBa2Cu3O7−δ thin films during depositions were studied using X-ray diffraction method, and their influences on superconducting properties were also investigated by examining Tc variation of the as-grown and post-annealed thin films. It was found that there exist several kinds of defects in the as-grown thin films, such as oxygen deficiency, cation disordering, and others like lattice dislocation, stacking faults, etc.. These defects could be relaxed by heat treatment at temperatures ranging from 400 to 900 °C, resulting in Tc enhancement more or less. Quantitative understandings of the relations among annealing temperature, structural relaxation, and Tc improvement has also been reached. The kinetics of the relaxation of the defects in the YBCO films was studied further using a high-temperature X-ray diffractometer. Interesting phenomena have been observed at around 400 °C due to abnormal desorption behavior of oxygen and at higher temperatures relating to cation ordering.

Epitaxial films of Sr2RuO4 have been grown in situ by pulsed laser deposition on (100) LaAlO3 and (100) LaSrGaO4 substrates. X-ray diffraction results show that the films are single domain and grow c-axis oriented on (100) LaAlO3 and a-axis oriented on (100) LaSrGaO4 substrates. X-ray ø-scans indicate epitaxial alignment of the film and substrate in-plane axes in both cases. Resistivity versus temperature measurements reveal that the as-grown c-axis oriented films are semiconducting and the a-axis oriented films are metallic. The metallic films grown so far were found to be non-superconducting down to 50 mK.

La0.8Sr0.2 MnO3 thin films with different microstructural characteristics were prepared by both pulsed laser deposition and a novel polymeric sol-gel process. For both techniques, polycrystalline films were obtained on Si(100) with a native oxide surface but epitaxial and highly oriented growth could be induced on lattice-matched LaAlO3(100) substrates. Whilst the overall magnetization and magnetoresistance behavior as a function of temperature and applied field were consistent with recent results, i.e. a semiconductor-to-metal transition accompanied by an antiferromagnetic-to-ferromagnetic transformation and large magnetoresistance at the resistance peak temperature, the properties of the films were significantly different depending on the microstructure, epitaxy and polycrystallinity. Detailed microstructural characterization of the films are presented and correlated with their magnetic/magnetotransport properties.

The ferrite of composition Cu0.5Fe2.5O4, prepared using a conventional solid-state processing route, was soaked at different temperatures between 1200°C and 1350°C, for an hour, in air. The samples were rapidly quenched into oil. Mössbauer, resistivity and x-ray lattice parameter data were obtained for the quenched phase. The maximum saturation magnetization value, obtained at 10K, was close to 5.0μB. The characterization of the sample showed Cu+1 to be partially disordered across both A and B sites of the spinel structure, with an excess occupancy of the A-site showing that under the proper conditions the ordered high 4πMs Cu+1 phase will be stable.

Thin magnetite (Fe3O4) films and Fe3O4/MgO multilayers have been epitaxially grown by Molecular Beam Epitaxy (MBE) on MgO(100) and MgAl2O4(100) substrates. The epitaxial growth on MgO(100) substrates, with a slightly larger bulk lattice parameter than that of magnetite, resulted in an in-plane expansion of the magnetite lattice, accompanied by a perpendicular compression. For films grown on MgAl2O4(100), with a smaller lattice parameter, the substrate misfit is relaxed by the incorporation of misfit dislocations at the interface. It is shown that the substrates have a large effect on the magnetic and electronic properties of the films.

The characteristic Verwey transition is shifted towards lower temperatures, broadened, and reduced in amplitude, more so for thinner films. This can not be quantitatively explained by substrate induced stress alone, but is more likely due to a rigid structural coupling between the magnetite film and the cubic lattice of the substrate. Hereby the orthorhombic deformation accompanying the Verwey transition may be supressed.

When growing at reduced oxygen pressure, the length scale for the rigid coupling will be reduced by the introduction of vacancies. This leads to more bulk-like resistivity and Verwey transition characteristics, but also to deviations from stoichiometry, as suggested by magnetization and Ferromagnetic Resonance (FMR) experiments.

Perovskite La1-xCaxMnO3-δ (LCMO) thin films with a wide range of x, i.e., 0.0 ≤ x ≤ 0.6, were deposited on MgO(001) substrates using a pulsed laser deposition (PLD) technique. Epitaxial La0.7Ca0.3MnO3-δ/MgO thin films were able to be grown under a condition such as 1.5 ∼ 2.1 J/cm2 of a laser fluence, 650 ∼ 750 °C of a substrate temperature, and 100 ∼ 300 mtorr of an oxygen pressure. X-ray pole figures and electron diffraction pattern showed that the LCMO films were grown epitaxially on MgO(001). Rutherford Backscattering Spectroscopy measurements investigated that the epitaxial LCMO films have compositions similar to those of targets, demonstrating the PLD is a useful technique to get films with complicated chemical compositions. Various physical properties, including resistance, R, magnetoresistance, ΔR/R(H=0) ≡ (R(H)-R(0))/R(O), and magnetization, M(T), were measured. The LCMO thin films with 0.2 ≤ x ≤ 0.5 had both semiconductor-metal and ferromagnetic ordering transitions, whose temperatures are located close to each other. These physical properties were explained in terms of the magnetic polaron model.

We have grown films of (LaSr)MnO3 (LSMO) and (LaCa)MnO3 (LCMO) using atomic layer-by-layer molecular beam epitaxy (ALL-MBE). Depending on growth conditions, substrate lattice constant and the exact cation stoichiometry, the films are either pseudomorphic or strain relaxed. The pseudomorphic films show atomically flat surfaces, with a unit cell terrace structure that is a replica of that observed on the slightly vicinal substrates, while the strain relaxed films show bumpy surfaces correlated with a dislocation network. All films show tetragonal structure and exhibit anisotropic magnetoresistance, with a low field response, (1/R)(dR/dH) as large as 5 T−1.

The successful growth of single crystal ferrites in thin film form is an important step towards their future incorporation into integrated circuits operating at microwave frequencies. We have successfully grown high quality single crystalline spinel ferrite thin films of (Mn, Zn)Fe2O4 and CoFe2O4 on (100) and (110) SrTiO3 and MgAl2O4 at low temperature. These ferrite films are buffered with spinel structure layers that are paramagnetic at room temperature. In contrast to ferrite films grown directly on the substrates, ferrite films grown on buffered substrates exhibit excellent crystallinity and bulk saturation magnetization values, thus indicating the importance of lattice match and structural similarity between the film and the immediately underlying layer. X-ray, RBS, AFM and TEM analysis provide a consistent picture of the structural properties of these ferrite films. We then use this technique to grow exchange-coupled bilayers of single crystalline CoFe2O4 and (Mn, Zn)Fe2O4. In these bilayers, we observe strong exchange coupling across the interface that is similar in strength to the exchange coupling in the individual layers.

Multilayers composed of oxides including Fe3O4, CoxFe3−x, O4, CoO, NiO and MgO have been grown epitaxially by MBE on MgO(100) single crystal substrates. These structures can be grown with a high crystallinity in the form of flat layers having sharp interfaces. RHEED studies which commonly yielded sharp streaks accompanied by Kikuchi lines show that, for instance, growth of CoO on Fe3O4 changes the RHEED pattern from that consistent with a spinel structure to that of a rocksalt structure within about one and a half unit cell of CoO. STM studies on a 400 Å Fe3O4 layer displaying atomic resolution enabled us to identify the origin of the reconstruction that one commonly observes in the RHEED and LEED patterns for magnetite. Regarding important fundamental magnetic parameters, relevant thickness dependencies were mapped out using localized magneto-optical Kerr effect experiments performed on several samples that routinely included one or multiple wedge shaped layers. These studies revealed the existence of a region in the Fe3O4 layer near the interfaces which exhibits no net magnetic moment, strain driven perpendicular orientated magnetization for the CoO/Fe3O4(100) and CoO/CoxFe3−xO4(100) bilayer systems, and information on the thickness dependence of the magnetic interlayer coupling across an MgO spacer layer.

In this paper, we discuss the change in the band gap of charge transfer insulators for different surface terminations. We have calculated the Madelung potential of the unreconstructed (100) surfaces of the rocksalt structured TM-O compounds (TM = Mn, Fe, Co and Ni). We also considered possible step defects on a (100) surface. The presented results are calculated in both the purely ionic case (TM2+O2−) and in the strong ligand p - cation 4s, phybridization (TM1+O1−) case. In both cases the charge transfer gap, Δ, for the surface is reduced compared to the bulk value. As a consequence of this reduction there is a large increase of the surface superexchange interaction, Jsur, and a decrease of the band gap.

The less than 1% lattice mismatch between MgO and NiO makes them ideal candidates for investigating the growth and stability of multilayered oxide films. Ultra-thin multilayers composed of alternating films of MgO and NiO were deposited onto a stoichiometric NiO(100) single crystal substrate at 250°C by evaporating Mg and Ni in 5×10−7 Torr of O2, respectively. The structure of these multilayers was determined using LEED. Reactivity and chemical composition studies of the MgO/NiO interfaces were carried out using XPS and UPS. The MgO/NiO multilayers grow epitaxially on NiO(100), as evidenced by LEED. XPS and UPS analysis indicates attenuation of the NiO or MgO peaks during growth which is consistent with discrete layering. Chemical analysis also reveals negligible intermixing of the MgO and NiO layers during deposition. Results pertaining to the thermal stability of the multilayers show that UHV annealing above 750°C results in significant diffusion of MgO into the NiO(100) substrate.

The AMO3 perovskites containing transition-metal atoms M have an MO3 array with M-O-M bond angles 180°-φ in which the angle φ increases with the mismatch of the A-O and M-O equilibrium bond lengths. The tight-binding bandwidths for the π-bonding t and σ-bonding e orbitals of d-orbital parentage are Wπ < Wσ. In the orthomanganites Ln1−xAxMnO3 with A an alkaline earth, the octahedral-site high-spin Mn3+ ions have a d-electron configuration t3e1:5Eg that is orbitally twofold-degenerate, and the on-site electron-electron coulomb energy for adding a fifth d electron is Uσ ≈Wσ > Wπ. Consequently the equilibrium reaction t3e1 ≈ t3 σ*1 for a first-order transition from localized-e to itinerant-σ* electrons is shifted to the left as the bandwidth Wσ ≈ 12 εσλσ2 cos φ is narrowed by increasing φ, by perturbations of the periodic potential of the MO3 array, and/or by increasing spin-disorder scattering; it is shifted to the right by oxidation of the MnO3 array, by application of a magnetic field, and by pressure. Cross-over from polaronic-e to itinerant-σ* electrons at a ferromagnetic Curie temperature Tc or a charge-ordering temperature Tco < Tc can give a “colossal” negative magnetoresistance provided the Fermi energy εF lies below a mobility edge μc in the narrow σ* band of the ferromagnetic phase. Cooperative Jahn-Teller distortions that remove the orbital degeneracy of a localized 5Eg configuration also introduce unusual antiferromagnetic ordering of the manganese-atom spins.

Thin films (∼1000 Å) of LaxCa1−xMnOδ (x=0.67) were deposited onto LaA1O3 (100) substrates at of 600 and 700°C. Varying the oxygen deposition pressure between 15 and 400 mTorr systematically changed the oxygen concentrations in the as deposited films. Asdeposited films exhibited an orthorhombic structure with an oxygen pressure dependent lattice parameter. The films were highly oriented as characterized by narrow x-ray ω-scans (FWHM ≤ 0.16 −0.70°). At low pressures, the films were preferentially (202) oriented while at high pressures deposited films had a (040) preferred orientation. A 900°C anneal in flowing oxygen for a film deposited at low oxygen pressures resulted in a decrease in the lattice parameter (associated with an increase in δ) and a change in the preferred orientation from (202) to (040). The resistivity as a function of temperature (R(T)) showed a significant variation as a function of growth conditions. At 600°C, the peak in the resistivity curve (Tm) varied between 73 and 93 K for P(O2) = 15 to 400 mTorr, while at 700°C, Tm was ∼150 K. For films deposited at 600°C, the resistivity was reduced by a factor of 103 for H = 9T and Tm was shifted to 150 K. The activation energy associated with the semiconducting phase was approximately the same for all as-deposited films (∼100 meV).

The properties encompassed by the family of complex metal oxides span the spectrum from superconductors to insulating ferroelectrics. Included in this family are the new colossal magnetoresistive perovskites with potential applications in advanced high density magnetic data storage devices based on single or multilayer thin films units of these materials fabricated by vapor phase deposition (PVD) methods. The realization of this potential requires solving basic thin film materials problems requiring understanding and controlling the growth of these materials. Toward this end, we have grown La0.7Ca0.3MnO3 and La0.7Sr0.3MnO3 on LaAlO3 single crystal substrates by pulsed laser and RF sputter deposition at temperatures from 500° C to 900° C and annealed at over 900° C for about 10 hours. The evolution of the microstructure of these films was studied by scanning probe microscopies and transmission electron microscopy (TEM).

The results of SPM characterization showed that at the lower end of the growth temperature range, the as-grown films were polygranular with grain size increasing with temperature. The 500° C as-grown films appeared to be amorphous while the 750° C film grains were layered with terrace steps often one unit cell high. In contrast, films grown at 900° C consisted of coalesced islands with some 3-D surface crystals. After annealing, all films had coalesced into very large extended layered islands. The change in microstructure was reflected in a decreased resistivity of coalesced films over their unannealed granular precursors. Previous reported work on the growth of La0.84 Sr0.16MnO3 and La0.8Sr0 2CoO3 grown demonstrated the sensitivity of the microstructure to substrate and deposition conditions. Films grown on an “accidental” vicinal surface grew by a step flow mechanism.

Filling dependence of magnetotransport properties was investigated in Ndl−xSrxMnO3 films. Hysteretic behavior was observed in temperature dependencies of resistivity for restricted filling of x-0.35 and 0.50, indicating the occurrence of a first-order phase transition at low temperatures. The transition gives rise to a high-resistivity state which collapses to low-resistivity state under an external magnetic field. Change in resistivity by applying magnetic field was more than a order of magnitude. Origin of colossal magnetoresistance (CMR) is briefly discussed in terms of such a first-order phase change induced by a magnetic field.

An investigation designed to display the intrinsic properties of perovskite manganites was accomplished by comparing the behavior of bulk samples with that of thin films; the results show the "colossal magneto resistance" at very low temperatures is not an intrinsic property of the thermodynamically stable 1/3 doped material. Epitaxial 1500 Å films of perovskite La0.67Ca0.33MnO3 and La0.67Sr0.33MnO3 were grown by solid source chemical vapor deposition on LaA1O3 and post annealed in oxygen at 950°C. Crystals were prepared by LASER heated pedestal growth. The magnetic and electrical transport properties are essentially the same. Below Tc/2 the intrinsic magnetization decreases as T2 (as can be expected for itinerant electron ferromagnets) while the intrinsic resistivity increases proportional to T2. The constant and T2 coefficients of the resistivity are largely independent of magnetic field and alkaline earth element (Ca, Sr or Ba). We identify three distinct types of negative magnetoresistance. The largest effect is observed near the Curie temperature and is likely to be due to magnetic critical scattering. There is also magnetoresistance associated with the net magnetization of polycrystalline samples. The high temperature (above Tc) resistivity of La0.67Ca0.33MnO3 is consistent with small polaron hopping conductivity with a transition at 750K, while La0.67Sr0.33MnO3 does not exhibit activated conductivity until about 500K, well above Tc. The limiting low and high temperature resistivities may place a limit on the maximum possible magnetoresistance of these materials.

Copper oxide films were grown using the pulsed laser deposition (PLD) method with either a copper metal or a copper oxide target. A variety of deposition pressures of oxygen (0.001–0.3 torr) and temperatures (350, 450, and 550°C) were explored. From x-ray diffraction analysis, epitaxial <100> CuO films were observed at 450°C and either at 0.01 or 0.1 torr for the CuO and the Cu targets, respectively. Deposition rates were measured for the two targets (Cu and CuO) as a function of laser fluence and background gas pressure and enabled a morphological study of films prepared at different laser fluences but of similar film thickness (∼0.3 μm). The number density of particulates as well as the smoothness of the underlying film surface of the CuO films varied significantly with the target composition. The CuO films from the Cu target were the smoothest and had the fewest particulates.