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Register a new userControlling ferroelectric hysteresis offsets in PbTiO$_{3}$ based superlattices
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Ferroelectric materials are characterized by degenerate ground states with multiple polarization directions. In a ferroelectric capacitor this should manifest as equally favourable up and down polarization states. However, this ideal behavior is rarely observed in ferroelectric thin films and superlattice devices, which generally exhibit a built-in bias which favors one polarization state over the other. Often this polarization asymmetry can be attributed to the electrodes. In this study we examine bias in PbTiO$_3$-based ferroelectric superlattices that is not due to the electrodes, but rather to the nature of the defects that form at the interfaces during growth. Using a combination of experiments and first-principles simulations, we are able to explain the sign of the observed built-in bias and its evolution with composition. Our insights allow us to design devices with zero built-in bias by controlling the composition and periodicity of the superlattices.
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The effect of octahedral tilting on the acentric structural transitions in AGaO$_{3}$/AGaO$_{3}$ [001], [110], and [111] superlattices (A, A = La, Pr, Nd) is studied using density functional theory. We find the displacive transitions are driven by two octahedral rotations modes (a$^{-}$a$^{-}$c$^{0}$ and a$^{0}$a$^{0}$c$^{+}$ tilting), with amplitudes that depend on the A and A chemistry and cation ordering direction. We find the ground states structures of the [001] and [111] ordered superlattices are polar. The coupling of octahedral tilting modes through a hybrid improper ferroelectric mechanism induces the polar displacements and produces the macroscopic electric polarizations.
We present measurements of conductance hysteresis on CH3NH3PbI3 perovskite thin films, performed using the double-wave method, in order to investigate the possibility of a ferroelectric response. A strong frequency dependence of the hysteresis is observed in the range of 0.1 Hz to 150 Hz, with a hysteretic charge density in excess of 1000 {mu}C/cm2 at frequencies below 0.4 Hz - a behaviour uncharacteristic of a ferroelectric response. We show that the observed hysteretic conductance, as well as the presence of a double arc in the impedance spectroscopy, can be fully explained by the migration of mobile ions under bias on a timescale of seconds. Our measurements place an upper limit of approximately 1 {mu}C/cm2 on any intrinsic frequency-independent polarisation, ruling out ferroelectricity as the main cause of current-voltage hysteresis and providing further evidence of the importance of ionic migration in modifying the efficiency of CH3NH3PbI3 devices.
Charge dipole moment and spin moment rarely coexist in single-phase bulk materials except in some multiferroics. Despite the progress in the past decade, for most multiferroics their magnetoelectric performance remains poor due to the intrinsic exclusion between charge dipole and spin moment. As an alternative approach, the oxide heterostructures may evade the intrinsic limits in bulk materials and provide more attractive potential to realize the magnetoelectric functions. Here we perform a first-principles study on LaAlO$_3$/PbTiO$_3$ superlattices. Although neither of the components is magnetic, magnetic moments emerge at the ferroelectric domain walls of PbTiO$_3$ in these superlattices. Such a twist between ferroelectric domain and local magnetic moment, not only manifests an interesting type of multiferroicity, but also is possible useful to pursuit the electrical-control of magnetism in nanoscale heterostructures.
In published papers, the Gibbs free energy of ferroelectric materials has usually been quantified by the retention of 6th or 8th order polarization terms. In this paper, a newly analytical model of Gibbs free energy, thereout, a new model of polarization-electric field hysteresis loops in ferroelectric materials has been derived mathematically. As a model validation, four patterns of polarization-electric field hysteresis loops of ferroelectric materials have been depicted by using the model. The calculated results indicated that the self-similar model can characterize the various patterns of hysteresis loops in ferroelectric materials through adjusting the external excitation or the synthetically parameter (e.g., electric, temperature, and stress, etc.) employed in the model.
We report on growth and ferroelectric (FE) properties of superlattices (SLs) composed of the FE BaTiO3 and the paraelectric (PE) CaTiO3. Previous theories have predicted that the polarization in (BaTiO3)n/(CaTiO3)n SLs increases as the sublayer thickness (n) increases when the same strain state is maintained. However, our BaTiO3/CaTiO3 SLs show a varying lattice-strain state and systematic reduction in polarization with increasing n while coherently-strained SLs with n=1, 2 show a FE polarization of ca. 8.5 uC/cm^2. We suggest that the strain coupling plays more important role in FE properties than the electrostatic interlayer coupling based on constant dielectric permittivities.
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