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Register a new userNanoscale switching characteristics of nearly tetragonal BiFeO3 thin films
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We have investigated the nanoscale switching properties of strain-engineered BiFeO3 thin films deposited on LaAlO3 substrates using a combination of scanning probe techniques. Polarized Raman spectral analysis indicate that the nearly-tetragonal films have monoclinic (Cc) rather than P4mm tetragonal symmetry. Through local switching-spectroscopy measurements and piezoresponse force microscopy we provide clear evidence of ferroelectric switching of the tetragonal phase but the polarization direction, and therefore its switching, deviates strongly from the expected (001) tetragonal axis. We also demonstrate a large and reversible, electrically-driven structural phase transition from the tetragonal to the rhombohedral polymorph in this material which is promising for a plethora of applications.
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We present evidence for a concomitant structural and ferroelectric transformation around $T_Ssim 360$ K in multiferroic BiFeO$_3$/LaAlO$_3$ thin films close to the tetragonal phase. Phonon excitations are investigated by using Raman scattering as a function of temperature. The low-energy phonon modes at 180-260 cm$^{-1}$ related to the FeO$_6$ octahedron tilting show anomalous behaviors upon cooling through $T_S$; (i) a large hardening amounting to 15 cm$^{-1}$, (ii) an increase of intensity by one order of magnitude, and (iii) an appearance of a dozen new modes. In contrast, the high-frequency modes exhibit only weak anomalies. This suggests an intimate coupling of octahedron tilting to ferroelectricity leading to a simultaneous change of structural and ferroelectric properties.
We demonstrate a direct correlation between the domain structure of multiferroic BiFeO3 thin films and exchange bias of Co0.9Fe0.1/BiFeO3 heterostructures. Two distinct types of interactions, an enhancement of the coercive field (exchange enhancement) and an enhancement of the coercive field combined with large shifts of the hysteresis loop (exchange bias), have been observed in these heterostructures, which depend directly on the type and crystallography of the nanoscale (2 nm) domain walls in the BiFeO3 film. We show that the magnitude of the exchange bias interaction scales with the length of 109 degree ferroelectric domain walls in the BiFeO3 thin films which have been probed via piezoresponse force microscopy and x-ray magnetic circular dichroism.
BiFeO3 thin films have been deposited on Pt/sapphire and Pt/Ti/SiO2/Si substrates with pulsed laser deposition using the same growth conditions, respectively. Au was sputtered as the top electrode. The microscopic structure of the thin film varies by changing the underlying substrate. Thin films on Pt/sapphire are not resistively switchable due to the formation of Schottky contacts at both the top and the bottom interface. However, thin films on Pt/Ti/SiO2/Si exhibit an obvious resistive switching behavior under forward bias. The conduction mechanisms in BiFeO3 thin films on Pt/sapphire and Pt/Ti/SiO2/Si substrates are discussed to understand the different resistive switching behaviors.
BiFeO3 thin films with various thicknesses were grown epitaxially on (001) LaSrAlO4 single crystal substrates using pulsed laser deposition. High resolution x-ray diffraction measurements revealed that a tetragonal-like phase with c-lattice constant ~4.65 {AA} is stabilized by a large misfit strain. Besides, a rhombohedral-like phase with c-lattice constant ~3.99 {AA} was also detected at film thickness of ~50 nm and above to relieve large misfit strains. In-plane piezoelectric force microscopy studies showed clear signals and self-assembled nanoscale stripe domain structure for the tetragonal-like regions. These findings suggest a complex picture of nanoscale domain patterns in BiFeO3 thin films subjected to large compressive strains.
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.
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