Guanyin Gao

Transparent and conductive oxide films with the perovskite structure: La- and Sb-doped BaSnO3

We report that (La0.07Ba0.93)SnO3 films grown epitaxially on SrTiO3(001) substrates by laser ablation at 760°C show a cubic perovskite structure of lattice constant 4.121A, excellent optical transmittance in the visible range, and a weak metallic behavior with low resistivity of about 4mΩcm within 10–300K. The transparent perovskite oxide films are n-type conductors, with carrier concentration and mobility at room temperature of about 2×1021cm−3 and 0.69cm2∕Vs, respectively, and a direct allowed band gap of 4.02eV. The effect of deposition temperature on structural, optical, and electric properties of the Sb-doped Ba(Sn0.93Sb0.07)O3 films was also demonstrated.

Interface ferromagnetism in (110)-oriented La 0.7 Sr 0.3 MnO 3 / SrTiO 3 ultrathin superlattices

We explore manganite interface magnetism in epitaxially grown La0.7Sr0.3MnO3(LSMO)/SrTiO3 ultrathin superlattices (SL) along (110) orientation. we show that robust ferromagnetism persists down to four monolayers LSMO(MLs) (~1.1 nm in thickness), of which 50% Mn is at the interface state. Above eight MLs, the magnetic moment is nearly saturated to the theoretical value of 3.7u_B, with an estimated interface moment of 3.2u_B. In comparison to (100)-oriented SLs which were previously shown to have a spin canted ground state, (110)-oriented SLs exhibit stronger low-dimensional ferromagnetism and better metallicity, suggesting a ferromagnetic interface spin state well suited for all-oxide spintronic devices. The underlining mechanism is qualitatively discussed.

Electric-field-modulated nonvolatile resistance switching in VO₂/PMN-PT(111) heterostructures.

The electric-field-modulated resistance switching in VO2 thin films grown on piezoelectric (111)-0.68Pb(Mg1/3Nb2/3)O3-0.32PbTiO3 (PMN-PT) substrates has been investigated. Large relative change in resistance (10.7%) was observed in VO2/PMN-PT(111) hererostructures at room temperature. For a substrate with a given polarization direction, stable resistive states of VO2 films can be realized even when the applied electric fields are removed from the heterostructures. By sweeping electric fields across the heterostructure appropriately, multiple resistive states can be achieved. These stable resistive states result from the different stable remnant strain states of substrate, which is related to the rearrangements of ferroelectric domain structures in PMN-PT(111) substrate. The resistance switching tuned by electric field in our work may have potential applications for novel electronic devices.

Lattice-mismatch-strain induced inhomogeneities in epitaxial La0.7Ca0.3MnO3 films

Epitaxial La0.7Ca0.3MnO3 (LCMO) films of 15–150nm thick have been grown on SrTiO3(001) [STO(001)] and (LaAlO3)0.3(Sr2AlTaO6)0.7(001) [LSAT(001)] substrates with lattice mismatches of 1.11% and 0.15%, respectively. Asymmetric x-ray reciprocal space mapping was used to determine the strain state evolution in the films. For LCMO/STO(001), as the film thickness increases, at about 30nm a change from fully to partially strained has been induced, and a further annealing at 900°C can fully relax the films. For LCMO/LSAT(001), however, due to the negligible lattice mismatch, even at 15nm the films are almost fully relaxed. Correspondingly, for the fully relaxed LCMO films a paramagnetic to ferromagnetic transition at TC of about 260K was observed, and for those partially relaxed, both the structure and magnetic transition are inhomogeneous, two thickness-dependent TC were detected for each film. The results show strong evidence that the transition temperature is correlated with the strain state in epitaxial LCMO ...

Anisotropic-strain-induced antiferromagnetic-insulating state with strong phase instability in epitaxial (La0.8Pr0.2)0.67Ca0.33MnO3 films

We grow epitaxial (La0.8Pr0.2)0.67Ca0.33MnO3 films simultaneously on the lattice-closely-matched substrates, cubic (LaAlO3)0.3(Sr2AlTaO6)0.7 [LSAT(001)] and orthorhombic NdGaO3 [NGO(001) and NGO(110)]. While all as-grown films show a ferromagnetic-metallic (FM) ground state as observed for the bulk target, the annealed films show quite different magnetotransport behavior as follows: on NGO(110) they show a robust FM ground state, on LSAT(001) and NGO(001) however, they show surprisingly a coexisted antiferromagnetic insulating state with high phase instability in a wide temperature range. The phase coexistence being easily induced via the control of anisotropic epitaxial strain suggests that the phase separation in manganites could be elastically driven, and thus can be strain-engineered for devices applications.

Control of the charge-ordering-insulating phase in epitaxial La1−xCaxMnO3 (x=0.30–0.45) thin films under the anisotropic strain

The control of charge-ordering-insulating (COI) phase in epitaxial La1−xCaxMnO3/NdGaO3(001) (x=0.30–0.45) thin films with essentially the ferromagnetic metal ground state as observed for the bulk counterparts has been realized via the anisotropic strain relaxation. This epitaxial system is special in that there is a negligible average lattice mismatch but a large anisotropic strain in between the film and the substrate. By changing the film thickness, postannealing temperature, along with the doping level for strain relaxation, the COI phase in the films can be tuned to either melt completely under 1 T, producing a huge low-field magnetoresistance (MR) in a wide temperature range (e.g., for the 20 nm film with x=0.33 and annealed at 780 °C, the MR can be over 70% at 0.2 T and 97% at 0.5 T in 10–200 K), or survive under a high magnetic field of 6 T. The results demonstrate the crucial role of anisotropic strain relaxation in inducing the inhomogeneity in manganites films, thus providing a forward understan...

Phase evolution and the multiple metal-insulator transitions in epitaxially shear-strained La0.67Ca0.33MnO3/NdGaO3(001) films

The ferromagnetic-metal (FM) to antiferromagnetic-insulator (AFI) transition, near 250 K, has been induced in epitaxially shear-strained La0.67Ca0.33MnO3/NdGaO3(001) films, although they are doped for a FM ground state. We showed that for these films the phase diagram can feature the five regions of paramagnetic-insulator, FM, AFI dominated, FM dominated, and the frozen state, evolving with decreasing temperatures. And the phase instabilities in the temperature/magnetic-field regime can lead to multiple metal-insulator transitions over the various phase boundaries, in addition to a huge low-field magnetoresistance in the wide temperature range. The results underline that with the elastic-driven phase separation and consequently the complex phase evolution, this epitaxial system could be employed for further understanding of the manganites, and also for thin-film devices applications.

All-oxide–based synthetic antiferromagnets exhibiting layer-resolved magnetization reversal

Making an oxide-layered antiferromagnet Antiferromagnetism, a state of matter where ordered neighboring spins point in opposite directions, can be engineered in layered heterostructures, which affords control over their properties. Doing so in oxide heterostructures is tricky because the necessary ferromagnetism of the constituent layers may not survive thinning to nanometer thicknesses. Chen et al. overcame this materials challenge by finding and growing the right combination of substrate, magnetic, and insulating layers to engineer antiferromagnetic coupling. The resulting superlattices, consisting of alternating layers of a ferromagnetic oxide and an insulating material, exhibit layer-by-layer switching of magnetization. Science, this issue p. 191 Superlattices made of layers of ferromagnetic La2/3Ca1/3MnO3 and insulating CaRu1/2Ti1/2O3 show antiferromagnetic coupling. Synthesizing antiferromagnets with correlated oxides has been challenging, owing partly to the markedly degraded ferromagnetism of the magnetic layer at nanoscale thicknesses. Here we report on the engineering of an antiferromagnetic interlayer exchange coupling (AF-IEC) between ultrathin but ferromagnetic La2/3Ca1/3MnO3 layers across an insulating CaRu1/2Ti1/2O3 spacer. The layer-resolved magnetic switching leads to sharp steplike hysteresis loops with magnetization plateaus depending on the repetition number of the stacking bilayers. The magnetization configurations can be switched at moderate fields of hundreds of oersted. Moreover, the AF-IEC can also be realized with an alternative magnetic layer of La2/3Sr1/3MnO3 that possesses a Curie temperature near room temperature. The findings will add functionalities to devices with correlated-oxide interfaces.

Annealing assisted substrate coherency and high-temperature antiferromagnetic insulating transition in epitaxial La0.67Ca0.33MnO3/NdGaO3(001) films

Bulk La0.67Ca0.33MnO3 (LCMO) and NdGaO3 (NGO) have the same Pbnm symmetry but different orthorhombic lattice distortions, yielding an anisotropic strain state in the LCMO epitaxial film grown on the NGO(001) substrate. The films are optimally doped in a ferromagnetic-metal ground state, after being ex-situ annealed in oxygen atmosphere, however, they show strikingly an antiferromagnetic-insulating (AFI) transition near 250 K, leading to a phase separation state with tunable phase instability at the temperatures below. To explain this drastic strain effect, the films with various thicknesses were ex-situ annealed under various annealing parameters. We demonstrate that the ex-situ annealing can surprisingly improve the epitaxial quality, resulting in the films with true substrate coherency and the AFI ground state. And the close linkage between the film morphology and electronic phase evolution implies that the strain-mediated octahedral deformation and rotation could be assisted by ex-situ annealing, and moreover, play a key role in controlling the properties of oxide heterostructures.

The thickness evolution of orthorhombic lattice distortions in heteroepitaxial La0.67Ca0.33MnO3/NdGaO3(110)Orobserved by x-ray reciprocal space mapping

La0.67Ca0.33MnO3 (LCMO) films of 6?60?nm thickness were grown epitaxially on orthorhombic NdGaO3(1?1?0)Or (NGO) substrates by the pulsed laser deposition method. Like NGO, the films when relaxed should also have an orthorhombic structure that can be described by a pseudocubic perovskite unit, with the angle ? between a and c axes deviating from 90?. Using high-resolution off-specular x-ray reciprocal space mapping, we clearly observed the angle deviations in pseudocubic LCMO(0?0?1)/NGO(0?0?1) and investigated the thickness evolution of lattice distortions in the films. At above 30?nm the films are angularly relaxed, while the thinner ones suffer from the shear strain. We argue that for this system although the lattice mismatch in between is negligible the shear strain is crucial for transport properties of the ultrathin LCMO films.