The electronic and magnetic structure at surfaces and interfaces in mixed valence manganites is an active area of research. To study localized properties of interface magnetism in heterostructures we have conducted both current in-plane and current perpendicular-plane measurements in multilayers and trilayer tunnel junctions, respectively. Initial study of the transport in multilayers, consisting of alternating La0.7Ca0.3MnO3 and SrTiO3 with different layer thickness provided information on vertical inhomogeniety in the manganite layers and disordered interfaces. Issues of lattice mismatched strain, structural inhomogeniety were addressed. Trilayer tunnel junctions have been fabricated with the aim of correlating tunneling magnetoresistance to the interface magnetism deduced from multilayer measurements. We describe devices incorporating a novel barrier material which have yielded single domain switchings and very high TMR values above 77 K.

We discuss the heteroepitaxial growth of La1−xSrxMnO3 films and CaTiO3 insulating barriers by molecular beam epitaxy. We find that the surface morphology and residual resistivity of the manganite electrodes is critically dependent on the film stoichiometry. The most important parameter is the concentration of La+Sr (cubic perovskite A-site cations) to that of Mn (B-site cation). If La+Sr is supplied in slight excess, the films grow with atomically flat surfaces, but the residual resistivity at 4.2K is high (as high as 6500 µΩ-cm), and Curie temperature (Tc) low (<300 K). If Mn is supplied in slight excess, the films have high Tc (370 K) and residual resistivity (35 µΩ-cm) better than bulk single crystal values, but the surface is no longer atomically flat. There appears to be a very narrow region of phase space where it is possible to have low resistivity, high Tc films with atomically flat surfaces. This is precisely where one would like to place heterostructure devices.

A comprehensive study of transport properties and magnetism on β-MnO2 reveals the strong coupling between the conduction electrons and the localized spins which are supposed to form a magnetic helix below TN ∼ 92 K. We also show the direct evidences of the helical magnetism by means of the measurements of the anisotropy in the magnetic susceptibility and the observation of x ray magnetic scatterings on a single crystal of β-MnO2. These results are consistent with proper-type helix model proposed by Yoshimori [J. Phys. Soc. Jpn. 14, p. 807 (1959)]. This model also qualitatively agrees with the anisotropy in magnetoresistance that appears below TN. The pitch of the magnetic helix is not commensurate to the lattice and it is slightly temperature dependent. The intensity of several Bragg peaks drastically changes at TN suggesting that the magnetic ordering is accompanied by a lattice distortion.

We have synthesized ceramics of A2FeReO6 double-perovskites A2FeReO6 (A=Ba, Ca). Structural characterizations indicate a cubic structure with a=8.0854(1) Å for Ba2FeReO6 and a distorted monoclinic symmetry with a=5.396(1) Å, b=5.522(1) Å, c=7.688(2) Å and β=90.4° for Ca2FeReO6. The barium compound is metallic from 5K to 385K, i.e. no metal-insulator transition has been seen up to 385K, and the calcium compound is semiconducting from 5K to 385K. Magnetization measurements show a ferrimagnetic behavior for both materials, with Tc =315 K for Ba2FeReO6 and above 385K for Ca2FeReO6. At 5K we observed, only for Ba2FeReO6, a negative magnetoresistance of 10% in a magnetic field of 5T. Electrical, magnetic and thermal properties are discussed and compared to those of the analogous compounds Sr2Fe(Mo,Re)O6 recently studied.

The properties of a new cubic perovskite phase of thin-film BaFexTi1−xO3 (0.5 ≤x≤0.75) are reported. This material is novel because the corresponding bulk compounds have hexagonal structure for comparable x. The films, grown by pulsed laser deposition on MgO and SrTiO3 substrates, are magnetic (ferro- or ferri-, with Tc > 500°C) and ferroelectric (Tc ∼ 200-300°C).

Dynamic susceptibility measurements at microwave frequencies (2 – 10GHz) are a sensitive probe of charge dynamcis in La5/3Sr1/3NiO4. Below the charge ordering temperature of 240K, a dielectric loss peak due to a relaxation mode with a large dielectric susceptibility is observed, and is associated with charge stripe formation. The dielectric response for Hω∥b (Eω ⊥ b) is well represented by ε(T) = εo/(1 – iωτ(T)), with εo, ∼ 50, and τ(T) = 2 × 10−9(sec) exp(−T/37K). Parallel conductivity σ(T) contributions dominate at higher temperatures and for Hω∥c (Eω ⊥ c). The dielectric loss peak observed indicates that the charge relaxation rates lie in the GHz frequency ranges.

Transport and structural properties of the colossal magnetoresistance pyrochlore TI2Mn2O7 are studied as a function of applied pressure up to ∼20GPa. This allows us to probe the effect of structural changes on the ferromagnetic transition and the transport properties. We observe a non-monotonous pressure dependence of the ferromagnetic transition temperature. We correlate this unusual variation with the structural parameters that, according to electronic band calculations, are key in controlling the properties of these materials.

Oriented and non-oriented thin films of silver(I) telluride, Ag2Te, were prepared by e-beam evaporation, vapor transport technique and Chemical Vapor Deposition (CVD). Crystallinity and orientation of the films were studied by Θ−2Θ XRD, rocking curve and pole figure measurements. The origin and conditions for the oriented growth are discussed. Special microdevice was prepared by photolitography from the oriented films of Ag2Te in order to investigate magnetoresistance (MR) in this material. It was proved that the reported earlier negative MR in Ag2Te films is a completely geometrical effect, which can be observed using non-linear arrangement of current and voltage contacts.

We have grown epitaxial thin films of metastable four-layered hexagonal (4H) BaRuO3 on (111) SrTiO3 by 90° off-axis sputtering techniques. X-ray diffraction and transmission electron microscopy experiments reveal that the films are single crystals of c-axis 4H structures with an inplane epitaxial arrangement of BaRuO3 [2110] // SrTiO3 [110]. Smooth multilayer growth has been observed in these films with a step height equaling the size of half unit cell. In-plane resistivity of the films is metallic, with a room temperature value of about 810µΩ-cm and slightly curved temperature dependence. Their magnetic susceptibility is paramagnetic. The metastable layered compounds can be very useful for understanding new solid-state phenomena and novel device applications.

Strain-induced large low-field magnetoresistance has been observed in very thin Pr0.67Sr0.33MnO3 films1. To better understand the role of strain in the low-field magnetotransport properties of manganite thin films, we have studied and compared very thin (3-20 nm) Pr0.67Sr0.33MnO3 (PSMO), La0.67Ba0.33MnO3 (LBMO), La0.67Sr 0.33MnO3 (LSMO) and La0.67Ca0.33MnO3 (LCMO) films grown on different substrates, such as LaAlO3(001) (LAO), NdGaO3(110) (NGO), and SrTiO3(001) (STO). Due to the lattice mismatch between the films and the substrates ranging from -2.6% to +1%, different strains can be imposed to the films. We have found that: (1) large low-field magnetoresistance(LFMR) behaviors are observed in PSMO, LCMO and LSMO thin films on LAO substrates when a magnetic field is applied perpendicular to the film plane, but the maximum LFMR is the largest in PSMO and LCMO samples; (2) most of the films grown on STO substrates show positive MR when a magnetic field is applied perpendicular to the film plane, and when the field is parallel to the film plane all films show negative MR regardless of the substrates; (3) the large low-field MR is strongly dependent on the film thickness and the composition of the manganites. The anomalous low-field MR effect will be discussed based on strain-induced magnetic anisotropy and domain rotation and movement.

We have grown epitaxial thin films of antiferromagnetic ruthenate Sr2YRuO6 on miscut (001) SrTiO3 by 90° off-axis sputtering. Sr2YRuO6 is a unique material that allows us to grow epitaxial ferromagnetic/antiferromagnetic heterostructures. Antiferromagnetic Sr2YRuO6 has the same pseudo-cubic perovskite crystal structure as the ferromagnetic conductive oxide SrRuO3. The Sr2YRuO6 perovskite crystal structure has Y and pentavalent Ru located on the octahedral sites and the pseudo-cubic lattice parameter of 4.08Å. The Neel temperature of bulk Sr2YRuO6 is known to be 26K. Four-circle X-ray diffraction analysis revealed the Sr2YRuO6 films are purely (110) normal to the substrate with two 90° domains in the plane. We have also grown epitaxial heterostructures of SrRuO3/Sr2YRuO6. These bilayers permit detailed studies of the magnetic exchange bias phenomena at these interfaces, including the role of uncompensated spins thought to arise from interface roughness. Magnetization measurements on the SrRuO3/Sr2YRuO6 heterostructures show a shifting of the hysteresis loop, indicating exchange bias. Such exchange-biased interfaces are important for electrode pinning in magnetic tunnel junctions.

We have successfully grown La0.65PbO.35MnO3 thin films by a RF magnetron sputtering method onto (100) LaAlO3 single crystal substrates. X-ray diffraction measurements are consistent with a (100) cubic orientation of the films. The fourfold symmetry showed by LEED(Low Energy Electron Diffraction) patterns indicate that the films have surface order. STM(Scanning Tunneling Microscopy) measurements indicate that the surface of the films were smooth, with approximate 5 nm roughness. XPS (X-ray Photoemission Spectroscopy) shows that the surface defect density in the films is comparatively low. The bulk magnetization of the films at 6K in 1 T magnetic field reached 77 emu/g and a Curie temperature near 354 K, close to maximum resistivity. A negative magnetoresistance of 47% was observed at 320K in 5.5 T magnetic field.

A careful analysis of high-resolution transmission electron microscopy images from La0.8Ca0.2MnO3 thin films indicates that the images can not explained based on either the classical Pnma structure or its monoclinic distortion. A cation ordered structure is proposed which could be responsible for the significantly higher Tc of the film (298K) compared with the bulk material of the same composition (190K).

We have studied the anisotropic magnetoresistance (AMR) of strained Pr0.67Sr0.33MnO3 thin films by measuring the MR as a function of the angle between the magnetic field direction and the substrate normal (out-of-plane). The results show that the compressive- and tensile-strained ultrathin films (5-15 nm) grown on LaAlO3 (001) (LAO) and SrTiO3 (001) (STO) substrates show unusually large out-of-plane AMR, but with opposite signs. In contrast, the almost strainfree films on the NdGaO3 (110) substrates show much smaller AMR over all the temperature and field ranges studied. Thick films on LAO and STO substrates also show much smaller AMR.

La1−x SrxMnO3(0.19≤x≤0.31) thin films were prepared on silicon wafers by sputter techniques. The effect of substrate temperature, chemical composition and post-deposition heat-treatment on the crystalline structure and electrical characteristics of the films was investigated. The films grown at a substrate temperature of 500°C were found to be of the pseudo-tetragonal system (0.97≤a/c≤1) and exhibited a strong tendency of {001} planes to lie parallel to substrate surface. With the increase of x, the electrical resistivity of the films decreased and the transition temperature between the metallic and semiconducting electrical transport behaviors shifted to high temperature. With a magnetic field of 0.18 Tesla, the maximum magneto-resistance ratio (MR%) of La0.69Sr0.31MnO3 polycrystalline thin films was about 390%.

We report studies on La1−xSrxMnO3 /SrTiO3/ La1−xSrxMnO3 (x=0.33) trilayer junctions made using 90° off-axis sputtering. Both (110) NdGaO3 and (001) (LaAlO3)0.3-(Sr2AlTaO6)0.7 (LSAT) are used for substrates. Optical lithography is used for junction formation. These sputtered trilayers show improved junction resistance uniformity over trilayers made using laser ablation. A magnetoresistance of ∼100% is observed for junctions on LSAT with 30 Å barrier at 13 K and around 100 Oe. The shape of junction magnetoresistance vs. field depends both on barrier thickness and on substrate type, suggesting that both inter-layer coupling and substrateinduced-strain play a role in determining the junction's micromagnetic state. These results indicate better junction interfaces can be obtained for manganite trilayer junctions by 90° off-axis sputtering.

We have grown Mn:LaGaO3 crystals (Mn=0.5%, 2%, 10%, and 50%) and studied their properties with the methods of optical and EPR spectroscopy. We have found Mn2+, Mn4+, and Mn5+ ions in low doped crystals (Mn=0.5% and 2%) and Mn3+ ions in crystals with higher Mn concentrations (10% and 50%). The formation of Mn3+ valence state in Mn:LaGaO3 is discussed.

We observed a dramatic peak in the 1/f noise at the metal-insulator transition (MIT) in low-Tc, manganites. This many-orders-of-magnitude noise enhancement is observed for both polycrystalline and single-crystal samples of La5/8−y. Pry, Ca3/8MnO3 (y = 0.35 – 0.4) and Pr1−xCaxMnO3 (x = 0.35 – 0.5). This observation strongly suggests that the microscopic phase separation in the low-Tc, manganites causes formation of a percolation network, and that the observed MIT is a percolation threshold. It is shown that the scale of phase separation in polycrystalline samples is much smaller than that in single crystals.

Pulsed UV laser illumination of doped LaMnO3 thin films deposited on vicinal cut SrTiO3 [ STO ] substrates causes unexpected transient voltages at room temperature in remarkable analogy to those observed for intrinsically anisotropic YBa3Cu3O7−x, [YBCO] thin films. The experimental data for La2/3Ca1/3MnO3 [LCMO] thin films are consistent with the description of thermoelectric fields caused by off-diagonal elements of the Seebeck tensor. Systematic measurements of the dependence of the laser-induced signal on the geometry of the sample, the temperature and doping dependence suggest that the anisotropy of the Seebeck tensor is due to combination of long range co-operative Jahn-Teller distortions and substrate-induced biaxial strain effects.

The traveling wave (TW) method has been utilized to investigate the transport mechanism in paramagnetic-insulator state of La0.75Sr0.11Ca0.14MnO3 films. The drift mobility of the films increased from 2.5 × 10−2 cm2/Vs at 310 K to about 9.2 × 10−2 cm2/Vs at 400 K. The Arrhenius behaviors of the conductivity and drift mobility indicate that the transport process in manganites above the Curie temperature is dominated by the thermally assisted hopping of small polarons.