Oxide electronics

Abstract:

A sintering process of Bi-Ca-Sr-Cu-O superconductor is reported. The electrical and microstructural analysis of the pellets shows the coexistence of B and C phases with transition temperatures of about 85 K and 105 K, respectively. © 1989 Società Italiana di Fisica.

Abstract:

In this work a procedure followed for the preparation of superconducting compounds in the Bi-Ca-Sr-Cu-O system using the cytrate polymer precursor route is reported. A preliminary characterization of the structure and superconducting properties of the samples so obtained is given through X-ray diffraction analysis, resistivity and a.c. susceptibility measurements. All the samples so obtained showed the presence of superconducting phases at 80 K and 110 K beyond other non superconducting ones.

Abstract:

We report resonant x-ray scattering measurements on a single crystal of the orbitally degenerate triangular metallic antiferromagnet 2H-AgNiO2 to probe the spontaneous transition to a triple-cell superstructure at temperatures below T-S = 365 K. We observe a strong resonant enhancement of the supercell reflections through the Ni K edge. The empirically extracted K-edge shift between the crystallographically distinct Ni sites of 2.5(3) eV is much larger than the value expected from the shift in final states, and implies a core-level shift of similar to 1 eV, thus providing direct evidence for the onset of spontaneous honeycomb charge order in the triangular Ni layers. We also provide band-structure calculations that explain quantitatively the observed edge shifts in terms of changes in the Ni electronic energy levels due to charge order and hybridization with the surrounding oxygens.

Abstract:

We present the discovery and refinement by neutron powder diffraction of a new magnetic phase in the Na1-xCaxMn7O12 quadruple perovskite phase diagram, which is the incommensurate analogue of the well-known pseudo-CE phase of the simple perovskite manganites. We demonstrate that incommensurate magnetic order arises in quadruple perovskites due to the exchange interactions between A and B sites. Furthermore, by constructing a simple mean field Heisenberg exchange model that generically describes both simple and quadruple perovskite systems, we show that this new magnetic phase unifies a picture of the interplay between charge, magnetic and orbital ordering across a wide range of compounds.

Abstract:

Whirling magnetic textures can have topological properties, enhancing their stability over and above that derived from energetic considerations. Such structures have been proposed as data carriers in next-generation post-Moore computing. Whilst abundantly observed in ferromagnets, their antiferromagnetic counterparts are more elusive. Interest in antiferromagnetic topological textures for device applications is growing, due to their predicted ultra-fast, deflection-free dynamics whilst being robust against external fields. In this thesis, I develop processes for imaging, nucleating and controlling topological textures in antiferromagnets, targeted towards their integration in next-generation racetrack-based oxide electronics. The prototypical canted antiferromagnet α-Fe2O3 is used throughout as an interesting test case, due to the family of topological textures present at room temperature that can be repeatedly nucleated via a Kibble-Zurek-like quench.

I developed analytical and micromagnetic models for topological textures in A-type antiferromagnets, focusing on the scaling of textures with relevant material parameters, allowing us to push towards the ultra-small sizes relevant for device applications. This was also used to predict the existence of the long sought-after topological antiferromagnetic skyrmions. I investigated freestanding crystalline α-Fe2O3 nanomembranes, a novel form of matter developed by my collaborators. One key conclusion of these experiments was that defects strongly affect the first-order Morin transition, whilst maintaining the Kibble-Zurek phenomenology observed in thin films attached to substrates. Magnetic fields cause domain repopulation in this canted AFM, but topological textures were observed to be stable in the presence of moderate field perturbations. Finally, freestanding crystal membranes can host relatively large strains compared to attached thin films or bulk crystals, which have similar lateral dimensions but the latter are drastically thicker. This was used to produce an athermal route to nucleate topological textures and tune domain populations, opening novel pathways for exploring Kibble-Zurek phenomenology in crystal membranes, as well as providing an interesting route towards device applications.