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Register a new userSuppression of magnetism in BiFeO$_3$ ultrathin epitaxial multilayers
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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.
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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.
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.
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.
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 $theta = pi /2$.
We report the observation of weak magnetism in superlattices of LaAlO3/SrTiO3 using beta-detected nuclear magnetic resonance. The spin lattice relaxation rate of 8 Li in superlattices with a spacer layers of 8 and 6 unit cells of LaAlO3 exhibits a strong peak near ~35 K, whereas no such peak is observed in a superlattice with spacer layer thickness of 3 unit cells. We attribute the observed temperature dependence to slowing down of weakly coupled electronic moments at the LaAlO3/SrTiO3 interface. These results show that the magnetism at the interface depends strongly on the thickness of the spacer layer, and that a minimal thickness of ~4-6 unit cells is required for the appearance of magnetism. A simple model is used to determine that the observed relaxation is due to small fluctuating moments (~0.002 muB) in the two samples with a larger LaAlO3 spacer thickness.
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