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The thickness-dependent epitaxial strains and phase transformations of (001)-VO2/TiO2 thin films are investigated systematically in a wide thickness range (from 9 to 150 nm). Under a thickness of 18 nm, the tensile in-plane strain is maintained, owing to the good lattice and the symmetry matching between the VO2 thin film and the TiO2 substrate, but the compressive out-of-plane epitaxial strain is gradually relaxed. The epitaxial strains co-stabilize the rutile phase (R phase) in this thickness range. Beyond a thickness of 18 nm, the out-of-plane lattice c exhibits a sudden elongation and reaches the bulk level of 2.8528 Å at a thickness of 20 nm, which indicates a structural phase transition (SPT). A further increase of the film thickness results in another new phase (tetragonal-like or T-like) with lattice distortion, which maintains the tetragonal symmetry in the thickness range of 20 to 55 nm. From a thickness of 60 nm, the monoclinic phase (M1 phase) appears, which indicates another SPT from T-like to the monoclinic M1 phase. This SPT is more favorable energetically, owing to the assistance of the strain relaxation in the thicker films. Additionally, the metal-insulator transition temperature positively increases as a function of the out-of-plane strain. This result is consistent with the fact that the tensile strain along the cR axis (V-V atom chain) is conducive for the stabilized insulating phase. This work highlights strain engineering as a crucial avenue for manipulating the phase transformations and properties in the correlated electron system. [ABSTRACT FROM AUTHOR]

Strain engineering is a popular approach for manipulating material properties through modifying crystal structures and/or electron-lattice interactions. In this work, we used a phase transition material, vanadium dioxide (VO) as an active and reconfigurable substrate to generate a controllable strain. Amorphous ferromagnetic Ni thin films were deposited on the (010)-VO/(0001)-AlO substrates. It is observed that the magnetic moments of the Ni thin film were modulated by interfacing it with the VO thin film. We observed a hysteresis in the magnetic moment-temperature curves in the vicinity of the metal-insulator transition of the VO thin films at a low magnetic field bias (< 30 Oe). This result can be attributed to magnetoelastic coupling between the Ni and VO thin films through a reversible structural phase transition. As the bias field was increased, the hysteresis was suppressed and eventually disappeared. The competition between the magnetoelastic energy and external Zeeman energy can explain these behaviors. Our observations suggest a promising approach to dynamically controlling the properties of functional thin films by strain engineering. [ABSTRACT FROM AUTHOR]

The VO2 thin films with sharp metal–insulator transition (MIT) were epitaxially grown on (001)-oriented Yttria-stabilized zirconia substrates (YSZ) using radio-frequency (RF) magnetron sputtering techniques. The MIT and structural phase transition (SPT) were comprehensively investigated under in situ temperature conditions. The amplitude of MIT is in the order of magnitude of 104, and critical temperature is 342 K during the heating cycle. It is interesting that both electron concentration and mobility are changed by two orders of magnitude across the MIT. This research is distinctively different from previous studies, which found that the electron concentration solely contributes to the amplitude of the MIT, although the electron mobility does not. Analysis of the SPT showed that the (010)-VO2/(001)-YSZ epitaxial thin film presents a special multi-domain structure, which is probably due to the symmetry matching and lattice mismatch between the VO2 and YSZ substrate. The VO2 film experiences the SPT from the M1 phase at low temperature to a rutile phase at a high temperature. Moreover, the SPT occurs at the same critical temperature as that of the MIT. This work may shed light on a new MIT behavior and may potentially pave the way for preparing high-quality VO2 thin films on cost-effective YSZ substrates for photoelectronic applications. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The non-volatile resistance states induced by converse piezoelectric effect are observed in ferromagnetic/ferroelectric epitaxial heterostructures of (011)-La2/3Sr1/3MnO3/0.7Pb(Mg2/3Nb1/3)O3-0.3PbTiO3 (LSMO/PMN0.7PT0.3). Three stable remnant strain states and the corresponding resistance states are achieved by properly reversing the electric field from the depolarized direction in ferroelectric PMN0.7PT0.3 substrate. The non-volatile resistance states of the LSMO film can be manipulated by applied electric-field pulse sequence as a result of the large coupling between the electronic states of LSMO film and the strain transferred from the ferroelectric substrate. The electrically tunable, non-volatile resistance states observed exhibit potential for applications in low-power-consumption electronic devices. [ABSTRACT FROM AUTHOR]

Tunable magnetic anisotropy can greatly facilitate the integration of perovskite oxides in emerging magnetic memory devices. Tailoring the metastable structures is a powerful and effective method to tune the magnetic anisotropy of perovskite oxides possessing a large spin–lattice correlation. Herein, we stabilized SrRuO 3 (SRO) at two metastable phases by tailoring the interfacial octahedral coupling between SRO and LaCoO 3 (LCO). Two metastable phases, distorted-rhombohedral and tetragonal structures, were stabilized in turn as the SRO thickness increased. Through the stabilization of these metastable phases, we realized a strongly lattice-dependent magnetic anisotropy and magnetic structure symmetry of SRO, employing its large spin–orbit coupling. Detailed octahedral rotation analysis demonstrated that when the SRO film was thin, the rotation phase mismatch between SRO and LCO played a decisive role in stabilizing the distorted-rhombohedral phase, while the competition between the rotation phase mismatch and tilt propagation relaxation induced the tetragonal phase when the SRO film was thick. These results demonstrate the feasibility of tailoring various structural symmetries and exotic functionalities through interface coupling. [ABSTRACT FROM AUTHOR]