Tunable magnetic anisotropy of CoFe2O4 nanopillar arrays released from BiFeO3 matrix

We deposited epitaxial BiFeO₃–CoFe₂O₄ (BFO–CFO) self‐assembled thin films on (001) SrTiO₃ (STO) substrates. We find that a combined annealing and etching process could remove the BFO matrix, thereby resulting in free‐standing CFO nanopillar arrays. Scanning electron and atomic force microscopies showed well separated CFO nanopillars, which were very similar to the original CFO ones in the self‐assembled structure. Finally, comparison of the magnetic hysteresis loops before and after removal of the BFO matrix showed a significant decrease of the coercive field and a dramatic decrease in the strain dominated magnetic anisotropy. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Convenient one-step synthesis of 5-carboxy-seminaphthofluoresceins

The one-step synthesis and characterization of a series of regioisomerically pure 5-carboxy-seminaphthofluoresceins (5-carboxy-SNAFLs) is reported. The optical properties were determined in aqueous buffer at around biological pH, and highly pH sensitive, large Stokes-shift fluorophores with emission in the deep-red to near-infrared region were identified.     

Spin degeneracy and conductance fluctuations in open quantum dots

The dependence of conductance fluctuations on parallel magnetic field is used as a probe of spin degeneracy in open GaAs quantum dots. The variance of fluctuations at high parallel field is reduced from the low-field variance (with broken time-reversal symmetry) by factors ranging from roughly 2 in a 1 microm (2) dot to greater than 4 in 8 microm (2) dots. The factor of 2 is expected for Zeeman splitting of spin-degenerate channels. A possible explanation for the larger suppression based on field-dependent spin-orbit scattering is proposed.     

Branch structure of -holomorphic curves near periodic orbits of a contact manifold

Let be a three-dimensional contact manifold, and a finite-energy pseudoholomorphic map from the punctured disc in that is asymptotic to a periodic orbit of the contact form. This article examines conditions under which smooth coordinates may be defined in a tubular neighbourhood of the orbit such that resembles a holomorphic curve, invoking comparison with the theory of topological linking of plane complex algebroid curves near a singular point. Examples of this behaviour, which are studied in some detail, include pseudoholomorphic maps into , where denotes a rational ellipsoid (contact structure induced by the standard complex structure on ), as well as contact structures arising from non-standard circle-fibrations of the three-sphere.

     

How Dihalogens Catalyze Michael Addition Reactions

We have quantum chemically analyzed the catalytic effect of dihalogen molecules (X₂=F₂, Cl₂, Br₂, and I₂) on the aza‐Michael addition of pyrrolidine and methyl acrylate using relativistic density functional theory and coupled‐cluster theory. Our state‐of‐the‐art computations reveal that activation barriers systematically decrease as one goes to heavier dihalogens, from 9.4 kcal mol^?1 for F₂ to 5.7 kcal mol^?1 for I₂. Activation strain and bonding analyses identify an unexpected physical factor that controls the computed reactivity trends, namely, Pauli repulsion between the nucleophile and Michael acceptor. Thus, dihalogens do not accelerate Michael additions by the commonly accepted mechanism of an enhanced donor–acceptor [HOMO(nucleophile)–LUMO(Michael acceptor)] interaction, but instead through a diminished Pauli repulsion between the lone‐pair of the nucleophile and the Michael acceptor's π‐electron system.     

The temperature‐dependent ballistic performance and the ductile‐to‐brittle transition in polymer networks

The temperature dependence of the ballistic impact performance of a series of transparent polymer networks is evaluated. A systematic series of homogeneous epoxy/propylene‐oxide‐based thermosets, a nanoscale phase‐separated epoxy/dual amine thermoset, and two homogeneous, completely aliphatic materials synthesized via ring‐opening metathesis polymerization are examined. The Vogel temperature (T_o) and the Kauzmann temperature (T_K) are critical parameters for scaling the temperature‐dependent ballistic impact performance of each class of materials. The ductile‐to‐brittle transition temperature in a series of propylene‐oxide amine‐cured epoxies occurs at the material T_K, corresponding to a sharp drop in fracture toughness and ballistic impact performance. Two aliphatic, ring‐opening metathesis polymerized materials are found to exhibit no clear transition from purely ductile to purely brittle behavior, but the temperature dependence is still scaled to a single curve when normalized by T_o. The cooperatively rearranging region (CRR) or the volume of this region is related to the breadth of temperatures over which these materials exhibit purely ductile deformation both quasi‐statically and at higher rates. The temperature‐dependent performance is discussed in the context of the configurational entropy. The breadth of the ductility window is related to the size of the CRR, calculated from calorimetric measurements at the resin T_g. Published 2019. This article is a U.S. Government work and is in the public domainin the USA. J. Polym. Sci., Part B: Polym. Phys. 2019 , 57, 511–523