Phase Transitions in Strain-Tuned SrCoO3-δ (0<δ≤ 0.5) Thin Films

Abstract

SrCoO3-δ (SCO) has been drawing large attention due to its intriguing topotactic phase transitions and promising applications in cutting-edge technologies. Recent theoretical calculations predict that SCO in thin film form can acquire many interesting electronic and magnetic properties under epitaxial strain and by undergoing phase transitions between perovskite (P) and brownmillerite (BM) phases. Therefore, the material s phase transitions tuned by both oxygen content and epitaxial strain are of great interest for many potential applications. In this thesis, fully strained SCO thin films were grown on different substrates, i.e. LaAlO3, SrTiO3, DyScO3 and SmScO3 by pulsed laser deposition (PLD). The as-grown thin films were characterized by XRD and AFM. The results show that the out-of-plane lattice constants of SCO thin films decrease almost linearly as the in-plane strain increases; while in in-plane directions the films are fully strained by the substrates. From AFM measurements both BM and P SCO exhibit smooth terraces surface with roughness less than 0.3 nm. The crystallographic phase transitions of the SCO thin films between BM and P phase and the associated oxygen stability were then investigated in a large range of epitaxial strains (-1.2% ~ 3.9%) as a function of temperature, chemical oxidation with NaClO and as a function of time by XRD. It is found at room temperature the oxygen stability for P SCO on different substrates orders as DSO ≈ STO > SSO ≈ LAO, and the oxygen mobility in SCO sequences DSO ≈ SSO > STO ≈ LAO. At high temperature the oxygen stability in SCO on different substrates sequences STO, DSO, LAO and SSO. After that, the oxygen content of SCO on SrTiO3 was modulated locally by NaClO etching and electrical poling. The oxidised BM SCO is found to exhibit distinct morphological changes by forming small nanosized islands with boundaries preferentially in [100] or [010] directions. The conductivity, or oxygen content, of each single island is confined by these textures which can be locally patterned even further with electric poling. In addition, a strain-induced ferromagnetic-to-antiferromagnetic phase transition was observed in SrCoO3−δ films grown on DyScO3, which provide a large tensile epitaxial strain, as compared with ferromagnetic films under lower tensile strain on SrTiO3. Magnetometry results demonstrate the existence of antiferromagnetic spin correlations and neutron diffraction experiments provide a direct evidence for a G-type antiferromagnetic structure with Néel temperatures between TN ∼ 135 ± 10 K and ∼325 ± 10 K, depending on the oxygen content of the samples.