In order to investigate the interface termination dependence of perovskite band alignments, we have studied the Schottky barrier height at La$_{0.7}$Sr$_{0.3}$MnO$_3$/Nb:SrTiO$_3$ (001) heterointerfaces. As the Nb:SrTiO$_3$ semiconductor was varied from TiO$_2$ termination to SrO termination by variable insertion of a SrMnO$_3$ layer, a large systematic increase in the Schottky barrier height was observed. This can be ascribed to the evolution of the interface dipole induced to screen the polar discontinuity at the interface, which gives a large internal degree of freedom for tuning band diagrams in oxides.
We have observed temperature-dependent reversal of the rectifying polarity in Au/Nb:SrTiO3 Schottky junctions. By simulating current-voltage characteristics we have found that the permittivity of SrTiO3 near the interface exhibits temperature dependence opposite to that observed in the bulk, significantly reducing the barrier width. At low temperature, tunneling current dominates the junction transport due both to such barrier narrowing and to suppressed thermal excitations. The present results demonstrate that novel junction properties can be induced by the interface permittivity.
Internal photoemission spectroscopy was used to determine the Schottky barrier height in rectifying SrRuO3/Nb-doped SrTiO3 junctions for 0.01 wt % and 0.5 wt % Nb concentrations. Good agreement was obtained with the barrier height deduced from capacitance-voltage measurements, provided that a model of the nonlinear permittivity of SrTiO3 was incorporated in extrapolating the built-in potential, particularly for high Nb concentrations. Given the generic polarizability of perovskites under internal/external electric fields, internal photoemission provides a valuable independent probe of the inte…
The transport and magnetic properties of correlated La{0.53}Sr{0.47}MnO{3} ultrathin films, grown epitaxially on SrTiO{3}, show a sharp cusp at the structural transition temperature of the substrate. Using a combination of experiment and theory we show that the cusp is a result of resonant coupling between the charge carriers in the film and a soft phonon mode in the SrTiO{3}, mediated through oxygen octahedra in the film. The amplitude of the mode diverges towards the transition temperature, and phonons are launched into the first few atomic layers of the film affecting its electronic state.
We have investigated electron transport in Nb doped SrTiO$_3$ single crystals for two doping densities. We find that the resistivity and mobility are temperature dependent in both whereas the carrier concentration is almost temperature invariant. We rationalize this using the hydrogenic theory for shallow donors. Further, we probe electrical transport across Schottky interfaces of Au on TiO$_2$ terminated n-type SrTiO$_3$. Quantitative analysis of macroscopic I-V measurements reveal thermionic emission dominated transport for the low doped substrate whereas it deviates from such behavior for t…
SrRuO3 (SRO), a conducting transition metal oxide, is commonly used for engineering domains in BiFeO3. New oxide devices can be envisioned by integrating SRO with an oxide semiconductor as Nb doped SrTiO3 (Nb:STO). Using a three-terminal device configuration, we study vertical transport in a SRO/Nb:STO device at the nanoscale and find local differences in transport, that originate due to the high selectivity of SRO growth on the underlying surface terminations in Nb:STO. This causes a change in the interface energy band characteristics and is explained by the differences in the spatial distrib…
Interface magnetic coupling and magnetization dynamic of La$_{2/3}$Sr$_{1/3}$MnO$_3$ single layer and (La$_{2/3}$Sr$_{1/3}$MnO$_3$/SrRuO$_3$)$_n$ (n = 1, 5) superlattice on SrTiO$_3$(001) substrate
We present a detailed investigation of the electrical properties of epitaxial La0.7Sr0.3MnO3/SrTi0.98Nb0.02O3 Schottky junctions. A fabrication process that allows reduction of the junction dimensions to current electronic device size has been employed. A heavily doped semiconductor has been used as a substrate in order to suppress its series resistance. We show that, unlike standard semiconductors, high-quality oxide-based Schottky junctions maintain a highly rectifying behavior for doping concentration of the semiconductor larger than 10^20 cm^(-3). Moreover, the junctions show hysteretic cu…
We observed a strong modulation in the current-voltage characteristics of SrRuO$_3$/Nb:SrTiO$_3$ Schottky junctions by Mn substitution in SrRuO$_3$, which induces a metal-insulator transition in bulk. The temperature dependence of the junction ideality factor indicates an increased spatial inhomogeneity of the interface potential with substitution. Furthermore, negative differential resistance was observed at low temperatures, indicating the formation of a resonant state by Mn substitution. By spatially varying the position of the Mn dopants across the interface with single unit cell control, …
Thin films of perovskite manganite La_{0.7}Ca_{0.3}MnO_{3} were grown epitaxially on SrTiO_3(100), MgO(100) and LaAlO_3(100) substrates by the pulsed laser deposition method. Microscopic structures of these thin film samples as well as a bulk sample were fully determined by x-ray diffraction measurements. The unit cells of the three films have different shapes, i.e., contracted tetragonal, cubic, and elongated tetragonal for SrTiO_3, MgO, and LaAlO_3 cases, respectively, while the unit cell of the bulk is cubic. It is found that the samples with cubic unit cell show smaller peak magnetoresista…
Colossal magnetoresistive (CMR) La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ (LSMO) thin films have been grown under tensile strains on (100)-SrTiO$_{3}$ substrates and compressive strains on (100)-LaAlO$_{3}$ and (110)-NdGaO$_{3}$ substrates by pulsed laser deposition. Using magnetic force microscopy (MFM), a "feather-like" magnetic pattern, characteristic of films with an in-plane magnetization, is observed for films deposited on both SrTiO$_{3}$ and NdGaO$_{3}$ while a "bubble" magnetic pattern, typical of films with an out-of plane magnetization, is recorded for LaAlO$_{3}$. We show that the shape of the…
Understanding band alignment and charge transfer at complex oxide interfaces is critical to tailoring and utilizing their diverse functionality. Towards this goal, we design and experimentally validate both Ohmic- and Schottky-like charge transfers at oxide/oxide semiconductor/metal interfaces. We utilize a method for predicting band alignment and charge transfer in ABO3 perovskites, where previously established rules for simple semiconductors fail. The prototypical systems chosen are the rare class of oxide metals, SrBO3 with B=V-Ta, when interfaced with the multifaceted semiconducting oxide,…
In this work we have studied the effect of light and electric field on the electrical properties of La0.7Sr0.3MnO3 (LSMO) film at 300 K. Taking advantage of it's charge, spin, orbital and lattice degrees of freedom, we have successfully shown that light and electric field together can be used to tune the resistance states of the manganite system. Using applied gate voltage (Vg) and light we were able to obtain a change in resistance of about +/-7.5%. Furthermore, incorporating an ultra thin interfacial LaTiO3 (LTO) layer provides valuable insight on the origin of the photoinduced effect. The o…
We have characterized the vertical transport properties of epitaxial layered structures composed of Pr$_{0.7}$Ca$_{0.3}$MnO$_{3}$ (PCMO) sandwiched between SrRuO$_{3}$ (SRO) bottom electrode and several kinds of top electrodes such as SRO, Pt, Au, Ag, and Ti. Among the layered structures, Ti/PCMO/SRO is distinct due to a rectifying current-voltage ($I$--$V$) characteristic with a large hysteresis. Corresponding to the hysteresis of the $I$--$V$ characteristics, the contact resistance of the Ti/PCMO interface reversibly switches between two stable states by applying pulsed voltage stress. We pr…
The two-orbital double-exchange model is employed for the study of the magnetic and orbital orders in ($R$MnO$_3$)$_n$/($A$MnO$_3$)$_{2n}$ ($R$: rare earths; $A$: alkaline earths) superlattices. The A-type antiferromagnetic order is observed in a broad region of parameter space for the case of SrTiO$_3$ as substrate, in agreement with recent experiments and first-principles calculations using these superlattices. In addition, also a C-type antiferromagnetic state is predicted to be stabilized when using substrates like LaAlO$_3$ with smaller lattice constants than SrTiO$_3$, again in agreement…
Strained Pr$_{0.5}$Ca$_{0.5}$MnO$_3$/La$_{0.5}$Ca$_{0.5}$MnO$_3$/Pr$_{0.5}$Ca$_{0.5}$MnO $_3$ trilayers were grown on (001)-SrTiO$_3$ substrates using the pulsed-laser deposition technique. The coupling at the interfaces of several trilayers has been investigated from magnetization and electronic transport experiments. An increase of La$_{0.5}$Ca$_{0.5}$MnO$_3$ layer thickness induces a magnetic ordering in the strain layers and at the interfaces leading to ferromagnetic behavior and enhanced coercivity, while resistivity shows metal-like behaviors. These effects are not observed in the parent…
Epitaxial La$_{2/3}$Sr$_{1/3}$MnO$_3$ (LSMO) films have been grown on SrTiO$_3$ (001) substrates via pulsed laser deposition. In a 22-nm thick LSMO film with a low residual resistivity of $ρ_0$ = 59 $μΩ$ cm, we found a zero-field dip in the magnetoresistance (MR) below 10 K, manifesting the weak antilocalization (WAL) effect due to strong spin-orbit coupling (SOC). We have analyzed the MR data by including the D'yakonov-Perel' spin-relaxation mechanism in the WAL theory. We explain that the delocalized spin-down electron subband states play a crucial role for facilitating marked SOC in clean L…
Thin films of Nd$_{0.5}$Ca$_{0.5}$MnO$_{3}$ manganites with colossal magnetoresistance (CMR) properties have been synthesized by the Pulsed Laser Deposition technique on (100)-SrTiO$_{3}$. The lattice parameters of these manganites and correlatively their CMR properties can be controlled by the substrate temperature $T_{S}$. The maximum CMR effect at 75K, calculated as the ratio $ρ(H=0T)/ρ(H=7T)$ is 10$^4$ for a deposition temperature of $T_{S}=680$ degC. Structural studies show that the Nd$_{0.5}$Ca$_{0.5}$MnO$_{3}$ film is single phase, [010]-oriented and has a pseudocubic symmetry of the pe…
Spin-dependent conductance across a manganate-barrier-manganate junction has recently been demonstrated. The junction is a La$_{0.67}$Sr$_{0.33}$MnO$_3$% -SrTiO$_3$-La$_{0.67}$ Sr$_{0.33}$MnO$_3$ trilayer device supporting current-perpendicular transport. Large magnetoresistance of up to a factor of five change was observed in these junctions at 4.2K in a relatively low field of the order of 100 Oe. Temperature and bias dependent studies revealed a complex junction interface structure whose materials physics has yet to be understood.
Epitaxial superlattices of ferromagnetic/paramagnetic La$_{0.67}$Sr$_{0.33}$MnO$_3$/SrIrO$_3$ materials have been prepared on SrTiO$_3$ (100) substrate using pulse laser deposition technique. An unexpected onset of interface magnetic interaction has been observed around 40 K. Interestingly, magnetic exchange bias effect has been observed in both field cooled and zero field cooled magnetization loops, however, the shifting of loop is opposite in both measurements. Exchange bias field vanishes as temperature increases to interface magnetic ordering temperature. Moreover, exchange bias field is f…
We have performed an in situ angle-resolved photoemission spectroscopy (ARPES) on single-crystal surfaces of La$_{0.6}$Sr$_{0.4}$MnO$_{3}$ (LSMO) thin films grown on SrTiO$_{3}$ (001) substrates by laser molecular beam epitaxy, and investigated the electronic structure near the Fermi level ($E_{F}$). The experimental results were compared with the band-structure calculation based on LDA + $U$. The band structure of LSMO thin films consists of several highly dispersive O 2$p$ derived bands in the binding energy range of 2.0 - 6.0 eV and Mn 3$d$ derived bands near $E_{F}$. ARPES spectra around t…
We have found that the current- voltage characteristics of La0.7Sr0.3MnO3(-delta)/Nb:SrTiO3 rectifying junctions are quantitatively well-described by (thermally-assisted) tunneling with an effectively temperature-independent Schottky barrier under no magnetic field, while those of the oxygen deficient junction remarkably deviate from such a simple behavior as magnetic field is applied. These results indicate a new form of magnetoresistance arising from magnetic field changes of the interface band diagram via the strong electron-spin coupling in manganites.
Thin film trilayer junction of La$%_{0.67}$Sr$_{0.33}$MnO$_3$ - SrTiO$_3$ - La$_{0.67}$Sr$_{0.33}$MnO$_3$ shows a factor of 9.7 change in resistance, in a magnetic field around 100 Oe at 14K. The junction magnetoresistance is bias and temperature dependent. The energy scales associated with bias and temperature dependence are an order of magnitude apart. The same set of energies also determine the bias and temperature dependence of the differential conductance of the junction. We discuss these results in terms of metallic cluster inclusions at the junction-barrier interface.
LaNiO$_3$ (LNO) is an intriguing member of the rare-earth nickelates in exhibiting a metal-insulator transition for a critical film thickness of about 4 unit cells [Son et al., Appl. Phys. Lett. 96, 062114 (2010)]; however, such thin films also show a transition to a metallic state in superlattices with SrTiO$_3$ (STO) [Son et al., Appl. Phys. Lett. 97, 202109 (2010)]. In order to better understand this transition, we have studied a strained LNO/STO superlattice with 10 repeats of [4 unit-cell LNO/3 unit-cell STO] grown on an (LaAlO$_3$)$_{0.3}$(Sr$_2$AlTaO$_6$)$_{0.7}$ substrate using soft x-…
A spin-tunnel-junction based on manganites, with La$_{1-x}$Sr$_x$MnO$_3$ (LSMO) as ferromagnetic metallic electrodes and the undoped parent compound LaMnO$_3$ (LMO) as insulating barrier, is here theoretically discussed using double exchange model Hamiltonians and numerical techniques. For an even number of LMO layers, the ground state is shown to have anti-parallel LSMO magnetic moments. This highly resistive, but fragile, state is easily destabilized by small magnetic fields, which orient the LSMO moments in the direction of the field. The magnetoresistance associated with this transition is…
Using polarized neutron reflectometry (PNR) we have investigated a YBa2Cu3O7(10nm)/La2/3Ca1/3MnO3(9nm)]10 (YBCO/LCMO) superlattice grown by pulsed laser deposition on a La0.3Sr0.7Al0.65Ta0.35O3 (LSAT) substrate. Due to the high structural quality of the superlattice and the substrate, the specular reflectivity signal extends with a high signal-to-background ratio beyond the fourth order superlattice Bragg peak. This allows us to obtain more detailed and reliable information about the magnetic depth profile than in previous PNR studies on similar superlattices that were partially impeded by pro…
