Abstracts Category : Other

Epitaxial engineering of ferrimagnetic double perovskites

by Vikas Shabadi

Double perovskite (DP) oxides of the type A2BBO6 (A: 12-coordinated large di/tri-valent cation; B/B: octahedrally coordinated transition metals) offer a unique material framework to engineer a wide range of physical functionalities. In its simplest form, the DP structure involves a cubic array of A-cations which is interspersed by corner-sharing BO6 and BO6 octahedra, often arranged in a rock-salt type order. The choice of the B/B cations and their coupling within the ordered DP structure are known to largely determine the electronic structure and the resulting functionality of the compounds. Compounds such as the 3d5-4d1 coupled Sr2FeMoO6 (which exhibits fully spin polarized charge carriers and large magnetoresistance at room-temperature) and the 3d3-5d3 coupled Sr2CrOsO6 (which shows a high ferrimagnetic ordering temperature and a positive temperature coefficient of coercivity) stand as prominent examples for the diversity of physical functionalities achievable in these compounds. Yet, it is interesting to note that a vast majority of all possible DP compounds remain experimentally unexplored, mainly due to the meta-stable nature of some compounds and/or due to challenging synthesis procedures. With recent advances in thin film technology, particularly with techniques such as pulsed laser deposition (PLD), the ability to stabilize complex multi-cation oxides by epitaxial strain under non-equilibrium growth conditions has been well established. Furthermore, the PLD process has also been noted to support spontaneous cation ordering driven by a contrast in size/charge of cations. These developments provide an effective alternative route to overcome the synthesis challenges associated with meta-stable DP compounds.In this work, we use the PLD based thin film approach to explore ferrimagnetic insulating phases among DPs. Such phases when stabilized as thin films can have wide range of possible device applications in the areas of spin-electronics and modern computing. In addition, such compounds can also be viable templates for achieving single phase type I multiferroism, if the A-sites are subsequently substituted with a ferroelectric active cation such as Bi3+. The study was carried out across two families of double perovskites, namely 3d-3d and 3d-5d (the nomenclature refers to the elemental periods from which the B and B cations are chosen). Within the 3d-3d family, we chose to explore Bi2FeCrO6 (BFCO), a compound theoretically predicted to be a robust ferrimagnetic-ferroelectric. Epitaxial thin films of BFCO grown via PLD on single crystal SrTiO3 (STO) substrates were phase pure and fully strained. Distinct and intense superstructure peaks (SPs) were observed in XRD scans along the pseudo-cubic [111] direction. Considering the low scattering contrast between Fe and Cr, intensity of the SPs appeared suspiciously high. Using the photon energy dependence of contrast between atomic scattering factors of Fe and Cr, a spontaneous chemical ordering at the B-site was ruled out. Detailed structural calculationsAdvisors/Committee Members: Alff, Lambert (advisor), Donner, Wolfgang (advisor).