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Register a new userDomain tuning in mixed-phase BiFeO3 thin films using vicinal substrates
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The structural and ferroelectric domain variants of highly-strained BiFeO3 films grown on vicinal LaSrAlO4 substrates were studied by piezoelectric force microscopy and high-resolution X-ray reciprocal space mapping. Through symmetry breaking of the substrate surface, ferroelastic domain variants in the highly-strained MC phase BiFeO3 can be greatly reduced. Single-domain film can be obtained on substrates with large miscut angle, which is accompanied by the reduction of structural variants in the mixed-phase nanodomains. These findings lead to better understanding of the phase evolution and polarization rotation process in the strain-driven morphotropic phase system.
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The novel strain-driven morphotropic phase boundary (MPB) in highly-strained BiFeO3 thin film is featured by ordered mixed phase nanodomains (MPNs). Through scanning probe microscopy and synchrotron X-ray diffraction, eight structural variants of the MPNs are identified. Detailed polarization configurations within the MPNs are resolved using angular-dependent piezoelectric force microscopy. Guided by the obtained results, deterministic manipulation of the MPNs has been demonstrated by controlling the motion of the local probe. These findings are important for in-depth understanding of the ultrahigh electromechanical response arising from phase transformation between competing phases, enabling future explorations on the electronic structure, magnetoelectricity and other functionalities in this new MPB system.
Local conduction at domains and domains walls is investigated in BiFeO3 thin films containing mostly 71o domain walls. Measurements at room temperature reveal conduction through 71o domain walls. Conduction through domains could also be observed at high enough temperatures. It is found that, despite the lower conductivity of the domains, both are governed by the same mechanisms: in the low voltage regime electrons trapped at defect states are temperature-activated but the current is limited by the ferroelectric surface charges; in the large voltage regime, Schottky emission takes place and the role of oxygen vacancies is that of selectively increasing the Fermi energy at the walls and locally reducing the Schottky barrier. This understanding provides the key to engineering conduction paths in oxides.
Recent works have shown that the domain walls of room-temperature multiferroic BiFeO3 (BFO) thin films can display distinct and promising functionalities. It is thus important to understand the mechanisms underlying domain formation in these films. High-resolution x-ray diffraction and piezo-force microscopy, combined with first-principles simulations, have allowed us to characterize both the atomic and domain structure of BFO films grown under compressive strain on (001)-SrTiO3, as a function of thickness. We derive a twining model that describes the experimental observations and explains why the 71o domain walls are the ones commonly observed in these films. This understanding provides us with a new degree of freedom to control the structure and, thus, the properties of BiFeO3 thin films.
We report a Raman scattering investigation of multiferroic bismuth ferrite BiFeO3 epitaxial (c-axis oriented) thin films from -192 to 1000C. Phonon anomalies have been observed in three temperature regions: in the gamma-phase from 930C to 950C; at ~370C, Neel temperature (TN), and at ~123C, due to a phase transition of unknown type (magnetic or structural). An attempt has been made to understand the origin of the weak phonon-magnon coupling and the dynamics of the phase sequence. The disappearance of several Raman modes at ~820C (Tc) is compatible with the known structural phase transition and the Pbnm orthoferrite space group assigned by Arnold {it et al.} cite{arnold:09}. The spectra also revealed a {it non-cubic} $beta$-phase from 820-930dc and the same {it non-cubic} phase extends through the $gamma$-phase between 930-950dc, in agreement with Arnold {it et al.} cite{arnold2:09}, and an evidence of a cubic $delta$-phase around 1000dc in thin films that is not stable in powder and bulk. Such a cubic phase has been theoretically predicted in cite{vasquez:prb09}. Micro-Raman scattering and X-ray diffraction showed no structural decomposition in thin films during the thermal cycling from 22-1000dc.
In this work we report on the controlled fabrication of a self-assembled line network in highly epitaxial BiFeO3 thin films on top of LaAlO3 in the kinetically limited grown region by RF sputtering. As previously shown in the case of manganite thin films, the remarkable degree of ordering is achieved using vicinal substrates with well-defined step-terrace morphology. Nanostructured BiFeO3 thin films show mixed-phase morphology. Besides typical formation following (100) and (010) axes, some mixed phase nanodomains are detected also in-between the regular line network. These particular microstructures open a playground for future applications in multiferroic nanomaterials.
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