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Register a new userEmergence of ferroelectricity at the morphotropic phase boundary of ultrathin BiFeO$_3$
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We demonstrate the robustness of polarization in ultrathin compressive strained BiFeO$_3$ single layers and heterostructures during epitaxial thin-film growth. Using in-situ optical second harmonic generation (ISHG), we explore the emergence of ferroelectric phases at the strain-driven morphotropic phase boundary in the ultrathin regime. We find that the epitaxial films grow in the ferroelectric tetragonal (T-) phase without exhibition of a critical thickness. The robustness of this high-temperature T-phase against depolarizing-field effects is further demonstrated during the growth of capacitor-like (metal|ferroelectric|metal) heterostructures. Using temperature-dependent ISHG post-deposition, we identify the thickness-dependent onset of the monoclinic distortion in the T-matrix and trace the signature of the subsequent emergence of the strain-relaxed rhombohedral-like monoclinic phase. Our results show that strain-driven T-phase stabilization in BiFeO$_3$ yields a prominent candidate material for realizing ultrathin ferroelectric devices.
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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.
Morphotropic phase boundaries (MPBs) show substantial piezoelectric and dielectric responses, which have practical applications. The predicted existence of MPB in HfO2-ZrO2 solid solution thin film has provided a new way to increase the dielectric properties of a silicon-compatible device. Here, we present a new fabrication design by which the density of MPB and consequently the dielectric constant of HfO2-ZrO2 thin film was considerably increased. The density of MPB was controlled by fabrication of a 10-nm [1 nm-Hf0.5Zr0.5O2 (Ferroelectric)/1 nm-ZrO2 (Antiferroelectric)] nanolaminate followed by an appropriate annealing process. The coexistence of orthorhombic and tetragonal structures, which are the origins of ferroelectric (FE) and antiferroelectric (AFE) behaviors, respectively, was structurally confirmed, and a double hysteresis loop that originates from AFE ordering, with some remnant polarization that originates from FE ordering, was observed in P-E curve. A remarkable increase in dielectric constant compared to the conventional HfO2-ZrO2 thin film was achieved by controlling the FE-AFE ratio. The fabrication process was performed at low temperature and the device is compatible with silicon technology, so the new design yields a device that has possible applications in near-future electronics.
In this work, we address the issue of peaking of piezoelectric response at a particular composition in the morphotropic phase boundary (MPB) region of (Pb0.940Sr0.06)(ZrxTi1-x)O3 (PSZT) piezoelectric ceramics. We present results of synchrotron x-ray powder diffraction, dielectric, piezoelectric and sound velocity studies to critically examine the applicability of various models for the peaking of physical properties. It is shown that the models based on the concepts of phase coexistence, polarization rotation due to monoclinic structure, tricritical point and temperature dependent softening of elastic modulus may enhance the piezoelectric response in the MPB region in general but cannot explain its peaking at a specific composition. Our results reveal that the high value of piezoelectric response for the MPB compositions in PSZT at x=0.530 is due to the softening of the elastic modulus as a function of composition. The softening of elastic modulus facilitates the generation of large piezoelectric strain and polarization on approaching the MPB composition of x=0.530. This new finding based on the softening of elastic modulus may pave the way forward for discovering/designing new lead-free environmentally friendly piezoelectric materials and revolutionize the field of piezoelectric ceramics.
Two-dimensional polarity is intriguing but remains in the early stage. Here a structural evolution diagram is established for GeS monolayer, which leads a noncollinear ferrielectric $delta$-phase energetically as stable as the ferroelectric $alpha$-phase. Its ferrielectricity is induced by the phonon frustration, i.e., the competition between ferroelectric and antiferroelectric modes, providing more routes to tune its polarity. Besides its prominent properties like large band gap, large polarization, and high Curie temperature, more interestingly, the morphotropic phase boundary between $alpha$- and $delta$-phases is highly possible, which is crucial to obtain giant piezoelectricity for lead-free applications.
Many years and great effort have been spent constructing the microscopic model for the room temperature multiferroic BiFeO3 However, earlier models implicitly assumed that the cycloidal wavevector q was confined to one of the three-fold symmetric axis in the hexagonal plane normal to the electric polarization. Because recent measurements indicate that q can be rotated by a magnetic field, it is essential to properly treat the anisotropy that confines q at low fields. We show that the anisotropy energy $-K_3 S^6 sin^6 theta cos 6 phi $ confines the wavevectors q to the three-fold axis $phi =0$ and $+-2 pi/3$ within the hexagonal plane with $theta = pi /2$.
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