Christianne Beekman

Phase Transitions, Phase Coexistence, and Piezoelectric Switching Behavior in Highly Strained BiFeO 3 Films

Highly strained BiFeO3 films transition into a true tetragonal state at 430 °C but remain polar to much higher temperatures (∼800 °C). Piezoelectric switching is only possible up to 300 °C, i.e., at temperatures for which strain stabilizes the stripe-like coexistence of multiple polymorphs.

Fabrication and characterization of topological insulator Bi2Se3 nanocrystals

In the recently discovered class of materials known as topological insulators, the presence of strong spin-orbit coupling causes certain topological invariants in the bulk to differ from their values in vacuum. The sudden change in invariants at the interface results in metallic, time reversal invariant surface states whose properties are useful for applications in spintronics and quantum computation. However, a key challenge is to fabricate these materials on the nanoscale appropriate for devices and probing the surface. To this end we have produced 2 nm thick nanocrystals of the topological insulator Bi2Se3 via mechanical exfoliation. For crystals thinner than 10 nm we observe the emergence of an additional mode in the Raman spectrum. The emergent mode intensity together with the other results presented here provide a recipe for production and thickness characterization of Bi2Se3 nanocrystals.

Raman study of the phonon symmetries in BiFeO3single crystals

In bismuth ferrite (BiFeO3), antiferromagnetic and ferroelectric order coexist at room temperature, making it of particular interest for studying magnetoelectric coupling. The mutual control of magnetic and electric properties is very useful for a wide variety of applications. This has led to an enormous amount of research into the properties of BiFeO

Big Data Analytics For Scanning Transmission Electron Microscopy Ptychography

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Unit cell orientation of tetragonal-like BiFeO3 thin films grown on highly miscut LaAlO3 substrates

. Nonetheless, one of the most fundamental aspects of this material, namely the symmetries of the lattice vibrations, remains controversial.We present a comprehensive Raman study of BiFeO

Transport properties of microstructured ultrathin films of La0.67Ca0.33MnO3 on SrTiO3

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Understanding strain-induced phase transformations in BiFeO3 thin films

single crystals with the approach of monitoring the Raman spectra while rotating the polarization direction of the excitation laser. Our method results in unambiguous assignment of the phonon symmetries and explains the origin of the controversy in the literature. Furthermore, it provides access to the Raman tensor elements enabling direct comparison with theoretical calculations. Hence, this allows the study of symmetry breaking and coupling mechanisms in a wide range of complex materials and may lead to a noninvasive, all-optical method to determine the orientation and magnitude of the ferroelectric polarization.

Strain driven anisotropic magnetoresistance in antiferromagnetic La 0.4Sr0.6MnO3

Electron microscopy is undergoing a transition; from the model of producing only a few micrographs, through the current state where many images and spectra can be digitally recorded, to a new mode where very large volumes of data (movies, ptychographic and multi-dimensional series) can be rapidly obtained. Here, we discuss the application of so-called “big-data” methods to high dimensional microscopy data, using unsupervised multivariate statistical techniques, in order to explore salient image features in a specific example of BiFeO3 domains. Remarkably, k-means clustering reveals domain differentiation despite the fact that the algorithm is purely statistical in nature and does not require any prior information regarding the material, any coexisting phases, or any differentiating structures. While this is a somewhat trivial case, this example signifies the extraction of useful physical and structural information without any prior bias regarding the sample or the instrumental modality. Further interpretation of these types of results may still require human intervention. However, the open nature of this algorithm and its wide availability, enable broad collaborations and exploratory work necessary to enable efficient data analysis in electron microscopy.

Focused-ion-beam induced damage in thin films of complex oxide BiFeO3

Synchrotron and lab-scale x-ray diffraction shows that tetragonal-like T′-BiFeO3 films on miscut LaAlO3 substrates (α < 5°) exhibit (00l)-planes tilted away from those of the substrate as predicted by the “Nagai model” (except for miscut < 0.2°). Tilts as large as 1° are achieved even in 100 nm thick films, strikingly larger than those observed in other perovskites. We attribute this to the large c/a ratio and the high crystalline coherency of the T′-BiFeO3/LaAlO3 interface. This coherency is possible through an observed “diagonal-on-diagonal” film/substrate alignment. Interestingly, the substrate miscut does not influence the relative population of monoclinic domains.

Large electric-field effects on the resistance of La0.67Ca0.33MnO3 microstructures

The authors have investigated the electrical transport properties of 8nm thick La0.67Ca0.33MnO3 (LCMO) films, sputter deposited on SrTiO3 (STO), and etched into 5μm wide bridges by Ar-ion etching. The authors find that even slight overetching of the film leads to conductance of the STO substrate, and asymmetric and nonlinear current-voltage (I-V) characteristics. However, a brief oxygen plasma etch allows full recovery of the insulating character of the substrate. The I-V characteristics of the bridges are then fully linear over a large range of current densities. The authors find colossal magnetoresistance properties typical for strained LCMO on STO but no signature of nonlinear effects (the so-called electroresistance) connected to electronic inhomogeneites. In the metallic state below 150K, the highest current densities lead to heating effects and nonlinear I-V characteristics.