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Register a new userLow Symmetry Phase in (001) BiFeO$_3$ Epitaxial Constrained Thin Films
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The lattice of (001)-oriented BiFeO$_3$ epitaxial thin film has been identified by synchrotron x-ray diffraction. By choosing proper scattering zones containing the fixed (001) reflection, we have shown that low-symmetry phases similar to a $M_A$ phase exist in the thin film at room temperature. These results demonstrate a change in phase stability from rhombohedral in bulk single crystals, to a modified monoclinic structure in epitaxial thin films.
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Piezoelectrics interconvert mechanical energy and electric charge and are widely used in actuators and sensors. The best performing materials are ferroelectrics at a morphotropic phase boundary (MPB), where several phases can intimately coexist. Switching between these phases by electric field produces a large electromechanical response. In the ferroelectric BiFeO$_3$, strain can be used to create an MPB-like phase mixture and thus to generate large electric field dependent strains. However, this enhanced response occurs at localized, randomly positioned regions of the film, which potentially complicates nanodevice design. Here, we use epitaxial strain and orientation engineering in tandem - anisotropic epitaxy - to craft a hitherto unavailable low-symmetry phase of BiFeO$_3$ which acts as a structural bridge between the rhombohedral-like and tetragonal-like polymorphs. Interferometric displacement sensor measurements and first-principle calculations reveal that under external electric bias, this phase undergoes a transition to the tetragonal-like polymorph, generating a piezoelectric response enhanced by over 200%, and associated giant field-induced reversible strain. These results offer a new route to engineer giant electromechanical properties in thin films, with broader perspectives for other functional oxide systems.
Ferroelectric switching in BiFeO$_3$ multiferroic thin films with intrinsic ``stripe-like and ``bubble-like polydomain configurations was studied by piezoresponse force microscopy. Using the local electric field applied by a scanning probe microscope tip, we observe reversal of both out-of-plane and in-plane components of the polarization, with the final domain state depending on the tip sweeping direction. In ``bubble-like samples, complete control of the polarization is achieved, with in-plane polarization change mediated and stabilized by out-of-plane polarization reversal. In ``stripe-like samples the intrinsic domain structure influences polarization switching and in-plane reversal may occur without out-of-plane change. The observed switching behaviour can be well correlated with the radial and vertical components of the highly inhomogeneous electric field applied by the tip.
Using the model system of ferroelectric domain walls, we explore the effects of long-range dipolar interactions and periodic ordering on the behavior of pinned elastic interfaces. In piezoresponse force microscopy studies of the characteristic roughening of intrinsic 71{deg} stripe domains in BiFeO$_3$ thin films, we find unexpectedly high values of the roughness exponent {zeta} = 0.74 $pm$ 0.10, significantly different from those obtained for artificially written domain walls in this and other ferroelectric materials. The large value of the exponent suggests that a random field-dominated pinning, combined with stronger disorder and strain effects due to the step-bunching morphology of the samples, could be the dominant source of pinning in the system.
Magnetite epitaxial thin films have been prepared by pulsed laser deposition at 340 C on MgO and Si substrates. One key result is that the thin film properties are almost identical to the properties of bulk material. For 40 - 50 nm thick films, the saturation magnetization and conductivity are respectively 453 emu/cm^3 and 225 1/(Ohm cm) at room temperature. The Verwey transition is at 117 K. The Hall effect indicates an electron concentration corresponding to 0.22 electrons per formula unit at room temperature. Normal and anomalous Hall effect both have a negative sign.
The present manuscript completes the study presented in two recent research articles [K. Koumpouras and I. Galanakis, textit{J. Magn. Magn. Mater.} 323, 2328 (2011); textit{ibid}, textit{J. Spintron. Magn. Nanomater.} 1, in press]. Preliminary first-principles calculations using the QUANTUM-espresso package [P. Giannozzi et al textit{J. Phys.:Condens. Matter} 21, 395502 (2009)] on the magnetic behavior of ultra-thin epitaxial multilayers between the BiFeO$_3$ magnetoelectric compound and various types of spacers are presented. As spacer we have considered i) InP semiconductor, ii) Fe which is a ferromagnet, and iii) metallic V. In all cases under study the growth axis of the multilayer was the [001]. Our results indicate that the magnetic properties are seriously downgraded for the ultrathin BiFeO$_3$ multilayers independent of the nature of the spacer and in some cases under study magnetism even vanishes. More extensive calculations are needed to establish a more clear view of the physical properties of the interfaces involving the BiFeO$_3$ compound.
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