The transport mechanism underlying the colossal magnetoresistance (CMR) of doped manganites is not yet understood and their technological applications are limited by the high fields (∼IT) required to obtain any significant MR. Following the development of a polymeric chemical synthesis route, we have investigated the O2p unoccupied density of states in sol-gel derived Lal-xSrxMnO3 (0 < x < 0.7) thin films grown epitaxially on LaA1O3, by electron energy-loss spectroscopy at sub-eV resolution. The spectra show a distinct prepeak in the OK edge at the Fermi level, the intensity of which correlates directly with the conductivity of the film. Similar correlation was also obtained for La0.7Sr0.3MnO3-z annealed in vacuum to obtain well defined oxygen content (z) in the film. This confirms that the charge carriers in these manganese perovskites have significant oxygen 2p hole character and suggests that the “double exchange” mechanism has to be modified. In another set of experiments we have studied room-temperature field amplification effects to enhance the low-field sensitivity of La0.7Sr0.3MnO3-z films sandwiched between two thin rectangular slices of either α-Fe or Mn-Zn ferrite. This field amplification leads to an enhanced low-field MR value as high as 6% at an external field of 500 Oe which is 6 times the value observed without the amplification.

The relevance of lattice distortion, polaron conduction, and double-exchange interaction to the occurrence of colossal magnetoresistance (CMR) is investigated by comparing the physical properties of magnetoresistive manganites and cobaltites. The samples studied in this work include epitaxial films and ceramics of manganites with both A- and B-site substitution, (La0.7Ca0.3MnO3, LaMn0.7 Ni0.3O3, LaMnO.5Ni0.5O3), as well as epitaxial films and ceramics of cobaltites (La0.5Ca0.5CoO3). The structural, chemical, electrical transport, magnetic, optical properties and tunneling spectroscopy are studied. Based on our experimental results, we conclude that both double-exchange interaction and strong electron-phonon coupling due to the Jahn-Teller effect are essential to the occurrence of CMR.

Very large increase in electrical conductivity by several orders of magnitude is obtained when an external magnetic field is applied to the colossal magnetoresistance (CMR) materials such as La-Ca-Mn-O. The magnetoresistance is strongly temperature-dependent, and exhibits a sharp peak below room temperature, which can be shifted by adjusting the composition or processing parameters. The control of lattice geometry or strain, e.g., by chemical substitution, epitaxial growth or post-deposition anneal of thin films appears to be crucial in obtaining the CMR properties. The orders of magnitude change in electrical resistivity could be useful for various magnetic and electric device applications.

The transition metal oxides La1-xAxMnO3 (A = Ba, Ca, or Sr) order ferromagnetically with Curie temperatures ranging from as low as 50 K to well above room temperature. Magnetic order in these compounds results in a concomitant metal-insulator transition. The feature displayed by the manganites that is most important technologically is the extremely large negative magnetoresistance that achieves its largest values near the magnetic ordering temperature. Qualitatively, this colossal magnetoresistance (CMR) phenomenon involves the suppression of the relatively sharp maximum in the resistivity that is centered at Tc. When considered collectively, the anomalous temperature-dependent transport properties, the CMR effect, and the magnetically ordered ground state indicate that a novel interplay between magnetism and electronic transport occurs in the manganites. General features of the magnetic-field and temperature-dependent electrical resistivity and magnetization as displayed by PLD-grown thin films are examined. Particular emphasis is placed on what these measurements tell us about the conduction process both above and below the magnetic ordering temperature.

Growth of LaMnO3 films that exhibit colossal magnetoresistance (CMR) has concentrated heavily on Ca doped materials. However, since the 33% Sr doped films are ferromagnetic at room temperature, they are ideal candidates for dual growth-magnetic structure studies using scanned probe techniques. In this study, interest was focused on the relations between growth/processing parameters, film morphology, and electronic/magnetic properties. In addition, films were grown on both LaA1O3 (LAO) and SrTiO3 (STO) to examine the results of stress induced by different substrate mismatches. La0.67Sr0.33MnO3 (LSMO) was grown using pulsed laser deposition (PLD) at temperatures between 500 °C and 800 °C. The film microstructure, crystallinity, and magnetic and electrical properties were characterized by room temperature scanning tunneling microscopy (STM), atomic force microscopy (AFM), magnetic force microscopy (MFM), x-ray diffraction, and temperature dependent transport and magnetization measurements. The growth trends follow those previously reported for Ca doped films. Grains increase in size with increasing temperature and coalesce into extended layers after annealing. Although topographic contributions complicate interpretation of some MFM data, local magnetic structure observed here is generally associated with film defects.

The relationship between structural distortions and resistivity in pulsed laser deposited La2/3Ca1/3MnO3 (LCMO) and La2/3Ca1/3MnO3 (LSMO) thin films was investigated by x-ray diffraction (XRD), transmission electron microscopy (TEM) and resistivity measurements. The growth defects inherent to annealed films were characterized, and then additional defects were introduced by radiation damage. Epitaxially grown films on (100) LaA1O3 substrates exhibited three primary types of growth defects: interface strain, column boundary mismatch, and column rotation. High resolution TEM measurements show well-ordered regions of film that offer low resistance paths for current flow around growth defects. A series of annealed samples were irradiated with 6 Mev Si+3 ions to produce 0.006 to 0.024 displacements per atom (dpa) in LCMO films, and 0.028 to 0.14 dpa in LSMO films. The peak resistance temperature (Tp) was found to depend strongly on defect concentration, varying from 235K ± 15K for the unirradiated LCMO films to 95K for 0.018 dpa. At 0.024 dpa, the LCMO film was insulating at all temperatures. LSMO films showed a downward shift in Tp from 300K for an unirradiated film to 250K for 0.028 dpa, and complete insulating behavior for 0.14 dpa.

Spinel Fe3O4 contains two sites for iron: tetrahedrally coordinated sites containing Fe3+ ions and octahedrally coordinated sites containing a mixture of Fe2+ and Fe3+ ions. Scanning tunneling microscopy performed on the (001) surface of Fe3O4, grown epitaxially on MgO, shows localized charge density at the tetrahedral sites. The images show that the p(1×1) surface reconstruction (also observed during molecular beam epitaxy of Fe3O4) is produced by a displacement of the two tetrahedrally coordinated Fe ions on the unit cell surface from their bulk positions toward each other. The octahedral Fe ions are imaged as extended rows of charge density, with no resolution of atom-size features along the rows. This slight corrugation of electron charge density along the octahedral sites is consistent with the original conjectures explaining the high electrical conductivity in bulk Fe3O4: electrons move by hopping between the Fe3+ and Fe2+ atoms along the octahedral rows of Fe ions.

We have observed both metallic and semiconducting behavior in epitaxial thin films of the metallic oxide CaRuO3 deposited under identical conditions on (100) SrTiO3 substrates of varying crystalline quality. X-ray diffraction studies showed that while semiconducting films with enlarged unit cells were obtained on single crystal SrTiO3 substrates, metallic films with lattice parameters close to the bulk material grew on poor crystalline quality SrTiO3 substrates. The films deposited on (100) LaAlO3 substrates consistently showed metallic behavior. Atomic force microscope images suggest that the semiconducting films had a coherent two dimensional nucleation. In contrast, three dimensional island-like incoherent growth was seen in the metallic films. It is believed that in the coherent films a strain induced substitution of the small Ru4+ cations by the larger Ca2+ cations occurs, breaking the conduction pathway within the three dimensional network of the RUO6 octahedra and leading to a metal-insulator transition. This unique phenomenon - which is not observed in bulk material - can be significant in technologically important epitaxial perovskite oxide heterostructures.

We show the importance of the role of strain in doped manganite perovskite materials by revealing the dominance of stress anisotropy effects over magnetocrystalline anisotropy effects in the magnetic anisotropy of these films. In order to measure magnetic anisotropy in the plane of the films, remnant magnetization was measured as a function of in-plane angle. The (110) La0.7Sr0.3MnO3 films reveal a twofold symmetry while the (100) films have fourfold symmetry. Temperature dependence of the magnetic anisotropy of La0.7Sr0.3MnO3 films reveals decreasing anisotropy as the Curie temperature is approached.

We have made a study of the use of NO2 as the source of oxygen in the MBE-growth of iron oxides thin films. It is found that NO2 is a much more efficient oxidising agent than molecular O2. As indicated by Mössbauer spectroscopy, performed on 57Fe probe layers, NO2 is not only capable of forming stoichiometric magnetite Fe3O41 but also all non-stoichiometric Fe3-δO4 phases. Even the metastable maghemite phase γ-Fe2O3 (Fe3-δO4 with δ=1/3) can be formed. All iron oxides grow layer-by-layer-like, as indicated by strong RHEED intensity oscillations. When small doses of NO2 are used, new wustite Fe3-xO and Fe3O4 phases are formed. In contrast to the Fe3-δO4 films, these phases have nitrogen incorporated into the crystal lattice. Similar compounds are obtained when NO is used as the source of oxygen. The use of N2O does not lead to the formation of iron oxides. It does, however, alter the growth mode of Fe on MgO(100). Whereas Fe deposited under UHV conditions forms 3D islands, the N2O acts as a surfactant and induces 2D layer-by-layer growth.

Epitaxial SrRuO3 films were prepared on (001) SrTiO3 substrates by pulsed laser deposition. The film structure was characterized by 4-circle x-ray diffraction and the electrical behavior by temperature dependent resistivity measurements. With variations in the deposition conditions, significant changes in both structural and electrical properties were observed. When deposited under conditions favoring appreciable energetic bombardment, the SrRuO3 films on SrTiO3 exhibited extended in and out-of-plane lattice constants and increased values of resistivity; in addition, a depression of the Curie temperature was measured. SrRuO3 deposited under less aggressive conditions displayed structures and properties more similar to those associated with bulk crystals.

The relationship between the magnetic and crystalline microstructure of SrRu03 thin films is analyzed using transmission electron microscopy. Regions with a stripe magnetic domain structure in different orientations are observed in Lorentz imaging mode when the specimens are zero-field-cooled through the ferromagnetic transition temperature, Tc ≈ 150K. The different orientations of the stripe regions correspond to different crystallographic domains as determined by electron diffraction and magnetic image contrast; all of the six possible orientations of the orthorhombic SrRuO3 structure grown epitaxially on a SrTiO3 cubic substrate are identified. The results show that the uniaxial anisotropy indicated for these multi(crystal)domain films is the same as that determined for single crystal films by bulk magnetization measurements, and is therefore primarily magnetocrystalline in nature.

Laser ablated La0.5Sr0.5CoO3-δ thin films have been studied by Doppler-broademng-detected positron annihilation using a variable-energy positron beam. The oxygen partial pressure during cooling from the growth temperature was altered through the range 760 torr to 10−5 torr to change the oxygen non-stoichiometry of the films.

The measured Doppler broadened lineshape parameter S was found to increase with increasing oxygen nonstoichiometry. For films cooled with an oxygen partial pressure of ≤ 10−4 Torr positron trapping to monovacancy type defects is infered. It is proposed that Sr ion oxygen vacancy complexes are likely trapping sites.

For the film cooled in 10−5 torr oxygen the magnitude of the increase in S, with respect to that measured from the film cooled in 760 Torr oxygen, showed positron trapping to vacancy cluster defects is occuring.

Thin films of the colossal magnetoresistive material La0.67Ca0.33MnO3 (LCMO) have been grown by off-axis sputtering from a single target of the LCMO material onto various substrates and studied by electrical transport, x-ray diffraction, scanning electron microscopy and atomic force microscopy. The LCMO films typically grow with the (002) direction oriented normal to the film. We find magnetoresistances at 6 T of ≈ 90%. We observe a scaling between the temperature of the resistivity peak (Tp) and the relative change in resistivity between room temperature and the resistance peak for Tp varying from 90 to 270 K. This scaling is consistent with an activated resistivity above the peak, with an activation temperature of ≈ 940 K. Films of LCMO grown on cubic zirconia exhibit an upturn in resistivity at low temperatures. Bilayers of LCMO and the superconductor YBa2Cu3O7-δ have been grown and compared to see the effects of having either the magnetic or superconducting layer first in the sequence.

Magnetic perovskites of the general form La1-xSrxMnO3 have successfully been synthesized using deposition by aqueous acetate solution (DAAS). This novel technique, which has the potential for depositing large area min films with high throughput and low cost, involves the preparation of an aqueous metal acetate precursor solution, drying the solution to generate a glassy gel, consolidating the gel, and then firing it for short periods of time (<2hrs) to produce crystalline lanthanum strontium manganate (LSMO). The DAAS method has been used to prepare both powders and thin films of LSMO. Powder samples of La0.83Sr0.17MnO3 annealed for 100 minutes at 1200°C were high purity, single phase, and exhibited excellent electrical and magnetic characteristics. Thin films of La0.7Sr0.17MnO3 deposited onto both sapphire and strontium titanate substrates and annealed at 900°C were also found to be crystalline, and substrate choice was found to influence thin film crystal structure. These films exhibited sharp metal-insulator transitions, and in the case of LSMO films on strontium titanate, magnetoresistance was observed at the unusually high temperature of ∼360K.

We report the studies of the Hall effect and magnetic properties in La0.5Ca0.5CoO3 thin films and ceramics. We use the local Hall probe and SQUID magnetometers to measure the magnetization M of the samples. Both below and above the Curie temperature, the Hall resistivity pxy(H) is shown to be proportional to M(H) (H is the applied magnetic field). Thus, our results provide convincing evidence for the anomalous Hall effect in these cobaltites. The value of the anomalous Hall coefficient Rs in La0.5Ca0.5CoO3 significantly exceeds those of other known single-phase ferromagnetic metals. We suggest that the coexistence of high- and low-spin configurations in the perovskite cobaltites, which gives rise to magnetic percolation behavior in La1-xCax.CoO3,may be responsible for the giant Ra.

The films of La0.8Sr0.2MnO3 (LSMO) were prepared on single-crystal SrTiO3(100) and polycrystalline yttria-stabilized zirconia (YSZ) substrates by dipping-pyrolysis process using metal naphthenates as starting materials. The crystallinity, grain alignment, morphology and electric properties of the films were compared. Epitaxial films were found to grow on SrTiO3, heat-treated at 1200°C, by x-ray diffraction, and to exhibit very smooth surfaces by SEM and AFM observations. On the other hand, the surfaces of the polycrystalline films grown on YSZ consisted of round-shaped LSMO grains. The resistivity of the epitaxial films was about two orders of magnitude lower than that of the polycrystalline films prepared under the same conditions. Preparation of epitaxial LSMO films on SrTiO3(110) is also presented.

We have grown epitaxial thin films of isotropie metallic oxide SrRuC>3 on both exact and vicinal (001) SrTiO3 substrates with miscut angle (α) up to 5.0° and miscut direction (β) up to 37° away from the in-plane [010] axis. The effects of both α and β on the epitaxial growth and domain structure of epitaxial SrRuC>3 thin films were studied by x-ray diffraction and atomic force microscopy (AFM). On vicinal SrTiO3 substrates with a large miscut angle (α = 1.7°, 2.0°, 4.1°, and 5.0°) and miscut direction close to the [010] axis, single crystal epitaxial (110)° SrRuO3 thin films were obtained. [The superscript o refers to the Miller indices based on the orthorhombic unit cell.] Decreasing the substrate miscut angle or aligning the miscut direction close to the [110] axis (β = 45°) resulted in an increase of 90° domains in the plane. The films grown on vicinal substrates displayed a significant improvement in crystalline quality and in-plane epitaxial alignment as compared to the films grown on exact (001) SrTiO3 substrates. AFM revealed that as the miscut angle increased the growth mechanism changed from two dimensional nucleation to step flow growth.

We have investigated the structural and compositional changes that are induced by the segregation of substrate Mg to the surface of 1μm-thick Fe3O4 films on MgO(001). The thin films have been grown with plasma-assisted MBE, and characterization with RHEED (reflection high-energy electron diffraction), x-ray diffraction (XRD), and Superconducting Quantum Interference Device (SQUID) magnetometry show slightly strained, single-crystalline Fe3O4 films. For the surface studies, we have combined Low-Energy Electron Diffraction (LEED) and Scanning Tunneling Microscopy (STM). Initial and final surface characterization employed X-ray Photoelectron Spectroscopy (XPS) and Ion Scattering Spec-troscopy (ISS) respectively. The surfaces of the MBE-grown samples are flat and show a (√2 × √2)R45° reconstruction with respect to the Fe3O4 surface unit cell. We observe the onset of Mg segregation to the surface at around 700 K, with long, narrow extensions of terraces being observed growing along the [110] and [110] directions. Upon prolonged heating at 800 K, massive Mg segregation to the surface is observed. Heating in an oxygen atmosphere induces a 1×4 surface reconstruction, and results in extremely long (≈ 1000 Å), wide terraces.

The growth and structural characterization of Fe3O4/MgO superlattices on MgO(001) and Fe-coated MgO(001) are compared. Long modulated coherence and sharp interface are revealed by X-ray diffraction and reflection high energy electron diffraction (RHEED). For the superlattice grown directly on MgO(001), high crystalline quality is comparable to the previous report on the oxide superlattice system. For the superlattice grown on Fe/MgO(001), both chemical and lattice spacing modulations are maintained which is different from the polycrystlline structure of the Fe3O4/NiO grown on Ni-based system. This superiority of the growth on Fe-coated surface results from both the smaller lattice mismatch and the oxidation state of Fe relative to MgO. The changes of the RHEED intensity during the growth of these superlattices are also measured. The evolution of the oxide interface during the growth of Fe3O4/MgO is quantitatively presented.