LaAlO3 Research Articles

Tunneling current in magnetic-ferroelectric-superconducting heterostructures

We report the tunneling current behavior of magnetic-ferroelectric-superconducting heterostructures for multistates non-volatile random access memory (NVRAM) elements. A heterostructure of La0.67Sr0.33MnO3 (LSMO) (50 nm)/PbZr0.52Ti0.48O3 (PZT) (5 nm)/Bi-Sr-Ca-Cu2-OX (BSCCO) (100 nm)/LaAlO3 (LAO) architecture was fabricated by pulsed laser deposition technique. The tunneling effects were investigated well above and below the superconducting phase transition temperature of the BSCCO bottom electrode. A divergent current and conductance paths were observed for polarization up and down direction above the coercive field of the ferroelectric tunnel barrier. This behaviour was significant below Ts where the moderate effect of the external magnetic field was also observed on the tunneling current. The dynamic conductance G(V) data fitted well with Brinkman's model for both polarizations up and polarization down states which suggest the presence of large tunnel electro-resistance.

Probing charge transfer during metal-insulator transitions in graphene-LaAlO3/SrTiO3 systems

Two-dimensional electron systems (2DESs) at the interface between LaAlO3 (LAO) and SrTiO3 (STO) perovskite oxides display a wide class of tunable phenomena ranging from superconductivity to metal-insulator transitions. Most of these effects are strongly sensitive to surface physics and often involve charge transfer mechanisms, which are, however, hard to detect. In this work, we realize hybrid field-effect devices where graphene is used to modulate the transport properties of the LAO/STO 2DES. Different from a conventional gate, graphene is semimetallic and allows us to probe charge transfer with the oxide structure underneath the field-effect electrode. In LAO/STO samples with a low initial carrier density, graphene-covered regions turn insulating when the temperature is lowered to 3 K, but conduction can be restored in the oxide structure by increasing the temperature or by field effect. The evolution of graphene’s electron density is found to be inconsistent with a depletion of LAO/STO, but it rather points to a localization of interfacial carriers in the oxide structure.

Largely Enhanced Mobility in Trilayered LaAlO3/SrTiO3/LaAlO3 Heterostructures.

LaAlO3 (LAO)/SrTiO3 (STO)/LaAlO3 (LAO) heterostructures were epitaxially deposited on TiO2-terminated (100) SrTiO3 single-crystal substrates by laser molecular beam epitaxy. The electron Hall mobility of 1.2 × 104 cm2/V s at 2 K was obtained in our trilayered heterostructures grown under 1 × 10-5 Torr, which was significantly higher than that in single-layer 5 unit cells LAO (∼4 × 103 cm2/V s) epitaxially grown on (100) STO substrates under the same conditions. It is believed that the enhancement of dielectric permittivity in the polar insulating trilayer can screen the electric field, thus reducing the carrier effective mass of the two-dimensional electron gas formed at the TiO2 interfacial layer in the substrate, resulting in a largely enhanced mobility, as suggested by the first-principle calculation. Our results will pave the way for designing high-mobility oxide nanoelectronic devices based on LAO/STO heterostructures.

LaAlO3:Cr3+, Sm3+: Nano-perovskite with persistent luminescence for in vivo optical imaging

Persistent luminescence (PersL) nanoparticles offer attractive capabilities for in vivo imaging applications as an optical excitation of the nanoparticles outside the animal before injection is possible. Therefore, long-lasting emission of light allows performing in vivo imaging with high signal to noise ratio and with high sensitivity. Here we report the development of LaAlO3 (LAO) nano-perovskite doped with Cr3+ and Sm3+, the trivalent chromium cation being the recombination center, while Sm3+ is used to enhance the persistent properties. Nanoparticles are synthesized by co-precipitation method and are tailored to present deep red persistent luminescence at 734 nm due to the Cr3+: 2E(2G) → 4A2(4F) transition after UV excitation. The intense deep red PersL matches well with the first biological window (650–950 nm). These nanoparticles have been successfully functionalized with polyethylene glycol (PEG) and the stealth obtained nanoparticles were tested for the first time for imaging on small animal in order to have access to their in vivo biodistribution.

Thickness Dependence of Magnetic Behavior of LaAlO3/SrTiO3 Heterostructures

AbstractThickness dependence of magnetic properties of LaAlO3 (LAO)/SrTiO3 (STO) heterostructures is systematically studied. It is found that the magnetic moment is rather robust at elevated temperatures in the samples grown under ultralow PO2 of 1 × 10−8 Torr, indicating room‐temperature ferromagnetism in the LaAlO3/SrTiO3 heterostructures. Spin‐glass‐like behavior is observed in the LAO/STO heterostructures with the LAO thickness ranging from 5 to 90 unit cells (2–35.2 nm). As the LAO layer becomes thicker, the blocking temperature of spin‐glass‐like behavior decreases and magnetic moment weakens, which may be related to the reduced density of oxygen vacancies at the interfacial TiO2 layer. It is believed that the well‐controlled magnetism by tuning the LAO thickness may open an avenue for the potential application of spintronics with LaAlO3/SrTiO3 heterostructures.

The Effects of Gd-Free Impurity Phase on the Aging Behavior for the Microwave Surface Resistance of Ag-coated GdBa2Cu3O7−δ at Cryogenic Temperatures

High-TC GdBa2Cu3O7−δ (GdBCO) superconductor has been popular for making superconductive tapes that have much potential for various fields of large-scale applications. We investigated aging effects on the microwave surface resistance (RS) of Ag-coated GdBCO layer on Hastelloy substrate, so called GdBCO coated conductors (CCs), and Ag-coated GdBCO films on LaAlO3 (LAO) single-crystal substrates at cryogenic temperatures and compared them with each other. Unlike the RS of Ag-coated GdBCO films showing significant degradation in 4 weeks, no significant aging effects were found in our Ag-coated GdBCO CCs aged 85 weeks. The reactive co-evaporation deposition and reaction (RCE-DR) method was used for preparing the Ag-coated GdBCO CCs. Such durability of the Ag-coated GdBCO CCs in terms of the RS could be explained by existence of a protective impurity phase, i.e., Gd-free Ba–Cu–O phase as confirmed by transmission electron microscopy study combined with the energy-dispersive X-ray spectroscopy measurements. Although the scope of this study is limited to the Ag-coated GdBCO CCs prepared by using the RCE-DR method, our results suggest that a solution for preventing the aging effects on transport properties of other kinds of Ag-coated GdBCO CCs could be realized by means of an artificially-grown protective impurity layer.

Growth and characterization of Bi3.15Nd0.85Ti2.95Hf0.05O12/La0.67Sr0.33MnO3 composite film with strong magnetoelectric effect by chemical solution deposition under moderate crystallization temperature

Using chemical solution deposition method, La0.67Sr0.33MnO3 (LSMO) and Bi3.15Nd0.85Ti2.95Hf0.05O12 (BNTH) layers were successively fabricated onto (001) LaAlO3 (LAO) substrates, and then BNTH/LSMO ferromagnetic-ferroelectric composite films with a 2-2 layered type structure were obtained. The LSMO ferromagnetic layer was epitaxially grown on the LAO substrate and showed the low resistivity of 5.0 × 10−3 Ω cm. It was not only used as a seed layer to induce the oriented growth of BNTH ferroelectric layers, but also as a bottom electrode to measure ferroelectric/magnetoelectric (ME) properties. The effects of crystallization temperature on the orientation degree, the surface morphology, and the dielectric and ferroelectric performances of the composite films were investigated. The composite film crystallized at 730 °C has the largest remnant polarization (Pr) of 35.0 μC/cm2, the lowest leakage current density of 1.35 × 10−7 A/cm2 and the high saturated magnetization (Ms) of 370 emu/cm3. The composite film also exhibits a strong ME response in the absence of DC bias magnetic field. Its ME voltage coefficient, αE, gradually increases from zero with the increasing of the AC magnetic field frequency, and eventually reaches 35.8 V/cm·Oe at 100 kHz.

Top and Split Gating Control of the Electrical Characteristics of a Two-dimensional Electron Gas in a LaAlO3/SrTiO3 Perovskite

A top gate field effect transistor was fabricated using polymethyl methacrylate (PMMA) as a gate insulator on a LaAlO3 (LAO)/SrTiO3 (STO) hetero-interface. It showed n-type behavior, and a depletion mode was observed at low temperature. The electronic properties of the 2-dimensional electron gas at the LAO/STO hetero-interface were not changed by covering LAO with PMMA following the Au top gate electrode. A split gate device was also fabricated to construct depletion mode by using a narrow constriction between the LAO/STO conduction interface. The depletion mode, as well as superconducting critical current, could be controlled by applying a split gate voltage. Noticeably, the superconducting critical current tended to decrease with decreasing the split gate voltage and finally became zero. These results indicate that a weak-linked Josephson junction can be constructed and destroyed by split gating. This observation opens the possibility of gate-voltage-adjustable quantum devices.

Oxygen vacancies modulated photoelectrical properties in LaAlO3/SrTiO3 interface

We demonstrate the photoconductivity and related relaxation characteristics at the metallic interfaces between SrTiO3 (STO) substrates and LaAlO3 (LAO) films deposited at oxygen pressures ranged from 2 × 10−3 to 2 × 10−6 Torr. The photoconductivity results show that all the LAO/STO interfaces exhibit both persistent and transient photoconductivity, which decrease rapidly with increasing oxygen vacancies in the heterostructures. Using double exponential model, the photoinduced relaxation processes are also analyzed. The interface with less oxygen vacancies shows slower resistance recovery process. Our results provide deeper insight into the optoelectric response of two-dimensional electron gas at complex oxides interface.

The effect of heat treatment temperature on superconductivity of Bi-2212/YBCO heteroepitaxial structure fabricated by chemical solution deposition approach

A planar-type heteroepitaxial junction with high-Tc superconducting YBa2Cu3O7−δ(YBCO) and Bi2Sr2CaCu2O8+δ(Bi-2212)electrodes has been prepared. Bi-2212/YBCO heteroepitaxial structure without an insulating layer was deposited on LaAlO3 (LAO) monocrystalline substrates by an all chemical solution deposition approach. Furthermore, the effect of heat treatment temperature on the superconductivity of Bi-2212/YBCO planar-type junction has been studied. Two superconducting transition steps in the R-T curves of Bi-2212/YBCO bilayer films prepared at 780 ℃ were clearly observed. The superconducting critical transition temperatures (Tc) of lower YBCO and upper Bi-2212 were about 91 K and 80 K, respectively. It was found that the critical current(Ic) was about 0.4 mA at 10 K and the superconductivity of planar-type junction strongly depend on the heat treatment temperature.

Self-biased magnetoelectric coupling effect in the layered La0.7Sr0.3MnO3/BaTiO3/La0.7Sr0.3MnO3 multiferroic heterostructure

The experimental observation and analysis of self-biased magnetoelectric (ME) coupling effect in the layered La0.7Sr0.3MnO3/BaTiO3/La0.7Sr0.3MnO3 (LSMO/BTO/LSMO) multiferroic heterostructure were reported, and the epitaxial layered LSMO/BTO/LSMO heterostructure was grown on (001) oriented LaAlO3 (LAO) single crystal substrate by using pulsed laser deposition technique. The microstructure, ferromagnetic and ferroelectric properties were studied. The exchange bias effect in the LSMO/LAO interface and the compressive strain gradient field in the heterostructure induced by lattice mismatch resulted in a built-in magnetic bias field (Hbias) in the LSMO layer, which provided limited ME coupling effect at zero applied DC bias magnetic field. It may potentially be used to micro-nano devices especially for multiple state memories.

Growth and Electrostatic/chemical Properties of Metal/LaAlO3/SrTiO3 Heterostructures.

The quasi 2D electron system (q2DES) that forms at the interface between LaAlO3 (LAO) and SrTiO3 (STO) has attracted much attention from the oxide electronics community. One of its hallmark features is the existence of a critical LAO thickness of 4 unit-cells (uc) for interfacial conductivity to emerge. Although electrostatic mechanisms have been proposed in the past to describe the existence of this critical thickness, the importance of chemical defects has been recently accentuated. Here, we describe the growth of metal/LAO/STO heterostructures in an ultra-high vacuum (UHV) cluster system combining pulsed laser deposition (to grow the LAO), magnetron sputtering (to grow the metal) and X-ray photoelectron spectroscopy (XPS). We study step by step the formation and evolution of the q2DES and the chemical interactions that occur between the metal and the LAO/STO. Additionally, magnetotransport experiments elucidate on the transport and electronic properties of the q2DES. This systematic work not only demonstrates a way to study the electrostatic and chemical interplay between the q2DES and its environment, but also unlocks the possibility to couple multifunctional capping layers with the rich physics observed in two-dimensional electron systems, allowing the fabrication of new types of devices.

Interface chemical and electronic properties of LaAlO3/SrVO3 heterostructures

We have studied the chemical and electronic properties of LaAlO3/SrVO3 (SVO) ultrathin films by combining hard x-ray photoemission spectroscopy and transport measurements. We compare single SrVO3 (SVO) ultrathin films and SrVO3 buried below a polar LaAlO3 (LAO) thin layer, both epitaxially grown on SrTiO3. While ultrathin films (4 unit cells) of SVO do show insulating behavior over the entire temperature range, the LAO/SVO interface has a resistivity minimum at 250 K. When increasing the SVO layer thickness, the minimum is observed to shift to higher temperatures, but the resistivity always stays smaller than that of comparable SVO single films. Hard x-ray photoemission spectroscopy reveals a surface or interface related V5+ component in the V 2p spectra for SVO films and LAO/SVO heterostructures, respectively, attributed to a strongly oxidized component. This chemical reconstruction is weaker in LAO/SVO heterostructures compared to single SVO films. We show that this dead layer in SVO ultrathin films has to be considered when the film thickness reaches the few unit-cells limit and propose solutions on how to prevent this detrimental effect.

Comparison study of the flux pinning enhancement of YBCO superconductor with BZO and BZO + Y2O3 mixed phase additions

Adding nanophase defects to YBa2Cu3O7-δ (YBCO) superconductor thin films is well-known to enhance flux pinning, resulting in an increase in current density (Jct). While many previous studies focused on single phase additions, the addition of several phases simultaneously shows promise in improving current density by combining different pinning mechanisms. This paper compares the effect of the addition of two insulating, nonreactive phases of barium zirconium oxide (BZO) and yttrium oxide Y2O3, both as a single addition of BZO and as a double addition in conjunction with Y2O3. Processing parameters vary the target composition volume percent of BZO from 2-6 vol. %, while maintaining 3 vol. % Y2O3, and the remaining vol. % YBCO. Pulsed laser deposition produced thin films on LaAlO3 (LAO) and SrTiO3 (STO) substrates at various deposition temperatures. Comparison of strong and weak flux pinning mechanisms, current densities, critical temperatures, and microstructures of the resulting films will be presented. Films produced from the 2 vol. % BZO + 3 vol. Y2O3 doped YBCO target at a deposition temperature of 825 °C attained the highest current density. The addition of second phase Y2O3 impacted the film microstructure, resulting in more isotropic behavior when compared to the YBCO films doped with only BZO.

Room-Temperature Giant Charge-to-Spin Conversion at the SrTiO3-LaAlO3 Oxide Interface.

The two-dimensional electron gas (2DEG) formed at the interface between SrTiO3 (STO) and LaAlO3 (LAO) insulating layer is supposed to possess strong Rashba spin-orbit coupling. To date, the inverse Edelstein effect (i.e., spin-to-charge conversion) in the 2DEG layer is reported. However, the direct effect of charge-to-spin conversion, an essential ingredient for spintronic devices in a current-induced spin-orbit torque scheme, has not been demonstrated yet. Here we show, for the first time, a highly efficient spin generation with the efficiency of ∼6.3 in the STO/LAO/CoFeB structure at room temperature by using spin torque ferromagnetic resonance. In addition, we suggest that the spin transmission through the LAO layer at a high temperature range is attributed to the inelastic tunneling via localized states in the LAO band gap. Our findings may lead to potential applications in the oxide insulator based spintronic devices.

Growth, microstructure and transport properties of ultrathin epitaxial La0.5Ca0.5MnO3 films prepared by laser molecular beam epitaxy

We present a structure, surface morphology and electrical study of ultrathin epitaxial La0.5Ca0.5MnO3 (LCMO) films (2-100 unit cells) grown by laser molecular beam epitaxy. The films are cube on cube epitaxial with respect to the SrTiO3 (STO) substrate confirmed by x-ray analysis. When the film thickness is increased, the resistivity decreases drastically while no important surface changes are observed. Single crystalline substrates: STO, LaAlO3 (LAO) and (La,Sr) (Al,Ta)O3 are used to investigate the strain related surface microstructure and electrical properties. Different surface morphology and transport behaviors are observed in the 10 unit cells thick LCMO films with different strain type. The comparison among the films on different substrates indicates that the films under compressive strain (on LAO) exhibit the smoothest surface, the lowest resistivity and the highest magnetoresistance, while those under larger tensile strain (on STO) show a contrary behavior. The strain-related transport behaviors in the multi-phase competing LCMO system are interpreted on the basis of different Jahn-Teller distortion and relative size of the ferromagnetic-metal region for the films under different strain.

Suppressed in a superconducting/ferromagnetic bilayer

We investigate the influence of a ferromagnetic layer (La0.7Sr0.3MnO3) on the upper critical field of YBa2Cu3O7−δ superconducting layer in a superconductor/ferromagnet (SC/FM) hybrid structure. The YBa2Cu3O7−δ/La0.7Sr0.3MnO3 (YBCO/LSMO) hybrid bilayers as well YBCO single layer were fabricated on (001) LaAlO3 (LAO) substrate using pulsed laser deposition.The temperature dependent upper critical field of type-II superconductors are generally described in the frameworks laid down by Werthamer–Helfand–Hohenberg and Ginzburg–Landau. We employ both formalism to estimate of YBCO from magneto-transport data in SC/FM bilayers of varying ferromagnetic layer thickness as well as for YBCO single layer. We find that the of YBCO in YBCO/LSMO bilayer gets suppressed as compared to that of single YBCO layer. Further to this, we also observe that the of YBCO in the bilayer system decreases with increasing ferromagnetic LSMO layer thickness. These two results are discussed in the light of magnetic proximity effect, spin-diffusion induced pair breaking and enhanced effective magnetic field experienced by the YBCO layer with increasing ferromagnetic LSMO layer thickness.

Fabrication of YBa2Cu3O7 twin-boundary-junction dc SQUID by using a focused-ion-beam pattern technique

We have fabricated YBa2Cu3O7 (YBCO) dc SQUIDs containing nanobridges across twin boundaries of LaAlO3 (LAO) substrates as Josephson elements by using a focused ion beam (FIB) etching method and measured their transport properties. The beam energy was 30 keV and the current was 1.5 pA for the nanobridge pattern. Each bridge with a nominal width of 200 nm crossed a twin boundary in the (100) direction. The SQUID loop had a 10 μm × 10 μm hole with a 5.7 μm average linewidth. The SQUID voltage showed modulations in response to the external flux with a maximum modulation depth of 350 μV at 77.0 K. HR-XRD spectra showed that the epitaxially grown YBCO film was twinned in commensurate with the twinning of the LAO substrate. Tilting of the c-axis of YBCO across the twin boundary is believed to play a role as a tunnel barrier.

Grating-patterned Bi0.85La0.15Fe0.95Mn0.05O3 epitaxial thin film prepared using photosensitive sol-gel method and its ferromagnetic and ferroelectric properties

Bi0.85La0.15Fe0.95Mn0.05O3 (BLFMO) gel films with benzoylacetone (BzAc) as a chelating reagent were fabricated on (00l) LaAlO3 (LAO) substrates using a photosensitive sol-gel method. The morphology, crystalline structure, electric and magnetic properties, and the grating-pattern of the films were investigated. The results showed that the epitaxial BLFMO films deposited on LAO single crystal substrates exhibit a higher degree of the c-axis orientation. The remnant polarization and saturation magnetization were found to be higher than those of the randomly oriented BLFMO films. Moreover, the gratings of the film with a period of 1 μm were obtained after exposure to two interference laser beams followed by leaching in ethanol. After treatment at 600 °C, the gel film turned into inorganic film gratings.

Hidden peculiar magnetic anisotropy at the interface in a ferromagnetic perovskite-oxide heterostructure

Understanding and controlling the interfacial magnetic properties of ferromagnetic thin films are crucial for spintronic device applications. However, using conventional magnetometry, it is difficult to detect them separately from the bulk properties. Here, by utilizing tunneling anisotropic magnetoresistance in a single-barrier heterostructure composed of La0.6Sr0.4MnO3 (LSMO)/LaAlO3 (LAO)/Nb-doped SrTiO3 (001), we reveal the presence of a peculiar strong two-fold magnetic anisotropy (MA) along the [110]c direction at the LSMO/LAO interface, which is not observed in bulk LSMO. This MA shows unknown behavior that the easy magnetization axis rotates by 90° at an energy of 0.2 eV below the Fermi level in LSMO. We attribute this phenomenon to the transition between the eg and t2g bands at the LSMO interface. Our finding and approach to understanding the energy dependence of the MA demonstrate a new possibility of efficient control of the interfacial magnetic properties by controlling the band structures of oxide heterostructures.