Dechao Meng

Interface engineering in epitaxial growth of layered oxides via a conducting layer insertion

There is a long-standing challenge in the fabrication of layered oxide epitaxial films due to their thermodynamic phase-instability and the large stacking layer number. Recently, the demand for high-quality thin films is strongly pushed by their promising room-temperature multiferroic properties. Here, we find that by inserting a conducting and lattice matched LaNiO3 buffer layer, high quality m = 5 Bi6FeCoTi3O18 epitaxial films can be fabricated using the laser molecular beam epitaxy, in which the atomic-scale sharp interface between the film and the metallic buffer layer explains the enhanced quality. The magnetic and ferroelectric properties of the high quality Bi6FeCoTi3O18 films are studied. This study demonstrates that insertion of the conducting layer is a powerful method in achieving high quality layered oxide thin films, which opens the door to further understand the underline physics and to develop new devices.

Growth of single-crystalline Bi6FeCoTi3O18 thin films and their magnetic–ferroelectric properties

High-quality single-crystalline thin films of n = 5 (n denotes the period of the Aurivillius structure) multiferroic Aurivillius oxides were obtained by determining and applying an optimal growth temperature of 605 °C and oxygen pressure of 20 Pa using pulsed laser deposition. The optimal growth ranges were narrower than 30 °C for the temperature and 10 Pa for the pressure. The optimized thin films exhibited obvious ferroelectric polarization switching in the out-of-plane direction as well as weak ferromagnetism with a saturation magnetization of less than 10 emu/cm3.

Self-modulated nanostructures in super-large-period Bi11(Fe5CoTi3)10/9O33 epitaxial thin films

Super-large-period Aurivillius thin films with a pseudo-period of ten were grown on (0 0 1) SrTiO3 substrates using the pulsed laser deposition method. The as-grown films are found to be coherently strained to the substrate and atomically smooth. X-ray diffraction indicates an average periodicity of ten, while analysis with the high resolution scanning transmission electron microscopy reveals a self-modulated nanostructure in which the periodicity changes as the film thickness increases. Finally, we discuss the magnetic and possible ferroelectric properties of the self-modulated large period Aurivillius films at the room temperature.

Soft X-ray absorption spectroscopy investigations of Bi6FeCoTi3O18 and LaBi5FeCoTi3O18 epitaxial thin films

High-quality single-crystalline Bi6FeCoTi3O18 and LaBi5FeCoTi3O18 thin films were prepared by pulsed laser deposition. X-ray diffraction characterizations indicate a more disordered lattice structure of the LaBi5FeCoTi3O18 film. The magnetic measurement results demonstrated significantly enhanced ferromagnetism in the LaBi5FeCoTi3O18 film. The modulation of oxidation and hybridization states caused by substituting Bi with La was studied using the soft X-ray absorption spectroscopy. The spectroscopic results revealed the reduced concentration of oxygen vacancies and the more distorted lattice structure in the LaBi5FeCoTi3O18 film, which explained the enhanced ferromagnetism.

DC current induced metal-insulator transition in epitaxial Sm0.6Nd0.4NiO3/LaAlO3 thin film

The metal-insulator transition (MIT) in strong correlated electron materials can be induced by external perturbation in forms of thermal, electrical, optical, or magnetic fields. We report on the DC current induced MIT in epitaxial Sm0.6Nd0.4NiO3 (SNNO) thin film deposited by pulsed laser deposition on (001)-LaAlO3 substrate. It was found that the MIT in SNNO film not only can be triggered by thermal, but also can be induced by DC current. The TMI of SNNO film decreases from 282 K to 200 K with the DC current density increasing from 0.003 × 109 A•m−2 to 4.9 × 109 A•m−2. Based on the resistivity curves measured at different temperatures, the MIT phase diagram has been successfully constructed.

Discerning lattice and electronic structures in under- and over-doped multiferroic Aurivillius films

Aurivillius type multiferroic thin films with controllable doping have not been studied. Here, we achieve accurate doping of (La,Bi)6Fe2−xNixTi3O18 epitaxial films using two-target pulsed laser deposition. An upper doping limit of x ∼ 0.4 for fabricating the single-phase structure is found. In over-doped films, secondary phases appear and the Ni valence is close to 2+. The under-doped films exhibit a single-phase and the measured electronic structure agrees with a stoichiometric phase. The multiferroic properties of the single-phase films with under-doping are probed. Our study reveals the doping limit in the Aurivillius-type multiferroic system and demonstrates the lattice-structure and electronic-structure difference between the under- and over-doped films.

Strain-induced high-temperature perovskite ferromagnetic insulator

Significance Ferromagnetic insulators are highly needed as the necessary components in developing next-generation dissipationless quantum-spintronic devices. Such materials are rare, and those high symmetric ones without chemical doping available so far only work below 16 K. Here we demonstrate a tensile-strained LaCoO3 film to be a strain-induced high-temperature ferromagnetic insulator. Both experiments and first-principles calculations demonstrated that the tensile-strain–supported ferromagnetism reaches its strongest when the composition is nearly stoichiometric. It disappears when the Co2+ defect concentration reaches around 10%. The discovery represents a chance for the availability of such materials, a high operation temperature, and a high epitaxial integration potential for making future devices. Ferromagnetic insulators are required for many new magnetic devices, such as dissipationless quantum-spintronic devices, magnetic tunneling junctions, etc. Ferromagnetic insulators with a high Curie temperature and a high-symmetry structure are critical integration with common single-crystalline oxide films or substrates. So far, the commonly used ferromagnetic insulators mostly possess low-symmetry structures associated with a poor growth quality and widespread properties. The few known high-symmetry materials either have extremely low Curie temperatures (≤16 K), or require chemical doping of an otherwise antiferromagnetic matrix. Here we present compelling evidence that the LaCoO3 single-crystalline thin film under tensile strain is a rare undoped perovskite ferromagnetic insulator with a remarkably high TC of up to 90 K. Both experiments and first-principles calculations demonstrate tensile-strain–induced ferromagnetism which does not exist in bulk LaCoO3. The ferromagnetism is strongest within a nearly stoichiometric structure, disappearing when the Co2+ defect concentration reaches about 10%. Significant impact of the research includes demonstration of a strain-induced high-temperature ferromagnetic insulator, successful elevation of the transition over the liquid-nitrogen temperature, and high potential for integration into large-area device fabrication processes.

Phase competition in the growth of SrCoOx/LaAlO3 thin films

The reversible topotactic phase transformation between brownmillerite SrCoO2.5 to perovskite SrCoO3 has attracted more and more attention for potential applications as solid oxide fuels and electrolysis cells. However, the relatively easy transformation result from small thermal stable energy barriers between the two phases leads to unstable the structures. In the paper, amounts of SrCoO3-δ films have been prepared by pulsed laser deposition at optimized growth conditions with the temperature range of 590-720°C. The X-ray diffraction (XRD) results demonstrated that a phase competition emerged around 650°C. The Gibbs free energies of two phases at high temperature revealed the difference of stability of these two phases under different growth temperature. The optical spectroscopies and X-ray photoelectron spectroscopies were used to verify the electronic structure and chemical state differences between the two phases with distinct crystal structures.

Electric-field-controlled nonvolatile magnetic switching and resistive change in La0.6Sr0.4MnO3/0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 (011) heterostructure at room temperature

Control over nonvolatile magnetization rotation and resistivity change by an electric field in La0.6Sr0.4MnO3 thin films grown on (011) oriented 0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 substrates are studied. By utilizing an in-plane strain induced by a side ferroelectric switching with pulsed electric fields from −2.5 kV/cm to +5 kV/cm along [011¯], a nonvolatile and reversible 90°-rotation of the magnetic easy-axis is achieved, corresponding to −69.68% and +174.26% magnetization switching along the [100] and [011¯] directions, respectively. The strain induced nonvolatile resistivity change is approximately 3.6% along the [011¯] direction. These findings highlight potential strategies for electric-field-driven spintronic devices.